NASA/TM-2010-215856

W, ,F 1

Advances in Measuring the Apparent Optical Properties (AOPs)

of Optically Complex Waters

John H Morrow, Stanford B. Hooker Charles R. Booth, Germar Bernhard, Randall N. Lind,

and .lames W Brown

December 2010
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NASA/TM-2010-215856

Advances in Measuring the Apparent Optical Properties (AOPs)

of Optically Complex Waters

John H Morrow

Biospherical Instruments Inc., San Diego, California

Stanford B. Hooker

NASA Goddard Space Flight Center, Greenbelt, Maryland

Charles R. Booth, Germar Bernhard, and Randall N. Lind

Biospherical Instrrtments Inc., San Diego, California

James W. Brown

CSTARS University of Miami, Miami, Florida

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

December 2010
Available from:

NASA Center for AeroSpace Information National Technical Information Service

7115 Standard Drive 5285 Port Royal Road

Hanover, MD 21076-1320 Springfield, VA 22161
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Table of Contents

Prologue ................................................................................................. 1

1. The Biospherical Surface Ocean Reflectance System (BioSORS) ..................................... 8

1.1 Introduction ....................................................................................... 8

1.2 The P RR-800 Sensors ............................................................................. 11

1.3 The P RR-800 Enhancements ...................................................................... 11

1.4 BioSORS ......................................................................................... 12

1.5 Data Processing .................................................................................. 13

1.6 Radiometric Intercomparisons ..................................................................... 14

1.7 Summary ......................................................................................... 15

2. The Submersible Biospherical Optical Profiling System (SuBOPS) ................................. 17

2.1 Introduction ...................................................................................... 17

2.2 Design Requirements .............................................................................. 18

2.3 The BioPRO System .............................................................................. 18

2.4 Evaluation of BioPRO ............................................................................ 20

2.5 SuB OPS-A Kite-Shaped Profiler ................................................................. 21

2.6 SuB OPS Evaluation .............................................................................. 22

2.7 Design Refinements ............................................................................... 23

2.8 Summary ......................................................................................... 26

3. Development of the Microradiometer .............................................................. 27

3.1 Introduction ...................................................................................... 27

3.2 Miniaturization ................................................................................... 28

3.3 Data Acquisition Sequence ........................................................................ 28

3.4 Performance Analysis ............................................................................. 28

3.5 Multiple Microradiometer Systems ................................................................ 31

3.6 Improvements and Enhancements ................................................................. 39

3.7 Microradiometer Specifications .................................................................... 40

3.8 Summary ......................................................................................... 41

4. The Compact-Optical Profiling System (C-OPS) ................................................... 42

4.1 Introduction ...................................................................................... 42

4.2 C-OP S Description ................................................................................ 43

4.3 Design ............................................................................................ 43

4.4 Operation ......................................................................................... 45

4.5 C-OPS Evaluation ................................................................................ 46

4.6 Advancements and Enhancements ................................................................. 48

4.7 Summary ......................................................................................... 49

5. Biospherical Shadowband Accessory for Diffuse Irradiance (BioSHADE) ........................... 51

5.1 Introduction ...................................................................................... 51

5.2 Description ....................................................................................... 52

5.3 Design ............................................................................................ 52

5.4 GPS Operation ................................................................................... 53

5.5 Shadowband Operation ........................................................................... 54

5.6 Shadowband Modeling and Testing ................................................................ 54

5.7 Summary ......................................................................................... 59

iii
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Table of Contents (Cont.)

6. Scalable Hydro-optical Applications for Light-Limited Oceanography (SHALLO) ................... 60

6.1 Introduction ...................................................................................... 60

6.2 Classes ........................................................................................... 61

6.3 Design ............................................................................................ 61

6.4 Operation ......................................................................................... 62

6.5 Testing and Advancements ........................................................................ 62

6.6 Future Designs .................................................................................... 64

6.7 Summary ......................................................................................... 65

7. The Telescoping Mount for Advanced Solar Technologies (T-MAST) ............................... 66

7.1 Introduction ...................................................................................... 66

7.2 Description ....................................................................................... 68

7.3 Design ............................................................................................ 69

7.4 Modifications and Operation ...................................................................... 69

7.5 Summary ......................................................................................... 71

8. The Cable Hauler for Optical In Situ Technologies (C-HOIST) ..................................... 72

8.1 Introduction ...................................................................................... 72

8.2 Description ....................................................................................... 73

8.3 Design ............................................................................................ 73

8.4 Operation ......................................................................................... 74

8.5 Testing ........................................................................................... 74

8.6 Summary ......................................................................................... 74

ACKNOWLEDGMENTS ................................................................................... 75

r

SYMBOLS ............................................................................................... 76

REFERENCES ........................................................................................... 77

1V
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

ABSTRACT

This report documents the scientific advances associated with new instruments built to measure the apparent

optical properties (AOPs) of optically complex waters. The principal objective is to be prepared for the launch

of the next-generation ocean color satellites with the most capable commercial off-the-shelf (COTS) instrumentation in the shortest time possible. The first step in the development of the new technology was to enhance

a COTS radiometer as the starting point for designing the new sensors and testing the modified sensor(s) in

the field. The follow-on steps were to apply the lessons learned towards a new in-water profiler which used

a new deployment system or backplane for mounting the light sensors (Chap. 2). Following an incremental

approach to manage risk, the next level of sophistication involved evaluating new radiometers that were emerging from a separate development activity based on so-called microradiometers (Chap. 3). The exploitation of

microradiometers for in-water sensors resulted in a state-of-the-art profiling system (Chap. 4), which includes a

sensor networking capability to control ancillary sensors like a shadowband or global positioning system (GPS)

device (Chap. 5). In terms of next-generation problem sets, one of the principal advantages of microradiometers

is their flexibility in producing, interconnecting, and maintaining state-of-the-art instruments for light limited

environments (Chap. 6), i.e., shallow and optically complex waters. The full problem set for collecting sea-truth

data—whether in coastal waters or the open ocean—involves other aspects of data collection that need to be improved if the uncertainty budget is to be minimized. New capabilities associated with deploying solar references

were developed (Chap. 7). Ancillary measurements, particularly observations of the inherent optical properties

(IOPs) of seawater are an important part of AOP measurements, but are frequently difficult to make in small

boats, because of the current size of IOP instruments. A compact solution for recovering in-water instrument

systems from small boats was also developed (Chap. 8).

Prologue

A number of international ocean color sensors have

been designed and launched in the last decade and a half to

support oceanographic studies and applications including

the following: the Ocean Color and Temperature Scanner (OCTS), the Polarization and Directionality of the

Earth's Reflectance (POLDER) sensor, the Sea-viewing

Wide Field-of-view Sensor (SeaWiFS), two Moderate Resolution Imaging Spectroradiometer (MODIS) instruments

launched on the Earth Observing System (EOS) Terra and

Aqua satellites (denoted MODIS-T and MODIS-A, respectively), and the Medium Resolution Imaging Spectrometer (MERIS). All of these sensors have contributed significantly to the general problem of inverting optical measurements to derive concentration estimates of biogeochemical

parameters, and several continue to provide regular coverage of the global biosphere.

The spaceborne sensors of particular interest here are

the SeaWiFS and the two MODIS instruments, and the

principal data product of interest is the so-called waterleaving radiance, Lw(A), where A denotes wavelength. For

the purposes of ground truth—more correctly sea-truthobservations, the Lw(A) values can be derived by extrapolating in-water measurements taken close to the sea surface

or obtained directly from above-water measurements. For

meaningful applications, an extremely high radiometric accuracy is required. The SeaWiFS Project, for example,

established a radiometric accuracy to within 5% absolute

and 1% relative, and chlorophyll a (Chl a) concentrat.iont

to within 35% over a range of 0.05-50.Omgm-3 (Hooker

and Esaias 1993).

The SeaWiFS and MODIS missions are of particular

importance here, because their calibration and validation

capabilities were developed in parallel and leveraged a number of joint developments which established many of the

requirements for ocean color research, e.g., the atmospheric

correction scheme. A particularly important joint accomplishment was the establishment of a separate site for vicarious calibration data, which involved a rotating deployment of specially built Marine Optical Buoy (MOBY) units

in a clear-water site off the coast of Lanai, Hawaii (Clark

et al. 1997).

The worldwide deployments of commercial off-the-shelf

(COTS) radiometers have typically been the primary

source of validation data for ocean color data products,

because they are one of the few mechanisms to sample the

dynamic range involved, and have been archived in the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS)

for use by the worldwide scientific community. Observations from COTS instruments have also been used for vicarious calibration at a similar level of efficacy to custom

hardware like MOBY (Bailey et al. 2008).

t In fact, field-to-satellite comparisons (or matchups) are made

with respect to the total chlorophyll a (TChI a) concentration, denoted TChI a].
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

The ability to use COTS hardware for vicarious calibration was also confirmed by the Bougie pour Vacquisition de

Series Optiques a Long Termet (BOUSSOLE) project in

the Ligurian Sea (Antoine et al. 2008). The primary difficulties with buoy solutions for AOP measurements are the

wave-induced motions, biofouling of the sensor apertures,

and vandalism from commercial and recreational boaters.

Both MOBY and BOUSSOLE rely on unique structural

choices for wave mitigation (spar and transparent-to-swell

designs, respectively), divers to keep the sensor apertures

clean, and remote locations to deal with boat traffic.

Despite the potential of increased contamination of the

optical data from superstructure perturbations, oceanographic towers offer the unique opportunity of a very stable

measurement platform enabling easy and complete control

of the deployment geometry (Zibordi et al. 1999). The

latter feature permits the deployment of above- and inwater optical instruments with no vertical tilt, and the exact identification of the solar illumination geometry needed

for an accurate avoidance or correction of superstructure

perturbations (Hooker and Zibordi 2005, and Zibordi et

al. 1999, respectively). In-water sensors are easily accessed

during periodic (monthly) short maintenance visits to minimize biofouling effects, and above-water sensors experience almost no biofouling, but are also easily serviced.

Ensuring the calibration and validation field data sets

are of uniform quality and have an uncertainty less than

5% requires a continuing commitment to quantifying the

uncertainties associated with the spaceborne and in situ

instrumentation. Assuming the uncertainties combine in

quadrature, the allowed uncertainty in the remote and in

situ optical data is approximately 3.5% for each (V/5_2/_2).

This means the individual sources of uncertainty for the

acquisition of sea-truth data must be on the order of 1-

2%, or what is referred to more generally as simply 1 %

radiometry.

The uncertainties for the in situ part of the total uncertainty budget have a variety of sources:

1. The measurement protocols used in the field,

2. The environmental conditions encountered during

data collection,

3. The absolute calibration of the field radiometers,

4. The conversion of the light signals to geophysical

units in a data processing scheme, and

5. The stability of the radiometers in the harsh environment they are subjected to during transport and

use.

Of these, only items 2 and 5 (above) are outside the control of the individual investigator. The negative aspects

of environmental conditions can be largely eliminated by

t Literally translated from French as the "buoy for the acquisition of a long-term optical series." "Boussole" is the French

word for "compass."

restricting data collection to ideal conditions (clear sky,

calm sea, little or no marine haze, and a small solar zenith

angle), and the potential for shipping damage can be minimized by using double-packed (box within a box), properly

designed, shipping containers.

For the SeaWiFS Project, the first step in the process

of controlling uncertainties in field data was establishing—

through consensus at a community workshop—and then

publishing, the SeaWiFS Ocean Optics Protocols (Mueller

and Austin 1992), hereafter referred to as the Protocols.

The Protocols are considered a work in progress and were

revised (Mueller and Austin 1995) and significantly expanded (Mueller 2000, 2002, and 2003) by having the scientific community decide which scientific areas would be updated. The Project continuously used the Protocols as the

requirements for all sea-truth observations, and although

the Protocols were initially established for the SeaWiFS

Project alone, all ocean color missions advocate adherence

to the Protocols to minimize uncertainties.

The uncertainty in calibrations is the most fundamental, because all the others are only quantifiable if the radiometers are properly calibrated. To maintain internal

consistency between calibrations of the in situ sensors (and

the SeaWiFS instrument itself), the SeaWiFS Project required calibration traceability to the National Institute of

Standards and Technology (NISI) and implemented an

ongoing series of SeaWiFS Intercalibration Round-Robin

Experiments (SIRREXs) to investigate and minimize calibration uncertainties.

In the progression from the first to the third SIRREX

(Mueller 1993, Mueller et al. 1994, and Mueller et al. 1996,

respectively), uncertainties in the traceability to NIST for

intercomparisons of spectral lamp irradiance and sphere radiance improved from 7-8% to 1-2%. The fourth through

sixth SIRREX activities further investigated laboratory

and field protocols (Johnson et al. 1996, Johnson et al.

1999, and Riley and Bailey 1998, respectively), and showed

calibrations at an uncertainty level of approximately 2%

were routinely achievable if the Protocols were carefully

implemented. This culminated in a detailed experiment to

quantify many sources of uncertainties not thoroughly investigated during previous activities at a single calibration

facility (Hooker et al. 2002a).

The uncertainties associated with data processing are

tied to the Protocols, but there are subjective aspects, like

the choice of the in-water extrapolation interval, which are

not completely resolved by a single protocol. The first

SeaWiFS Data Analysis Round Robin (DARK 94) investigated data processing uncertainties. In addition, it showed

differences in commonly used data processing methods for

determining primary optical parameters from in situ light

data, which were about 3-4% of the aggregate mean estimate (Siegel et al. 1995). The focus of the second DARR

(DARK 00) was to determine if these results could be improved (Hooker et al. 2001). In terms of overall spectral

averages, many of the DARK 00 intercomparisons were to
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

within 2.5%, and if the processing options were made as

similar as possible, agreement to within less than 1% was

possible.

Uncertainties associated with sensor stability have elements both within and beyond the control of an individual

researcher. Instrumental drift due to filter deterioration

and physical stresses, which can cause shifts in the optical

alignment and electrical characteristics of a device, must be

tracked even if a concerted effort is made to minimize these

problems. The most accessible technology for monitoring

the temporal characteristics of instrument response is a

portable source (Hooker 2002). All of the devices currently

available are derived from the SeaWiFS Quality Monitor

(SQM) which was jointly developed by NASA and NIST

(Johnson et al. 1998). The SQM is capable of monitoring the stability of light sensors to within 1% in the field

(Hooker and Maritorena 2000) and was sufficiently successful to be commercialized by two different companies. More

recently, the Portable Universal Radiometer Light Source

(PURLS), which is based on tunable, light emitting diodes

(LEDs), is being developed by Hydro-Optics, Biology &

Instrumentation Laboratories (Bellevue, WA) to replace

the SQM.

Prior to the field commissioning of the original SQM,

many aspects of sensor performance were maintained by

the instrument manufacturer and were not routinely investigated by the individual investigator. The first operational deployment of the SQM demonstrated the importance of independent evaluations of commercial equipment,

however, wherein large changes in the responsivity of some

of the radiometers—as much as approximately 25% over

a one month period—were detected (Hooker and Aiken

1998). More recently, SIRREX-8 showed that the immersion factors supplied by a commercial manufacturer were

more than 10% in error at some wavelengths (Zibordi et al.

2002), and there are other examples of the need for independent confirmation of performance specifications in the

literature (e.g., Mueller 1995, and Hooker and Maritorena

2000).

The optical parameters do not account for all of the

validation requirements for many satellite data products.

The determination of chlorophyll a concentration, for example, is central to the SeaWiFS Project pigment objective

of agreement to within 35%, which is based on inverting

the optical measurements to derive pigment concentrations

using a bio-optical algorithm. The in situ pigment observations will always be one of two axes to derive or validate the pigment relationships, so it seems appropriate to

reserve approximately half of the uncertainty budget for

the in situ pigment measurements. Assuming the sources

of uncertainty combine independently (i.e., in quadrature),

an upper accuracy range of 20-25% is probably acceptable,

although 15% would allow for significant improvement in

algorithm refinement (Hooker et al. 2005).

The inquiry into the validation requirements for parameters other than the optical variables began with an

intercomparison of four laboratories using different high

performance liquid chromatography (HPLC) methods for

determining [TChl a] and showed the overall accuracy of

the four methods was 7.9% (Hooker et al. 2000a). One

method did not separate mono- and divinyl Chl a, and if

the samples containing significant divinyl Chl a concentrations were ignored, the four methods had an average accuracy of 6.7%. More recent results in very different oceanic

water types confirm [TChl a] can be estimated with an accuracy to within approximately 7% for a quality-assured

laboratoryt (Hooker et al. 2005, 2009, and 2010).

Careful implementation of the Protocols can reduce

many uncertainties, but it cannot remove them all—many

can only be designed out of the instrumentation. To accomplish the latter and to incrementally improve the capabilities of the currently available field equipment, the SeaWiFS Project undertook a long-term development activity for above- and in-water optical sampling systems. Aspects of the design effort were refined based on the lessons

learned from the aforementioned round robins associated

with understanding the sources of uncertainties in field instruments.

As part of SeaWiFS calibration and validation field

campaigns, the SeaWiFS Project combined data acquisition exercises with specific experiments to investigate data

acquisition and data processing uncertainties. The experiments were designed to compare a variety of deployment

techniques used to measure the in situ light field and to

incrementally improve the methods and instrumentation

employed. Both above- and in-water sensors and methods

were evaluated, although the latter predominated. The

culmination of the experiments was a demonstration that

the in situ part of the SeaWiFS uncertainty budget (3.5%)

could be satisfied with a dedicated effort of recurring calibrations, stability monitoring, and strict adherence to the

Protocols for both the in-water (Hooker and Maritorena

2000) and above-water (Hooker et al. 2004) approach.

Calibration and validation activities require match-up

data, that is, contemporaneous observations by the satellite and an in situ instrument. Variables that explicitly

account for the global solar irradiance, EdW, A), at the

time of data collection—so-called apparent optical properties (AOPs)—are used for match-up analysis, because

derivations of LW (A) in identical waters—but different illumination conditions—will differ. The variability can be

removed, in part, by normalizing Lw (A) by the solar irradiance to compute the remote sensing reflectance, Rrs(A)•

An additional refinement is made by adjusting Rrs (A) by

the time dependent mean extraterrestrial solar irradiance

to produce the so-called normalized water-leaving radiance

(Gordon and Clark 1981), [Lw(A)]N'

t A quality-assured laboratory satisfies the so-called quantitative level of performance metrics as proposed by Hooker et

al. (2005), refined by Hooker et al. (2009), and adopted by

the SeaHARRE community.

3
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Accounting for the illumination conditions makes normalized variables the primary input parameters for inverting Chl a concentration from in situ optical measurements

(O'Reilley et al. 1998), which means they are central variables for validation exercises. For vicarious calibration,

a final—more exact—computation includes correcting the

observations for the angular (bidirectional) dependence of

Lys (Mueller and Morel 2003), which are also used for validation and routine data processing.

The central theme in the discussion presented here is

the incremental pursuit of more accurate field observations,

either in the data collection part of the problem set or in

the data processing. The current challenge in ocean color

remote sensing is to extend the accomplishments achieved

in the open ocean and coastal zone into much shallower waters, e.g., estuaries and rivers. This requirement is driven

by the current focus of satellite observations, which are inexorably tied to launching new missions based on novel research topics and assuring the quality of the ensuing satellite data.

The long-term NASA programmatic requirements for

ocean color remote sensing are articulated in an Advanced

Science Plan, On the Shores of a Living Ocean: The Unseen Worldt. The designated mission themes from this

plan, along with the corresponding high-priority research

questions, highlight the science and mission concepts the

calibration and validation activity must help enable.

The mission themes span a range of scales and applications: a) global separation of pigments and ecosystem

components, b) high spatial and temporal resolution of

near-shore waters, c) active assessment of plant physiology and composition, and d) determination of mixed layer

depths. The corresponding research questions span equally

large scales:

• How are oceanic ecosystems and their attendant

biodiversity influenced by climate or environmental

changes, and how will these evolve over time?

• How do carbon and other elements transition between oceanic pools and pass through the Earth

system; and how do biogeochemical fluxes impact

the ocean and planetary climate over time?

• How (and why) are the diversity and geographical distribution of coastal marine habitats changing,

and what are the implications for human health?

• How do hazards and pollutants impact the hydrob

raphy and biology of the coastal zone and human

activities, and can the effects be mitigated?

These questions require more interdisciplinary science and

a greater amount of observations in the land–sea boundary

t The Advanced Science Plan, which was drafted by a subset of

the science community led by the Ocean Biology and Biogeochemistry (OBB) Program Manager, is available from the following Web site: http://oceancolor.gsfc.nasa.gov/DOCS.

than any other ocean color mission in the past. More importantly, a higher accuracy in field observations is needed,

because the types and diversity of data products will involve significantly more optically complex water masses

than usual.

The purpose of this report is to document some of the

activities initiated by the OBB Calibration and Validation Office (CVO) to reduce uncertainties in the measurement of AOPs with COTS radiometers—both from a data

acquisition and data processing perspective. The principal objective is to be prepared for the launch of the nextgeneration of ocean color satellites with the most capable

COTS instrumentation in the shortest time possible. The

latter is required to ensure that the scientists involved can

start collecting the baseline observations needed to begin

formulating and testing the myriad details associated with

hypotheses, algorithms, and databases for the new missions.

To fulfill the need for new sensors, a next-generation capability called Optical Sensors for Planetary Radiant Energy (OSPREy) is being developed (Hooker et al. 2010).

The primary objective of the OSPREy activity is to establish an above-water radiometer system, composed of

multiple sensors mounted on an offshore-platform, as a

lower-cost alternative to existing in-water buoys for the

collection of sea-truth observations. The OSPREy sensor

suite is being developed by Biospherical Instruments, Inc.

(BSI) in partnership with NASA.

The OSPREy concept requires next-generation designs

for both above- and in-water AOP sampling systems. The

first step in the development of OSPREy was to enhance

a COTS radiometer as the starting point for designing

the new sensors and testing the modified sensor(s) in the

field. The Biospherical Surface Ocean Reflectance System

(BioSORS), an above-water radiometric system manufactured by BSI (San Diego, California), was the very first

step (Chap. 1). The follow-on steps were to apply the

lessons learned from the BioSORS work towards a new

in-water profiler called the Submersible Biospherical Optical Profiling System (SuBOPS). SuBOPS combined the

incremental changes in radiometry with a new deployment

system or backplane for mounting the light sensors. The

capabilities of the new profiler were intercompared with

legacy instruments—the Biospherical Profiler (BioPRO)

being the most similar to SuBOPS in terms of spectral

and performance capabilities (Chap. 2).

Following an incremental approach to manage risk, the

next level of sophistication involved evaluating new radiometers that were emerging from a separate Small Business Innovation Research (SBIR) development activity—

based on so-called microradiometers (Chap. 3)—to determine if the currently available profiling reflectance radiometer (PRR) series of sensors, or the recently developed microradiometers, should be the foundation for the OSPREy

sensor suite. In the end, enhanced microradiometers were

selected, because of their unprecedented capabilities across

4
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

a wide dynamic range in measurement requirements. The

exploitation of microradiometers for in-water sensors was

realized with the state-of-the-art Compact-Optical Profiling System (C-OPS), which was intercompared with the

SuBOPS instrumentation (Chap. 4). Part of the attraction

of sensor systems built from microradiometers includes

their adaptability to sensor networking, for example, controlling ancillary sensors like a shadowband or global positioning system (GPS) device (Chap. 5).

In terms of next-generation problem sets, one of the

principal advantages of microradiometers is their flexibility in providing a multitude of system configurations and

upgrade paths for shallow and optically complex waters

(Chap. 6). The full problem set for collecting sea-truth

data—whether in coastal waters or the open ocean—

involves other aspects of data collection that need to be improved if the uncertainty budget is to be minimized. New

capabilities associated with deploying solar references were

developed (Chap. 7). Ancillary measurements, particularly observations of the inherent optical properties (IOPs)

of seawater are an important part of AOP measurements,

but are frequently difficult to make in small boats, because

of the current size of IOP instruments. A compact solution for recovering in-water instrument systems from small

boats was also developed (Chap. 8).

A summary of the material presented in each chapter

is given below.

The Biospherical Surface Ocean

Reflectance System (BioSORS)

As part of a long-term perspective of supporting both

current and next-generation calibration and validation activities for ocean color satellites, incremental modifications

to existing COTS sensors were undertaken to reduce the

uncertainties in AOP measurements. A principal concern

was improving observations made in shallow, optically complex waters, wherein above-water methods have an advantage because they are not subjected to self-shading effects.

The primary objective of the BioSORS development was

to fit the capabilities of the state-of-the-art COTS PRR

series of sensors into a smaller form factor that is more

compatible with existing deployment capabilities, and to

characterize the new sensors more fully than is the usual

practice. The results of these two activities are presented.

2. The Suhmersible Biospherical Optical

Profiling System (SuBOPS)

A recurring difficulty with in-water AOP measurements

in optically complex waters is adequately resolving the

presence of one or more near-surface layers. In some cases,

for example close to rivers or sources of melting ice, the layers can have significantly different water properties. The

SuBOPS profiler tested the new idea of using a kite-shaped

profiler to obtain slower and more stable profiles over traditional rocket-shaped devices, which must fall more rapidly

to maintain vertical stability. The performance of the

new profiler is presented and includes intercomparisons

with legacy instruments, in particular the BioPRO system, which was the most similar to SuBOPS in terms of

spectral and performance capabilities. The results show

the much slower descent speed and higher data rate for

SuBOPS yields a vertical sampling resolution of less than

1 cm in near-surface waters.

3. Development of the Microradiometer

A microradiometer consists of a photodetector, preamplifier with controllable gain, high resolution (24 bit)

analog-to-digital converter (ADC), microprocessor, and an

addressable digital port. In other words, it is a fully functional networkable sensor, all of which resides on one small,

thin, circuit-board assembly that is sleeved inside a metal

cylinder. With the addition of the front-end optics (collector, window, and filter stack), the basic form factor resembles a shortened pencil. The microradiometer design

was developed in response to a need for smaller, faster,

and potentially less expensive radiometers, which could be

easily scaled to either more or fewer channels and more

easily deployed in coastal waters. Because each microradiometer channel has an individual ADC, no multiplexer is

required, and no cabling is needed, thereby eliminating a

source of electronic leakage and improving reliability. The

metal cylinder provides additional isolation from electromagnetic interference sources (e.g., radio frequencies). The

photodiode current is converted to voltage with an electrometer amplifier with originally two, but subsequently

three gain settings, and the resulting voltage is directly

fed to the ADC. The entire assembly, including the photodetector, is located on a single circuit board measuring

0.35 x 3.0 in'. Each microradiometer is also equipped with

a temperature sensor located close to the photodetector.

Clusters of microradiometers can be matched with frontend optics to form small, fast, less expensive, multiwavelength radiometers for a variety of measurements. Each

cluster is managed by an aggregator that allows the array of individual radiometers, plus any ancillary sensors,

to function as a solitary device.

4. The Compact-Optical Profiling System (COPS)

The C-OPS instrument successfully integrates a number of new technologies, each focused on different aspects of

the practical problem of resolving the optical complexity of

the near-shore water column. Although C-OPS represents

a significant improvement over BioPRO and other legacy

profilers, C-OPS was designed from inception specifically

to operate in shallow coastal waters and from a wide variety of deployment platforms. In terms of the mechanics

of operating the instrumentation and its behavior during

descent, the most significant improvement was to change

the basic design for mounting the light sensors from a

rocket-shaped deployment system, used in legacy profilers, to the kite-shaped backplane developed for SuBOPS
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

(Chap. 2). This change allowed the flotation to be distributed as a primary hydrobaric buoyancy chamber along

the top of the profiler, plus an adjustable secondary set

of one or more movable floats immediately below. The

primary set provides the upward buoyant thrust to keep

the profiler vertically oriented. The secondary set, coupled with an adjustment mechanism perpendicular to the

flotation adjustment axis is used to ensure the two light

sensors are level. The hydrobaric buoyancy chamber can

contain one to three air-filled bladders, which compress

slowly and allow the profiler to loiter close to the sea surface, thereby significantly improving the vertical sampling

resolution in near-surface waters. Electronically, the system is self-organizing; when initially powered, the aggregator queries each sensor to determine optimal power required for operation over the existing length of the cables

and the population of detectors available to the configuration. Typically, each sensor geometry (Ed , Eu , and

L,,) is composed of 19 microradiometer detectors, clustered using the MMS hierarchical architecture coordinated

through a master aggregator (Chap. 3). Although the use

of microradiometers provides improvements in a variety

of operational specifications compared to SuBOPS (e.g.,

reduced electronic noise and slightly faster data acquisition rates), most notable is the reduction in instrument

diameter: C-OPS light sensors use a 2.75 in (7 cm) outside

diameter housing, which is 27% smaller in diameter than

SuBOPS.

5. Biospherical Shadowband Accessory

for Diffuse Irradiance (BioSHADE)

BioSHADE is an accessory for shipboard radiometers

that is used to measure the optical properties of the atmosphere while providing the usual global irradiance measurement of a solar reference. It meets the need for improving the self-shading correction applied to in-water AOP

measurements, wherein atmospheric complexity makes it

more difficult to properly model the diffuse irradiance. It

also provides for the capability of several atmospheric data

products that are useful to the atmospheric correction part

of calibration and validation activities. BioSHADE integrates seamlessly into instrument systems based on the

microradiometer architecture, although it can be configured for use with the PRR-800 series of solar references

or other irradiance sensors with the same form factor. It

is composed of the following components: a) the stepping

motor and controller unit housing an Mdrive 17 from IMS

Systems, b) a data aggregator derived from the microradiometer product line, c) a power regulation subsystem, d)

the housing for the electromechanical components, e) the

shadowband subassembly, and f) the radiometer mounting components. The Biospherical GPS (BioGPS) is a

12-satellite GPS receiver designed to be operated simultaneously with radiometric instruments. The system can

be fully integrated with instrumentation based on the microradiometer architecture, wherein the GPS is operated

over a single cable, linking radiometers, BioGPS, and the

BioSHADE accessory. The latter ensures the accessory

components and the radiometric sensors can be operated

using standard cabling and deck box configurations.

6. Scalable Hydro-optical Applications for

Light-Limited Oceanography (SHALLO)

Based on microradiometer detectors, a suite of instruments has been produced that are specifically designed to

improve near-surface, near-shore, and above-water AOP

measurements, particularly in light-limited environments.

The hierarchical organization inherent in the use of clusters

of microradiometers affords an unprecedented ease in interconnecting, modifying, or upgrading the instruments as

scientific objectives or financial resources evolve. Purposebuilt instruments have the potential to reduce instrument

size, complexity, and costs while retaining the optimal field

configuration for optical and ancillary sensors. This philosophy is made more powerful by establishing an outline

of modular capabilities and a step-wise upgrade pathway

for the instrument classes. Unlike legacy sensors, changing, replacing, or repairing a filter or filter-photodetector

combination for a microradiometer sensor does not require

disassembly of the entire electro-optics section of the instrument. The modularity of the design means it is not

a significantly time-consuming and tedious procedure to

upgrade a sensor. In addition, the added risk of unintended damage to associated components is minimized, as

is the time required to test the subassemblies and recalibrate the sensor. The purpose of this chapter is to introduce a framework for the evolution and expansion of

AOP microradiometer instruments into novel deployment

opportunities, particularly in light-limited environments,

while providing upgrade scenarios to match evolutions in

resource allocations and science objectives.

The Telescoping .Hount for Advanced

Solar Technologies (T-NIAST)

The solar reference data collected with an in situ AOP

observation must be at the highest point possible on the

measurement platform and free from obstructions and reflection sources. Although this is easy to state, it is not always a straightforward operation to implement. On many

research vessels, the highest spaces are usually already occupied with the ship's equipment and such spaces are frequently inaccessible at sea (because of safety concerns).

Consequently, AOP observations are frequently made with

the solar reference located in a less than ideal location. A

quantification of the consequences of improperly siting the

solar reference are presented along with field evaluations of

a new Telescoping Mount for Advanced Solar Technologies

(T-MAST). Field trials show T-MAST is an excellent solution for this problem while providing access to the sensor(s)

for cleaning, servicing, and dark current measurements.
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

8. The Cable Hauler for Optical In Situ

Technologies (GHOIST)

Because of their large size and weight, IOP instruments

are usually mounted inside a large metallic frame and require significant resources to deploy over the side of a ship,

e.g., a hydraulic winch and an A-frame. In comparison,

modern free-fall AOP profilers, like C-OPS, are sufficiently

small in size and weight to be deployed by hand. For small

boat operations, AOP instruments are easily accommodated, but IOP sensors usually are not. Most small boats

are equipped with a davit for deploying small packages

into the water, which, when combined with a COTS system used by the commercial fishing industry, provides a

solution to the IOP instrument frame problem. The capabilities of this new deployment system, called the Cable

Hauler for Optical In Situ Technologies (C-HOIST), is presented along with results from the field commissioning of

the prototype. The significant advantages of C-HOIST are

as follows: a) the power head uses 12 VDC power, which

is available on most small boats; b) there is no need for

hydraulics; c) the payload can be raised or lowered very

slowly; and d) it can be used with standard synthetic line

of any length—there is no need for an integral drum of

cable.

7
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Chapter 1

The Biospherical Surface Ocean Reflectance System (BioSORS)

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

JOHN H. MORROW

Biospherical Instruments, Inc.

San Diego, California

.TAMES W. BROWN

CSTARS University of Miami

Miami, Florida

ABSTRACT

As part of a long-term perspective of supporting both current and next-generation calibration and validation

activities for ocean color satellites, incremental modifications to existing COTS sensors were undertaken to

reduce the uncertainties in AOP measurements. A principal concern was improving observations made in shallow,

optically complex waters, wherein above-water methods have an advantage because they are not subjected to

self-shading effects. The primary objective of the BioSORS development was to fit the capabilities of the

state-of-the-art COTS PRR series of sensors into a smaller form factor that is more compatible with existing

deployment capabilities, and to characterize the new sensors more fully than is the usual practice. The results

of these two activities are presented.

1.1 Introduction

The largely successful application of in-water techniques

in Case-1 waters (Hooker and Maritorena 2000) has not

been exploited to the same degree with above-water methods (approximately 76% of the SeaBASS data is in-water

data), although there are specific examples of accomplished

above-water field campaigns (Hooker et al. 2004). The

above-water approach for calibration and validation activities remains attractive, however, because a) the data can

presumably be collected more rapidly and from a ship underway, and b) the frequently turbid, optically complex

(both vertically and horizontally), and strongly absorbing

waters in shallow Case-2 (optically complex) environments

impose significant limitations on in-water measurements.

From a measurement perspective, the above-water

problem is more restrictive, because presently there is no

reliable mechanism for floating an above-water system away

from a research vessel, which is easily and effectively accomplished for an in-water system. Although Hooker et

al. (2003) used an extensible deployment system to permit

surface viewing farther away from an offshore platform,

almost all above-water measurements are made in close

proximity to the sampling platform. Careful attention to

the metrology of the required geometry and strict adherence to the protocol being used can virtually eliminate the

degradations from platform perturbations (Hooker and Zibordi 2005).

The above-water technique described here is an extension of the so-called Modified Fresnel Reflectance Glint

Correction method in the Protocols (Mueller and Austin

1995) and includes a more accurate representation of the

sea surface reflectance and bidirectional effects (Hooker et

al. 2004) and is called the Q02 method. The Q02 approach

is based on three measurements: the spectral global irradiance from the sky and Sun, Ed (0+ , A); the spectral indirect

(or sky) radiance reaching the sea surface, Li ( 0

+ , A); and

the (total) radiance above the sea surface, LT (0+ , A). The

latter is composed of the radiance leaving the sea surface

from below (LW ), the direct sunlight reflecting off the surface (the sun glint), and the indirect skylight reflecting off

the surface (the sky glint).

1.1.1 Legacy Instruments

One of the significant differences between above- and

in-water methods is the need for maintaining the metrology with the former, that is, the pointing angles associated
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

with the solar geometry must be accurately determined

and periodically adjusted. This requirement was the primary motivation for the development of the SeaWiFS Surface Acquisition System (SeaSAS). The SeaSAS frame was

a unique device consisting of a pedestal and two rails with

sensor mounting plates (Fig. 1) connected to a gear box.

When not in use, the two rails can be locked together in

the horizontal position, which prevents large accelerations

during adverse environmental conditions. The gear box

enabled the two rails to move in a scissor-like fashion (i.e.,

when the sea-viewing sensor was moved upwards, the skyviewing sensor moved downwards the same amount). The

entire assembly could be rotated (and locked) in the horizontal (azimuthal) plane, and a graduated band permitted

positioning to within 0.5°. An external module, especially

designed for the SeaSAS instrument, measured the vertical (two-axis) tilt and horizontal (compass) pointing of the

radiometers, so the stability of the pointing angles could

be monitored during data acquisition (Hooker et al. 1999).

Fig. 1. The SeaSAS frame with the rails oriented at

approximately 40° with respect to nadir and zenith.

The pointing module is attached to the upper right

rail (vertically orientated).

The SeaSAS frame was very large, because the radiometers were large, and could not be mounted on lightweight

superstructures or used very easily on small boats. The

follow-on revisions maintained the accuracy and flexibility

in pointing of the original, but were successively smaller.

The first of these was the SeaWiFS Underway Surface Acquisition System (SUnSAS), which was used on both small

and large boats (Hooker and Lazin 2000). SUnSAS (Fig. 2)

made the same measurements as SeaSAS and had an azimuthal band that permitted pointing accuracy to within

1°. The surface-viewing radiometer viewed the sea surface

through a square aperture that could be blocked with a

calibrated plaque, so it could also measure the radiance of

the plaque, Lp (0*, A). The plaque option was a a required

part of one of the above-water methods (Carder and Steward 1985) in the Protocols, but it had to be gray to prevent

saturation of the seven-channel sensors used at the time.

Fig. 2. The SUnSAS frame on the bow of a ship.

The long white cylinder is the data logger, which is

integral with the LT (and Lp ) sensor, and the Li

sensor is the smaller silver sensor in the foreground.

Both devices are attached to hinged plates that can

be secured at the desired viewing angle.

The micro Surface Acquisition System (microSAS) deployment frame was based on the SUnSAS design (Hooker

et al. 2000b). The primary differences were a result of trying to make the entire instrument package more than an

order of magnitude smaller, so it could be fitted inside a

cardanic gimble (thereby minimizing the negative effects

of ship motion during sampling). The mechanical frame

was a compact instrument mounting system wherein the

light sensors were clamped to two small plates, which could

be tilted to the desired nadir and zenith angles. A vertical

tilting and horizontal heading sensor package was fitted inside the gimble ballast, and an azimuthal indexing system

with a vernier scale and locking collar permitted accurate

horizontal pointing (to within 0.50).

All SAS deployments required manual pointing of the

instrumentation, which meant the data could not be collected autonomously. The SeaWiFS Photometer Revision

for Incident Surface Measurements (SeaPRISM) was based

on a commercial, fully automated, eight-channel sun photometer that has been successfully used as part of the

9
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Aerosol Robotic Network (AERONET) with many deployed in remote (island) locations (Holben et al. 1998).

The normal robotic system measured the direct sun irradiance plus the sky radiance, and transmitted the data

over a satellite link. To make the instrument useful for

SeaWiFS calibration and validation activities, a capability

was added to measure the radiance leaving the sea surface

in wavelengths more suitable for the determination of Chl a

concentration (Hooker et al. 2000b).

The primary advantages of a SeaPRISM instrument

that is mounted on an offshore platform versus an in-water

moored system (based, for example, on a buoy), are as

follows:

1. Offshore platforms that are capable of accommodating above-water instruments are ubiquitous features

of the coastal environment, so there are no extra

infrastructure costs (and many offshore structures

have power systems already installed);

2. A reduction in the vulnerability of the sensors (the

structure, and, thus, the entire sensor system, is not

easily harmed by recreational or commercial activities);

3. An increase in the pointing stability of the sensors

(the sensors are not subjected to the ocean wave

field);

4. An almost complete elimination in the fouling of

the optical surfaces (the primary source of fouling is

wind-blown particles and rather simple stowing procedures of the radiometers can minimize this problem); and

5. A simplification in maintaining and cleaning the

equipment (most offshore structures provide easy

access for authorized personnel).

The primary disadvantages of the SeaPRISM concept tivere

as follows:

a. It had a small number of channels (eight), which

relied on a filter-wheel design (so the channels were

not sampled simultaneously);

b. It used a single sensor (so there was a time delay

between measurement targets); and

c. The technology was rather old.

Note that the use of a single sensor is also an advantage,

because there are no intercalibration issues to worry about.

1.1.2 Next- Generation Instruments

Despite any detractions, SeaPRISM has proved to be

a capable platform for algorithm validation and coastal

monitoring work (Zibordi et al. 2004). To exploit the

lessons learned from SeaPRISM, a next-generation capability called OSPREy is being developed (Hooker et al. 2010).

The primary objective of the OSPREy activity is to establish an above-water radiometer system, composed of multiple sensors mounted on an offshore-platform, as a lowercost alternative to existing in-water buoys for the collection

of sea-truth observations. The use of multiple sensors permits synchronous (simultaneous) and asynchronous measurements when needed, and provides a built-in backup

capability (if one sensor fails, the synchronized sampling

might be lost, but an asynchronous alternative can still

be executed). The extra sensors also permit continuous

monitoring of the global irradiance, and the addition of

a shadowb and attachment permits the derivation of extra atmospheric data products. A conceptual depiction of

an OSPREy calibration and validation site is presented in

Fig. 3.

Fig. 3. An OSPREy system showing two sensor

sets on top of an offshore tower measuring the atmosphere and ocean, an ocean color satellite sampling the on- to off shore (green to blue) productivity gradient, the Sun and Moon as calibration targets, a communications satellite for data (including

meteorological) telemetry, solar panels and a wind

generator for power, a shore-based support facility,

and a small boat for monthly validation—including

next-generation in-water AOP profiles—and maintenance visits.

The goal of the OSPREy project is to collect highquality data satisfying the accuracy requirements for ocean

color vicarious calibration and algorithm validation. This

means the measurements will have a documented uncertainty satisfying the established performance metrics for

producing climate-quality data records (CDRs). The plan

is to enhance COTS radiometers to improve their accuracy

and extend their spectral range into shorter and longer

wavelengths. Greater spectral diversity in the ultraviolet

(UV) will be exploited to separate the living and nonliving components of marine ecosystems; UV bands will also

be used to flag and improve atmospheric correction algorithms in the presence of absorbing aerosols. The shortwave infrared (SWIR) is expected to improve atmospheric

correction, because the ocean is radiometrically blacker at

these wavelengths.

10
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

The OSPREy concept requires next-generation designs

for both above- and in-water AOP sampling. The first step

in the development of OSPREy was to enhance a COTS

radiometer as the starting point for designing the new sensors and testing the modified sensor(s) in the field. The

philosophy behind the development plan was to make a series of incremental advances and apply the lessons learned

from the prior step to the next one. Within this architecture of managed risk, BioSORS was the very first step.

1.2 The PRR-800 Sensors

The distinguishing features of BioSORS sensors are as

follows: a) very high spectral coverage (19 wavelengths

with 10nm bandwidths); b) high-speed, low-noise, threestage amplification and 16-bit digitization circuitry with

each detector individually amplified by an electrometergrade field-effect transistor (FET) operational amplifier

with variable gain (10', 3 x 107 , and 1010 V A- 1 ) that

covers more than nine decades of light levels with minimum detectable signals of less than 10 fA and 1.3 μX for

the least significant bit; c) very wide dynamic range (the

system does not saturate at natural light levels even when

the radiance aperture is pointed directly at the solar disk);

d) narrow radiance half-angle field of view (FOV) of 3.25°;

and e) an irradiance cosine departure of less than 2% from

0-65° and less than 10% from 65-85°.

1.3 PRR-800 Enhancements

The aforementioned PRR-800 specifications are accurately known because the development of the PRR 800 series of sensors included a more extensive amount of instrument characterization than is usually adopted in commercial instrument manufacturing. Specific examples of this

extra effort include a) spectral response function determination, b) center wavelength determination and bandwidth

of an individual photodetector as assembled in the instrument to an accuracy of 0.1 nm, and c) out-of band blocking

at greater than six decades. Additional details concerning

the enhancements performed for the BioSORS instrumentation is described in the following sections.

widths (3 mm) produce 1.3 nm half bandwidth because of

the low-aberration, coma-canceling design, and the use of

2,400 grooves per millimeter gratings.

During operation, the instrument is mounted to the

output of the spectral tester, and the illumination wavelength is computer controlled (Fig. 4). The advantage to

this system is that the spectral response of the assembled

and unmodified instrument can be measured directly over

many orders of magnitude, rather than having to measure the spectral characteristics of the parts and convolve

a spectral response from the components. The relative

response of each detector, set between 0-100% of the signal levels recorded during the spectral response, is used to

determine the full-width at half-maximum (FWHM) bandwidth, which is 10±1 nm for BioSORS sensors.

100

10-1

N

Q102

M

o^

10-3

E 10- 0z

10-

10 -6 _ _

200 400 600 800

Wavelength [nm]

Fig. 4. A semi-log plot of a subset of wavelengths

(in nanometers) for the results of the spectral test

of the BioSORS irradiance sensor. Features three

• 320

• 412

! '

465

^'

• 555!.

•665 #• ^•

• 710 !'

r,

decades below the peak do not contribute significantly to the measurement.

1.3.2 Linearity

The linearity of each BioSORS radiometer was tested

by mounting it to a moveable carriage on a 3m optical

rail, and repeatedly changing the distance between the instrument and a 1,000 W FEL lamp. With this approach,

departures from linearity of down to 0.1% over four decades

of response can be detected. For reference, nonlinearities

in any channel in a BioSORS sensor exceeding 0.5% would

be reason to open the instrument and look for a problem

(perhaps caused by a bad resistor or a gain change issue).

1.3.3 Cosine Collector

1.3.1 Spectral Response

Characterization of the spectral response function for

BioSORS was accomplished using a high-resolution, variable wavelength source based on a monochromator. The

unit was designed and built at BSI. The lamp unit is modular and flexible, with multiple sources that are easily selected. The primary source is a xenon lamp, but a mercury

arc lamp is used to quickly verify the wavelength scale.

To achieve the highest spectral purity, the system uses a

quartz prism monochromator as a predisperser feeding a

grating double monochromator. These monochromators

feature 0.5m focal lengths and a stacked over-under de- The directional response of the irradiance collector on

sign with zero tracking error. Relatively wide working slit BioSORS was extensively characterized by mounting it on

11
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

consists of As an additional test for nonspecific stray light, Bioa 1,000 W

operation,

a directional test apparatus. The apparatus

an automated microstepping rotary stage and

FEL lamp mounted to an optical bench. In

the instrument is mounted such that the cosine collector

is positioned at the axis of rotation. Using the FEL as a

source at 165 cm, the directional response is collected from

—110° to +110°. Typically, the BioSORS irradiance collector agrees with the cosine law to within 2% from 0-65°

and to within 10% from 65-85°, with an undetectable azimuthal variation. For reference, the integrated directional

response error in measured irradiance of a uniformly diffuse light field is less than 1.5% (worst case).

1.3.4 Stray Light

Stray light consists of extraneous photons that are included in the measurement, but are actually outside the

desired FOV. FOV is difficult to measure directly, and

it is often specified using a center-point, ray-trace design

scheme rather than as a measurement. The impact of FOV

and stray light are tightly linked and it is as important to

understand the FOV of the instrument as it is to understand the sources of stray light.

The wide dynamic range of BioSORS means that a radiance sensor can be mounted on the directional test apparatus used for cosine characterization without having to

adjust for saturation by the 1,000 W FEL lamp. Using the

directional test apparatus, the measured full-angle FOV

(FAFOV) of the BioSORS design averaged 6.5°. Stray

light would appear as shoulders or other features outside

the FAFOV (there are no obvious sources of stray light in

the final BioSORS design (Fig. 5).

100-

a^

cn ,

c

Q102

rr

70

N

E 10-4-

Z

10-6

-90 -30 30 90

ti

Angle [0]

Fig. 5. A semi-log plot of a subset of wavelengths

for the directional response of a BioSORS radiance

sensor after final stray light and FOV revision.

SORS was tested using two scenarios on the BSI radiance calibration apparatus: a) three different instrumentto-plaque distances of 26 (deemed to be ideal), 33.5, and

44.6 cm; and b) two lamp-to-plaque distances of 295 and

195 cm. As a test of stray light, at shorter distances, more

of the plaque surface is available to contribute photons

from outside the FAFOV. In the absence of stray light,

as the instrument is moved closer to the plaque, the radiance is expected to not change until the FOV of the sensor

includes the shadow cast by the instrument itself. In all

cases, the instrument was positioned at 45 0 relative to the

plaque. At all instrument-to-plaque and lamp-to-plaque

distances, the instrument was rotated, which has the effect of changing the area of the plaque that is viewed as

the channels move to the edge of the housing. With the

exception of the UV channels (because of the low signal),

all measurements were within 1% of the mean.

1.4 BioSORS

A preliminary testing of all the modifications detailed

in Sect. 1.3 was conducted on the rooftop of the BSI building in San Diego, California. The radiometers were configured exactly as they were to be used in the field, except

they were pointed only at the sky. The data collected during an approximately 24h time period confirms the very

wide dynamic range of the PRR 800 technology (Fig. 6),

most notably because the data do not saturate even when

the sensor is pointed at the solar disk (top), which for the

relatively wide FOV of the sensor includes sky radiance

contributions.

104

L 102

E

100

U

X10 2

Q) U

C

cO

CU 10-4

Cc

10-6

0 6 12 18 24

Elapsed Time [h]

Fig. 6. A time series of zenith sky radiance over an

approximately 24 h time period starting in mid-day.

Also shown (top) are direct solar disk measurements

taken in the middle of the following day.

340 nm

• 395 = 1

490

560

• 670

• 780

• 875 i r j

1A *030

•

•,• • ••• • OR •,'^I

M,,

Direct

Solar

Clouds Disk

Clouds

Sky Sky

340 nm 589 nm Sodium

• 395

490

jRefle

589

670

• 710 ted City Lig ts'

12
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

The sky radiance data in Fig. 6 show the effect associated with clouds passing within the FOV of the sensor, both during Sun and sky measurements. The data

also show an interesting subtlety associated with cloudreflected city lights at night, which are seen as bright reflections in the data phis the presence of a 589 nm sodium

line associated with sodium-vapor street lights.

1.5 Data Processing

Regardless of preconceptions, there are no a priori reasons to select water-leaving radiances from an above-water

method, Lw (A), over an in-water method, Lair (A), when

sampling in the open ocean (which usually has a deep, optically simplistic mixed layer). There are, however, differences between the two approaches that are relevant to the

problem of sampling in the coastal environment. In particular, one of the acknowledged—but largely unproven—

advantages of the above-water method is it should be more

capable in shallow coastal waters than existing in-water

systems. One reason for this perception is all above-water

methods are based on pointing a radiance sensor at the

sea surface (and at a prescribed nadir angle, i9, here 40°),

so the total radiance at the sea surface LT (A), is measured. The acquisition occurs without disturbing the sea

surface, so the net effect of the vertical complexity of the

water column is precisely captured by the total radiance

measurement.

Unwanted radiance contributions must be avoided with

both above- and in-water methods. The latter is easily accomplished by floating a free-fall profiler away from the

deployment platform. This is not easily done with an

above-water method, which must include a prescription

for avoiding the glint in the sun plane. This is frequently

accomplished by pointing the LT sensor an angle 0' away

from the Sun (here either 90 0 or 135°), while remembering

not to point the sensor into any perturbations (shadows or

reflections) from the sampling platform.

1.5.1 In-Water Processing

The basic in-water processing follows the wellestablished methodology of Smith and Baker (1984). Inwater radiometric quantities in physical units are denoted

T and represent the upwelling radiance (L,,) or either

the downward or upward irradiance (Ed or Eu , respectively). The in-water values were normalized with respect

to Ed (01 , A, t), with t explicitly expressing the time dependence, according to

q3 (z, A , to) _ T (z , A, t) E

d (0+ , A, to)

 (1)

where 93(z, A, to) identifies the radiometric parameters as

they would have been recorded at all depths z at the same

time to, and to is generally chosen to coincide with the

start of the cast. For simplicity the variable t is omitted

in the following text.

Subsurface primary quantities q3(0 - , A) were obtained

from the exponent of the intercept given by the leastsquares linear regression of In [T(z, A)] versus z within

an extrapolation interval specified by zl < zo < z2 . The

negative value of the slope of the regression fit is the diffuse attenuation coefficient KT (A). The KL,. value is used

to extrapolate the uptivelled radiance through the upper

layer to determine Lu (0-, A) at null depth z = 0- . The

water-leaving radiance is obtained using

Lw (a) = 0.54 Lu, (0-, a), (2)

where the constant 0.54 accurately accounts for the partial reflection and transmission of the upwelled radiance

through the sea surface, as confirmed by Mobley (1999).

It is important to note the formulation given in (2) assumes there are no artificial perturbations to the L,,, (z, A)

measurement (a commensurate assumption is made for the

above-water approach), and if they are present and unavoidable, corrections to the data are required. Artificial

in-water perturbations, for which spectral corrections are

routinely applied, is the so-called self-shading effect (Gordon and Ding 1982, and Zibordi and Ferrari 1995).

The appropriateness of the extrapolation interval, i.e.,

whether or not it satisfied the linear decay of In [Vz, A)]

in a chosen near-surface layer, was evaluated on a castby-cast basis by successive trials using a specific optical

quantity (usually Lu or Ed ) and a specific wavelength. A

direct comparison of the extrapolated Ed (0- , A) value with

the independently measured solar Ed (0+ , A) value which

should closely agree—identified profiles characterized by

poor quality (usually because of the presence of large optical stratifications within the extrapolation interval). The

use of the red channel (A = 665nm), where seawater is

characterized by high absorption and the data show a fast

decay to radiometric noise levels as a function of depth,

helped in excluding noise from the extrapolation interval.

1.5.2 Above-Water Processing

The aforementioned Q02 method (Sect. 1.1) is used

here because for a meaningful comparison with the (nadirviewing) in-water sensors, the above-water methodology

needs to be corrected to account for the bidirectional dependency of the upward radiance field below the surface

with that exiting the surface. The basic equations for this

transformation (Morel and Gentili 1996, and Mobley 1999)

are an established part of the Protocols and have been

successfully incorporated into above-water measurements

(Hooker and Morel 2003, Hooker et al. 2003, and Hooker et

al. 2004), so only only a brief summary is presented here.

The basic difficulty in above-water radiometry is to remove the reflected sky light from the surface measurement,

and it is this procedure that distinguishes most abovewater methods from one another. For the Q02 method, the

13
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

sky glint is removed by measuring L i in the same plane as

LT , and at a zenith angle z9' equivalent to 0 (Y = 180- 9),

as shown in Fig. 7.

a Top View b Side View

>> (North) 

Sun Sun

(Zenith)

_ Z

00

i ^

Radiometer x

99 ^^0

jr

2 (Nadir)

where jr where ff _ ,r - 6

2

Fig. 7. The coordinate systems used for instrument

pointing: a) looking down from above (the z-axis is

out of the page), and b) looking from the side (the

y-axis is out of the page). The 0 coordinate is the

solar azimuth angle, 0 is the solar zenith angle, and

19 is the radiometer pointing angle with respect to

the vertical axis, z. The perturbations (or tilts) in

vertical alignment, which can change the pointing

angles, are given by cp.

Note that 0 is measured with respect to an arbitrary reference, in this case due north, and ^0 is measured with

respect to nadir (the direction pointing straight down to

the sea surface). The angle Y corresponds to the angle ^9

measured with respect to the zenith (the direction pointing

straight up from the sea surface).

The Li and LT measurements can then be used to derive the water-leaving radiance (omitting the pointing angles for convenience):

Livs (A) = LT(A) - pLi( A ), (3)

where the S95 notation acknowledges the principal source

of the formulation, the SeaWiFS 1995 revision of the Protocols (Mueller and Austin 1995), and p is the surface reflectance factor. If the interface was level, p would be the

Fresnel reflectance (and equal to a constant 0.028 for a 40'

viewing angle and relative solar azimuth of 90-1350 ). Generally, the sea surface is wind roughened, so p depends on

the capillary wave slopes, and, thus, on wind speed (Austin

1974 and Mobley 1999). The wavelength dependence originates from the normal dispersion of the refractive index

of water, which is weak and can be neglected. An updated version of the S95 method using a more accurate p

value derived from the Mobley (1999) results, which include a wind speed (W) dependence, is referred to as the

S01 method (Hooker et al. 2003).

The radiance bidirectionality is parameterized by the

so-called Q function, which takes a particular value, denoted Q,,, for the nadir-viewing measurements. For abovewater measurements, the angular parameters are imposed

by the pointing angles of the sensors, as well as the surface effects of reflection and refraction. When dealing exclusively with Case-1 waters, the functional dependence of

the variables can be simplified. In particular, it is assumed

that the inherent optical properties are universally related

to the chlorophyll a concentration (Morel and Prieur 1977),

Ca.

Because a nadir-transformed, above-water estimate of

LW is equivalent to the in-water value, a corrected abovewater formulation can be produced (Morel and Mueller

2002):

LWO2 (^) _ Ro Q(A, 0, 0', e', Ca) L

aW), (4) l ( R(01 , W) Q,,,(A,0,Ca)

where the Q terms are evaluated at null depth (z = 0-),

0' is the above-water viewing angle (t9) refracted by the

air-sea interface, and the factor R merges all the effects

of reflection and refraction (the Ro term is evaluated at

nadir, i.e., 0' = 0). All the correction terms are computed

here from look-up tables (Morel et al. 2002).

1.6 Radiometric Intercomparisons

Within the context of the generalized difference between above- and in-water methods, an above-water acquisition effort ven if platform perturbations are recognized

and minimized—is still more likely to contain platformcontaminated acquisition sequences than free-fall profiles

from a simultaneous in-water method. There are exceptions to this generality (discussed below), but it remains

more true than not, and is one of the reasons why in-water

calibration and validation exercises predominate. Using

these arguments as an overall rationale for prioritizing the

data used in this study, the analytical perspective adopted

here is as follows:

1. Use the in-water observations as the reference measurement for evaluation purposes.

2. When matching the above- and in-water results, use

nearly simultaneous (within ±5 min) data acquisition

sequences to minimize environmental influences.

3. Use radiometers with absolute calibrations from separate calibration facilities (the usual case for most investigators).

Although the discussion contained within the aforementioned rationale readily supports a decision to use the inwater measurements as the analytical reference, another

reason for doing so is that the in-water data processor used

14
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

here was evaluated in a data processor round robin, and

its capabilities (or more correctly, uncertainties) are well

quantified (Hooker et al. 2001).

The statistical approach used here is associated with

providing an evaluation of an above-water method using

the simultaneous in-water measurements as reference values. Toward that end, the primary analytical variable is

the relative percent difference (RPD), V), between the two

simultaneous results. The results of interest are the different determinations of water-leaving radiances, so the RPD

formulation is given as

op) = 100 Y X X (5)

where Y is the observed value and X is the reference value.

For above- and in-water intercomparisons, Y and X

in (5) are denoted by Lw and Lw, respectively. Because

there are two sources of above-water data, BioSORS and

microSAS, additional distinctions are provided by which of

the two instruments is used. In addition, the two abovewater instruments can be intercompared, in which case,

the results from the established microSAS instruments are

used as the reference value (X) in (5).

The field testing of the prototype took place in 2004

during the Biogeochemistry and Optics South Pacific Experiment (BIOSOPE) cruise (Claustre et al. 2008). The

BIOSOPE activity emphasized a significant diversity of

AOP sampling from different instruments, and permitted

comparisons with traditional above- and in-water techniques. The latter provided the opportunity to compare

the capabilities of BioSORS against instruments with a

well-established performance history. The first data products to be intercompared were obtained from simultaneous

samplings of the new system with the older, seven-channel

microSAS instrumentation (Fig. 8).

Fig. 8. The BioSORS (left) and microSAS (right)

instruments mounted on the bow platform of the

research vessel (R/V) Atalante during BIOSOPE.

The wavelength configuration for BioSORS during the

BIOSOPE field commissioning campaign spanned the ultraviolet and near-infrared domains, but included many of

the relevant ocean color remote sensing bands for a variety

of satellites: 320, 340, 380, 395, 412, 443, 465, 490, 510,

532, 555, 560, 625, 665, 670, 683, 710, 780, and 875 nm.

The microSAS radiometers, in comparison, only contained

the bands 412, 443, 490, 510, 555, 560, and 865 nm. The

derived Lw (A) values obtained during simultaneous sampling and involving only the common wavelengths (within

1 nm) between BioSORS and microSAS are presented in

Fig. 9. The two sensor systems agreed to within 2.9% (on

average) for the center wavelengths that were the same.

4.5–

i

U)

E

N 3.0-

E

U

u

J 1.5-

U)

CL

0

U)

_o

co

0.0 1.5 3.0 4.5

510

^ 555

560

microSAS Lw [μW cm- 2 nm- 1 sr- 1 ]

Fig. 9. An intercomparison of microSAS and BioSORS in predominantly oligotrophic (Case-1) waters for six wavelengths given in nanometers. The

one-to-one line is shown as solid, and the leastsquares linear fit to all the data as dashed (regression information is given in the inset panel).

Comparisons between BioSORS and the in-water AOP

profilers were also made, but the number of casts involved

are significantly less because the two different types of sampling were frequently far apart in time.

1.7 Summary

Above-water methods for calibration and validation activities are attractive, because turbid, optically complex

and strongly absorbing waters in shallow (nominally Case2) environments impose significant limitations on in-water

sensor systems. Above-water instrumentation requirements are well understood and are frequently based on

three spectral measurements—the global solar irradiance,

the indirect (sky) radiance reaching the sea surface, and

the global radiance above the sea surface. The data can

f(x)=1.011x-0.006

R2 = 0.998

412

443

• 490

15
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

be collected rapidly and from a wide variety of platforms,

including moving vessels and stationary towers, but careful attention to instrument metrology (viewing angles and

measurement geometry) and strict adherence to deployment protocols are required. This requirement was the primary motivation for the development of the early, pedestalmounted legacy instruments, such as SeaSAS, which emphasized control of the pointing angles and relationship to

the Sun that are critical in reducing measurement uncertainties.

All subsequent SAS deployment systems used manual

sensor pointing, which meant the data could not be collected autonomously. SeaPRISM was based on an existing commercial, fully automated, eight-channel sun photometer that was successfully used as part of AERONET.

In addition to controlling the geometry of the measurements, among the benefits that autonomous operation by

SeaPRISM demonstrated was a significant reduction in the

fouling of the optical surfaces and the utility of abovewater structures as a convenient platform for deployment.

The primary disadvantages of the SeaPRISM concept came

from the small number of wavebands (eight) and the lack

of synchronous sampling inherent with a single sensor and

a filter-wheel design.

BioSORS is a modification of existing high-performance

radiance and irradiance instruments (PRR-800) originally

used for in-water observations to the above-water approach.

The distinguishing features of the BioSORS radiometers

are a) wide spectral coverage using 19 wavebands (all 10 nm

wide); b) high-speed, low-noise, three-stage amplification

and 16 bit digitization and three additional stages of variable gain; c) a very wide dynamic range (9 decades), so the

system does not saturate at natural light levels even when

the radiance aperture is pointed directly at the solar disk;

d) a narrow radiance FOV of 3.25° specifically targeting

above-water measurements; and e) an excellent irradiance

cosine response. In addition to a instrument characterizations in the laboratory for stray light, spectral coverage,

and FOV, time series measurements of sky radiance in a

large metropolis (San Diego) show subtleties, such as cloud

reflections of sodium-vapor street lights.

Field intercomparisons highlighted the significant diversity of AOP approaches from different instruments, and

permitted comparisons with traditional above- and inwater techniques. During BIOSOPE, for example, BioSORS was compared with instruments that have wellestablished performance histories, such as the older, sevenchannel microSAS instrumentation. The derived values

obtained during simultaneous sampling and involving only

the common wavelengths (within 1 nm) between BioSORS

and microSAS agreed to within 2.9% (on average) for the

matching center wavelengths.

To exploit the lessons learned from SeaPRISM deployments, a next-generation capability called OSPREy is being developed to establish an above-water autonomous system. Composed of multiple sensors mounted on a stable

platform, OSPREy is proposed as a lower cost alternative

to existing in-water buoys for the collection of sea-truth

observations. To minimize risk, the philosophy behind the

OSPREy development plan is to make a series of incremental advances and apply the lessons learned from the

prior steps to the ensuing one. The first step in the development of OSPREy is to enhance a COTS radiometer as

the starting point for designing the new sensors and testing

the modified sensor(s) in the field. Within this architecture of managed risk, BioSORS was the very first step.

Originally conceived using BioSORS technology, OSPREy

will take advantage of even newer high performance AOP

instruments developed with NASA funding called microradiometers (Chap. 3).

16
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Chapter 2

The Submersible Biospherical Optical Profiling System (SuBOPS)

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

RANDALL N. LIND AND JOHN H. MORROW

Biospherical Instruments, Inc.

San Diego, California

JAMES W. BROWN

CSTARS University of Miami

Miami, Florida

ABSTRACT

A recurring difficulty with in-water AOP measurements in optically complex waters is adequately resolving the

presence of one or more near-surface layers. In some cases, for example close to rivers or sources of melting ice,

the layers can have significantly different water properties. The SuBOPS profiler tested the new idea of using a

kite-shaped profiler to obtain slower and more stable profiles over traditional rocket-shaped devices, which must

fall more rapidly to maintain vertical stability. The performance of the new profiler is presented and includes

intercomparisons with legacy instruments, in particular the BioPRO system, which was the most similar to

SuBOPS in terms of spectral and performance capabilities. The results show the much slower descent speed

and higher data rate for SuBOPS yields a vertical sampling resolution of less than 1 cm in near-surface waters.

2.1 Introduction

A number of legacy instruments that were the predecessors of SuBOPS were developed as part of the SeaWiFS Project. The perspective at the time was to produce cheaper—but appropriately accurate—sensors that

could be used in open-ocean (optically deep) waters for

algorithm validation exercises. The first of these was the

Low-Cost NASA Environmental Sampling System (LoCNESS), so named because it was built out of the (relatively

inexpensive) modular components typically used with traditional winch and crane deployment systems (Aiken et

al. 1998). The LoCNESS profiler was an extremely capable open-ocean unit (Hooker and Maritorena 2000), which

included a three-sensor version to measure the upwelled radiance plus upward and downward irradiance as a function

of depth, Lu (z, A), Eu (z, A), and Ed (z, A), respectively.

Although used successfully under controlled circumstances in coastal waters at an offshore platform (Zibordi

et al. 1999), LoCNESS was difficult to use in small-boat

operations or in optically shallow (Case-2) waters, and

the light sensors were not mounted on the same horizontal

plane—they were separated by the length of the profiler. A

smaller version of LoCNESS, called the miniature NASA

Environmental Sampling System (miniNESS), was built to

determine if light sensors could be mounted on the fins (almost in the same horizontal plane) in a more compact configuration without degrading the quality of the measured

signals. Intercomparisons of miniNESS with traditional

profilers established the efficacy of the new concept during

open-ocean cruises, and then subsequently during coastal

campaigns at an offshore tower (Hooker et al. 1999).

The success of miniNESS led to a new design effort

called the micro NASA Environmental Sampling System

(microNESS) to further decrease the overall size and weight

of the profiling package. At the same time, there was a

strong desire to replace the analog cabling (associated with

traditional profilers) with digital interfaces. The change

to digital interfaces was particularly important when combined with the desired size reduction because it helped

ensure a) a decrease in power requirements, b) a smaller,

lighter profiler with a lower descent speed and, thus, a

higher vertical sampling resolution, c) a reduction in the

perturbation caused by the instrument to the in situ light

field, and d) a profiling system that could be easily deployed from a small boat. The latter three are particularly

important for coastal, optically shallow (Case-2) applications.

17
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

2.2 Design Requirements

Within the context of sampling protocols and data processing, working at the 1% uncertainty level requires the

elimination—or at least the significant minimization during sampling or correction during data processing—of any

degradation influencing the accuracy of the observations.

The perspective here is on coastal sampling, and although

the near-shore environment can be optically similar to

open-ocean (deep water) conditions, the chance of encountering greater turbidity and optical complexity increases

as the water depth decreases. Another unique aspect of

shallow water sampling is damage from bottom impacts,

or the possible loss of an instrument from entanglement

with sea floor debris or structures, is significantly greater.

The principal theoretical difficulty of collecting highquality AOP observations in turbid waters is the degradation of the data from self-shading effects, the severity

of which is proportional to the diameter of the light sensors (Gordon and Ding 1992). The need to use small sensors is offset, in part, by the aforementioned desire to have

the maximum spectral information possible—whether for

satellite match-up analyses or biogeochemical research objectives. Fitting all the needed electronics and optics for a

large number of wavelengths in a cost-effective design inevitably increases the diameter of the housing. In the end,

a compromise between affordability and size is chosen.

The primary practical problem with obtaining good

AOP observations in shallow waters is properly resolving

the optical complexity of the water column. The nearshore environment often contains multiple layers of distinct

water types that are optically different. The presence of

one or more optically different near-surface layers can alter

the usual log-transformed linear decay of the measured optical properties, and, thus, significantly degrade the accuracy of the (estimated) surface values. In these situations,

it is crucial that the vertical sampling of the water column

discriminates the layers with enough data points that the

data processing scheme can accommodate their influence.

When one or more layers are very close to the sea surface,

this is a hard requirement to satisfy, because of the turbulence of the air–sea interface (from surface gravity and

capillary waves) and the bulkiness of the instruments.

The surface waves add a significant amount of noise to

the data from wave-focusing effects, and they can cause excessive tilting of the nadir-viewing sensors, as well as splash

effects (e.g., foam and bubble formation). The physical size

of the instruments dictate the mounting options—which

must permit unobstructed viewing for all the sensors—

and this usually prevents all the measurements from being taken in the same horizontal plane. Consequently,

there are usually depth offsets between the various sensors,

which means some portion of the upper water column is

not sampled by all of the sensors. The most extreme offsets

occur when sensors are placed on opposite (vertical) ends

of the mounting assembly (as is the case with BioPRO).

The hazard associated with the sea floor, either from

bottom impacts or debris entanglements, cannot be removed by simply not sampling very close to the bottom.

As the water depth decreases to very shallow values (defined here as being less than 10 m), bottom contributions—

either from light reflections or the optical influence of distinct turbidity layers—can have an increasingly important

effect on near-surface parameters, so this part of the water

column needs to be measured.

Regardless of how these myriad problems are addressed,

the chosen deployment system must be able to make a vertical profile of the water column that is free of any perturbations from the sampling platform, usually a boat. For

coastal campaigns, small vessels are ubiquitous, so there is

the added challenge of getting good data from an actively

moving platform. Although winch and crane systems can

be adapted to small boats, they usually do not have significantly large cranes (as found on deep-ocean research vessels), so it is unlikely an optical package can be deployed

an appreciable distance away from the ship.

2.3 The BioPRO System

A reliable procedure for obtaining a vertical profile of

light parameters devoid of ship perturbations is to use a

free-fall instrument that is floated away from the vessel

before measurements are started. This technique has been

used extensively in a variety of sensor systems based on a

common instrument design developed for open-ocean waters (McClain et al. 2004), i.e., deep water and extensive

mixed layers). The light sensors are connected in line with

power and telemetry modules to form a long cylinder. A

radiance sensor is positioned on the nose to measure the

upwelled radiance, L,,, (z, A), and an irradiance sensor on

the tail to measure downward irradiance, Ed (z, A). The

addition of weight to the nose and buoyant (foam) fins to

a tail bracket produces a rocket-shaped package that falls

through the water column with minimal tilts (less than 50

over most of the profile).

Adjustments to the amount of weight and fin surface

area are used to fine tune the stability of the package during descent, but fundamentally, the most stable configuration occurs at the highest descent velocities. The power

and telemetry cable extends through the field of view of

the irradiance sensor, but the small diameter of the cable

minimizes any negative effects on the measured light field.

The addition of a temperature (and, if possible, a conductivity) probe, provides a good description of basic water

column properties. To ensure the data are acquired during

stable illumination conditions, a separate irradiance sensor

is mounted as high on the ship's superstructure as possible

(so the data are not degraded by shadows or reflections)

to measure the total solar irradiance, Ed(0+,A).

BioPRO is a 19-channel (A19) device based on a rocketshaped design, although the fins are not buoyant. The

tail buoyancy comes from a foam collar attached to the

18
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

main body at the end of the profiler, and the two fins

are solid plastic (Fig. 10). The overall length is approximately 59.7 cm, so it is a rather compact design (typical free-fall profilers in use for open-ocean sampling at

the time BioPRO was designed were about 1.24-1.78m

in length). Using only the collar weight and adding no

additional weights, the descent speed is about 30 cm s-1.

To achieve greater stability, weight is added resulting in a

typical descent speed of 40-60 cm s-1.

Ed (0', A 19, ^)

Ed (z, A19, Buoyant

(2:) _,,^ Collar

Solid Plastic

Fin

Temperature

Sensor

Adjustable

Weighted

Collar

L. (z. A 1J , ^;) BOPRO

Fig. 10. A schematic of the BioPRO profiling system showing the three types of light measurements

involved. The two solid plastic fins provide some

protection for the buoyant collar during deployment

and recovery. The numeric bullets clarify the number of individual light sensors and are used in all

subsequent instrument diagrams. The use of different gray colors are to provide contrast for easier

interpretation of the schematic.

One advantage of BioPRO, in terms of sampling capabilities, is it can be deployed easily by only two people,

so data collection can commence quickly when light conditions are optimal. BioPRO is most frequently deployed

from the stern of the vessel, and whenever possible, the

ship maintains a small or impulsive headway speed of approximately 0.5 kts or less (depending on surface currents,

winds, and waves). The profiling instrument is carefully

lowered into the water and repeatedly dropped and recovered within the near-surface layer until it has drifted

clear of any possible perturbation effects from the vessel.

When the profiler reaches the desired distance from the

stern (usually 30-50m), it is ready for deployment and

can be dropped or released.

Once released, the nose weight orients (or rights) the

profiler into a vertical alignment, and the entire package

starts to descend (the first few meters of data frequently

have large tilts and are ignored). The greater the nose

weight, the shorter the righting time, but there is also an

increased chance of large back-and-forth oscillations, because the weighted nose and buoyant fins establish a natural pendulum. The cable is almost neutrally buoyant and

has a low coefficient of drag, so the profiler falls freely

through the water column. The desired depth is usually

the 1% light level, but deeper casts to completely sample some other aspect of the water column are frequently

made.

The most important aspect for collecting good data

with a free-fall profiler is to prevent the telemetry cable

from ever coming under tension, because even brief periods of tension can adversely affect the vertical orientation

(two-axis tilt) and descent velocity of the profiler. To ensure this does not occur, the operator leaves a few coils

of cable at the surface. A tangle-free and continuous feed

of cable into the water is also needed, so all of the cable

(approximately 125 m for BioPRO) is laid out or flaked

on deck prior to each deployment in such a manner as to

minimize any entanglements. The profiler descends at approximately 50-60 cros- 1 so a coastal ocean cast can be

acquired very quickly.

The Recommended Standard 485 (RS-485) signals (or

RS-422 for some instruments) from the in-water profiler

and the above-water solar reference are combined in a deck

box and converted to RS-232 communications for computer logging. The deck box also provides the (computercontrolled) power for all the sensors and was designed to

avoid instrument damage due to improper power-up sequences over varying cable lengths. The RS-232 data are

logged on a Macintosh laptop using software developed at

the University of Miami Rosenstiel School for Marine and

Atmospheric Science (RSMAS) in partnership with NASA.

The software time stamps the two data streams (inwater and above-water measurements), which are both

acquired at a 12 Hz data sampling rate, and simultaneously writes them to disk. The data is stored as American

Standard Code for Information Interchange (ASCII), tabdelimited (spreadsheet) files. The software controls the

logging and display of the data streams as a function of

the data collection activity being undertaken: dark data

(caps on the radiometers), down cast, etc. The selection of

the execution mode automatically sets the file name, so all

the operator has to do is push buttons to initiate and terminate data acquisition. All of the telemetry channels can

be displayed in real time, and the operator can select from

a variety of plotting options to visualize the data being

collected.

The spectral configuration of AOP profilers are necessarily tied to the remote sensing and scientific research

objectives. The former are further divided between verifying the vicarious calibration of the satellite sensor, and

validating (or creating) the data products derived from

bio-optical algorithms that require a well-defined set of

wavelengths; whereas the latter permit a wider diversity

of choices, although for many ocean color investigations

19
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

they are most frequently composed of blue-green band ratios of the remote sensing reflectance (O'Reilly et al. 2000).

Whether for ground truth observations, algorithm validation, or the aforementioned in situ stability analyses, it is

desirable that the center wavelengths and bandwidths for

the individual channels agree as closely as possible.

The BioPRO band set is a compromise between research and remote sensing requirements and is composed

of the following wavelengths (each is 10 nn wide and the

channels that are specifically selected for remote sensing

applications are underlined): 320, 340, 380, 395, 412, 443,

465 490 510 532 555 560 625, 665 670 683 710 780

and 860 nm.

Although the calibration and validation activities for

the remote sensing objectives can be restricted primarily

to the near-surface layer of the water column, generalized

inquiries into the biogeochemical properties of the ocean

require water column sampling throughout the euphotic

depth (i.e., to as deep as the 1% light level). The optical profiling systems discussed here were all designed to

contribute to extensive sampling of the euphotic layer.

2.4 Evaluation of BioPRO

The BioPRO design was evaluated by intercomparing

it to microNESS, which was especially designed to have a

smaller size and weight—and to a certain extent—a slower

descent speed. The microNESS profiler was conceived as

a successor system to miniNESS, which was built to determine whether or not light sensors could be mounted

on the fins (in very nearly the same horizontal plane) in

a more compact configuration without degrading the stability of the rocket-shaped profiler and, thus, the lightfield measurements. The compactness was achieved by

significantly reducing the overall length of the instrument,

which reduced the righting moment (the distance from the

weighted nose to the buoyant fins). Intercomparisons of

miniNESS with traditional profilers established the efficacy of the new concept during open oceanic cruises, and

then subsequently during coastal field campaigns (Hooker

et al. 1999).

The success of miniNESS led to the microNESS design effort (Fig. 11), the main objective of which was to

further decrease the overall size and weight of the profiling package. This was achieved by replacing many metal

components with synthetics, switching from a four-fin to a

two-fin design, and reducing the size of the electronics and,

thus, the diameter of many components. The solar reference has seven channels, so the in-water light sensors have

redundant 412 nm channels, which are used for self-shading

severity analysis. The emphasis on coastal sampling prescribed a high-precision depth sensor, which is fitted to the

nose of the instrument. Although capable of slower descent

speeds than traditional free-fall profilers, the rocket form

factor required relatively high speeds for stability. This,

coupled with the relatively slow data rate (6 Hz), meant

near-surface effects (e.g., wave focusing) were significantly

aliased during vertical profiling.

(j) Ed (O

+

 A7)

Ed (z, A4, (P) (Z 1 02 Ed (z, A4, V)

L,(z,A4,^0)3 (3 L,(z,A4,V)

Batteries and Buoyant

Temperature Probe Fins

DATA-HUB.; Digiquartz Pressure

w/tilt sensors Sensor (0.01 % FullWeight----I_-- Scale Accuracy)

microNESS

Fig. 11. A schematic of the microNESS profiling system showing the two four-channel, all-in-one

light sensors (Lu, and Ed are in the same housing)

mounted on the edge of the buoyant fins.

Another microNESS design objective was to replace the

traditional analog cabling with digital interfaces. This objective was particularly important, because when combined

with the desired size reduction, it would help ensure the following: a) lower power requirements and, thus, a battery

option; b) a smaller, lighter profiler (a 1.0 in and

an in-air weight of 4kg); c) a reduction in the light field

perturbation caused by the instrument (the main-body diameter was reduced to 4.6 cm); and d) a profiling system

easily deployed from a small boat. The microNESS profiler was a capable system (Barlow et al. 2003), although it

suffered from elevated uncertainties in the 510 nm channel

(as did some other profilers with a similar heritage).

An intercomparison of BioPRO and microNESS based

on common wavelengths is presented in Fig. 12. Because

the data were not simultaneous, like the BioSORS evaluation (Fig. 9), changes in the solar geometry between

the casts are accounted for by using the normalized waterleaving radiance, [Lw (A)] N. Predominantly open-ocean,

oligotrophic (Case-1) waters were used to ensure the water

types were not a limiting factor in the comparisons—the

objective was to discern the performance of the instruments, without having to qualify the results as a function of the water masses. The unbiased percent difference

(UPD 1) for each channel between the two profilers averages -7.6% to 0.3%, with an overall average of -2.2% (for

the Fig. 12 wavelengths), which is to within the calibration uncertainty. The largest difference corresponds to the

510 nm channel, which was a problematic wavelength with

The UPD is defined as 200(Y - X)/(Y + X), where X is

the reference instrument or data, which in this case is the

microNESS profiler.

20
3

0

0 1 2 3

z

2

J

O

rr

C 1

_o

M

J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

the class of radiometers used with microNESS (Hooker and

Maritorena 2000). A least-squares linear regression of the

data (Fig. 12 inset panel) shows almost one-to-one correspondence with over 95% of the variance explained.

microNESS [LWPAN

Fig. 12. An intercomparison of microNESS and

BioPRO in predominantly oligotrophic (Case-1) waters for six wavelengths given in manometers. The

units for [Lw (A)] N are μW cm` nm` sr-1 . The

one-to-one line is shown as solid, and the leastsquares linear fit to all the data as dashed (regression information is given in the inset panel).

One reason for the good agreement in Fig. 12 is the extrapolation intervals of the two profilers were rather similar. In addition, the vertical resolution (i.e., the number

of acceptable data points) in each extrapolation interval

was adequate for open-ocean (deep mixed layer) conditions: approximately 4 cm for BioPRO and about 7 cm for

microNESS. The reason for the difference between the two

profilers is the BioPRO instrument has a higher data rate,

so it collected approximately 43% more data on average.

2.5 SuBOPSA Kite-Shaped Profiler

The SuBOPS free-fall profiler is based on the same

radiometers used with BioSORS and BioPRO (i.e., the

PRR-800 series of optical sensors packaged into 3.5 in housings). The changes that were made to create the new design were motivated by the desire to collect the best possible data in coastal (Case-2), optically complex waters.

A free-fall design permits the avoidance of ship-induced

perturbations, but it does not deal with the problems of

self-shading perturbations, optical complexity, near-surface

effects, and bottom hazards. The SuBOPS profiler was designed with the goal of minimizing these effects in the most

cost-effective manner possible. This was accomplished by

using the following design principles:

1. Repackaging the electronics and optics in a smaller

3.5 in (8.9 cm) housing in keeping with in-water designs already successfully used in coastal campaigns

(Hooker et al. 1999, Doyle et al. 2003, Hooker et al.

2004, and Hooker and Zibordi 2005);

2. Decreasing the overall length for irradiance (and radiance) from 22.0 in (55.9 cm) to 13 in (33.0 cm) and

mounting the light sensors as close as possible to the

same horizontal plane;

3. Increasing the stability of the instrument package

during descent (minimizing the vertical tilting of the

sensors as they fall through the water column) while

improving the vertical resolution of the sampling by

a) maintaining a higher data sampling rate ( 12 Hz),

and b) reducing the descent speed by switching from

a rocket-shaped device (BioPRO and its predecessors) to a kite-shaped design; and

4. Relying on a slow descent speed ( about 20 cm s- 1 or

less) and a bottom-pointing spar to minimize damage from accidental bottom impacts.

Note that the reduction in size also reduces the total weight

and makes the profiler easier to handle from small boats.

A schematic of the SuBOPS profiler is presented in Fig. 13.

Ed(0+ , A19, W)

Fixed Buoyant

Disks Above

Ed (z, A 19 , Adjustable Disks

/Fender

L,,( z , A 19; 4^)

Temperature ^^ weighted

Sensor Bottom Spar

Perforated

Backplane SuBOPS

Fig. 13. A schematic of the SuBOPS profiler. Any

needed weight is added to a bottom-pointing spar

(or stinger), which also protects the Lu aperture in

the case of an accidental bottom impact.

Of the design changes adopted in creating SuBOPS, the

most significant—in terms of the mechanics of operating

the instrumentation and its behavior during descent—was

to change the basic design for mounting the light sensors

from a rocket-shaped deployment system to a kite-shaped

21
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

backplane. This change allowed the buoyancy to be distributed as a primary set of small disc floats (used in the

commercial fishing industry) along the top of the profiler,

plus an adjustable secondary set of one or more floats immediately below (Fig. 14). The primary set provides the

upward buoyant thrust to keep the profiler vertically oriented, and the secondary set is used to ensure the two light

sensors are level (the Ed sensor is slightly bigger and heavier than the Lu sensor, because it contains more electronics

and the ancillary tilt and temperature sensors).

Fig. 14. The early SuBOPS prototype in preparation for redeployment in Mission Bay (California)

after having the weighted bottom spar adjusted (not

blackened for clarity). Note the aluminum fenders

protecting the sensors at the bottom of the profiler

(not blackened pending final dimensioning).

SuBOPS can be trimmed for very slow descent speeds

(less than 15 cm s- 1 ). During trials in a swimming pool,

the profiler descended so slowly it would come to rest on

the cable catenary while being vertically oriented to within

1°. The vertical orientation specification for the new backplane was vertical tilts to within 2.5° for the entire water

column (under favorable conditions). Although higher sea

states can cause sib lificant tilts during the early descent

of the profiler, once these perturbations subside, the rest

of the profile typically meets this specification. Note that

vertical tilts are not so damaging as long as the profiler is

going back and forth through zero, because larger tilts can

be filtered out in the data processing scheme.

The light sensors are mounted against a thin perforated

plate or backplane. The perforations help reduce the tilting influence of water currents by allowing some of the

water to pass through the plate (as well as the open area

associated with the disc floats). The distribution of the

floats within the interior of the profiler protects them from

accidental ship impacts during profiler deployment and recovery. The light sensors are likewise protected by fenders

mounted at the top and bottom of each radiometer.

The power-telemetry cable attaches to a Y-cable connected to the two light sensors, and a strain relief on the cable attaches to a four-point cable harness (Fig. 14). When

the cable is pulled, the harness extends away from the

backplane and the profiler kites up to the surface (if submerged), and this direction is in the direction of the pitch

axis. The direction perpendicular to the pitch axis is the

roll axis. The latter is adjusted by moving the secondary

row of floats. A pitch bias is counterbalanced by attaching

weight to the appropriate side of the backplane. Additional refinements to the kite-shaped backplane and more

deployment detail are provided in the description of the

C-OPS instrumentation (Chap. 4).

2.6 SuBOPS Evaluation

The intercomparison results between SuBOPS and BioPRO for eight common wavelengths are shown in Fig. 15

(six of the wavelengths are common to Fig. 12). Predominantly eutrophic (Case-2) waters were used because the

focus of SuBOPS was to begin the process of making better

AOP observations in coastal waters. The average UPD for

each channel between the two profilers ranges from —3.5%

to 3.6%, with an overall average of 1.3% (for the wavelengths plotted in Fig. 12), which is to within the calibration uncertainty. A least-squares linear regression of

the data (Fig. 15 inset panel) shows an almost one-to-one

correspondence with over 95% of the variance explained.

0 380

0 412

0 443

0 490

9 510

0 555

a 560

0 665

fez

Apd

f(x) = 0.966x + 0.019

R2 = 0.967

0.0 0.5 1.0 1.5

BioPRO [LW(k)]N

Fig. 15. An intercomparison of BioPRO and SuBOPS in predominantly eutrophic (Case-2) waters

for eight wavelengths, given in nanometers. The

units for [Lys (a)] N are μW cm-2 nm-1 sr-1 . The

one-to-one line is shown as solid, and the leastsquares linear fit to all the data as dashed (regression information is given in the inset panel).

1.5

z

1.0

J

U) 0-

ipp 0.5

U)

0.0

22
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Deeper, but nonetheless turbid, waters were used for

the intercomparisons presented in Fig. 15, because of the

difficulty of avoiding bottom impacts with BioPRO in shallow water. Most casts were executed in calm conditions,

to minimize BioPRO near-surface tilting when it was released, thereby minimizing wave-induced tilting. Although

these conditions might be considered nearly optimal, they

are not as ideal as the open-ocean (deep mixed layer) conditions for the prior BioPRO intercomparison (Fig. 12), so

there is greater variance in the distribution of the data.

The most highly attenuated parts of the spectrum were

in the UV and near-infrared (NIR.) channels for which

diffuse attenuation coefficient (Kd ) values were 3-4m-1.

These channels are not shown in Fig. 15 and had average UPD values ranging from —7.0% to 27.8%. The large

differences are primarily associated with the inability of

the BioPRO profiler to collect sufficient data in the nearsurface waters to provide good data products for highly

attenuated channels. Even though the extrapolation intervals for BioPRO extended approximately 29% more than

SuBOPS, the slower descent speed of SuBOPS resulted in

about 16% more data in the extrapolation interval. The

start of the extrapolation interval was shallower for the

SuBOPS data set, which is important for the most highly

attenuated channels.

2.7 Design Refinements

After establishing the prototype, the SuBOPS instrumentation was used to test and refine various aspects of

the new deployment capability. One of the first aspects to

be refined was the flotation, because the original flotation

could not be machined or shaped into smaller pieces. Consequently, the smallest integral unit was one flotation disk,

which was too coarse an element for quickly fine tuning

the descent of the profiler. If only a small amount of flotation was needed, an entire disk needed to be added, which

meant that more weight needed to be added to reduce the

net buoyancy, and this process required fine tuning with a

series of test casts. Although this system worked, it was

time consuming and there were water masses for which an

ungainly amount of buoyancy and weight was needed to

trim the profiler to lower descent speeds.

A new flotation material that could be machined and

cut into pieces without compromising its strength was substituted for the original flotation disks. The new material

was very strong, which meant that the top-most portion

on the profiler could be hollowed out. This allowed an

elongated compressible bladder filled with air to be fitted

into the cavity, which significantly improved the loitering

time of the profiler closer to the sea surface by providing a

so-called hydrobaric buoyancy system. The volume of the

bladder is set so the profiler barely sinks. As the depth increases, the bladder is compressed more and more and the

descent rate increases, until it reaches terminal velocity.

A design objective of the hydrobaric buoyancy system

was to acquire 200 or more samples in the upper 2 m of

the water column, i.e., a vertical sampling resolution of

at least 1 cm. The prototype system very nearly satisfied this requirement, and the extent of the accomplishment is seen by comparing this new capability to another

legacy profiler, the SeaWiFS Profiling Multichannel Radiometer (SPMR). This comparison is made with respect

to the SeaWiFS Free-Falling Advanced Light Level Sensors

(SeaFALLS) profiler, which was a modified SPMR having

slightly larger fins for greater stability and finer trim tuning (Fig. 16).

) Ed (0'. A 13, ^0)

Ed( z , A13, 1F) ^)

Fluorometer

Buoyant

CT Probe Fins

Weight

L, (ti .. A13, w) SeaFALLS

Fig. 16. A schematic of the SeaFALLS profiler. A

unique aspect of this instrument was the fluorometer mounted on one of the fins.

As shown in Fig. 17, the new SuBOPS unit fitted with

the hydrobaric buoyancy system has a vertical sampling

resolution at 2 m of about 90 samples per meter. The total number of samples collected at 5, 10, and 15 m depth is

N5 =519, N10 =829, and N15 =1,138, respectively. In comparison, the rocket-shaped SPMR has a high descent rate,

which yields an average sampling resolution of about 6-7

samples per meter, and the total number of samples collected at 5, 10, and 15 m depth is an order of magnitude

less than the SuBOPS profiler.

If profiler orientation is used to reject data that exceeds

a vertical tilt. of 5 0 , the sampling resolution of the SPMR

is further degraded in the near-surface waters, because the

profiler has a large righting moment. That is, because of its

length (about 1.24 m), it takes time (which is to say, depth)

for the SPMR to become vertically oriented. SuBOPS, on

the other hand, starts out more vertically oriented, and

because it initially sinks very slowly, not much depth is lost

before it is reliably sinking in a near-vertical orientation.

Because of the shape of the backplane and the fourpoint harness, when the SuBOPS unit is retrieved from

depth, it has a tendency to kite up to the surface. That is,

23
15

0 15 30

Velocity [cm/s]

0.01 0.1 0.3

PAR

0-

5-

E

Q

a>

0

10-

15

30 70 110

Velocity [cm/s]

10 30

PAR

eovp- j

N5=519

Tetn inal

Vel ity

N10=829

SuBOPS N15=1,138

0-

5-

T

Q -

10-

Ter

Velocity

final

;

N5=38

^•

tir•

J

N10=70

SPMR N15=102

Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Samples Per Meter Samples Per Meter

50 100 600 5 10 60

Fig. 17. A comparison of the descent velocity, vertical sampling resolution (samples per meter), and inwater photosynthetically available radiation (PAR) for SuBOPS (left) and an SPMR (right) in open-ocean

(Case-1) waters during clear-sky conditions. Only the upper 15 m of the profiles are shown to provide greater

detail of near-surface differences.

it moves upward in a vertical orientation with small vertical tilts in many cases. In fact, it is not unusual for the majority of the up cast to have vertical tilts less than 100 . This

is significantly different from legacy rocket-shaped profilers, which usually had an attachment point at the end of

the instrument, so when the instrument was retrieved, the

profiler started to lay over and tilted significantly. A notable exception was the microNESS design, which had a

small diameter main housing, a large flat fin area, and an

extended side attachment mounting plate (Fig. 11), so it

also had a tendency to kite upwards when first retrieved.

The kiting motion is particularly advantageous when

trying to keep the profiler at the surface, perhaps because a

temporary perturbation needs to pass. For example, when

waiting for a cloud to pass over the Sun, the profiler can

be bobbed at the surface by applying short sharp tugs on

the cable. With this technique, the instrument tends to

kite up to the surface and stay the same distance away.

This is desirable on a boat, because in many cases the

boat must maneuver if the profiler starts getting too close,

which mixes up the water and takes time. By bobbing

the profiler at the surface, the instrument can be kept in

a ready-to-deploy mode without much effort.

A tangible expression of the improved resolution that

is achieved with SuBOPS is the significant improvement in

capturing the high frequency perturbations associated with

wave-focusing effects close to the sea surface, as expressed

in the Fig. 17 PAR data. A notable consequence of the low

SPMR sampling resolution is a significantly aliased sampling of wave-focusing effects during clear-sky conditions—

the phenomenon appears as noise. For the SuBOPS data,

however, the central tendency of the in situ light field close

to the sea surface becomes evident. A picture of the SuBOPS prototype fitted with the new flotation capability is

presented in Fig. 18. Note the use of two fenders on each

radiometer to improve protection during deployment and

recovery. The red foam is a custom-blended, low-density

polyurethane resin specially formulated to have a very high

resistance to penetration by solvents.

Fig. 18. The SuBOPS instrumentation with the

custom-blended foam, which can be machined and

shaped (the weighted spar is shown not wrapped in

black tape for clarity of presentation).

A recurring problem in shallow water, particularly in

rivers or tidally influenced coastal waters, is the presence

24
Fig. 19. The SuBOPS instrumentation with the

mechanism for adjusting the pitch of the radiometers (left numeric scale) shown in greater detail.

J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

of currents. Currents push against a profiler as it descends

and create a tilt bias as a function of depth. Another

source of tilt bias is the power-telemetry cable, which when

released, starts to sink and pulls against the cable harness

attaching the cable to the backplane. Currents and cable

tension tend to act along the same axis, because the profiler

naturally aligns itself with the current in the direction of

the cable when it is held at the surface.

If the axis joining the two radiometers along the long

dimension of the backplane is considered the roll axis, then

the cable and current forces act perpendicular to that axis,

which is the pitch axis. Roll biases are basically caused by

the unequal weights of the two light sensors, and are easily

adjusted for by using the movable floats in the second tier

of flotation below the main flotation housing. Pitch biases

can be more significant and are removed by tilting the

sensors in opposition to the bias.

The mechanism used to overcome a tilt bias is based on

a two-point system, wherein one point close to one end of

the radiometer is fixed and a second point close to the other

end of the radiometer is allowed to pivot. The amplitude

of the opposing pitch is set by the distance that the sensor

is moved relative to the flat backplane (Fig. 19). Both

sensors need to be moved the same amount, so there are

numeric scales on both radiometers to ensure they are both

to the same bias angle (the numeric scale is a convenience,

because the distances are easily set and confirmed using a

small metal ruler or a caliper).

floats of variable thicknesses, allows the user to trim SuBOPS to a wide range of descent velocities (10-100 cm s-1)

while maintaining vertical tilts to within 5°. If there is

more than one current system within the water column, or

if one in particular is especially strong in one depth regime,

it may not be possible to find a pitch adjustment that adjust for the ill effects of the current while permitting good

tilts outside the influence of the current. It should be possible, however, to find a setting wherein more good data

are collected than if the tilt adjustment was not available.

The original mechanical configuration of BSI optical

sensors had the pressure transducer and temperature probe

fitted in the Ed sensor. The normal location for the latter was on the end cap, which meant it was positioned

rather deep in the water column, even when the profiler

was close to the surface. This meant that the shallowest

observations did not have simultaneous water temperature

(T) data, which was a significant disadvantage in optically

complex waters, because such waters are frequently characterized by one or more near-surface layers of differing

optical properties. For example, in an estuary it is rather

typical to have a thin layer of fresh river water overlaying

the saltier oceanic water below. To ensure near-surface

layers were detectable by more than just their light signatures, a temperature probe was added to the Lu end cap,

which meant it was very close to the same plane as the Ed

cosine collector (Fig. 20).

Also shown in Fig. 19 is the weighted spar (which is

not wrapped in black tape for clarity of presentation), with

stainless steel nuts and washers to finely tune the descent of

the profiler. The combination of using small weight adjustments, a pitch bias, a compressible bladder, and movable

Fig. 20. The SuBOPS instrumentation with the

extra T probe fitted inside a perforated shroud for

protection and mounted on the L,,, sensor end cap

(bottom right). The multicolored radiance aperture

for COPS (Chap. 4) is visible in the background

(top right).

25
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

2.8 Summary

In-water legacy systems are not always well suited for

properly resolving the optical complexity of shallow waters, such as near-surface layers, principally because of

their large instrument size, proximity of the sampling platform, or high rate of descent. The exceptional performance

of the 19-channel PRR-800 made it the ideal foundation

for a new profiler specifically designed to meet the unique

challenges presented by deployments in optically shallow

waters. The first steps in this development were a reduction in the diameter of the housing and minor modifications in the rocket-shape of the existing PRR-800 free-fall

collar. The resulting BioPRO system was adaptable and

stable enough to deploy in a variety of environments, could

utilize the available self-shading algorithms, but was ultimately too long and descended too quickly to use in optically shallow waters.

Comparisons of BioPRO and microNESS (a rocketshaped legacy profiler) showed that a radically new approach was needed. Inspired by the split PRR-800, the new

profiler divided the downward irradiance and upwelling radiance capabilities of the PRR and separated them on

either side of a wide, thin plastic back plane, forming a

unique, kite-shaped profiler. The new SuBOPS backplane

featured semi-rigid foam disks along the top, and stainless steel weights at the bottom, so buoyancy (and, thus,

the descent rate) could be finely adjusted. The descending

pitch, roll, and rate of fall was controlled by adjusting the

amount and position of floats and weights attached to the

backplane. At the end of the cast, SuBOPS kites up to the

surface by simply hauling in on the power-telemetry cable.

With PRR electronics supplying acquisition rates of 12 Hz

for all 19 channels, descent speeds of less than 20 cm s-1,

and vertical resolutions for coastal profiles of 1.0-1.5 cm,

observed instrument tilts were regularly less than 5° from

the vertical.

Inspired by the excellent results obtained with the prototype unite, three additional innovations were incorporated to the SuBOPS free-fall back plane:

• Replacing the semi-rigid foam flotation disks with

a machined, rigid foam material;

• Adding a flotation chamber filled with multiple elongated compressible air bladders to produce a hydrobaric profiling system; and

• Engineering a pivot point in the back plane that

allows an adjustment in the mounting angle of the

light sensors relative to the backplane.

Even though all three of these innovations improved the

amount of high-quality data collected during a profile, the

hydrobaric buoyancy system provided the largest increase

in data and vertical sampling resolution.

The custom-blended rigid foam is easily machined and

impervious to water even after many profiles, ensuring consistent buoyancy from profile to profile, as well as campaign

to campaign. A SuBOPS instrument equipped with hydrobaric profiling has a significant capability to loiter very

near the surface and sink very slowly (less than 5cros-1).

As the bladder compresses, the descent rate increases until

the amount of buoyancy supplied by the air bladder vanishes and terminal velocity is reached. The final improvement with SuBOPS allows the system to be deployed in

situations where the telemetry cable is pulling the pitch

of the instrument outside of the usable tolerance, such as

in tidal currents or riverine outflows. As much as 8° of

positive or negative pitch bias in the backplane can be

accommodated by pivoting the V-blocks that fasten the

instruments to the back plane. Vertical profiles using this

system have been acquired at temperate and arctic stations

in less than 3m of water depth with free-fall velocities of

less than 10 cros-1 , which provides vertical resolutions of

less than 1 cm.

26
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Chapter 3

Development of the Microradiometer

CHARLES R. BOOTH, JOHN H. MORROW, AND RANDALL N. LIND

Biospherical Instruments, Inc.

San Diego, California

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

ABSTRACT

A microradiometer consists of a photodetector, preamplifier with controllable gain, high resolution (24 bit.)

analog-to- digital converter (ADC), microprocessor, and an addressable digital port. In other words, it is a fully

functional networkable sensor, all of which resides on one small, thin, circuit-board assembly that is sleeved

inside a metal cylinder. With the addition of the front-end optics (collector, window, and filter stack), the basic

form factor resembles a shortened pencil. The microradiometer design was developed in response to a need for

smaller, faster, and potentially less expensive radiometers, which could be easily scaled to either more or fewer

channels and more easily deployed in coastal waters. Because each microradiometer channel has an individual

ADC, no multiplexer is required, and no cabling is needed, thereby eliminating a source of electronic leakage

and improving reliability. The metal cylinder provides additional isolation from electromagnetic interference

sources (e.g., radio frequencies). The photodiode current is converted to voltage with an electrometer amplifier

with originally two, but subsequently three gain settings, and the resulting voltage is directly fed to the ADC.

The entire assembly, including the photodetector, is located on a single circuit board measuring 0.35 x 3.0 in 2.

Each microradiometer is also equipped with a temperature sensor located close to the photodetector. Clusters

of microradiometers can be matched with front-end optics to form small, fast, less expensive, multiwavelength

radiometers for a variety of measurements. Each cluster is managed by an aggregator that allows the array of

individual radiometers, plus any ancillary sensors, to function as a solitary device.

3.1 Introduction

Greater radiometric accuracy, decreased costs, and the

need for multidisciplinary research are all technologyforcing challenges to be addressed by the next generation of

optical instruments. To develop a commercially successful

marine spectroradiometer, the device must also appeal to

the majority of investigators working in this field. This effort focused on the development of a smart photodetector,

called a microradiometer, and its ancillary interface modules. It is not an exaggeration to suggest that a microradiometer can be used anywhere that a filter-photodetector

has been used in the past. Thus, it is anticipated that this

microradiometer module will become the cornerstone of a

wide variety of instruments that help meet these needs.

When research objectives require clusters of microradiometers, the final instrument can be assembled easily

by selecting the desired filter assemblies and optical front

ends. It is unnecessary to modify the sensitivity of each

channel for optimal dynamic range because the sensitivity

is dynamically configured. Furthermore, the design eliminates many of the electronic interconnections required in

existing radiometer designs, which increases redundancy

and improves reliability. The principal benefits to the microradiometer development are:

• Increased radiometric accuracy in the field;

• Decreased instrument size and weight;

• Lower power consumption;

• Enhanced flexibility in the configuration of aboveand in-water instruments; and

• Reduced costs associated with production, calibration, maintenance, reconfiguration or modification,

and field research.

The culmination of these benefits is the entire analog signal

path is of minimum length, free of connectors or cables,

and is totally contained in a shielded metal sleeve.

As aquatic research shifts toward coastal monitoring

and shallow waters, deployments from small research vessels becomes a necessity. The principal advantages of the

27
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

microradiometer concept are a) greater accuracy, which

permits improved data products in a challenging environment; b) smaller size, which minimizes self shading for

in-water instruments; c) reduced weight, which facilitates

hand deployment from small boats; and d) lower power

consumption, which permits battery-operated scenarios.

Table 1 shows how clusters of microradiometers can be

arranged in tubular pressure housings using the MMS concept.. The combination of the number of microradiometers

and the sensor-housing diameter yields different packing

efficiency (Pe ) values. Although a 19-channel instrument

is not the most efficient, the multiwavelength sensors have

rather comparable Pe values. The 19-channel design was

chosen for the first C-OPS instrument (Chap. 4), to make

comparisons with SuBOPS, the principal legacy instrument (Chap. 2), as comprehensive as possible—the channel

configuration was exactly the same for the two profilers.

Table 1. The housing diameter (HD) in centimeters for different clusters of microradiometers as a

function of the number of photodetectors, Np. The

packing efficiency, Pe , for an Np and HD combination is the ratio of Np times the cross-sectional area

of a microradiometer (with clearance) to the cylinder cross-sectional area (minus sidewall thickness).

Np 1 4 7 13 19 31 37

HD :L1.3

.0

2.9

0.7

3.8

0.7

5.3

0.6

6.9

0.6

7.6

0.7

8.5

0.7

Alternative definitions for Pe are possible, but the one

used here attempts to balance the need for maximizing the

number of microradiometers while minimizing the size of

the sensor housing, so the self-shading effect is constrained

as much as possible. In other words, an optimal tradeoff is sought between the need for the maximum amount

of spectral information versus the desire to minimize sensor size and self-shading effects. The latter is particularly

important for high-quality data products in turbid waters.

Using clusters, a single radiometer approximating the

size of a fountain pen or a high-performance, hyperspectral sensor with a diameter less than 9 cm (3.5 in) can be

built. From a packing efficiency perspective, double-ended

7-channel sensors, with Ed on one end and Lu on the other,

would be preferred, because this would provide 14 channels

in a 3.8 cm form factor. Two sensors are typically used in

a free-fall design, so there is no increase in the number of

sensors, just their configuration.

The underlying electronics for all sensor scenarios presented in Table 1 are the same (as is the rated depth

for a standard housing, which is 100-150m), so savings

from true economies of scale can be realized specially for

machine-made components where per-unit costs decrease

significantly as the numbers produced increase. A depth

rating to 350 m is anticipated, but this has not tested or

marketed. There is no depth rating on the electronics.

Different packing fractions will require different housing

components, and each will require some level of redesign.

Furthermore, each packing fraction will require different

aggregator boards, either some different, or all different.

The microradiometers themselves are the same.

3.2 Miniaturization

One early issue in the development of microradiometers concerned the difference between the size of circuit

elements that could be manipulated by hand during testing (such as with jumpers, test leads, etc.) and those that

are physically too small to manipulate easily without special equipment. The final microradiometer has components

that are so small, they must be machine assembled, necessitating batch sizes of 100 or more. Consequently, an

intermediate hybrid prototype stage was needed to permit

adequate circuit construction and access by hand.

For the hybrid prototype (Fig. 21), the analog front end

for data acquisition was built in the final small form factor,

while the digital processing and programming portion—

where final miniaturization takes place—was scaled to allow human assembly and testing. The comparability of

testing of the hybrid with the final design was increased

using the same advanced printed circuit board materials

and board masking techniques as the final design.

Fig. 21. A hand-made microradiometer prototype.

The pencil-shaped board protruding to the right

(with the photodetector) is the final form factor,

which will be machine assembled (not shown are

the bottom-side circuits). The larger area behind

the protrusion is scaled for human manipulation.

When hybrid testing was completed, the final microradiometer circuit board was laid out for machine manufacturing using the Rogers RO4003C glass reinforced

hydrocarbon-ceramic laminate, a lead-free compatible process, coupled to a Parylene encapsulation conformal coating. The entire assembly, with photodetector, is located

on a single circuit board measuring 0.35 x 3.0 in (Fig. 22).

A fully assembled microradiometer, with metal sleeve and

28
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

front-end optics, has a dimension of 0.435 x 3.79 in2 . The

size of the photodetector is a primary driver of the final

outside diameter of the microradiometer. Other factors

that impact the final outside diameter are the wall thickness limitations of the material and machining for shield

tubes and filter sleeves.

Fig. 22. An individual microradiometer (bottom)

and then one sleeved in brass with fore-optics assembled (top). The metal cylinder provides mechanical support and isolation from electronic noise.

3.3 Data Acquisition Sequence

A significant hurdle in developing microradiometers was

creating the internal command and control firmware needed

to drive the data acquisition. Although designed as standalone intelligent photodetectors, up to 200 microradiometers can exist in a multiple-component system. In practice,

system expansion is only limited by data rates, and it is

useful to understand the sequence of events involved in

data acquisition. At the start of data acquisition, all microradiometers are issued a Sample command that starts

sampling with the desired conversion time and averaging.

When a second Sample command is issued, the current

sample (or sample average) is transferred into a buffer contained within each microradiometer. Data is transmitted

to the host computer via an RS-485 or RS-232 protocol.

The maximum acquisition rate is, therefore, limited by

data transmission and the number of channels configured.

Several of the commands available can be used with a

wildcard tag (*), so a population of microradiometers can

be controlled. For data acquisition, two of these are:

• ADC Sample Rate (4-125 Hz); and

• Averaging Time tip to 4.2 min at 125 Hz sampling

The following aspects of individual microradiometers are

also configured:

• Input Source, each microradiometer has three sets

of signal sources, although normally only one is

used—alternate sources may include grounded input and reference voltage, or temperature;

• Instrumentation Amplifier, not used except for

bridge sensors (e.g., temperature and pressure);

• Dark Correction, the microradiometer enters a

mode where it is presumed to be dark, and it averages several readings at all gain settings, which are

stored internally as the signal offset;

• Range Configuration, high gain, medium gain,

low gain, auto gain; and

• Gain Ratio, radiometer can alternately sample a

source with multiple gains, and compute the ratio

and standard deviation of the ratios (but only between adjoining gains).

3.4 Performance Analysis

Performance analysis covers linearity tests over a variety of current levels and at different temperatures. Dark

offsets and noise levels are also measured at a variety of

current levels. The results of the analyses are compared

with reference instruments. An important aspect of the

performance obtained is the choice of the photodetector.

For the results presented here, the Hamamatsu 51226 series was chosen because of the anticipated wavelengths for

C-OPS (Chap. 4), along with a sensible weighting of the

advantages and disadvantages of logical alternatives.

The Hamamatsu 51336 and 51337 series, for example,

has a better temperature coefficient, but it also has increased infrared (IR) sensitivity relative to the 51226 series, potentially adding out-of-band blocking requirements

in certain scenarios. Additionally, it has a decreased shunt

resistance relative to the 51226 series, which can amplify

front-end low frequency operational amplifier noise. There

are additional intricacies in the decision making to select

the photodetector, but these two represent the main tradeoffs that were considered in choosing the 51226 series for

C-OPS.

The 51336 series is being used for the 1,020nm channel for OSPREy. This photodiode was qualified for this

task after an extensive round of testing, similar to what is

presented here. It should also be noted that any system

should be subject to a full end-to-end characterization with

all of its specified components to evaluate linearity and stability. Different photodiodes cannot be arbitrarily placed

in circuits without actually testing the performance of the

resulting compound system.

3.4.1 Linearity Characterization

Linearity data were acquired using the flux from a light

emitting diode (LED) coupled into an optical fiber assembly that divides the flux from the LED into four output

fibers. One of the optical fibers leads to a silicon photodiode attached to a Keithley 6485 Picoampmeter, and

the remaining three fibers are directed to microradiometers under test.

The test sequence sets the LED at a variety of different

intensities (constant current mode) and then measures the

electrometer and microradiometers simultaneously. The

test can be run with either random light levels or with

a slow ramp, the ramp giving the smoothest results and

the random tests may show up autoranging artifacts more

easily. The system was fully automated with the LED

29
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

operating under computer control (Fig. 23). Test results

indicate that this device was linear to within ±1% over a

dynamic range of eight orders of magnitude and did not

saturate at the highest intensity level.

1.06

0

ro 1.02

o=

c

i

U

O

^ 0.98

n

0.94

0.0 6.0x10-' 1.2x10-4 1.8x10-4

Current [A]

Fig. 23. Measurements of the microradiometer amplifier using final design components and measurements of a reference electrometer.

1.06-

0

c^ 1.02-

c

m

1-

U

O

= 0.98-

a0.94

1X10-121 x10-91 x10-61 x10-3

Current [A]

Fig. 24. Same as Fig. 23, but plotted on a semi-log

scale. Measurements of the two radiometers agree

to within ±1% over the full range, except at signal

levels below 5 x 10 -11 A, which are affected by noise.

Microradiometers are typically highly linear throughout their range. At the high end, as the signal levels

decrease relative to the maximum, the reported noise increases (Fig. 24) because of a combination difficulty controlling LED brightness at levels below 1% of its maximum brightness, limitations in settling time for autoranging, limitations of the electrometer, and the inherent sensitivity of the microradiometers. Microradiometers are also

typically linear at the low sib ial end (Fig. 24), i.e., less

than 10 nA of photocurrent.

To verify that the devices would not saturate in full

sunlight, a microradiometer was assembled with a 24 KQ

gain resistor and operated on the roof of the BSI facility

with a 665 nm filter installed using a radiance front. end.

The device was pointed at the Sun, and a reading of 52 μA

was obtained. This reading is well below the maximum

photocurrent for the device. Using a magnifying lens, saturation was produced at a level of 171 μA. This is considerably larger than any natural photocurrent expected

anywhere on Earth.

3.4.2 Temperature Characterization

There are several areas where temperature is expected

to affect microradiometers: "dark" offset, sensitivity, and

stability at the gain switchpoints. To investigate dark offset and gain switchpoint offset, the microradiometers were

run over a series of temperature cycles over time. To investigate the sensitivity or gain as a function of temperature,

the 19 microradiometers from the first production run were

assembled and tested for temperature response. The host

data acquisition software was programmed to acquire data

at 0.2 Hz. Typically, every 3 min the data acquisition was

paused and each microradiometer was polled for temperature. During the series, temperature ranged fiom below

10°C to above 50°C.

3.4.2.1 Auto or High Gain Offset

The dark offset for a microradiometer is generated by

a combination of offsets of various electronic components.

At high gain, the normally largest contributor is the operational amplifier (electrometer amplifier or op-amp), which

has two major contributors to the total offset: input offset

voltage, and input offset bias current. Input bias current is

normally the largest contributor at high gain, and it is multiplied by the amplifier feedback resistor, which is 109 at

high gain and 106 at low gain. The amplifier that is used in

the microradiometers is specified with typically very small

input bias current (10- 14A), but this may reach 4x 10 -12 A

at 85°C. The bias current typically doubles every 10°C

above 25°C snaking high temperature operation more sensitive to varying temperature. The range of changes in

dark offset voltage with changing temperature during the

high gain tests was considerable, ranging from 90 μV to

3,472 μV. Two microradiometers showed negative temperature coefficients, and the majority showed some degree of

positive temperature coefficient.

Temperature behaviors are notoriously stochastic. The

AD7799 24 bit ADC has an input voltage offset temperature coefficient of 10 nV °C -1 , which is much smaller than

the op-amp coefficient of 1.3 μV °C -1 (but may range over

.K

•

•

.^ % •

•

r •

30
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

±2 μV and may not even be monotonic). Another accurate way to look at temperature coefficients is to use the

separate logged temperature data, which is periodically

extracted from the microradiometers. In these data, there

is little evidence that the offset is not tracking temperature, as expected. It is evident that there is a considerable

difference in the temperature effect between different microradiometers. In particular, the high gain temperature

drift has the most impact at temperatures above 30°C.

3.4.2.2 Low Gain

After completing the testing on auto gain, which selects high gain for the microradiometers, the devices were

programmed to run on low gainn to be able to examine the

expected temperature stability and performance near the

points where the circuit switches gains. If the device is zeroed at 20°C and its offset drifts up by 30 μV with respect

to the measured signal, the low gain reading will have a

maximum temperature-induced error of 30 μV/ 3400 μVor

0.8% at the switching point. Absent any other errors on

the high-gain side, this will result in a 0.8% discontinuity

in the transfer function.

For example, a typical microradiometer temperature

coefficient has a slope of -0.00124 V °C -1 , which corresponds to a measured value of -1.24 μV °C- 1 , or a change

of -0.0365%°C- 1 relative to the gain change point. This

implies the microradiometer would have a low gain, offsetinduced error of -0.365% for data recorded 10°C above the

temperature at which the radiometer was zero corrected.

Note that this temperature coefficient is consistent with

the specifications in the manufacturer's data sheets ("average drift 1.3 μV °C-`) for the operation amplifier that

is used in this circuit.

If certain applications suggest that an error that approaches these levels at the gain change point is objectionable, then a simple engineering change can be made that

increases the low gain level by, for example, a factor of

5. This then allows the gain change point to be raised to

17 mV, which would also reduce the potential nonlinearity,

due to temperature shift of the low gain offset, by a factor

of 5. It is important to dark correct all radiometers near

the temperature at which they will be used. Fortunately,

temperatures of 53°C should only be seen in heated transfer radiometers, and the changes at lower temperatures are

not as significant.

3.4.2.3 Gain Sensitivity

Gain changes due to temperature may arise from

changes in the resistor that sets the gain, change in the

reference voltage used by the ADC or by the ADC itself, or changes in the responsivity of the photodetector.

The ADC specifications state 1.0 ppm °C -1 . The reference used has a stated stability of 10 ppm °C -1 , typical,

and 25 ppm °C-1 , max. The 1 GSl resistors are specified

at 100 ppm °C-1 , and the 1 MQ resistors are specified at

25 ppm °C-1 . The microradiometer systems presented in

this report use the Hamamatsu S1226 photodiode, which

has a very low responsivity temperature coefficient—almost

zero for 400-600 nm. At longer wavelengths, the coefficient

starts to increase up to 0.5%°C- 1 at 1,000nm.

The lineator determines the optical gains at 660 nm, for

which the temperature coefficient of the detector is approximately +0.05% O C- 1 . Temperature testing reveals what

every field researcher is already aware of: if all instrument

is allowed to overheat in sunshine and is then deployed in

the (usually) much colder ocean, temperature effects will

be visible in the data. If a shaded location is not available,

covering the instrumentation with a white sheet while outside, and keeping the sheet wet in high temperatures, is

a simple but reliable way to prevent excessive solar insolation. The testing also implies that thermally regulated

instruments will be the most stable. In most cases, the

majority of this error could be addressed by temperature

correction.

3.5 Multiple Microradiometer Systems

Microradiometers are standalone photodetectors, but

clustering multiple microradiometers together to build a

high-performance instrument system requires additional

electronics. The exceptional expandability afforded by the

use of microradiometers results from an inherent organizational hierarchy that was designed into the system from

the outset. Adding to the ease of expandability is the fact

that the basic building block of a more complicated sensor

is the microradiometer, which is a sensor itself.

Depending on the complexity of the integration, the

basic system architecture for instruments built from microradiometers involves three major classes of electronic

devices, which are as follows: a) the microradiometers; b)

the aggregator components (Fig. 25); and c) the master

aggregator components (primarily used in the deck box,

which is the controller of larger sensor systems).

a^

^l

."I

Fig. 25. The aggregator assembly (to the right of

the brass microradiometers on the left) within an

MMS radiance sensor. The clear acrylic housing is

for demonstration purposes only.

Microradiometers are the operational optical sensing

units, each with a microprocessor, photodetector, optical

filter package, data acquisition system, and communications electronics (Fig. 26). They can be used as individual standalone sensors, but power must be supplied from

31
"N -

Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

an external source along with a communications interface.

For single-channel applications, e.g., measuring PAR, very

small handheld sensors are rather simple to make. For

larger sensors with multiple channels, a practical problem

is the packing efficiency of the cylindrical microradiometers, so certain channel numbers become critical in minimizing the size of the sensor (Table 1). The total number

of channels that can be installed in a sensor need not be installed when the sensor is first built. One of the advantages

of the microradiometer architecture is it is relatively easy

to replace or add microradiometers to reconfigure or complete a sensor. This flexibility allows a phased approach

for resource allocations.

Computer Microradiometer

Power Supply

Fig. 26. Individual (or sometimes multiple) microradiometers may be addressed from a computer

using a small, external power supply with a communications interface. This is the simplest configuration for single-channel sensors.

Aggregators are used to bundle larger collections of

microradiometers (Fig. 27) and ancillary sensors (such as

pressure or temperature) in individual instrument sensors.

Aggregators control the data flow to and from the microradiometers. They also have onboard removable data storage, which is provided by a microsecure digital (microSD)

card, and power control. They also provide additional

sensing capabilities including two-axis tilt angles, input

voltage and current, plus internal humidity and temperature.

Computer Aggregator Aggregator radiometers :i

Power Supply Ancillary

Sensor

Fig. 27. Adding an aggregator assembly permits

the control of data acquisition from multiple microradiometers plus one or more ancillary sensors.

To accommodate the dual objectives of supporting microradiometer clusters plus other smaller form factors, four

logic boards are needed. The required functionality is contained physically on two small rectangular circuit boards,

0.88 x 2.5 in 2. One board is dedicated to generating power

supplies for the aggregator and microradiometers, as well

as providing transient and over-voltage protection. The

other board is dedicated to microprocessor, communications, and memory functions.

The two circuit boards mount parallel to one another

with a 1 in round printed circuit assembly (PCA) connector at each end (new production units use round boards

as described in Sect. 3.6). One end provides downstream

microradiometers with power and communications. The

other end handles external communication (full or half duplex RS-485, and RS-232) and raw power, in addition to

providing a serial peripheral interface (SPI) bus for peripherals such as the pitch and roll module. The devices are

conformal coated to reduce stray currents that can degrade

performance, and their gains and offsets are automatically

adjusted during calibration at the factory. Ancillary devices can also be based on the microradiometer design to

create easily interfaced temperature, atmospheric pressure,

or other types of sensors.

Figure 28 shows a partially assembled, 19-channel cluster attached to a base aggregator board. Six more will be

installed on the aggregator (three each on the outer rows

with four microradiometers) for a total of 19 channels. Optical filter assemblies will be threaded into the metal sleeves

for wavelength selection (although, as discussed above, the

instrument could be completed later if resources were a

limiting factor). Table 1 gives the housing diameters that

can be achieved using microradiometer technology. The 7-

and 19-channel models are available as COTS instruments

for measuring either radiance or irradiance.

Fig. 28. Completed microradiometers are individually sleeved in brass and may be assembled in arrays

on an aggregator board. Shown are 13 individual

microradiometers.

When multiple sensors are in operation, a deck box

with an embedded master aggregator operates as a highspeed controller (Fig. 29). The master aggregator sets the

32
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

sampling rates and coordinates the polling of all the attached aggregators with their microradiometer assemblies.

This is the type of architecture used in aquatic profiling

systems, such as COPS, or for above water radiometry,

such as OSPREy. Indeed, Fig. 29 can easily be interpreted

as an Ed and Lu pair with a pressure transducer (top two

microradiometers plus ancillary sensor), and a solar reference (bottom microradiometers).

Computer Microradiometers

Deck Box C Ancillary

(MasterAggregator) Sensor

Aggregator

Fig. 29. When multiple sensors are being used, a

master aggregator located in the deck box operates

as a high-speed controller.

Figure 30 shows a functional multichannel radiance sensor, complete with the optical front end, 19 brass-encased

individual microradiometers, and one aggregator assembly.

The actual pressure housing for this radiometer, which is

made from aluminum (not clear acrylic), is 2.75 in (7 cm)

in outside diameter. The different colors of the 19 apertures correspond to the different wavebands selected for

each microradiometer.

Fig. 30. A 19-channel radiance sensor built from

microradiometers and using the architecture diagrammed in Fig. 27. The clear acrylic housing is

for demonstration purposes only.

3.5.1 Irradiance Sensors

The MMS architecture is expandable, and up to 200

microradiometers can be used in a multiple-component system with up to 10 data aggregators. In practice, system

expansion for a radiance sensor is only limited by data

rates, because each microradiometer, after application of

the fore-optics is, in fact, a radiance sensor. Irradiance sensors are considerably more constrained, however, because

each microradiometer has to properly view the solitary diffuser used in the construction of the cosine collector.

The fore-optics used with microradiometer irradiance

sensors are patterned after the PRR cosine collector. This

collector has outstanding directional response and mechanical stability. It was modified once before, but only slightly,

to accommodate the smaller-diameter housings required by

BioSORS and SuBOPS. For those applications, the primary modification was to scale down from the 4 in diameter of the original PRR housing, to 3.5 in for the BioSORS

and SuBOPS sensors. Although the MMS irradiance sensor is smaller still, the modified PRR design proved robust

in the past and was the logical starting place to begin

designing an MMS irradiance sensor. Note that the size

reduction and sensor modifications associated with producing the BioSORS and SuBOPS sensors were part of

the first steps in the incremental progression of designing

and developing the microradiometer architecture.

The PRR cosine collector was specifically constructed

to try and meet the instrument design specifications of the

Protocols in an economical and robust design. To cover a

wide wavelength range, the irradiance diffuser consists of

a raised trapezoidal quartz piece covered with a thin sheet

of vacuum-formed Teflon, which acts as a diffuser (Morrow et al. 1994). Thin Teflon diffusers have been used

for many years, especially in the construction of UV instruments (Booth et al. 1994). For the PRR, up to 19

filter-photodetector assemblies (FPA) are set in a circular

array at the base of the assembly arranged to view the

same area at the bottom of the diffuser. To improve the

azimuthal response, in addition to the integrating cavity,

a secondary Teflon sheet diffuser is installed at the base

of the reflecting cavity and the detector array is aimed at

this diffuse light source.

As with all flat diffusers, the Fresnel reflectance of the

exterior Teflon surface increases at large incidence angles,

which significantly decreases the flux. To mitigate this

problem, the side walls of the trapezoid dome both increase

the signal and provide a surface with a reduced angle of incidence, thereby increasing the response from larger zenith

angles. An occluding ring is used to further tune this relationship. The PRR cosine collector agrees with the cosine

law to within ±5% for incidence angles less than 70°, and

±10% for the entire working envelope. A cosine collector

should not acquire light from angles greater than 90°, so

an outer rim raised to the level of the top of the diffuser

acts as an additional occluding ring. Symmetrical diagonal notches in the outer rim of an above-water irradiance

sensor allows the release of any water that might become

trapped in the recessed area between the rim and the diffuser.

33
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

To cover a wide spectral range, the Teflon diffuser is

coupled to a reflecting cavity, which was carefully designed

to function as an optical integrator and to also provide the

necessary strength for operation underwater at depths up

to 350 m. Underwater, the integrated error in measured irradiance of a uniformly diffuse light field is less than 2.5%

throughout the visible spectrum, and less than 1.5% for

the corresponding surface reference used in air. In comparison, experimental uncertainties from factors such as

alignment of the instrument in the test fixture were observed to contribute as much as 2% uncertainty at 60° and

6% at 80°.

3.5.1.1 The Lens Solution

A microradiometer is a complex assembly of filter, photodetector, acquisition electronics, and microprocessor, all

packaged in a thin metal cylinder. For multiple waveband

instruments, it is not possible to tilt the individual microradiometers to orient them at the center of the secondary

diffuser and still maintain a small sensor diameter, because

microradiometers are too long. Several options were considered to solve this problem. The final design uses a single

lens to control the FOV and viewing angle, centering each

microradiometer on the same area of the lower intermediate diffuser. A conceptual illustration of what an irradiance sensor looks like, using the components discussed

here, is presented in Fig. 31.

Fig. 31. A cut-away illustration of the primary

features of an MMS irradiance 19-channel sensor.

In order to choose the lens, the original PRR design

was modeled using Zeemax ray-trace software from ZEMAX Development Corp. (Bellevue, Washington). The

software was first used to set the baseline design. The

positional nomenclature for FPA reflects the relationship

between the detector and the center of the collector. Given

the concentric circular arrangement of detectors, a detector

positioned on the outer ring of detectors is referred to as

an edge detector, because it is closest to the outer edge of

the array (and the outer edge of the housing). The centermost detector is called the center detector and all others

are middle detectors occupying intermediate positions in

the array.

The angle of incidence (AOI) to the FPA is important,

because interference filters change their passband and outof-band properties with changing AOI. A comparison of

the traditional PRR optical design with an early proposed

lens design is shown in Fig. 32. The six spot plots display the distribution of the radiance intensity of the rays

as traced from the rear diffuser impinging on the photodetector, as a function of changes in the AOI. The lens design results in a larger distribution, and the edge pattern

(lower right.) has significant vignetting and distortion with

respect to the baseline image above it. This distortion has

two aspects: a) the wider pattern indicates a greater range

of angles of flux on the detector (and, thus, through the

filters); and b) the vertical asymmetry warns that the position of the photodetector in the array may influence the

resulting cosine response, particularly if the field is asymmetrically illuminated.

-W.•

Fig. 32. A comparison of the traditional PRR

optical design with a proposed lens design. The

plot shows the radiance intensity distributions of

the rays from the rear diffuser hitting the photodetector as a function of incidence angle. In these

renderings, the PRR baseline is the top row and

the candidate lens design is the bottom row.

To better understand the implications of the asymmetrical collector illumination using the lens approach, a PRR

cosine collector with an empty instrument body was assembled, and a macro lens with a digital camera was installed

to take an image of the second cosine collector while under

worst case illumination (50 cm from an FEL lamp source).

In this instance, the worst case was grazing illumination of

ETER

34
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

the cosine collector where approximately 50% of the collector was illuminated.

Figure 33 shows the image taken of the secondary diffuser of a PRR instrument when the primary diffuser (the

cosine collector) is illuminated at an extreme grazing AOI.

The left image is untouched, and shows a slightly illuminated crescent on the left side. The image on the right

is contrast enhanced (to the maximum) to accentuate the

gradient.

Fig. 33. A digital photograph of the bottom of the

secondary diffuser when exposed to extreme grazing illumination angles. The right image has the

contrast artificially enhanced.

Color distribution histograms for Fig. 33 show the distribution of color across the surface of the diffuser is affected by the asymmetries. Unlike a normally toned photograph, the ideal histogram shapes in this case would be

narrow and Gaussian. Although this was found to be true

for the blue channel, the red channel displayed two forms

of asymmetry. Horizontal and vertical sections through

the image of the collector taken with only the red channel

active, showed a semicircle of brighter light on the left side

(Fig. 33).

These results indicate the red channel is less diffused

than the shorter wavelengths, because the diffuser is less

diffuse in the longer wavelengths. As a practical matter,

this indicates detectors with longer wavelengths should be

positioned at the center of the detector array, and the

shorter wavelength detectors at the outer positions where

the diffuser is almost completely effective. The use of secondary baffles near the filter to constrain the angular distribution was explored, but this approach was discarded,

because it caused too much asymmetry in the imaging of

the secondary collector.

A variety of COTS lenses with different focal lengths

were modeled using Zeemax. The results were evaluated in

terms of the amount of asymmetry in the collection of flux

from the second diffuser, the range of AOI on the filter,

and by total throughput. The final design achieved approximately 20% better collection at the edges, with only a

slightly broader AOI distribution. Figure 34 illustrates the

relative position of the optical elements comprising the irradiance collector and fore-optics. The commercially available plano-convex lens uniformly projects the flux exiting

the secondary diffuser to allow a parallel, radial arrangement of microradiometers.

Fig. 34. A magnified illustration of the primary

components of an MMS 19-channel irradiance sensor.

3.5.1.2 Irradiance Testing

A full calibration sequence was used to determine how

well the irradiance configuration performs. The sensitivity

was measured using the standard BSI calibration protocol

with the irradiance sensor placed 50 cm from an FEL standard of spectral irradiance traceable to NISI. Data were

recorded at a sample rate of 5 Hz, for a period of 60 s, first

with the direct beam blocked and then with the beam unblocked. Analysis of the resulting data show an ADC resolution is 0.5 μV and minimum detectable photocurrents

of less than 10 -15 A.

Sensor performance is not just measured using light

data—the dark data are equally important. The noise

equivalent irradiance (NEI) is the irradiance level equal

to the standard deviation of the dark signal. The main

gain element, the feedback resistor, is 1 GQ in the MMS

and 10 GQ in the PRR-800. Because of the lower resistance used with microradiometers, a higher NEI is expected than for the PRR. The noise levels of the MMS

sensor is about 1.5 times higher than a PRR, which is

expected given the factor of 10 difference in feedback resistors used at the highest gains in the two instruments.

The average NEI value for the wavelengths tested was

4.2 x 10-5 μW cm-2 nm 1.

One key part of the field test was to measure the global

solar irradiance (Fig. 35), which was done at the BSI rooftop calibration facility. While the tests were marred by a

computer crash at 0100 local time, the remainder of the

testing was nominal and was completed without incident.

The data were originally recorded at 5 Hz. At this rate,

SOLID QUARTZ

INTEGRATING

CAVITY

o ONDARY

DIFFUSER

LENS

OR SURFACE

19x uRADIOMETER

ASSEMBLY

35
10 0

45

10 - 2 35

in m

N

W 10 a 25 a

m

Q 0 a

10 -6

15

t^

s

Temp.

e

10 .8 5

0 12 24 36 48 60

Elapsed Time [h]

PAR

0^

Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

432,000 scans per day were recorded, with each scan containing the average of 25 ADC operations, or around 106

observations per day, per channel. No data were discarded

before they were subsequently binned into 1 min (boxcar)

averages. The small hump in the PAR data corresponds

to nighttime and is caused by the full Moon, which is

clearly seen on the cloudless nights. Although a notable

feature in the nighttime data, a temperature-controlled instrument would be better suited for accurately monitoring

the monthly lunar cycle.

Fig. 35. The global solar irradiance, presented as

PAR (blue), and the internal sensor temperature

(red) over approximately a 60 h time period starting

close to local noon. The data gap is from a failure

in the computer used to log the data.

3.5.2 Radiance Sensors

Radiance sensors built using the MMS architecture are

considerably less complex than irradiance sensors, because

the basic building block—the microradiometer with frontend optics—is essentially a fully functional radiance sensor.

The design of the radiance MMS also has roots in the successful PRR radiance design, but is carefully rescaled to

a more compact size. Consequently, there are no significantly new aperture components for the radiance sensor,

such as the lens that had to be added to the irradiance

sensor design.

The radiance design uses a clear quartz pressure window designed for a working depth of up to 350m. The

upper aperture plate, along with the distance from the

plate to the detector plane, determines the FOV, while

the lower aperture plate and light traps reduce out.-of--field

stray light. The sensor has an 18° FAFOV in air (13.4° in

water). A cut-away illustration showing the relative position of the components is presented in Fig. 36 (compare to

Fig. 31).

URE WINDOW

APERTURE PLATE

'ACER ! LIGHT TRAP

R APERTURE PLATE

=R ASSEMBLY

OTOD ODE

%DI011AETER

SFMBIJFS

Fig. 36. A cut-away plane view of a 19-channel

radiance sensor based on microradiometers.

To determine how microradiometers function in the

MMS architecture to measure radiance, a full calibration

sequence was performed. The sensitivity was measured

using the standard BSI calibration protocol, with a calibrated 100 cm' Spectralon plaque at 287.1 cm from an FEL

standard of spectral irradiance traceable to NIST. The instrument viewed the plaque at an angle of 45°. Data was

recorded at a sample rate of 5 Hz, for a period of 60 s, first

with the direct beam blocked and then with the beam unblocked. The computed responsivity of the MMS radiance

sensor shows it is slightly more sensitive than the PRR-800

designs.

As noted earlier for irradiance sensors, useful performance information is also derived from dark data. The

noise equivalent radiance (NER) is the radiance level equal

to the standard deviation of the dark signal. The main

gain element, the feedback resistor, is 1 GO in the MMS

and 10 GO in the PRR-800. Because of the lower resistance used with microradiometers, a higher NER is expected than for the PRR. A comparison of the two sensor types, however, shows they are very similar in performance. The average NER value for the wavelengths tested

was 3.6 x 10-6 μW cm-2 nm-1 sr-1 for the microradiometer.

3.5.3 Ancillary Sensors

The following ancillary sensors are regularly integrated

into profiling instruments based on clusters of microradiometers: a) water pressure, b) water temperature, c)

vertical two-axis tilts (pitch and roll), d) internal nitrogen pressure; and e) internal temperature. Three distinct approaches to data acquisition are used, depending on the sensor type. Water temperature and water

36
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

pressure are integrated directly with microradiometer-style

data acquisition hardware, but benefit from an onboard

programmable gate array (PGA). To the aggregator, they

appear to be simply another microradiometer sharing the

firmware command set and data bus. The pressure measurement uses a piezo-resistive silicon, absolute, pressure

sensor that is temperature compensated and offset corrected. The sensor is available in a variety of ranges from

0-5 psia up to 0-500 psia. Water temperature is recorded

using a custom-manufactured platinum resistance temperature detector (RTD) with a stainless steel probe, which

meets or exceeds the International Electrotechnical Commission (IEC) standard 60751.

There are several sensors that are used for engineering monitoring purposes, including internal housing temperature and pressure. The housings are back-filled with

dry nitrogen, and a pressure sensor is used to monitor the

state of the nitrogen fill. Similarly, the internal electronics

temperature is monitored as a diagnostic during extreme

temperature conditions and as an additional indicator of

overcurrent events. These so-called housekeeping- channels

are integrated to a low resolution, 10-bit ADC located on

an aggregator PCA.

Instrument two-axis tilt (pitch and roll) is monitored

using a low-power, dual-axis 1.7g accelerometer with an

SPI digital interface. Data are harvested from the SPI and

integrated into the data stream as an ancillary channel.

3.5.4 MMS Deck Box

Monitoring for sensor problems and determining their

cause(s) is an important function for ensuring successful

field deployments and good science. In-water instruments

can sometimes fail because of housing leaks. Deployment

conditions in the field sometimes require cables that are

hundreds of meters long, and experience has shown time

and again that field failures are frequently caused by connector or cable problems. In anticipation of these issues,

strategies and protocols can be used to reduce the complexities of deploying instruments in the field. Part of the

mitigation strategy adopted with MMS instruments is as

follows:

• Each microradiometer can monitor its supply voltages.

• Each aggregator can measure each of the regulated

voltages it produces.

• Each aggregator can measure its input voltage and

the current it is drawing.

• Each aggregator has a built-in pressure sensor to

monitor the atmosphere in the instrument and detect leaks.

• The master aggregator (usually in the deck box)

automatically queries each aggregator attached to

it, and verifies the voltage levels at the aggregators,

and then adjusts the voltage to run the subordinate aggregators at the optimum voltage, thereby

eliminating power supply issues caused by increased

loads or resistances.

• The master aggregator can turn off power to either the surface or underwater sensor assemblies if

it detects abnormal power draw or lack of proper

response from the remote sensors.

The deck box (Fig. 37), which usually contains the master aggregator, also has a four-line liquid crystal display

(LCD). The LCD provides feedback to the operator when

power is applied. Specifically, the display shows how many

sensors are found, the battery voltage (in the case of battery power), and warnings of unusual voltage or current

levels. This is all done, even if the computer used for data

acquisition is not connected. When the data acquisition

software is operational, the host can be configured to display messages on the deck box LCD.

Siosphe.=Lu r, Lruments Inc,

Microradiomeler

Master Controller

•

F4

9M

r i

%bo A

4 ..

m^

n4

Fig. 37. The front panel of the deck box used

with optical instruments built with the MMS architecture. The LCD is the green rectangle in the

bottom-middle portion of the panel.

The deck box is configured to communicate with the

computer using either RS-232 or RS-485 protocols, both

at 115,000 baud. The RS-232 signaling was selected as

the primary deck box-to-computer protocol because, unlike RS-485, it is native to many computers. In the future,

microradiometer deck boxes will be built with a Universal

Serial Bus (USB) interface. Compared to USB, RS-232

is much closer to the raw logic level communications from

the microprocessors used in the deck box and other system

components. Although USB is becoming increasingly common, RS-232 is enormously robust and has worked well for

many years.

Communications between the deck box and remote ab

gregators use RS-232 or RS-485 signaling and either fullor half-duplex. Initially, full-duplex RS-485 was chosen

based on the extensive experience with it. Subsequently,

37
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

a half-duplex version has been qualified and is being field ADC units were allowed to run freely, which means they

tested. Half-duplex is attractive because it reduces the are not quite synchronous, or, depending on the viewpoint,

number of conductors in connectors at the sensors, which

should increase reliability. Cable size and weight will also

be reduced.

In addition to the direct deck box status display, each

microradiometer has a set of diagnostic flags that can be

reported on request. In a similar fashion, the aggregators

also have a set of flags to provide alerts regarding a variety

of communications and sampling problems. Aggregators

also report the voltage and current at the sensor inputs,

internal temperature, and in more recent units, pressure

of an internal inert gas fill.

3.5.5 MMS Operational Details

During the development of the MMS architecture, several methods of running the microradiometers were considered, including choosing the sample rate of the ADC (rates

from 4-500 Hz were available). Microradiometer prototypes at a variety of sample speeds were tested and compared to sampling theory and the published data sheets for

the components. The choice of 125 Hz operation was based

on the range choice that the microprocessor makes when

sampling the analog input. If the microprocessor detects

that the sensitivity is too lour , then the gain of the circuit

is increased, and the following three samples are rejected

to allow for a settling time of the analog front end, and the

next sample is processed.

After the readings are made, and if more than three

consecutive readings on the same range (either high or low)

have been made, then the offset stored in memory for that

range setting is subtracted from the ADC reading, and the

resulting number is multiplied by the scale factor for the

range. Each of these parameters, the number of readings

to skip, the range delay after changing ranges, the scales

for each gain, and the offsets for each gain, can be set by

the user. In addition, automated routines in each microradiometer allows for the determination of the gains and

offsets assuming appropriate conditions at the front end

(like light or dark) are present.

Because of range changing, and the need for settling

time plus the desire not to report missing data if a range

change occurs, a fast sampling rate was needed. Testing

revealed that even given averaging, noise increased at rates

over 125 Hz, so the 125 Hz rate was selected, because it

allowed at least one sample in every data transmission if

there was only one gain range event. Missing data (coded

as -999) can occur when there are multiple range changes

occurring during the sample average period, but this is

exceedingly difficult to avoid in any autoranging system.

A truly synchronous system could have been designed,

with all microradiometers sampling at exactly the same instant, but this would have required a master clock propagated from the master aggregator; the overhead for this requirement was considered undesirable. Consequently, the

are synchronous to within 8 ms. The latter is considerably

faster than the bandwidth of the system.

A key aspect of being able to operate a system built

with microradiometers is that it must run dependably at

the selected sampling rate. Ensuring this task executed

reliably required considerable testing and code revisions.

In the course of documenting the system performance presented here, the system consisted of three 19-channel sensors (57 microradiometers), plus a coordinating master aggregator operating at a 20 Hz. Each microradiometer was

continuously converting in autoranging mode at a rate of

125 Hz. This means that there were over six ADC cycles

performed during each sample report. The individual microradiometers asynchronously convert and store a running

average of the measured signal.

An example of the sequence used to document system

performance is as follows:

1. The master aggregator issues a W command to the

subordinate aggregator to start polled sampling.

2. The receiving aggregator zeros the sampling bufferaccumulator as sampling continues. In the case of

20 Hz sampling, 50 ms after the original start command, a second start command is sent from the

master aggregator, passed down through each attached aggregator, to the microradiometers.

3. Each microradiometer then transfers its accumulated reading to the transfer buffer, zeroes its accumulator, and continues sampling, all without sib

nificantly interrupting its continuous sampling at

125 Hz.

4. Following this second start command, each aggregator serially requests readings from the microradiometers that are attached to it, placing the data

into a transfer buffer. At the same time, the master

aggregator asks the first attached aggregator for its

data and waits for a reply.

5. After the data arrives, it queries the next aggregator, and so on, until it has all the data, at which

point it issues a new start command and the sequence repeats.

Note that this entire sequence must happen at the required

sample rate.

To run as quickly as possible, timing gaps must be

used very efficiently. For the current MMS architecture, it

should be possible for four sensors to run at the same nominal (15 Hz) rate. If faster operations are needed, there are

several housekeeping characters in the microradiometerto-aggregator communications scheme that could be eliminated. In any case, four 38-channel sensors operating

at 15 Hz can be accommodated with standard 115 Kbaud

communications and existing hardware. To increase beyond that, 230 Kbaud communications may be required.

38
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

3.6 Improvements and Enhancements

The original two-gain microradiometer was introduced

in 2008. While performance was excellent for marine applications, when the application of microradiometers was

recently expanded to include sun photometry in support

of the OSPREy project, the maximum signal level that

the device could accommodate proved too low. Because

the sun photometry application involves having the same

radiometer also view the very bright Sun as well as the

much darker sea surface (to derive a water-leaving radiance), more dynamic range was required and this led to

the three-gain microradiometer (3GμR).

The original microradiometer gains were separated by

a nominal factor of 1,000, while the 3GμR employs three

gains separated by a factor of 200. The analog front end

3GμR was redesigned with different components, but the

physical package outline and connections remained identical as shown in Fig. 38 (compare to Fig. 22).

list .......0 w timu^ullw

Fig. 38. A side view of a 3GμR microradiometer

(top) showing the two-sided circuit board design,

and a sleeved version with fore-optics attached (bottom). The ruler is marked in centimeters.

Adding a third gain stage had several advantages:

• The difference between the gains was reduced from

1,000 to 200, which decreases the stabilization time

needed after a gain change.

• The voltages at which gains increase were increased

by a factor of 2.5 resulting in a decrease of the

medium and low gain offset uncertainty.

• The photocurrent signal range was increased to accommodate 40 times higher saturating signal levels

(160 μA).

The circuit architecture of both the two- and threegain microradiometers is very similar. The photodetector is connected to a high performance operational amplifier. The amplifier is configured as a current-to-voltage

converter with the gain set using a resistor and capacitor

combination selected by an analog switch. The resulting

voltage is digitized by a 24 bit ADC normally running at

125 Hz. The 3GμR has an ADC resolution of 0.5 μV, and

at maximum gain, a current resolution of less than 1 U.

A microprocessor controls the switch and ADC, implements autoranging, and calculates a running average. By

operating at 125 Hz, range changes can usually be made

(and time allowed for settling) while maintaining an uninterrupted stream at rates as high as 20 Hz. Before each

reading is averaged, it is corrected for offset and gain and is

expressed as a floating point number, which is then added

to the average. If a range change occurs during an averaging period, readings can be dropped while still being able

to calculate and report an average, which minimizes data

gaps at signal regions where gain changes occur.

The firmware contains modes for automatically determining dark offsets at each gain, gain ratios when exposed

to appropriate light levels, and configuring various operation modes such as ranging mode, switch points, and other

parameters. It is also possible to program the system for

manual gain, different ADC sample rates ranging from 4—

125 Hz, and sample averaging periods from over 20 Hz to

periods up to a minute.

Both versions of the microradiometer contain a temperature sensor, and circuitry to monitor the supply voltages.

Measurements of single 3GμR devices indicate that when

configured for radiance measurement (14° FOV) at 490 nm,

the saturating radiance is 8.5 mW cm-2 nm-1 sr-1 , and

the detection limit is 0.14 pW cm -2 nm-1 sr-1 at 1 Hz.

This represents a range of approximately 6 x 1010

Another advancement since the production of the prototype MMS instrument, is the reduction in size associated with the aggregator assembly. A so-called shortstack aggregator design repackages most of the components

from the original aggregator, deleting the data storage

card, but adding connectors for additional microradiometers connected via cable (enabling a radiometer design with

arrays at each end), and for the addition of data storage

mounted on an additional small circuit board. A picture

of the new aggregator, which is about 4.7 cm shorter than

the original (Fig. 25), presented in Fig. 39.

Fig. 39. The new aggregator board assembly (left)

shown with a 19-channel array populated with seven

microradiometers for clarity. From left to right are

the connector board providing interface with external devices and RS232, RS485, and microradiometer data busses. The second board is the microprocessor board, followed by the power supply board,

the microradiometer interface and connector board,

and finally, the microradiometers in brass cylinders

with fore-optics attached.

39
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Although compact, numerous test points are provided

as part of the new design. The new aggregator configuration also provides a physical pathway to route a rigid

optical fiber assembly, a provision that the OSPREy radiometers use to combine spectrograph and microradiometer technologies in the same instrument. An internal pressure sensor is also included that allows integrity monitoring

of the 30 psi dry nitrogen fill of the instrument cavity. This

aggregator retains the salve power conditioning circuitry,

real-time clock, RS-232, RS-485, two-axis tilt sensor, and

firmware as the original aggregator.

3.7 Microradiometer Specifications

Each microradiometer contains its own complete control and acquisition system composed of a microprocessor, 24 bit ADC, voltage reference, temperature sensor,

and electrometer front end. The electrometer is configured with three gains controlling the conversion of current

to voltage.

A cluster of microradiometers, typically sampling different wavelengths, is controlled by an aggregator which

synchronizes the polling and acquisition of signals from

each of the individual microradiometers to ensure simultaneous sampling at all wavelengths. The aggregator also

contains the power conditioning circuitry and data communications interfaces, and may also be equipped with internal data storage (1 GB microSD card) to support remote

data logging.

The principal specifications of sensors built with microradiometers are as follows:

• Detectors: Silicone (Si), indium gallium arsenide

(InGaAs), or gallium arsenide phosphide (GaAsP).

The form factors for the three types of detectors

are 13 mm' for the former, and 7 mm' for the latter

two, respectively.

• Photocurrent-to-Aloltage Conversion: Electrometer

amplifier with three gain stages-1, 200, and 40,000.

• ADC: 24-bit bipolar, 4-125 Hz data rates.

• Dynamic Range: A little more than nine decades

(usable).

• Linearity: Measured on all microradiometers over

a signal current range of from 1 x 10 -12 to 1 x

10-5 A using a programmable light source. Typically, errors are to within 1% compared to a reference system electrometer. Gain ratios are individually measured using a computer-controlled optical source and are programmed into each microradiometer.

Speed: ADC sample rate is programmable from 4-

125 Hz, and is normally set to 125 Hz, with averaging over the sampling period performed internally

by the microradiometer.

Response Thne: Exponential change with a time

constant less than 0.01 s. The time required for gain

change is less than 0.1 s.

Electronic Sensitivity: ADC resolution is 0.5 μV

with a current resolution of less than 1 fA. The saturation current is 160 μA and the minimum detectable signal is less than 10- 15 A.

Noise: The Si detector typically has 15-20 fA of

noise when the ADC is sampling at 125 Hz with the

internal microradiometer averaging of 25 samples,

resulting in a data rate of 5 Hz.

• Optical Sensitivity: Sensitivity depends on the spectral region and the choice of irradiance or radiance

entrance optics (Table 2).

Table 2. The optical sensitivity of microradiometers expressed as noise equivalent signals at 5 Hz

for radiance (μW cm -2 nm-1 sr- 1 ) and irradiance

(μW cm- 2 nm-1 ). Radiance is adjusted for immersion in water. Typical wavelengths spanning the

normal spectral range of C-OPS profiling systems

are shown.

Channel Radiance Irradiance

320 [nm] 2.9 x 10-6 9.0 x 10-5

395 5.0 x 10-6 6.9 x 10-5

490 1.8 x 10-6 2.3 x 10-5

683 9.9 x 10-7 1.1 x 10-5

780 6.8 x 10-7 8.0 x 10-6

• Dark O$sets: Dark offsets are measured and set

at the time of calibration for each gain level. Offsets can also be automatically measured and applied

in the field to accommodate different temperature

regimes.

• IVlicroradiometer Power: f5 VDC at 4 mA total.

• Optical Filters: 10 nm FWHM multicavity iondeposited interference filters selected for greatest

out-of-band blocking and minimum fluorescence and

maximum long-term stability.

• Spectral Range: 250-1,650 nm (1,100-1,650 nm requires InGaAs detectors).

• System Sampling Rate: System sampling rate depends on the number of sensors and channels being controlled. For a standard in-water system—

composed of three 19-channel sensors measuring Ed,

L,,, and Ed (0+ ), plus pressure, two-axis tilts in both

irradiance sensors, and water temperature—a system sampling rate of 12 Hz is reliably achieved.

• Radiance Size: A 19-channel in-water radiance sensor has a diameter of 6.9 cm, a length of 25 cm, and

a weight of 1.6 kg.

• Irradiance Size: A 19-channel in-water irradiance

sensor (with a pressure transducer, two-axis tilt sensors, and a water temperature probe) has a diameter of 6.9 cm, a length of 34 cm, and a weight of

1.7 kg.

40
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

3.8 Summary

Microradiometers represent a next-generation change

in the technology available to acquire high-quality optical

measurements for field and laboratory applications. The

MMS architecture was developed with the intent of immediately using it in a next-generation in-water profiler, i.e.,

C-OPS. Consequently, much of the design effort was evaluated from the perspective of the imagined new instrument.

One of the anticipated benefits of using the microradiometer technology was the opportunity for an unprecedented

amount of simplicity in assembly and repair. Both of these

accomplishments were expected to reduce the cost of acquiring and maintaining sensors with a state-of-the-art observational capability.

Prior to C-OPS, changing, replacing, or repairing a filter or filter-photodetector combination required disassembly of the entire electro-optics section of the instrument,

which was a time-consuming and tedious procedure with

the added risk of unintended damage to associated components. Significant effort was subsequently required to test

the subassemblies and then recalibrate the sensor.

In contrast, changing a filter or replacement of an entire

microradiometer with COPS requires only a few hours to

unplug the component and replace it before recalibrating.

In addition to the savings in time (and money), the potential for unintentional damage is minimized with the microradiometer design, because so much of the electronics is

modular. Consequently, there are significantly fewer connectors and cables that must be dealt with. Compare, for

example, the partially disassembled PRR-800 irradiance

sensor shown in Fig. 40 with the microradiometer equivalent shown in Fig. 31. Note also the number of cables and

connectors in Fig. 40, most of which carry low-level analog

signals, versus the absence of these in Fig. 39.

Fig. 40. A PRR-800 irradiance sensor with pressure housing removed. Visible are individual coaxial

cables conducting the detector photocurrents to a

stack of five input amplifier boards (with four channels per board). Completing the stack is an ancillary input board for temperature, pressure, and angle, the microprocessor board, and the multiplexer

digitization board.

41
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Chapter 4

The Compact-Optical Profiling System (C-OPS)

JOHN H. MORROW, CHARLES R. BOOTH, AND RANDALL N. LIND

Biospherical Instruments, Inc.

San Diego, California

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

ABSTRACT

The C-OPS instrument successfully integrates a number of new technologies, each focused on different aspects

of the practical problem of resolving the optical complexity of the near-shore water column. Although C-OPS

represents a significant improvement over BioPRO and other legacy profilers, C-OPS was designed from inception

specifically to operate in shallow coastal waters and from a wide variety of deployment platforms. In terms of the

mechanics of operating the instrumentation and its behavior during descent, the most significant improvement

was to change the basic design for mounting the light sensors from a rocket-shaped deployment system, used in

legacy profilers, to the kite-shaped backplane developed for SuBOPS (Chap. 2). This change allowed the flotation

to be distributed as a primary hydrobaric buoyancy chamber along the top of the profiler, plus an adjustable

secondary set of one or more movable floats immediately below. The primary set provides the upward buoyant

thrust to keep the profiler vertically oriented. The secondary set, coupled with an adjustment mechanism

perpendicular to the flotation adjustment axis is used to ensure the two light sensors are level. The hydrobaric

buoyancy chamber can contain one to three air-filled bladders, which compress slowly and allow the profiler to

loiter close to the sea surface, thereby significantly improving the vertical sampling resolution in near-surface

waters. Electronically, the system is self-organizing; when initially powered, the aggregator queries each sensor

to determine optimal power required for operation over the existing length of the cables and the population

of detectors available to the configuration. Typically, each sensor geometry (Ed , Eu , and LJ is composed of

19 microradiometer detectors, clustered using the MMS hierarchical architecture coordinated through a master

aggregator (Chap. 3). Although the use of microradiometers provides improvements in a variety of operational

specifications compared to SuBOPS (e.g., reduced electronic noise and slightly faster data acquisition rates),

most notable is the reduction in instrument diameter: C-OPS light sensors use a 2.75 in (7 cm) outside diameter

housing, which is 27% smaller in diameter than SuBOPS.

4.1 Introduction

The C-OPS instrument is a culmination of several new

technologies, each focused on different aspects of resolving

the optical complexity of the near-shore water column. Although C-OPS represents a significant improvement over

BioPRO and other legacy profilers, it was designed from

inception specifically to operate in shallow coastal waters

and from a wide variety of platforms ranging from offshore

towers, small boats, and ocean-class research vessels. Electronically, the system is self-organizing; when initially powered, the aggregator queries each instrument to determine

the optimal power required for operation over the existing length of the cables and the population of detectors

available to the configuration.

Typically, each radiometer (Ed , Ev, , and LJ is composed of 19 individual microradiometer detectors, which

are controlled as a single cluster using the MMS hierarchical architecture (Chap. 3). The operation of each cluster is

coordinated through a master aggregator (Fig. 29), which

is usually the (power and telemetry) deck box. Although

the use of microradiometers affords improvements in a variety of operational specifications compared to SuBOPS

(e.g., reduced electronic noise and slightly faster data acquisition rates), the most notable enhancement is the reduction in size and weight for the optical sensors. The

C-OPS sensors have a 2.75 in (7.0 cm) diameter, which is

27% smaller in diameter than the 3.5 in (8.9 cm) PRR-800

sensors used with SuBOPS.

42
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

4.2 C-OPS Description

To function well in shallow coastal waters, the C-OPS

instrument uses the basic SuBOPS design featuring two

parallel sensors mounted symmetrically on either side of a

kite-shaped free-fall backplane (Fig. 41). The sensors use

microradiometer clusters housed in a 2.75 in (7 cm) in diameter aluminum cylinder that is 27% smaller in diameter

than SuBOPS. The pressure transducer for C-OPS is installed on the upper surface of the upwelling radiance or

irradiance instruments.

The original twin V-block pairs used to mount the SuBOPS sensors were changed to single elongated blocks for

ease in adjusting the pitch orientation of the sensors. As

with SuBOPS, roll adjustment is accomplished by controlling the distribution of secondary floats laterally. The lateral floats were redesigned with a slot, so they can slide

easily into position and fiberglass nuts are used to hold

them in place. A temperature sensor can be mounted on

either end cap, but the Lu, sensor end cap is preferred,

because this permits measurements very close to the sea

surface.

Ed(0+,A1s,,P)

Hydrobaric Buoyancy

Chamber Above

Ed (z A, y) ^2 Adjustable Disks

/Fender

L,,(z, A 1s, gyp)

Temperature Flexible

Sensor Perforated Weighted Spar

Backplane C-OPS

Fig. 41. A schematic of the first C-OPS instrument; newer models have the temperature probe

mounted on the L,, endcap to permit measurements

as close to the sea surface as possible. Any needed

weight is added to a bottom-pointing flexible spar.

The rigid spar used with SuBOPS could be damaged

during the recovery of the profiler, so it was replaced by

a plastic flexible spar for C-OPS. The flexible spar somewhat protects the Lv, aperture in the case of an accidental

bottom impact, because it hits the bottom first, but the

primary protection for C-OPS is that it is usually tuned

for slow descent rates (typically about a 20 cm s-1 terminal velocity). The flexibility of the material has additional

advantages as a counterpoise in buoyancy control. As with

SuBOPS, the main rigid foam buoyancy element features

a hollow chamber that can hold 355 mL of air in compressible bladders (Fig. 42) or a combination of bladders and

incompressible foam.

Fig. 42. The C-OPS main buoyancy chamber hollowed out and fitted with two bladders plus a solid

foam insert. The removable flotation pieces for adjusting the roll stability of the backplane, with one

piece to the left and one to the right, can be seen

just below.

4.3 Design

The C-OPS instrument architecture was designed to be

inherently flexible. The core elements can be added or removed to accommodate a wide range of research activities.

Microradiometers are used as the photodetector elements

in all of the light sensors; aggregators ensure that ancillary sensors (e.g., temperature, pressure, pitch, and roll)

featured in an instrument are integrated into a single data

stream with the microradiometers. As a complete system

for the discussion presented here, C-OPS consists of a solar reference with BioSHADE accessory (Chap. 5), two inwater light sensors (selected from Ed , Ems , and Lv, ), a deck

box, and the cabling that connects them all together. Figure 43 shows the interrelationship between the different

components used in COPS.

Fig. 43. The C-OPS components configured together showing how the above- and in-water components communicate as sensor groups over corresponding dedicated cables.

43
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Table 3. The specifications for the data-telemetry (sea) cables used with the BioPRO, SuBOPS, and C-OPS

free-fall optical instruments. Because SuBOPS was the transition instrument between the legacy (PRR-800)

sensors and the new microradiometer sensors, the new cable characteristics were specified using SuBOPS and

then evaluated with C-OPS. The three internal synthetics involved are polyethylene (PE), polyurethane (PU),

and polypropylene (PP). Conductor specifications are given in American Wire Gauge (AWG) sizes. Bulkhead

connectors on the light sensors and deck box are SubConn Micro 6-pin male connectors with female locking

sleeves, and SubConn Micro 6-pin female connectors with female locking sleeves, respectively.

Cable Characteristic Red Polyester Overbraid Cable Pale Red Polyurethane Cable

Manufacturer Cortland Cable Company Storm Products

Power Conductors 2 x 20 AWG with PE Insulation 2 x 20 AWG with PP Insulation

Telemetry Conductorst 4 x 24 AWG with PE Insulation 4 x 24 AWG with PP Insulation

Outer Diameter 0.320 in (nominal) 0.320 in (nominal)

Strength Member Kevlar (1,000 lb breaking rating) Kevlar (500 lb breaking rating)

Outer Jacket Black PU with Red Polyester Overbraid Red Low-Density PU

Connectors SubConn Micro In-Line (MCIL6) SubConn Micro In-Line (MCIL6)

Instruments BioPRO and SuBOPS SuBOPS and C-OPS

t The four data telemetry conductors are packaged as two individual sets of twisted, shielded pairs.

All C-OPS instruments require a master aggregator

(the deck box) to provide power, coordinate data streams

from the individual instruments, and deliver data to a computer. There is no requirement to use all components at

all times, and because the deck box polls all elements at

the time the system is started, it is self organizing; data

acquisition does not require a description of the configuration before recording takes place. In the most common

C-OPS configuration, a 19-channel solar reference is used

to measure spectral global irradiance. A BioSHADE with

the optional Biospherical GPS (BioGPS) unit (Chap. 5)

can be attached to the reference to add a shadowband capability to the data stream as well as position and time.

Once the system is configured, the RS-485 signals from

the above- and in-water components are combined in the

deck box and converted to RS-232 communications for

computer logging. The RS-232 data are recorded on a laptop computer using commercial software developed by BSI,

or custom-built software developed by RSMAS in partnership with NASA. The latter is a continuing testbed for

evaluating desired changes in the Protocols associated with

deploying the instrumentation.

The deck box provides computer-controlled power to

avoid any damage to the instrumentation from improper

power-up sequences over varying cable lengths. Shielded

six-conductor cables, up to 500m long for the sea cable

and 150m long for the solar reference surface cable (both

frilly loaded with 19-channel sensors and all accessories),

are used to provide power and return the data from the

instruments to the deck box. A standard C-OPS design

has a maximum deployment depth of 150 m. The slow descent speed can result in large horizontal displacements of

the profiler if subsurface currents are substantial. When

coupled with the offset distance needed to avoid platform

perturbations, this means cables significantly longer than

150m are needed to achieve the maximum deployment

depth.

Experience with early free-fall designs (e.g., BioPRO

and SuBOPS) revealed that the cable was an important

component in stability and other behaviors of the instrumentation during descent. The original in-water (sea) cable was designed with an emphasis on strength and ruggedness. Because the profiling systems were deployed by hand,

a red polyester outer braid was used to provide a coarse,

but easy to handle, surface texture to provide a grip similar

to rope (Table 3).

The diameter of instrumentation cable is controlled by

the number of conductors required in the application, the

presence of a Kevlar strength member, and a waterproofing and protective polyurethane layer. The PRR-800 (BioPRO) electronic design required power and ground, and

also included two individual sets of twisted shielded conductors to accommodate the data stream. The shielding is

particularly desirable for the solar reference, because it is

frequently mounted in the presence of shipboard communications equipment, which are usually active transmitters.

The red color made it significantly easier to see the profiler

at a distance from the deployment platform during deployments. For use with SuBOPS, the SubConn Microseries of

wet-pluggable connectors replaced all connectors originally

used for the instrument and deck box.

The red braided cable was robust and easily gripped,

which made it easy to use in a wide variety of conditions.

Unfortunately, it was also somewhat stiff, and at times,

difficult to manage in the time compressed environment of

many field campaigns. A recurring problem was the memory of the original winding, which made it cumbersome to

coil the cable quickly into a bucket. Although the stiffness

of the cable provided a somewhat stabilizing influence on

free-fall profilers, from a rudder-like effect, the overall influence on instrument pitch was problematic, and the cable

was redesigned for the specific purpose of free-fall deployments.

44
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

The new sea cable is a paler shade of red. The design retained the original specifications of the conductors

in order to remain compatible with legacy instruments, as

well as the Kevlar strength member (Table 3). The individual conductor insulation was changed to polypropylene

to support high temperature terminations. Additionally,

a water blocking compound was incorporated to provide

roundness and prevent water propagation in the event of

an outer jacket breach. The red braid was removed entirely

and a new, low-density, polyurethane, 0.050 in thick outer

jacket provides abrasion resistance and waterproofing.

4.4 Operation

The deployment and operation of the C-OPS instrument is situationally dependent on weight distribution and

buoyancy, which is itself contingent upon water density.

The initial adjustments control platform orientation (pitch

and roll). Subsequent adjustments are used primarily to

set the proper buoyancy for the system, which determines

the magnitude and depth of the terminal velocity, and

pitch adjustments to provide bias offsets for currents and

the power-telemetry cable. (The pitch axis is perpendicular to the axis connecting the two light sensors and is in

the direction of the cabling harness.)

Preparing a C-OPS instrument for use in the field begins with ensuring that the solar reference is properly sited,

i.e., the cosine collector has an unobstructed view of the

sky, and is cabled to the deck box. The importance of this

part of the deployment procedure is presented in Chap.

8. The steps necessary to prepare the C-OPS in-water instrumentation for deployment are as follows: a) connect

the power telemetry cable to the Y-cable on the backplane

and attach the strain relief to the harness; b) install the

desired number of air bladders within the main buoyancy

chamber; c) add the desired amount of weight to the counterpoise at the bottom of the instrument; and d) adjust

the distribution of the secondary flotation. The instrument is subsequently placed in the water and the descent

and vertical tilt observed.

Weight or buoyancy is adjusted as needed to ensure

that the instrument is tuned to be almost neutrally buoyant, but biased slightly negative. At this point, the relationship between the near-surface behavior of the instrument and the ultimate terminal velocity is controlled by

the ratio of air flotation to weight. The larger this ratio,

the longer the system will tend to loiter near the surface.

The greater the additional weight, the faster the terminal

velocity of the system at depth. These two factors control

the wide range of sampling strategies that can be applied

to situations such as use in very shallow coastal water or

deep oceanographic stations.

When the buoyancy adjustment is complete, the instrument is allowed to drop 1-3 m below the surface (water

depth permitting) and the pitch and roll orientations are

noted. Secondary flotation is adjusted laterally as needed

to zero the roll values. Usually, it is possible to achieve less

than f0.5° difference from zero in roll, depending on surface conditions. In calm waters, roll adjustment to within

0.1 0 is normal. Pitch adjustment uses the geometric relationship between the bottom of the V-block mounts and

the backplane. Pivot nuts are loosened and the pitch angle

is adjusted on the irradiance side of the instrument to a

value that cancels the backplane bias measured during the

initial deployment. A plastic dial caliper is used to measure the V-block position relative to the backplane, and

then the radiance V-block is adjusted to match. This ensures that the two instruments are parallel relative to the

backplane.

The entire attitude adjustment process, from attaching

the cable to the initial test profiles, lasts approximately

15-30 min (depending on the experience of the operator

and the in situ environmental conditions). After the initial tuning, the instrument performance is highly robust

and rarely needs adjustment unless the density of the water changes significantly (such as moving from a marine to

riverine environment, or moving from the open ocean to

the marginal ice zone) or the in situ current field changes.

Currents pose the most significant challenges, but the main

point is to ensure good vertical tilts (less than 5°) close to

the sea surface (top 1-5 m). Subsurface currents can degrade data quality, but they usually do not significantly

degrade the final data products, so the most important

adjustments are those influencing the near-surface behavior of the profiler.

The COPS (and SuBOPS) instruments are usually deployed from the stern of a research vessel (although bow

deployments are also made to avoid the turbulent mixing

that occurs at the stern of a boat). The instrument is first

lowered into the water, and allowed to drift away from the

ship or to have the ship drift away from the instrumentation. The latter is frequently accomplished by the windage

on the vessel. If the profiler does not drift away, short, impulsive headway maneuvers (or bumps) of approximately

0.5 kts are used to create enough propeller wash to push

the profiler or boat away. Ships equipped with thrusters

can usually maneuver away from the profiler in a variety

of orientations.

The objective of the short impulsive maneuvers is to

position the profiler well clear of any possible shadows or

reflections caused by the deployment platform. In most

cases, three vertical profiles (or casts) are acquired, so the

profiler needs to be even farther away to allow for some

loss of distance when the instrument is pulled back to the

surface between casts. In most circumstances, if the profiler is initially placed 30-50 m away from the deployment

platform, three casts can be executed without any need for

significant repositioning.

A cast is executed by simply releasing the telemetry

cable and paying out cable at a sufficient rate to prevent it

from ever coming under tension. Although the harness of

a kite-shaped backplane keeps the instrument in a mostly

45
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

vertical orientation (so it is not subjected to a significant

righting event), the release of the tension associated with

the cable still results in oscillations, which can be accentuated or dampened by surface waves. Even brief periods

of tension can adversely affect the attitude and descent of

the profiler. To ensure this does not occur, the operator

leaves some slack cable at the surface. Care is taken to

not leave too much free cable in the water, because the cable can move under the ship and become entangled in the

propeller or stern thruster intake (if present). To ensure a

tangle-free and continuous feed of cable into the water, all

of the cable (usually 100-300 m) is coiled in a large bucket

or laid out on deck prior to each deployment sequence in

such a manner as to minimize entanglements.

Once the profiler reaches the desired depth, which is

usually set by the 1% light level or the proximity of the

sea bottom, the cable is pulled in to bring the profiler

to the surface. Because C-OPS behaves like a kite when

the cable is under tension, the profiler has a tendency to

ascend vertically (or pop up) to the surface without moving

significantly closer to the deployment platform (depending

on environmental conditions). Small bumps by the ship or

a C-HOIST unit (Chap. 8) can be used to haul the cable in

faster. If a winch is used, care must be taken to ensure the

diameter of the drum is sufficiently large to not damage

the cable (C-HOIST does not use a drum).

During a cast, the solar irradiance is monitored for

constancy to ensure data collection occurs during stable

atmospheric conditions. In addition, the vertical tilt of

the profiler is continuously checked to make sure the vertical tilts are to within 5°, particularly near the surface.

At depth, the terminal velocity is noted and compared to

the desired specification. The amount of negative buoyancy determines the descent speed, as well as the ability

of the profiler to sink through high shear features, like the

thermocline or a subsurface current.

Once data collection activities are completed, the profiler is washed off with fresh water. The optical apertures

are dried with a paper tissue by blotting the surfaces; wiping should be avoided because it can cause scratches. If the

cable bucket is large enough, the instrument is wrapped in

a white cloth to protect it from solar radiation and placed

inside the bucket. Both the bucket and the instrument

are then properly secured. If severe weather is expected,

the cable and instrument should be brought inside for safe

keeping.

4.5 C-OPS Evaluation

The field commissioning of the C-OPS instrument was

in predominantly mesotrophic (Case-1) waters. The objectives of the field campaign involved more than just an

evaluation of C-OPS, which was done on a not-to-interfere

basis with the other cruise priorities. In addition, the

station work did not permit simultaneous deployment of

equipment, so time differences between when one instrument sampled and when the other sampled was on the

order of 24-64 min (with an average of 42 min).

The lack of simultaneity in instrument deployments was

not considered a significant detraction to the field commissioning exercise, because a) the measurements were being

conducted in Case-1 waters with longer space and time

scales for homogeneity, and b) the primary purpose was

to test the capabilities of the new profiler under realistic

conditions with adequate information to make informed

decisions about how to proceed with problems should they

arise. Because the continuing refinement of the SuBOPS

deployment system was the basis for the C-OPS design, the

intercomparison was also an evaluation of how well the engineering concepts associated with kite-shaped deployment

systems can be adapted to instruments with differing sizes

and weights.

The results of the intercomparison of C-OPS and SuBOPS during the field commissioning campaign is presented

in Fig. 44 (for the same eight wavelengths presented in

Fig. 15, for which six of the wavelengths are common to

Fig. 12). The average UPD for each channel between the

two profilers ranges from -5.2% to 6.5%, with an overall

average of 1.8% (for the wavelengths plotted in Fig. 44),

which is to within the calibration uncertainty. A leastsquares linear regression of the data (Fig. 44 inset panel)

shows almost one-to-one correspondence with over 95% of

the variance explained.

0.9-

Z

0.6-

U)

CL

O 0.3-

U

0.0

0.0 0.3 0.6 0.9

SuBOPS [LWR)]N

Fig. 44. An intercomparison of the SuBOPS and

C-OPS instruments in mostly mesotrophic coastal

(Case-1) waters for eight wavelengths, which are

given in nanometers. The units for [Lw(A)] N are

μvv cm-2 mm-1 sr-1 . The one-to-one line is shown

as solid, and the least-squares linear fit to all the

data as dashed (regression information is given in

the inset panel).

f(x) = 1.002x + 0.005 V

R2 = 0.972

file

380

412

443

490

j' 510

555

t. 560

0 665

46
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

The larger variance in Fig. 44 with respect to the BioPRO and microNESS intercomparison (Fig. 12), which was

also in Case-1 waters, is caused by the greater variability of

the site and a larger time difference between the measurements by the two profilers. The increase in environmental

variability for the C-OPS and SuBOPS intercomparison

was caused by an atypical, and rather rapid, evolution

in the near-surface waters (upper few meters of the water column). In some cases, the TChl a concentration was

changing by more than 5% in a 45 min time period.

The operational evaluation of C-OPS was in predominantly turbid (Case-2 waters with very short time and

space scales for homogeneity (Fig. 45). For this activity,

the two profilers have almost exactly the same backplane

configuration, buoyancy capabilities, and pitch adjustment

mechanisms—the primary difference is the use of microradiometers for C-OPS and the PRR-800 technology for

SuBOPS. The level of turbidity is well characterized by

the average Kd values: 10.7 at 320 nm, decreasing to 0.78

at 560 nm, and then increasing to 3.3 at 780 nm. For this

field campaign, simultaneous measurements were permitted during controlled circumstances (e.g., while the ship

was anchored and the sea state was relatively calm), so

the difficulties associated with spatial inhomogeneity were

somewhat offset by an ability to sample the water masses

contemporaneously.

Fig. 45. The C-OPS (left) and SuBOPS profilers (right) deployed simultaneously for operational

evaluation. The two instrument systems are shown

in a river system with a water depth of approximately 6 m.

Although the C-OPS microradiometers have some design features that are a legacy of the technology used in

SuBOPS, the microradiometers are essentially new sensors. Consequently, it is appropriate to consider simpler

intercomparisons in the operational testing, like the performance of the solar references. This is doubly attractive

in this case, because the instruments were deployed simulrelated to true instrument performance issues rather than

environmental factors.

An intercomparison of the SuBOPS and C-OPS solar

references during the operational evaluation is shown in

Fig. 46 for an expanded set of wavelengths than was used

in the previous intercomparisons to show more spectral

information, but still containing most of the same wavelengths. The average UPD for each channel between the

two radiometers ranges from -2.5 to 3.9%, with an overall

average of 1.3% (for the wavelengths plotted in Fig. 46),

which is to within the calibration uncertainty. A leastsquares linear regression of the data (Fig. 46 inset panel)

shows almost one-to-one correspondence with over 95% of

the variance explained.

160-

120-

7

w° _-

O 80—

U

40

40 80 120 160

SuBOPS Ed(0+,k)

Fig. 46. An intercomparison of the SuBOPS and

C-OPS solar references in primarily coastal (Case-2)

waters for nine wavelengths given in nanometers.

The units for Ed (0') are μW cm 2 nm-1 . The oneto-one line is shown in solid, and the least-squares

linear fit to all the data in dashed (regression information is given in the inset panel).

The intercomparison of the in-water sensors for the operational evaluation of C-OPS and SuBOPS is presented

in Fig. 47 (for the same wavelengths shown in Fig. 46).

Ignoring first the far red and NIR plus the far UV, the

average UPD for each channel across the 380-625 nm part

of the spectrum ranges from -7.0% to 5.6%, with an overall average of -0.8%, which is to within the calibration

uncertainty. Considering now all the wavelengths shown

in Fig. 47, the average UPD does not change significantly,

it is -0.3%, but the range increases in the red and NIR

wavelengths and covers -17.4% to 19.7%. A least-squares

linear regression of the data (Fig. 47 inset panel) shows

almost one-to-one correspondence with almost 95% of the

380

• 412

443

• 490

• 510

• 555

• 625

• 670

• 710

lip

f(x) = 1.034x - 2.162

R2 = 0.993

taneously, so any differences will be much more closely variance explained.

47
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

1.2-

Z

0.8-

J

U)

n

0 0.4- U

0.0

0.0 0.4 0.8 1.2

SuBOPS [LWOAN

Fig. 47. An intercomparison of the SuBOPS and

C-OPS instruments in primarily eutrophic coastal

(Case-2) waters for the nine wavelengths in Fig. 46.

The units for [Lw(A)] N are μW cm-2 nm-1 sr-1.

The one-to-one line is shown as solid, and the leastsquares linear fit to all the data as dashed (regression information is given in the inset panel).

If other wavelengths are considered for the intercomparison, the range continues to increase with the addition

of the farthest UV and NIR bands, and reaches -50.7%

at 780 nm. The reason for this increase is the aforementioned near-surface layer evolution at the sampling site.

Although the two profilers were very similar in their capabilities (e.g., they both had vertical tilts to within 1.5°),

the C-OPS profiler loiters at the surface better than SuBOPS, so it collected more data in the area that was changing the most. This translated to the C-OPS extrapolation

intervals having almost 70% more data (on average) than

SuBOPS. For the highly attenuated wavelengths—the far

UV, red, and NIR part of the spectrum—this is a very

important difference between the two profilers.

4.6 Advancements and Enhancements

While retaining the features of the hydrobaric chamber,

the C-OPS backplane was modified to achieve greater terminal velocities by removing a significant amount of the

backplane material (which is heavy). The resulting Dshaped frame also eliminates the counterpoise in favor of

a horizontal weight.-bearing rod that is not within the field

of view of upward irradiance instruments. There is sufficient flat surface normal to typical retrieval forces that the

instrument will still kite up to the surface for deployments.

Using this arrangement, typical terminal velocities exceed

55 cin s-1 below 25 m in deep-water profiles.

Although a standard COPS profiler can be configured

to measure any two-sensor combination of L,,, , E, and

Ed, a specialized backplane providing simultaneous profiling for all three sensor types is being tested. The new

design permits the acquisition of all three principal light

field components (Fig. 48), so the Q-factor (E,,/L,,) can

be measured simultaneously with Ed . The Q-factor is an

important parameter for understanding the bidirectional

aspects of the underwater light. field. Although Q(A) is

well understood for Case-1 waters and can be computed

using look-up tables based on the solar geometry and the

chlorophyll a concentration (Morel and Gentili 1996), no

such capability exists for Case-2 (optically complex) waters.

Fig. 48. A schematic of the C-OPS deployment

system modified for the simultaneous deployment of

three sensors: Ed (left), Lu (center), and E,, (right).

To pursue the goals of simplifying free-fall deployments

and removing cable-induced perturbations to profiles, a

new sea cable for C-OPS was designed. The objective of

the redesign effort was to maximize flexibility and minimize memory via careful selection of jacketing and conductor stranding. Additionally, a half-duplex communication scheme was implemented to allow the removal of one

of the twisted pairs used for data telemetry. All internal

cable jackets were switched to low-density polyethylene,

which is much more flexible than polypropylene; however,

this change was made with the tradeoff that heat termination of the cable is no longer supported. The stranding

of all conductors was increased to the maximum readily

commercially available configurations, which significantly

increased the flexibility of the resulting cable. The Kevlar

strength member, internal water block, and low-density

polyurethane outer jacket were all retained. The outer

jacket was thinned slightly to add more flexibility while

still providing adequate protection to the internal conductors and a large enough diameter for comfortable handling.

With respect to the entries in Table 3, the following

characteristics apply to the new cable:

• 1Vlanufacturer is Storm Products;

• Power conductors are 2 x 20 A-VVG with low-density

PE insulation;

• Telemetry conductors are 2 x 24 A%VG and have lowdensity PE insulation;

• Outer diameter is 0.250 in (nominal);

• Strength member is Kevlar (500 lb breaking rating);

f(x) = 0.998x + 0.003

R2 = 0.947 • •

•

• 380

•^ 412

443

• 490

510

• • 555

625

• 670

• 710

48
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Table 4. A comparison of the specifications for the C-OPS (microradiometer) and the BioPRO (PRR-800) profiling

systems, with emphasis on the in-water components. The ES notation denotes a solar reference measuring Ed(0+)

Profiler dimensions do not include the diameter of the sensors. The listed ancillary sensors are in addition to a pressure

transducer, which is assumed common to both profiling systems. Profiler dimensions are given as width (W), height

(H), and depth (D). The ES notation denotes an Ed (0+ ) sensor.

Specification G OPS BioPRO

Free-Fall Dynamics Kite-shaped with hydrobaric buoyancy Rocket-shaped with buoyant collar

Descent. Speed 5-75cros-1 50-100 cros-1

Sampling Depth 150 m (300 m optional) 350 m

Trim Adjustment Individual pitch and roll adjustment Simultaneous pitch and roll adjustment

Vertical Stability 2.5-5.0° (typical) 3.0-7.0° (typical)

Sampling Resolution 1 cm (less than 1 cm in 0-5 m) 4 cm

Photocurrent-to-Voltage

Conversion

Electrometer amplifer with three gain

stages: 1, 200, and 40,000

Electrometer amplifer with three gain

stages: 1, 300, and 100,000

Analog-to-Digital

Conversion

24 bit bipolar sigma-delta running at

4-125 Hz

Shared 16 bit ADC with 1, 16, and 256

gain preamp running at 40 kHz

Dynamic Range 9.5 decades (usable) 9.0 decades (usable)

System Data Rate (one

and three sensors)

19 channels at greater than 30 Hz and

57 channels at 15 Hz

19 channels at greater than 20 Hz and

57 channels at 15 Hz

Minimum Detectable

Signal

Less than 10 -1' A (0.5 μV ADC resolution)Less than 10 -15 A (1.3 μV ADC resolution while on 256 voltage gain)

Spectral Range 250-1,650 nm j 250-875 nm

Sensor Diameter 7.0 cm 10.2 cm

Sensor Length 34 cm (Ed or ES ) and 25 cm (Lu ) 55.9 cm (Ed and Lu ) and 37.8 cm (ES)

Sensor Weight 1.7 kg (Ed or ES ) and 1.6 kg (Lu ) 4.8 kg (Ed and Lu ) and 3.0 kg (E,)

Profiler Dimensions 48.7 cm W x 36.0 cm H x 8.9 cm D 30.5 cm W x 55.9 cm H x 14.0 cm D

Profiler Weight 6.8 kg in air (Ed and LJ 5.9 kg in air (Ed and Lu)

Maximum Depth 150m (300m available) 350m (recommended maximum)

Cosine Collector

Error

±3% for 0 < 600 ; ±5% for 60 < 0 < 70°;

and ±10% for 70 < 0 < 85 0

±2% for 0 < 60°; and ±10% for 60 < 0

< 850

Ancillary Profiler

Sensors

Water temperature, pitch and roll,

internal pressure, and humidity

Water temperature, and pitch and roll

t 1,100-1,650 nm requires InGaAs detectors.

• Outer jacket is red low-density PU;

• Connectors are SubConn micro in-line (MCIL6 or

MCIL4); and

• Instrument is C-OPS.

The new cable is connector and conductor compatible with

the old cable. What this means is that all old cables will

work fine with systems that have been configured to work

with the new cable. They can be used as spares, extensions, or primary deployment cables. The new cable will

not work with an old system, however. To convert an old

system to a new system, a switch must be flipped in both

the deck box and each light sensor. It is not a difficult

process, but it is something that should be done at the

factory because of O-ring liability and N 2 purges, etc.

4.7 Summary

The C-OPS instrument successfully integrates a number of new technologies, each focused on different aspects

of the practical problem of resolving the optical complexity of the near-shore water column. Structured around

19 high-speed microradiometer optical sensors, C-OPS was

specifically designed to be compact enough to deploy from

small or large vessels by hand.

The profiling system includes separate sensors to measure vertical profiles of spectral downward irradiance, and

upwelling radiance or irradiance using a unique, variabledescent, free-fall backplane. A comparison of C-OPS with

the legacy BioPRO (PRR-800) profiler is presented in Table 4.

49
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

The system is capable of deployments in fresh and ma- sion). A mission such as this will likely highlight coastal

rine waters, in depths ranging from 2-150m. A multi- and open-ocean processes, placing renewed emphasis on

chamber, hydrobaric buoyancy system provides very slow making high-quality measurements with equal efficacy in

initial descent rates with ultimate terminal velocities of both the near-shore and open-ocean environments.

5-75 cm s-1 , with a typical attitude control to within approximately 2.5 0 from vertical. Sample speeds of 15 Hz

covering more than nine decades of dynamic range ensure

that a representative sample is collected even in the shallowest of waters. Unlike earlier versions, the newest counterpoise design uses naval brass weights mounted inboard

of the frame, eliminating any potential intrusion into the

nadir field-of-view.

Surface loitering, faster terminal velocity at depth, and

high data rates result in sufficient sampling to investigate

optically diverse, near-surface thin layers, or produce statistically relevant data sets on surface effects. Direct benefits of this new sampling capability include lower uncertainties in the data products across the full dynamic range

of the sampling problem set; better accuracy in separating the living and nonliving components of seawater; and

an improved understanding of the interaction between the

ocean and atmosphere.

The underlying microradiometer technology is notably

more compact and more easily expanded than legacy systems (Fig. 49), ensuring a cost-effective expansion path

for both AOP profiling instruments and novel systems occupying new roles in the future. The C-OPS technology is an important initial step toward supporting a coupled ocean—atmosphere observing system (i.e., a calibration and validation capability for a combined satellite misFig. 49. Three generations of AOP profilers, which

were deployed during one of the Bermuda Atlantic

Time Series (BATS) cruises in 2009 (left to right,

respectively): Natasha McDonald holding a MicroPro, which was based on the microNESS instrument; Stanford Hooker holding the first C-OPS unit;

and Vincenzo Vellucci holding an SPMR. A second

SPMR is on deck in the foreground at left.

50
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Chapter 5

Biospherical Shadowband Accessory for Diffuse Irradiance

(BioSHADE): A Marine Shadowband and GPS Accessory

GERMAR BERNHARD, CHARLES R. BOOTH, AND JOHN H. MORROW

Biospherical Instruments Inc.

San Diego, California

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

ABSTRACT

BioSHADE is an accessory for shipboard radiometers that is used to measure the optical properties of the

atmosphere while providing the usual global irradiance measurement of a solar reference. It meets the need for

improving the self-shading correction applied to in-water AOP measurements, wherein atmospheric complexity

makes it more difficult to properly model the diffuse irradiance. It also provides for the capability of several

atmospheric data products that are useful to the atmospheric correction part of calibration and validation activities. BioSHADE integrates seamlessly into instrument systems based on the microradiometer architecture,

although it can be configured for use with the PRR-800 series of solar references or other irradiance sensors

with the same form factor. It is composed of the following components: a) the stepping motor and controller

unit housing an Mdrive 17 from IMS Systems, b) a data aggregator derived from the microradiometer product

line, c) a power regulation subsystem, d) the housing for the electromechanical components, e) the shadowband

subassembly, and f) the radiometer mounting components. The BioGPS is a 12-satellite GPS receiver designed

to be operated simultaneously with radiometric instruments. The system can be fully integrated with instrumentation based on the microradiometer architecture, wherein the GPS is operated over a single cable, linking

radiometers, and the BioSHADE accessory with the BioGPS. The latter ensures the accessory components and

the radiometric sensors can be operated using standard cabling and deck box configurations.

5.1 Introduction

BioSHADE is an accessory for shipboard radiometers

that attaches to a solar reference. It is used to measure

the optical properties of the atmosphere as part of a separate data acquisition sequence. BioSHADE meets the need

for improving the self-shading correction applied in coastal

waters wherein atmospheric complexity makes it more difficult to properly model the diffuse irradiance. It was designed to be used during the time period when a free-fall

profiler is being hauled up to the surface. This normally

takes only a few minutes (depending on water depth and

environmental conditions), so the design parameters established for making the shadowband measurement were

based on a 90 s acquisition event. When not being used, the

solar reference is able to make an uncontaminated global

irradiance measurement..

A GPS can provide vital cruise track and time-of-event

records for post-processing optical data collected aboard

ship. The U.S. GPS consists of a constellation of medium

Earth-orbit satellites operated by the Department of Defense, but having public access. Each satellite in the constellation continually transmits a microwave stream containing time and position information. After locking onto

multiple satellites, a GPS receiver converts small differences in the arrival times of these transmissions into a calculated position on the ground. The accuracy and precision of the resulting geographic coordinates usually satisfies most oceanographic sampling requirements.

BioGPS is a 12-satellite GPS receiver designed to be

operated simultaneously with BSI radiometric instruments.

The system can be fully integrated with optical instruments based on the MMS architecture, wherein the GPS

is operated over a single cable linking radiometers and the

BioSHADE accessory with the BioGPS. When PRR systems, such as BioSORS, are deployed, BioGPS is used in

a stand-alone configuration, remotely delivering data to a

logging computer via an independent cable.

51
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

5.2 Description

BioSHADE is composed of several components: a) the

stepping motor and controller unit housing an Mdrive 17

from IMS Systems, b) a data aggregator derived from the

BSI microradiometer product line, c) a power regulation

subsystem, d) the housing for the electromechanical components, e) the shadowband subassembly, and f) the radiometer mounting components. The motor is controlled

by an aggregator, which is a microprocessor-controlled

module also derived from the BSI microradiometer product line. The aggregator allows seamless integration of the

BioSHADE accessory into a network of sensors built with

this technology. The power regulation subsystem provides

the needed optimum power to the motor (12.2 V) and the

aggregator (6.4 V), as well as power (6.4 V) to other microradiometer and BioGPS systems. The mounting system is

currently based on 1 in national pipe tapered (NPT) type

316 stainless steel (SS) pipe and fittings, although alternative mounting configurations are possible.

The BioGPS combines a high sensitivity GPS microcontroller with a microradiometer aggregator module. The

aggregator allows the data stream from the GPS module

to be integrated into any system conforming to the MMS

architecture, e.g., from a multichannel solar irradiance sensor. The aggregator has two communications ports, one

for upstream communications to the deck box and subsequently, the data acquisition computer, and the other for

downstream communications.

5.3 Design

The design of the BioSHADE accessory is optimized

for 19-channel microradiometer solar references operated

in a shipboard environment. The use of a shadowband

on a ship requires a different optical design than for terrestrial shadowband photometers. Most notably, it is not

possible to position a partial shadowband in accordance

with a fixed geographic location, because the heading of

the ship is usually not constant, nor is the vertical orientation (pitch and roll) of the device. The dimensions

and geometry of the shadowband assembly, therefore, are

constrained by the following requirements:

• When the centers of the Sun, shadowband, and diffuser align, the cosine collector must be completely

shaded from all parts of the solar disk.

• Because the shadowband is moving at a constant

speed, the size of the shadow should be sufficiently

large such that the diffuser is shaded for at least

seven sample points. (It was determined that this

number of sample points is sufficient to ascertain

unambiguously that the shading of the Sun is complete.)

• The width of the shadowband should be as small

as possible to minimize the area of the sky that is

shaded. This requirement helps reduce the uncertainties involved in calculating the direct irradiance

component of the global irradiance.

• For practical reasons, the diameter of the shadowband should not be larger than 10 in (25 cm).

• A 1800 swath of the shadowband should be completed in approximately 60 s (3° s-1 rotation rate),

which when coupled with the rest of the command

set, should result in a total data acquisition scenario

of about 90 s.

• The shadowband should be out of the FOV of the

cosine collector when moved to its lowest position

for accurately measuring the global irradiance.

Using these requirements, the nominal inside diameter of

the band is 8.56 in.

The band is made out of 0.06 in (1.5 mm) thick aluminum that is 1.0 in (2.5 cm) wide and anodized black.

The size of the main housing is 3.5 in (8.9 cm) in diameter

and 10.5 in (26.7 cm) long. A depiction of the BioSHADE

accessory in operation with a microradiometer solar reference and BioGPS unit is shown in Fig. 50.

Fig. 50. A shadowband assembly schematic with

a 19-channel microradiometer solar reference in the

left foreground, the motor housing with supporting

electronics at the right, and an optional BioGPS

unit at the left rear. The narrow cylinder at center

is the mounting post (1 in NPT 316SS pipe).

The minimum width of the shadowband is constrained

by the size of the cosine collector, which is 0.885 in (2.25 cm)

in diameter for BioSORS and C-OPS. Because the angular size of the solar disk is about 0.54°, the shadowband

must be slightly wider. The requirement that the diffuser

52
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

should be shaded for at least seven sample points further

increases the width.

Based on these considerations, the following dimensions were chosen: the shadowband is 1.00 in (2.54 cm)

wide. The distance from the collector to the band depends on the orientation of the band. It varies from 3.78 in

(9.60 cm) to 4.28 in (10.87 cm), because the pivot point is

0.5 in (1.27 cm) below the plane of the irradiance collector

(also referred to as the horizon) to satisfy the last requirement above. When the band is in the zenith, it shades a

15°-wide segment of the sky. In the normal mode, the radiometer is sampling at a rate of 15 Hz. The measurement

of seven sample points, therefore, requires 0.47s during

which the shadowband moves 1.2°.

The BioSHADE motor is a size 17 stepper motor with

51,200-step resolution, which is coupled to the shadowband

with a flexible coupling (Fig. 51). The system has an expected range of motion of approximately 220°. The motor

is equipped with a home sensor used in initial setup. Motor power is switched and provisions are made to measure

internal voltage and current to allow automatic compensation for variable cable lengths and the resulting uncertainty

of voltage drops. The supply voltage ranges from 12.75-

16.00 VDC depending on the configuration and length of

the cable. The system consumes up to 440 mA when fully

configured with the actual voltage required. Most cases

can be handled with a specification of 1 A.

Fig. 51. The BioSHADE drive components (left),

with the stepper motor above the white end cap;

above the motor is the aggregator (vertically oriented green circuit board), which serves as the motor controller and electronic interface. A magnified

view (right) shows the coupler in more detail. The

unit is upside down in both panels.

To keep the shadowband at or below the horizon when

stowed, the band needs space to rotate below the plane of

the cosine collector. For the stowed angle below the horizon to be reasonably large (e.g., 40°), the clearance of the

band with respect to the solar reference must be reasonably

large. The needed clearance produces an eccentricity in the

geometry of the band with respect to the sensor, which is

shown in Fig. 52. Not shown in the figure, but used in the

formulation of data products produced from the shadowband measurements, is the shadowband angle, v, which is

defined as the angle between the horizontal plane of rotation and the direction of the shadowband (measured from

the zero position, which is described below).

Fig. 52. A side-view illustration of the COPS solar reference with the BioSHADE, showing the distance between the shadowband and the diffuser for

vertical and horizontal orientation of the shadowband. The BioSHADE housing is behind the solar

reference (left) and the BioGPS is to the right.

5.4 GPS Operation

BioGPS can be used by itself or as part of more complicated systems, which need not be MMS instruments,

although cabling and data acquisition is simplified when it

is used as part of a microradiometer system. Five different

modes of operation are available:

1. Fully integrated microradiometer system with a synchronized shadowband and GPS data stream controlled MMS deck box;

2. Automated shadowband movement and GPS reporting controlled by an MMS deck box;

3. Manual shadowband movement and GPS reporting

controlled by a data acquisition computer;

4. Streaming GPS reporting using ASCII data; and

5. Polled GPS reporting using ASCII data.

The GPS data stream provides the following parameters: a) Universal Time Coordinated (UTC) to the nearest second, b) longitude, c) latitude, and d) the number

of satellites visible. In the event that a GPS fix is invalid,

-999 is reported for each parameter.

Because the MMS architecture requires synchronized

data streams at rates exceeding 12 Hz, the response times

53
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

of individual sensors are critical. In contrast, the GPS

engine used in BioGPS typically updates its position once

per second. This slow refresh rate has implications in the

way the data is buffered by the BioGPS aggregator.

The BioGPS aggregator controls the sampling of the

GPS, and also responds to polled requests for data from

the controlling deck box. The service time of the requests

is typically less than 10 ms to maintain a 15 Hz polled data

stream from multiple radiometers, and other devices such

as a shadowband. This need for very short lag times means

that querying the GPS and then waiting for a response

would greatly slow down the system. Consequently, the

GPS data stream is serviced by interrupts, because the

desired sample rate (nominally 15 Hz or 67 ms per frame)

is too short to request and receive data in many instances.

The best-case turnaround time period is approximately

35 ms, with many lasting 100 ms or longer, which is too

long to wait for a reply without disturbing the sample rate.

The data volume of MMSs can be substantial. For example, three 19-channel radiometers are projected to have

332 bytes transmitted in each data frame (104 bytes per

sensor plus about 20 bytes from the deck box). At 15 Hz,

this is 4,980 bytes per second, or for a system running continuously, 480 MB per day. Although many radiometric

systems have intermittent duty cycles (e.g., C-OPS), others will have more frequent sampling scenarios (e.g., OSPREy), so data volume can be an important problem.

The GPS data stream conforms to the National Marine

Electronics Association (NMEA) data format, which is an

ASCII string of up to 80 characters. The BioGPS command structure allows the full ASCII data stream to be retrieved. To minimize the flow of redundant data, however,

the NMEA string can optionally be converted to short precision (4-byte) floating point. numbers. Because fresh GPS

data is only available every second, the data stream can

be multiplexed, with the parameter actually transmitted

being rotated through the four types of parameters (time,

longitude, latitude, and number of satellites). This feature

further reduces the amount of data to be transmitted.

5.5 Shadowband Operation

The precise position of the shadowband is unknown

when the controller unit is powered up. When under the

automatic control with a C-OPS deck box, the motor is

turned on and initialized at startup. First, the motor is

moved counterclockwise (CCW) approximately 20 0 , then

back and forth 15° while deck box pourer levels are adjusted

to compensate for any voltage drop in long cables.

The second part of the initialization sequence, which

is issued by the data acquisition software, involves the following command sequences:

1. Read the motor position.

Move negative 12,000 steps from the current read

position. This move ensures that the shadowband

will find the home position in Step 3, regardless of

its initial position. On some occasions, the shadowband may run into stops that are integral to the

device's mounting brackets, causing the motor to

slip (which is acceptable).

3. Move into the home position (this is always in the

clockwise, CW, direction).

4. Move to the 0 (zero) position in the negative direction (CCW), approximately 30,000 steps. The

exact value is the motor offset and will vary from

unit to unit depending on the location of the home

position.

5. Set this position to 0. This should have the band

horizontal and even with the top of the irradiance

collector.

6. Move the shadowband from 0-180° (which is the

aforementioned v angle), with farther extension to

a typical value of 220°, where the band is completely

below the horizon. This latter position is also the

stowed position when not in use.

7. Move the shadowband back to 0 0 ; repeat Steps 6

and 7 as long as desired.

Extension of the motion cycle as part of the sixth step also

provides an extended period of time with the band out of

the field of view of the collector, so the data acquisition sequence ends with a global irradiance measurement. If the

shadowband data is collected immediately after a COPS

down cast, the global irradiances from the two events can

be used to verify atmospheric stability across the measurement sequences. A sequential movement of the shadowband over the COPS solar reference during clear-sky conditions is shown in Fig. 53.

5.6 Shadowband Modeling and Testing

In the following discussion, BioSHADE was operated

with a 19-channel C-OPS solar reference radiometer on the

roof calibration facility at BSI. Weather conditions were

ideal during the test period, offering several cloudless days

with low aerosol loading. This analysis focuses on measurements taken on 7 October 2008, one of the clearest

days of the test period.

Figure 54 shows solar reference measurements during a

full cycle of shadowband irradiance at 320 nm (blue data

set, right axis) and 780 nm (red data set, left axis) measured on 7 October 2008 between 17:52 and 17:56 UT. During this period, the solar zenith angle (SZA) was about

45.5°. Vertical lines indicate times when the motion of the

shadowband reversed. The band is in its maximum position at points G1 and G3 , and in the zero position at point

G 2 . As the band moves upward from point G 1 , it will eventually be horizontal (point A l). From this position onward,

a portion of the sky will be shaded and the irradiance measured by the microradiometer will decrease. The change

in signal is much larger at 320 nm than 780 nm, because

sky radiance is much greater at shorter wavelengths (at

780 nm, the predominant portion of the signal is due to

direct sunlight).

54
26

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J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

W_

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Fig. 53. A sequential movement of the shadowband over the C-OPS solar reference (center white cylinder) on

a clear-sky day. The shadowband controller is the right-most cylinder, and the GPS is the left-most. The solar

reference is slightly above the other two devices to ensure an unobstructed view of the sky. The primary cable

connection is to the controller; the other devices plug into the controller to receive power and send data. The

black cylinder between the BioGPS and the solar reference is a solar reference for another instrument system.

G2G3

F 1

A2

t

Al

^.

B1

E1,

a . 320 nm j C 1 D1 : 780

20 90 160 230

Elapsed Time [s]

Fig. 54. The spectral irradiance measurement at

320 and 780 nm using the prototype BioSHADE accessory and the COPS solar reference. The data

were acquired on 7 October 2008 between 1752 and

1756 UTC. Vertical lines indicate times when the

motion of the shadowband reversed. Labeled points

are explained in the text.

The collector of the solar reference is cosine weighted

and the effect of shading the sky increases as the band

moves from the horizon toward the zenith. At point 13 1 in

Fig. 54, the band starts to block the direct component of

the solar illumination, and the irradiance rapidly decreases

until the entire cosine collector is shaded (point C1 ). At

point D 1 , the band is no longer fully occluding the diffuser and direct sunlight begins to fall on a portion of the

collector, so the irradiance increases. There is very little

change in irradiance between points C1 and Dl , because

the width of the band is set to ensure a reasonable number

of totally occluded data points are acquired. At point. Fl,

the band is horizontal and the instrument once again measures global irradiance. The time required for the band to

move between points A l and F1 , which represents a full

1800 swath, is 64 s.

5.6.1 Direct Irradiance Calculation

The main purpose of the BioSHADE accessory is to

provide measurements of the optical properties of the atmosphere and, in particular, to allow the calculation of

the direct-horizontal spectral irradiance (i.e., the irradiance on a horizontal plane from direct solar illumination),

Eb (0+ , A, t). The calculation requires a variety of irradiance

measurements made before, during, and after the passage

of the shadowband over the cosine collector on the solar

reference.

To calculate Eb (0+ , A, t), the solar zenith angle (0) plus

the following defined quantities are needed:

E,,, (0+ , A, t) The direct-normal spectral irradiance (irradiance on a plane perpendicular to the

detector—Sun direction).

Ei (0+ , A, t) The diffuse spectral irradiance (irradiance

from the sky on a horizontal plane).

Ed (0+ , A, t) The global spectral irradiance (irradiance

from the Sun and sky on a horizontal

plane).

Ek (0+ , A, t„) The hypothetical (i.e., it is not measured

directly) spectral irradiance at the solar

reference for the segment of the sky that

is shaded by the shadowband at time t„

when the band is at shadowband angle v.

Ep (0+ , A, t„) The spectral irradiance at the solar reference when the band is at shadowband angle v and not blocking direct sunlight.

E,,(0+ , A, t,) The spectral irradiance at the solar reference when the centers of the solar disk,

shadowband, and diffuser are aligned and

direct sunlight is completely occluded.

cb (A) The angular response error of the solar reference, defined as the ratio of angular response at incidence angle 79 to cos(V).

90 G1

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Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

ci (A) The angular response error of the solar reference when exposed to isotropic radiation.

cd (A) The angular response error of the solar reference when measuring global irradiance.

From these definitions, the following equations can be

derived from inspection:

E, (0+, A, t) = Eb ( 0

+, 

A, t) > (6) Cos (0)

Ep (O+ , A, tv) = Ed (0+ , A, tv) — Ek(O+ , A, tv), (7)

and

E,,,(0+ , A , tv) = Ed (0+ , A , tv) — Ek (0+ , A , ti,)

— Eb (0+ , A, t). (8)

The six quantities Ed (0+ , A, t), Eb (0+ , A, t), En (0+ , A, t),

E'i(O+ , A , t), Ek(0+ 7 A , t„), Ep (0+ , A, t,,), and E,,(0+ , A, tv)

cannot be measured directly by the solar reference, because the solar reference is affected by a small angular

response error (or cosine error). Quantities affected by

this error are indicated by the prime symbol ('). For example, if the shadowband is below the horizon, the signal of the solar reference is proportional to E'd (0+ , A, t)

cG(A)Ed(O+ , A, t).

From (7) and (8), and the definition of CO), it can

be concluded that Eb (0+ , A, tm) = cb (A) Eb (0+ , A, tm) =

Ep(0+ , A, tm) — E' (0+ , A, tm), where tM indicates the time

when the centers of the Sun, shadowband, and collector are

all aligned. This leads to the expression for the calculation

Of Eb(0+,A,tM):

Eb (0+ tm) = E

p (0+ , a, tm) E^(0+ >

 A, tm)

(9) cb(A)

The challenge is that Ep(0+ , A, tm) is not directly accessible

from the measurements acquired with the shadowband accessory, but must be extrapolated from observations taken

at times when all parts of the collector are exposed to direct sunlight. (These are times to the left of point B 1 or

to the right of point El in Fig. 54).

To determine Ep(0+ , A, tm), an analysis software package was developed based on the formulation presented in

(9). The software also determines performance metrics of

the shadowband system, so the quality of the results can

also be assessed. The software procedure that was developed is illustrated graphically in Fig. 55, which also indicates the relative change of the measured irradiance from

one sample point to the next. This quantity is used to

determine the times when the shadowband starts to shade

the cosine collector from the direct illumination of sunlight (points B and E), and when the occlusion is at the

maximum (point. M).

20 35 50

Elapsed Time [s]

Fig. 55. A portion of the spectral irradiance at

780 nm from Fig. 54. The time M indicates when

the centers of the Sun, shadowband, and collector were in line (as determined with the analysis

software), and T is the intersection of the dashed

line with spectral irradiance at 780 nm. The other

points and the fitted lines are explained in the text.

A graphical depiction of the determination process for

the E1,(0+ , A, ti1,1 ) parameter is shown in Fig. 55. The

Ep(0+ , A, tm) value (the blue circle on M near the top

of the plot), is extrapolated via a linear regression using all measurements between points b' and b. Additionally, Ep (0+ , A, tm) was extrapolated from measurements

between points e and e', as shown by the green circle,

just below the blue circle, on M. The difference between

the two extrapolated values, denoted Ep,, (0+ , A, tm) and

EpE (0+ , A, tm), respectively, is a measure of the uncertainty for calculating Ep(0+ , A, tm) and presents the principal limitation of any shadowband approach for measuring

Eb (0+ , A, t). As Fig. 55 indicates, the difference between

EP',, (0+ , A, tm) and EP'-- A, tm) is rather small. The difference tends to be larger at shorter wavelengths, however,

because of the larger values of Ek (0+ , A, t„) and their dependence on the band position.

The variation of E' (0+ , A, tm) and EpE (0+ , A, tm) at

A = 780 nm was investigated for several consecutive shading events during stable atmospheric conditions. The relative difference was on the order of 0.2%. The average

of the two values exhibited little noise, from one event to

the next. Consequently, this value was used for the calculations of Eb (0+ , A, t) presented below. A similar analysis was

performed for A = 320 nm, for which the relative difference

was typically 1.5%, but the average of the two extrapolated

values varied by less than 0.3% from one shading event to

the next. This suggests Ep(0+ , A, tm) can be calculated

from all spectral channels with high accuracy.

56
11.8-

11.4-3.3

11.0 3.2

0 25 50 75

Elapsed Time [min]

Fig. 57. Measurements of Ea(0+ , A, tm) at 780 and

320 nm (red and blue, respectively).

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J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

5.6.2 Sliaded Irradiance

150-

0

N

U)0 100-

c

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50-

E

Z

0

0 4 8 12

Elapsed Time [h]

Fig. 56. The number of data points (red) contributing to the EQ(0 + , A, tm) values as a function

of SZA (black). The two parameters share the same

axis, but with different units.

The ability of the system to determine E,, (0 + , A, tm)

with high confidence was estimated by analyzing a time

series of Ea(0+ , A, tm) over several shading events (results

are presented in Fig. 57). At 320 nm, there is virtually

no event-to-event variation beyond the upward slope resulting from the decrease of SZA during the evaluation

period. At 780 nm there is some variation, which is likely

partly due to real changes of the atmosphere (e.g., change

in aerosol loading); however, the event-to-event variation

at this wavelength is also smaller than 0.7%.

Figure 58 shows the diurnal cycle of E'a (0+ , A, tm) for

all 19 wavelengths of BioSHADE. There is some asymmetry, with lower values generally in the morning, indicating

that the atmosphere's aerosol loading has changed over

the course of the day. The largest spectral irradiance is

observed at 412 nm. Measurements at wavelengths larger

than 600nm are comparatively low because the largest contribution stems from direct irradiance, which is blocked.

2

E

C

N

E1

aDU

C

C^

c^i

0

(D

0 4 8 12

Elapsed Time [h]

Fig. 58. Measurements of EQ(0+ , A, tm) using the

following gradient (respectively): light-to-dark violet 320, 340, 380, 395 nm; light-to-dark blue 412,

443, 465, and 490 nm; light-to-dark green 510, 532,

555, and 560 nm; light-to-dark red 625, 665, 670,

and 683 nm; and light and dark gray 710 and 780 nm.

•

•

•

•

•

•

t

t •

•

AO

t

The accurate determination of the Ea (0+ , A, tM ) parameter is also required for the calculation of Eb (0+ , A, t).

The value Ea (0+ , A, tm) is defined here as the average of

all sample points between points C and D, where the relative point-to-point change is less than 0.2% (see Fig. 55

for the definition of points C and D). The number of points oN

contributing to this average is shown in Fig. 56 for measurements on 7 October 2008 performed for SZA values WT

from 33-85°. The number of points increases with SZA.

This can be explained as follows: the axis of the shadowband was approximately oriented in the East—West direction. At a large SZA (or close to sunrise and sunset),

the Sun is either in the East or West. In this case, the

angular movement of the band across the disk of the Sun

is slower than when the axis is perpendicular to the line

of the collector and Sun, explaining the large number of

sample points contributing to Ea (0 + , A, tm).

57
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

5.6.3 Direct Normal Irradiance

Figure 59 shows the diurnal variation of direct-normal

irradiance E,,, (0+ , A, tm) calculated using (6). Data are not

perfectly symmetrical about the solar noon, most probably due to changes in aerosol loading over the course of the

day. There is a "hump" in data measured at SZA larger

than 800 at wavelengths larger than 600 nm. This feature

is likely related to an incomplete correction of the system's

cosine error. (At the time of this analysis, a detailed cosine correction of the instrument was not yet available; the

cosine correction used was based on a generic function for

microradiometers, which may not be appropriate for this

particular instrument at long Wavelengths.)

150

E

rN

E

U

100

mU

c

c^

c^

50

E

O

Z

U

4]

0 n

0 4 8 12

Elapsed Time [h]

Fig. 59. Spectral direct-normal irradiance for 7

October 2008. The SZA is shown in black using_ the

y-axis numerical values, but in degrees.

5.6.4 Global Irradiance

Figure 60 shows the diurnal variation of cosine-error

corrected global irradiance, Ed (0', a, t), measured at times

associated with points G1 , G2 , and G3 (Fig. 54 defines the

latter three points). The value Ed ( 0+ , A, t) was calculated

from uncorrected global irradiance data Ed(0+ , A, t) using

the following formulation:

Ed (0+ , A, t) _ [Ed( 0+ , A, t) — c,E,( O+ , A, t)

(10)

+ ciEb(0+ , A , t)] ci 1,

where the angular response error terms are from Sect.

5.6.1. Values of Eb(0+ , A, t) in (10) were interpolated from

Eb (0+ , A, tM ) to the times associated with points G;.

150

E

c

N

100

O

U

c

co

cz 50

c^

_O

C.'3

0

0 4 8 12

Elapsed Time [h]

Fig. 60. Spectral global irradiance Ed (0+ , A, t) for

7 October 2008. A cosine-error correction was applied.

5.6.5 Langley Plots

The change of direct-normal irradiance with SZA is

determined by the Beer-Lambert's law:

E,,, (0+ , A , t) = Eo(A , t )

exp-T(a)'n(e), (11)

where Eo(A,t) is the direct-normal irradiance outside

Earth's atmosphere, T(A) is the spectral optical depths of

all scatters and absorbers in the atmosphere, and rn(9) is

the relative optical airmass, which is a function of SZA and

can be approximated with:

m(B) = [ cos(B) + 0.50572 (96.07995-8)-1.6364] -1. (12)

Taking the logarithm of (11) leads to:

ln[E, (0+ , A, t)] = ln[Eo(A, t)] - T(A)m(B)• (13)

If T(A) does not change over the course of the day,

ln[E.,(0+ , A, t)] becomes a linear function of rn(B). Plotting

measured data of ln[E,,(0+ , A, t)] versus m(9) allows for the

determination of E0 (A, t) by extrapolation to m(0) = 0,

and to estimate T(A). The optical depth T(A) is the sum

of the Rayleigh optical depth TR(A), aerosol optical depth

TA (A), and the optical depth TX (A) of other scatterers and

absorbers. By calculating TR (A) and TX (A), the aerosol optical depth TA (A) can be determined. Plots of E,,(0+,A,t)

versus m(B) are known as Langley plots.

Langley plots from irradiance data collected with the

BioSHADE accessory were used to determine the potential for deriving Eo(A,t) and 7-A(A). A Langley plot for

58
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

data measured on 7 October 2008 is presented in Fig. 61,

which shows In[E,(0+ , A, t)] varies fairly linearly with airmass rn(0) for rn(0) < 3 (where 0 is less than 70°) with similar values in the morning and afternoon, as expected. For

an airmass between 3-8 (where 0 is between 70° and 82°),

measurements made in the afternoon are lower (measurements at 320 nm and 340 nm are below the detection limit).

For even higher airmass values, the pattern reverses. These

variations are likely caused by changes in aerosol loading

throughout the day, but could also be caused by systematic

instrument errors.

6

3

lL.l

c

0

-3

0 5 10 15

Air Mass

Fig. 61. A Langley plot for data measured on 7

October 2008 using the same spectral correspondence for the colored symbols that was established

for Fig. 58.

5.7 Summary

Analysis of BioSHADE data has confirmed that the

instrument meets its design specifications. The number

of sample points recorded while the shadowband is completely blocking the Sun is sufficient for accurate retrievals

of Eo, (0+ , A, tM ). Furthermore, Ep (0+ , A, tM ) can be accurately determined by extrapolating measurements taken

immediately before and after the times when the shadowband starts to cast a shadow of the Sun on the instrument's collector. Precise knowledge of Ea. (0 +

, A, tm) and

Ep (0+ , A, tM ) allows for the direct-horizontal and directnormal irradiance to be calculated with confidence, and to

construct Langley plots. The instrument size and recording speed also meet specifications.

The GPS unit provides UTC, longitude, and latitude

every second. These parameters are fully integrated into

the data stream of the microradiometer system and available for processing. This ensures that proper record traceability exists for sampling times and station position during data acquisition. Although not common, it is not unheard of for a research vessel to have GPS problems, so

a redundant data stream can be very useful. The latter

is particularly true for calibration and validation exercises

wherein an important objective is to provide matchups between the in situ and remote sensing observations. In addition, many small vessels do not automatically record position as a function of time or have displays located where

scientific work is being done, because of space and resource

limitations.

The BioSHADE and BioGPS systems can be fully integrated with instrumentation based on the microradiometer

architecture, wherein the two devices are operated over a

single cable, linking radiometers, BioGPS, and the BioSHADE accessory. The latter ensures the accessory components and the radiometric sensors can be operated using

standard cabling and deck box configurations.

due., s •^• • ' ..b . .

`1 •.^.l.^ • •' Start (am)

Solar •~'w • ; • • •

Noon .• i • r • •

Sib •

s

End (pm)

End (pm)

59
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Chapter 6

Scalable Hydro-optical Applications for

Light-Limited Oceanography (SHALLO)

JOHN H. MORROW

Biospherical Instruments, Inc.

San Diego, California

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

GERMAR BERNHARD AND RANDALL N. LIND

Biospherical Instruments, Inc.

San Diego, California

ABSTRACT

Based on microradiometer detectors, a suite of instruments has been produced that are specifically designed

to improve near-surface, near-shore, and above-water AOP measurements, particularly in light.-limited environments. The hierarchical organization inherent in the use of clusters of microradiometers affords an unprecedented

ease in interconnecting, modifying, or upgrading the instruments as scientific objectives or financial resources

evolve. Purpose-built instruments have the potential to reduce instrument size, complexity, and costs while

retaining the optimal field configuration for optical and ancillary sensors. This philosophy is made more powerful by establishing an outline of modular capabilities and a step-wise upgrade pathway for the instrument

classes. Unlike legacy sensors, changing, replacing, or repairing a filter or filter-photodetector combination for

a microradiometer sensor does not require disassembly of the entire electro-optics section of the instrument.

The modularity of the design means it is not a significantly time-consuming and tedious procedure to upgrade

a sensor. In addition, the added risk of unintended damage to associated components is minimized, as is the

time required to test the subassemblies and recalibrate the sensor. The purpose of this chapter is to introduce

a framework for the evolution and expansion of AOP microradiometer instruments into novel deployment opportunities, particularly in light-limited environments, while providing upgrade scenarios to match evolutions

in resource allocations and science objectives.

6.1 Introduction

The development of AOP field instrumentation requires

an optimization of a number of competing design elements.

For commercial instruments, reducing instrument cost is

always an issue, but it is not necessarily the most important design factor given that the ability for a sensor to fill

multiple roles is essential in a multidisciplinary research

environment. The requirement to reduce the impact of

the instrument on the measurement itself drives a consistent need to reduce the size (and resulting perturbations)

of the device, in particular for in-water sensors.

In contrast, and as part of next-generation planning

for remote sensing platforms, there is increased interest

in expanding the spectral resolution of the measurement

beyond the visible domain most commonly measured by

legacy instruments, into the UV and SWIR. This expansion requires the following enhancements with respect to

prior designs already in use:

• Development of cosine collectors that adequately

diffuse light across ever wider spectral bands (e.g.,

305-1,670 nm);

• Love-noise electronics for detecting very low flux levels;

• Extreme out-of-band blocking, so small signals in

the UV and SWIR can be measured accurately in

the presence of much larger visible flux;

• Highly sensitive sensors with a very wide dynamic

range, so there is no saturation within a greatly unbalanced spectrum or, in the case of radiance sen60
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

• STAR sensors are the smallest sensors available (1.5 in OD).

• Above- or in-water with up to 7 channels (305-1,670 nm).

• Fully functional with XTRA and EPIC radiometers plus some

EPIC devices (e.g., GPS).

• Small size significantly reduces self-shading effect (if in water).

a

• XTRA sensors provide wide spectral coverage (2.75 in OD).

• Above- or in-water with up to 8-19 channels (305-1,670 nm).

• Fully functional with STAR and all EPIC devices, i.e., pointing

units, GPS, shadowband, and ancillary (meteorological) sensors.

b

• Small size reduces self-shading effect (if in water).

• OXR sensors are insulated and temperature stabilized (6 in OD).

• 8-18 fixed-wavelength channels (305-1,670 nm) plus a hyperspectral spectrograph (245-785 nm or 310-1,100 nm).

• Designed for laboratory use only (calibration transfer), but is also

designed to be shipped to remote calibration facilities.

• EPIC sensors are insulated and temperature stabilized (6 in OD).

• Above water 8-19 fixed-wavelength channels (305-1,670 nm).

•' • Hyperspectral spectrograph (same as OXR) and video camera.

`^ • 7-position filter wheel permits hyperspectral polarimetry, Sun

a

viewing, plus improved dark currents and stray-light correction.

Fig. 62. The four different microradiometer instrument classes as represented by a radiance instrument.

The aperture ends are drawn to scale, but the colors are arbitrary.

sors viewing the surface of the water, no saturation

while measuring sun glint; and

• Small sensor diameters to facilitate their use in a

more diverse set of applications (e.g., robotic systems) and to minimize systematic errors (e.g., those

caused by self-shading).

Temporal and thermal stability are important design

elements that control not only data acquisition electronics,

but material selection, component location, and conformal

coating. Lastly, an outstanding instrument design will not

acquire the best data if a deployment protocol that focuses

on both the environment encountered in the field, and the

data products that will be produced, is not rigorously applied in every campaign.

6.2 Classes

To address the objectives outlined above, four instrument classes have been established. A common feature of

the classes is they all have state-of-the-art performance,

because they are all based on microradiometer technology (Chap. 3). The four classes are as follows: a) Standardized Technologies for Applied Radiometry (STAR),

b) Expandable Technologies for Radiometric Applications

(XTRA), c) OSPREy Transfer Radiometer (OXR), and

d) Enhanced Performance Instrument Class (EPIC). Figure 62 presents the four classes and summarizes their design features, which are presented in more detail below

(Sect. 6.3).

Instruments within each class are available with radiance or irradiance front-end optics and each class is optimized for a specific measurement environment. Instruments within every class can be custom configured to meet

the requirements for the measurement task to be performed. Several instruments can be networked together

in a straightforward manner, thereby offering very flexible

deployment options. Data from all instruments and ancillary sensors (e.g., pressure, temperature, shadowband,

and GPS) are generally controlled by a single deck box

and computer, resulting in a single, time-synchronized data

stream.

6.3 Design

STAR radiometers are the smallest sensors available

(1.5in OD) and are ideally suited for applications where

self-shading tends to degrade data quality or where small

lightweight sensors are needed. Examples include in-water

profiling of radiance and irradiance in shallow, optically

complex waters or above-water measurements from an unmanned aerial vehicle (UAV). STAR sensors feature seven

channels with user-selectable wavelengths, ranging from

305-1,640 nm.

XTRA radiometers provide a wide spectral coverage

with up to 19 fixed-wavelength channels, which are se61
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

lectable from the wavelength range of 305-1,640 nm. To

accommodate the additional number of channels, the OD

is 2.75 in, which is still small enough to limit self-shading

effects in many water types to an acceptable level of correction. XTRA sensors are anticipated to be the work horses

for most field campaigns and are compatible with STAR

and all EPIC devices, including pointing units, GPS, shadowband accessories, and meteorological sensors.

OXR radiometers are specifically designed for highaccuracy transfers of radiometric scales between standards

of spectral irradiance, such as 1,000W tungsten-halogen

lamps used by the National Institute of Standards and

Technology (NISI) or other national and commercial laboratories. In addition to 8-18 fixed wavelength channels,

these radiometers feature a hyperspectral spectrograph

with a wavelength range of either 345-785 nm or 310-

1,100nm. The microradiometer data are used to keep

the spectrograph calibrated, so the hyperspectral data can

be used to accurately establish measurements between the

fixed channels or at other bandwidths. The instruments

are temperature stabilized and designed for laboratory use.

For example, they are black anodized for minimal reflectivity, with the goal to reduce stray light in the laboratory.

The OXR design is rugged enough for shipping to remote

calibration facilities without the risk of affecting instrument sensitivity.

EPIC radiometers have a similar design as the OXR,

but are optimized for outdoor (above-water) deployment.

Like the OXR, they are temperature controlled, and feature 8-19 microradiometer channels and a spectrograph.

In addition, radiance units have a filter-wheel assembly,

placed in front of the optical fiber of the spectrograph.

Although customized filter sets are possible, the sevenposition filter wheel is typically equipped with the following: a) an opaque filter for accurate dark current measurements; b) a cut-on filter used for stray light correction;

c) neutral density filters to expand the dynamic range of

the spectrograph, and d) three polarization filters to determine the polarization state of both the sky and radiation

emanating from a water surface.

EPIC radiometers will not saturate when pointed directly at the Sun (unless a customized filter set does not

permit it), but are sensitive enough to measure lour light

levels. Examples include measurements of irradiance in the

UV-B domain at low Sun angles, water-leaving radiance

in the near infrared, and measurements of direct moon

light. Pointing devices and motorized shadowbands are

available as accessories for EPIC instruments. With these

fully-integrated devices, the units can be used for Sun photometry or applications associated with SeaPRISM sensors

(Hooker et al. 2000b and Zibordi et al. 2006). These applications allow, for example, the determination of aerosol

optical depth as a function of wavelength and water-leaving

radiance.

point the system accurately at the center of the Sun. The

camera can further provide a picture of the radiance fieldof-view, so the presence of clouds across the solar disk or

floating debris on the sea surface can be properly detected.

Picture recognition algorithms, which are integral to the

system control software, are used to flag data accordingly.

When equipped with a shadowband, EPIC irradiance radiometers can alternatingly measure diffuse and global irradiance, from which the direct solar irradiance is determined.

6.4 Operation

The expandability of these instrument systems is unprecedented, because microradiometers are the modular

building blocks for the sensors. In comparison to sensor

systems produced in the not-too-distant past, it is significantly easier to make changes to microradiometer instruments as scientific objectives and financial resources evolve.

This philosophy is made more powerful by establishing a

scale of modular capabilities for the instrument classes. In

the case of the STAR and XTRA sensors, five system configurations are envisioned (Fig. 63).

The simplest—or basic—system uses the minimum

number of sensors and manual pointing to provide a lowcost starting point for above-water radiometry. The addition of a second sensor provides redundancy and simultaneous sampling, which yields enhanced quality assurance

(QA) and data products. Automated pointing from the

EPIC pan-tilt unit adds unattended sampling scenarios,

which when combined with ancillary sensors like the shadowband, yield a variety of new data products. As the multiple sensor system is made more complete, redundancy

minimizes risk and sampling scenarios, using synchronous

and asynchronous protocols, further enhances data products and QA. Spectral diversity also can be optimized

by using different spectral configurations in the multiple

sensor systems, while remembering that common wavelengths will permit redundancy for the most important

wavelengths and provide better QA opportunities.

The culmination of autonomous control is the inclusion of additional sensors, so the automated functions can

be used safely on a moving platform, in this case a research vessel. The extra sensors also allow for improved

QA, which results in better data products. A bow mounting system provides an easy mechanism to quickly remove

the sensor suite in the event of severe weather (so it cannot

be damaged), as well as the capability for quick and accurate reinstallation. Additional mounting options for tall

(but accessible) superstructures or telescoping masts provide alternative solutions for measurements that are not

contaminated by the presence of the sampling platform.

6.5 Testing and Advancements

A video camera is integrated into EPIC radiance in- The principal testing of the microradiometer design elestruments and is used in lieu of a quadrant detector to ments are presented in Chap. 3. The first complete system

62
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Basic System Measurements, Data Products, and Mission Advantages

- _^ • 7 or 8-19 Channels

• One radiance sensor

• STAR (7 bands) or XTRA (8-19 bands) sensors (30S-1,670nm)

• Direct sea, sky, and reflectance plaque radiance measurements

and one solar reference (the latter permit enhanced QA opportunity)

a • Manual or fixed pointing • Tilt sensor in solar reference (GPS and shadowband optional)

Simultaneous System All of the Above, and in Addition:

f

• One irradiance and two

radiance sensors

• Sea and sky simultaneous sampling improves data products

• GPS recommended for moving platforms (shadowband optional)

T]•

• Shadowband optional

Manual or fixed pointing

• Some redundancy (radiance sensor loss permits basic sampling)

• Enhanced QA and data products from synchronous sampling

Automated System All of the Above, and in Addition:

1

f'

,am

• Synchronous and asynchronous sampling

Fixed platform (e.g., off-

• Unattended sampling scenarios (night darks), ancillary sensors

recommended (hazards), and multiple protocols (viewing angles) 

• Shadowband recommended (improved cloud detection)

c ^ shore tower) mounting • Stowing of radiance sensors significantly minimizes fouling

4

d _..^ ^.

Multiple System

• Up to 38 channels of

radiance and irradiance

• Fixed platform mounting

•Permits full redundancy

All of the Above, and in Addition:

• Redundancy minimizes risk (data loss from sensor malfunction)

or spectral diversity enhances data products

• Maximum synchronous and asynchronous sampling scenarios

• Ancillary sensors for detecting hazards (e.g., high wind and rain)

Moving Platform System All of the first three Systems, and in Addition:

A

- Designed Designed for mostly unended operations att• Additional detection scenarios for hazards (e.g., sea state)

• Enhanced QA and QC functions (digital compass in GPS)

c • Ancillary sensor package • Bow mounting system permits easy removal (as well as, quick and

e for hazards detection accurate reinstallation) of sensors to prepare for severe weather

Fig. 63. Configuration examples of STAR and XTRA sensors showing the progression in capability and

sophistication from a basic system to a moving platform system. The gold coloring denotes fully-integrated,

automated components.

built from microradiometers was C-OPS, and Chap. 4 provides significant information regarding the performance of

this new in-water profiling system. The capabilities of the

shadowband and GPS accessories are presented in Chap. 5.

Although the solar reference part of C-OPS represents an

above-water capability, it does not include any radiance

measurements.

The first microradiometer radiance sensor was developed as part of the OSPREy activity, and the first prototype radiance sensor was an OXR. The primary objective of the OSPREy project is to establish an above-water

radiometer system as a lower-cost alternative to in-water

buoys for the collection of sea-truth observations. The OSPREy system can also be used close to shore and ideally

complements in-water AOP instruments, such as C-OPS

(Fig. 3). Introductory information regarding OSPREy is

provided in Sect. 1.1.2, and complete details are provided

by Hooker et al. (2010), so only additional abbreviated

summary information is provided here.

Building on the modular approach inspired by microradiometers, the OSPREy system uses temperaturecontrolled EPIC instruments equipped with radiance front

ends that are installed on automated pointing devices (pantilt units) to measure the radiance emanating from the

sea surface, as well as the Sun and sky. These systems

are complemented by other EPIC irradiance instruments,

which are outfitted with computer-controlled shadowbands

to measure global and diffuse irradiance. Data collected by

these instruments supports calculations of a large number

of oceanic and atmospheric data products such as waterleaving radiance, aerosol optical depth, and the ratio of

direct-to-global irradiance. Data collected in the NIR are

particularly useful for interpreting the radiative environment in turbid coastal waters.

Stowed Rotated

Shadowband Shadowband

rGlobal Diffuse

Irradiance Irradiance

L- B

A ^.

f

Fig. 64. The OSPREy system concept showing

the two dyads of sensors (denoted A and B), which

each contain one irradiance sensor (with shadow

band attachments) and one radiance sensor. The

pedestal mounting is for illustrative purposes, as

are the pointing angles and shadowband positions.

A high level of redundancy is an important part of the

OSPREy concept. Typically, a system is based on two

duplicate radiometer sets, called dyads (Fig. 64 and also

63
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Basic System Measurements, Data Products, and Mission Advantages

• 7 or 8-19 Channels • Star (7 channels) or XTRA (8-19 channels) from 305-875 nm

• Hydrobaric buoyancy • In-water downward or upward irradiance and upwelled radiance

control • Two-axis tilt sensor in solar reference

a • Two-axis trim adjustment • Derivation of standard suite of ocean color data products

Extended System

• BioSHADE shadowband

All of the Above, and in Addition:

• Extended atmospheric characterization (direct and diffuse solar

with BioGPS accessory irradiances can be computed)

• Synchronous or asyn- • Improved self-shading correction

p chronous sampling • Time and position of all sampling locations

Bidirectional System All of the Above, and in Addition:

• Upward and downward • Direct measurement of the afactor for bidirectional effects

irradiance sensors • Enhanced data products from reflectance measurements

• Redundant derivation of • Improved data corrections plus more extensive QA and QC from

_C] standard data products alternative data products

Automated System All of the Above, and in Addition:

Automated and adaptive • Remote trimming of ascent and descent sampling options

buoyancy control • Automated buoyancy control also permits enhanced one-person • 

Fail-safe automatic return operation for manual deployment scenarios

d to surface capability Multiple buoyancy control options for fine-scale trim adjustment

Moving Platform System All of the Above, and in Addition:

1 Designed for unattended • Embedded microcomputer deck box controller

^1) or attended operations • Optional remote telemetry for onsite programming and enhanced

Research vessel or buoy QA and QC functions

e sampling options Data collection scenarios using temporal schedules and events

Fig. 65. Configuration examples of STAR or XTRA sensors for in-water profiling observations showing

the progression in capability and sophistication from a basic system to a moving platform system (a-e,

respectively).

discernible in Fig. 3), which can be operated synchronously

and asynchronously to enhance data quality and minimize

data gaps. Note that the dyad concept is shown in Fig. 63d

and represents an example of the upgrade pathways resident with all microradiometer instrument systems. All

measurements are traceable to the irradiance scale maintained by NIST via an OXR and a portable light source

for monitoring instrument calibration in the field at regular (e.g., monthly) intervals. Because the OSPREy system

is installed above water, it is less affected by biofouling

than a permanently installed in-water system.

OSPREy systems are modular and can be configured to

meet different measurement tasks and funding constraints.

The highest quality implementation of an OSPREy system uses sensor triads, wherein two irradiance sensors are

used to ensure the highest quality cosine response across

the full spectral range of 305-1,640 nm. The triad configuration will be deployed in a multidisciplinary role for

satellite calibration and validation activities. The operational (dyad) system optimizes radiance capabilities with

redundant synchronous or asynchronous capabilities. In

addition to all of the standard system features, the maximum (triad) system adds a wider spectral coverage and

maximum number of data products and redundancies.

The configuration options associated with OSPREy systems also make theirs useful for applications beyond AOP

measurements. For example, a single radiance radiometer mounted on a pointing device can replace a SeaPRISM

system and will provide significantly more spectral information, additional new data products, and higher quality

data.

6.6 Future Designs

Originally envisioned for autonomous deployment on

moorings and drifting buoy systems, the Compact Environmental Radiometer Buoyancy Enhancements for Rateadjustable Underwater Sampling (CERBERUS) profiler is

a three-sensor (Ed , Eu , and Lu ) modification to the hydrobaric free-fall backplane currently in use with the SuBOPS and C-OPS instruments. It consists of a buoyant

plastic backplane equipped with rigid foam flotation elements separating three 19-channel microradiometer sensors (Fig. 48). In addition to the innovations characteristic

of the standard free-fall system (hydrobaric air bladders,

adjustable flotation, plus pitch and roll control), CERBERUS features an automated bladder buoyancy system

(rated to 25 m) that controls the rate of descent or ascent.

Similar to the microradiometer optical instruments, the

buoyancy system is controlled by an embedded microprocessor located in the deck box controller.

The modularity first established for above-water systems is also present for in-water instrumentation for which

five configurations are envisioned (Fig. 65). The simplest—

or basic—system uses a minimum number of XTRA sensors to provide a low-cost starting point for deriving the

standard suite of ocean color data products. The addition

64
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

of a BioSHADE and BioGPS provides atmospheric data

products, improved self-shading correction, plus time and

position of all sampling locations. The use of the threesensor (CERBERUS) backplane permits the direct measurement of the Q-factor, enhanced data products, and

improved QA and QC. Remote trimming of ascent and

descent sampling options is achieved by adding an automated and adaptive buoyancy control module. The latter

also permits enhanced one-person operation and a fail-safe

automatic return to the surface capability, if the sea cable is severed. Although the backplane is automatically

brought to the surface, recovery is not guaranteed, because

it must still be located. Nonetheless, this is a far more desirable situation than having all the in-water components

sink to the bottom (if the water depth is deep enough, this

could lead to implosion of the sensor cylinders).

The culmination of autonomous control is to tether the

in-water backplane to a moving platform, for example, a

buoy. This system is designed for unattended or attended

operations, which are controlled by an embedded microcomputer deck box controller. Optional remote telemetry

can be used for onsite programming and enhanced QA and

QC functions. The data collection scenarios are based on

temporal schedules or specific events detected during normal operations.

6.7 Summary

Four radiometric instrument classes (STAR, XTRA,

OXR, and EPIC) have recently been developed and introduced to the market to measure AOPs using both aboveand in-water techniques. Each class is optimized for specific measurement tasks with an unprecedented degree of

affordable expandability. STAR and XTRA sensors have a

small diameter to minimize self-shading effects for in-water

deployments and to fit them into novel platforms (e.g.,

UAVs). Temperature-stabilized OXR and EPIC radiometers excel in terms of measurement accuracy, spectral resolution, and built-in quality-control features. Instruments

from all four classes can be networked together, resulting

in very flexible deployment configurations. For example,

above- and in-water sensors plus additional ancillary sensors, pointing devices, and shadowbands can be combined

to support multidisciplinary missions such as satellite calibration and validation exercises, coastal monitoring, or

UAV overflights.

65
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Chapter 7

The Telescoping Mount for Advanced Solar Technologies (T-MAST)

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

ABSTRACT

The solar reference data collected with an in situ AOP observation must be at the highest point possible on the

measurement platform and free from obstructions and reflection sources. Although this is easy to state, it is

not always a straightforward operation to implement. On many research vessels, the highest spaces are usually

already occupied with the ship's equipment and such spaces are frequently inaccessible at sea (because of safety

concerns). Consequently, AOP observations are frequently made with the solar reference located in a less than

ideal location. A quantification of the consequences of improperly siting the solar reference are presented along

with field evaluations of a new Telescoping Mount for Advanced Solar Technologies (T-MAST). Field trials show

T-MAST is an excellent solution for this problem while providing access to the sensor(s) for cleaning, servicing,

and dark current measurements.

7.1 Introduction

Whether made using above- or in-water light sensors,

the most significant problem with making AOP measurements is minimizing the perturbations from the sampling

platform the light sensors are deployed on or from. In the

case of large platforms, the reflections from the structure

above and below the water line brighten the ambient light

field, whereas the shadow cast by the platform darkens it.

The latter affects instruments that are deployed directly

into the shadow, but also those in near proximity to it, because photons that would normally be scattered into the

adjacent unshaded waters have been blocked by the structure causing the shadow. In all cases, corrections can be

produced, but they require significant modeling efforts involving a large dynamic range in solar illumination, sky

conditions, and viewing geometries, which is not practical unless a platform is used for extensive periods of time.

The simplest expedient, therefore, is simply to avoid the

perturbation areas by sampling outside them.

In the case of the sampling platform being a research

vessel, the in-water problem is easily solved by floating

the sampling system far away from the ship and collecting data as the profiling package falls freely through the

water column (Fig. 66). Currently, there is no reliable

mechanism for floating an above-water system away from

a ship, so the measurements are usually made on the bow

of the vessel, which is a point reliably far away from the

superstructure with good fields of view of the water. In

both cases, the solar reference measurement is made at the

highest point possible free from obstructions and reflection

sources. If properly implemented, this avoids the platform

perturbations, but it does not deal with all the perturbations. The data collected by the free-fall profiler is also

subjected to self-shading, the correction for which is based

on the in-water properties, the size of the sensors, and the

above-water solar illumination. Indeed, the absence of a

self-shading problem with the above-water approach is one

advantage for this type of measurement.

Solar Reference (mounted ^^

Sun on the beam for abovewater AOP measurements

as high above superstructure (made on the bow)

and stack as possible) I

i

e Superstructure

Albedo

X Hull

i Free-Fall Albedo

Profiler

(-50 m on

astern) O -

Fig. 66. A schematic representation of the platform perturbations associated with a ship and deployment locations for AOP measurements.

How far a free-fall profiler needs to be deployed away

from a ship is a function of not only avoiding the light

field perturbation, but also of incorporating the influence

of the ambient currents, which can carry the instrumentation back into the perturbation field. A sensible compromise is to use a distance of approximately 50m for a large

ship and about 30 m for a smaller vessel. Kite-shaped profilers tend to pop upwards when they are hauled in, so the

66
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Fig. 67. Stern (left), side (middle) and bow (right) views of the R/V Roger Revelle showing a typical location

for a solar reference towards the stern and the preferred (but more difficult) location on the bow. Although not

as high as the tallest mast on the ship, the latter is substantially above and far array from the most substantial

part of the superstructure, and provides significant shelter for the reference even in high sea states.

relative position of the profiler when it returns to the surface after being retrieved usually gives a good indication

of local current effects. If the profiler returns not too far

from where it was released, the distance from the ship need

not be adjusted; if the profiler returns much closer to the

ship, then a farther release distance is likely appropriate.

Very large, so-called ocean-class, vessels are needed for

many types of oceanographic research. Arctic field campaigns, for example, require large icebreakers. A significant difficulty with icebreakers is the oversized box-shaped

superstructure that is placed forward of the typical location in ocean-class research vessels. It is very difficult to

measure the solar irradiance whichis a requirement for

AOP measurements—on large vessels, because the light

sensor needs to be far away from the light-field perturbations caused by the associated superstructure and the

contamination caused by the ship's exhaust stack. Usually, this means the solar reference needs to be mounted

on the highest point of the ship. Unfortunately, on many

research vessels, the highest spaces are usually already occupied with the ship's equipment and such spaces are frequently inaccessible at sea (because of safety concerns).

The CVO participated in CLIVAR 16S to not only fill

in the current undersampling of high latitudes, but also to

understand ghat problems might be degrading AOP data

and, hopefully, provide solutions. Although the R/V Roger

Revelle provides many advantages for oceanic sampling, it

is not very attractive for optical measurements: the highest

point on top of the main mast is not readily available to

scientists, and the bow mast cannot be accessed at sea.

Consequently, a solar reference is usually mounted in a

less than ideal location (Fig. 67).

The importance of properly siting a solar reference is

quantified by comparing the bow and stern references on

CLIVAR I6S. The bow sensor is assumed to provide the

best data (i.e., the closest to truth), because it is mounted

at the highest elevation and the farthest from superstructure perturbations, so the RPD (5) is computed as

100Ed(O+,aB— Ed(0+,A), (14)

EB (0+ , A)

where Ed and Ed are the global solar irradiances measured

by the boor and stern sensors, respectively.

Figure 68 presents the RPD between the bow and stern

solar references. If properly sited, two solar references

should agree to within the calibration uncertainty (about

2.5%). The stern sensor exceeds this threshold about 49%

of the time and has only a few examples wherein all the

data agree with the bow sensor to within 2.5%. In many

instances, the differences are quite large, worse than f 15%.

0%

-25

0 25 50 75

Cast Number

Fig. 68. The RPD between the bow and stern solar

references on the R/V Roger Revelle (the former is

the reference in the RPD calculations).

20-

0

a_

I

-10-

Superstructure

Reflections

' Brighten Stern

Irradiances I •

6:

j

N•

i

±2.s

T --- I •

oo*

•+o

All As 4C

Good

•

Superstructure

Shading

Darkens Stern

Irradiances

67
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

The most troubling aspect of the Fig. 68 results, however, is the introduction of a persistent bias as the ship

steams farther towards the South Pole and the recurring

overcast conditions lead to a steady worsening in negative

RPD values. The occasional sunny stations show up as

large positive excursions. This type of significant bias can

have a seriously detrimental effect on data products that

use the solar reference data for normalization, e.g., Rrs (A)

or [LTv(A)IN'

7.2 Description

For ships that do not have or permit access to high superstructure locations free of significant perturbations, the

only solution for the data bias problem is to either a) use

contaminated data, or b) install a device that will elevate

the solar sensor to a height were contamination is not possible. The latter is a potentially difficult requirement on

a large vessel, because of the height of the superstructure.

There are also the difficulties of wind loading, ice loading,

ship motion, and the corrosive environment of conditions

at sea. If a device is going to be practical, it needs to be

easy to install and easy to take down—especially if foul

weather is forecast.

The solution for the data bias problem presented here

was to have a telescoping mast currently being used by

the US military (Fig. 69) and have it modified for use on

a ship. The masts are made by Floatograph Technologies

(Silver Spring, Maryland), and are available in a wide variety of sizes. The masts are also offered in two different

classes of ruggedness: heavy duty (steel) and light duty

(aluminum). Installations to-date include 50 ft and 60 ft

steel masts, and a 25 ft aluminum mast. The 50 ft and

25 ft masts were used on the Canadian Coast Guard Ship

(CCGS) Amundsen, and the 60 ft and 25 ft masts were used

on the United States Coast Guard Cutter (USCGC) Healy.

Fig. 69. A telescoping mast, with surveillance

equipment on top, is mounted to the bumper of a

humvee.

The installation of the telescoping masts on the CCGS

Amundsen took place in 2009 and was in cooperation with

the Laboratoire d'Oceanographie de Villefranche (LOV) in

France and the University of Laval in Canada as part

of the Malinat field campaign to the Canadian waters of

the Beaufort Sea. The installations on the USCGC Healy

took place in 2010 as part of the Impacts of Climate on

Ecosystems and Chemistry of the Arctic Pacific Environment (ICESCAPEt) expedition to the U.S. waters of the

Chukchi Sea.

For both the Malina and ICESCAPE campaigns, the

EM25 telescoping mast was installed on a smaller vessel

that was launched from the ice breaker (Fig. 70). The small

boats deployed from the CCGS Amundsen and USCGC

Healy were rather similar, and both could have the bow

lowered for immediate access to the sea. The latter was

useful for deploying free-fall optical sensors and was critical for finer-scale sampling, because the icebreakers and the

large sampling systems deployed from them significantly

mixes the upper portion of the water column to a depth

of many meters. The small boat, in comparison, was allowed to drift into the areas to be sampled and minimally

perturbed the near-surface layer.

Fig. 70. The EM25 telescoping mast extended on

the small boat (port side, stern) launched from the

CCGS Amundsen during C-OPS deployment operations. Note the red cable extending from the port

bow and the white streak (top right corner) from

the cable being hauled in.

Although a simpler mast arrangement could have been

used with the small boats, the deployment and recovery

scenarios for the smaller vessels on both icebreakers required a telescoping design to ensure the collapsed height

t Information about the Malina field campaign to the Canadian waters in the Beaufort Sea is available from the following

Web site: http://www.obs-vlfr.fr/Malina/.

f Information about the ICESCAPE field campaign in 2010

to the U.S. waters in the Chukchi Sea is available from the

following Web site: http://www.espo.nasa.gov/icescape/.

68
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

was below the height of the wheelhouse of the small boats

being launched.

7.3 Design

The masts described here are models FM50, FM60, and

EM25. The former two are heavy-duty steel masts and

the latter is a light-duty aluminum mast. The FM masts

are rated for 50 lb payloads on top of the mast, while the

EM mast is rated for a 25 lb payload. All of the masts

can be used in winds up to 60 mph and are equipped with

guy lines to stabilize the upper parts of the mast against

bending (Fig. 71). For the deployments described here,

the guy lines were only used with the large masts, because

the small masts were only extended for the short periods

of time associated with the small boat operations.

0

i^

I

Y 'CC

Fig. 71. The FM60 telescoping mast mounted on

top of the USCGC Healy with Kevlar guy lines attached.

Another distinction of the large masts is they break

clown after being collapsed, so they lie horizontally and fit

into a cradle. This places the entire mast at an accessible

height, so the sensors mounted at the top can be cleaned or

serviced. The latter also permits caps to be put on the radiometers, so dark measurements can be made. The FM60

has a partially detachable ladder as part of the base unit,

so it is possible to access the payload when the mast is collapsed, without having to break it down into the horizontal

resting position (Fig. 72).

Fig. 72. The PM60 telescoping mast collapsed,

with the solar references being cleaned prior to the

recording of dark measurements (caps on).

7.4 Modifications and Operation

Technical drawings of the FM50 and EM25 masts are

presented in Figs. 73 and 74, respectively. The masts were

used primarily as originally designed, but some modifications were made to accommodate their use in the marine

environment:

• Some of the hardware was replaced with stainless

steel (SS).

• The top stage had a 1 in national pipe tapered

(NPT) coupler welded to it, so the standard 1 in

NPT 316SS pipe used for mounting solar references

in the field could be attached directly to the top of

the mast.

• The winches were replaced with an SS marine compliant winch.

• The base of the small EM25 mast was modified, so

it could be bolted against standard ship railing using mounting plates that would compress the mast

against the railing.

The last item proved important for the small boat deployments during both Malina and ICESCAPE, because the

69
.50

1.75

8.00

2.75 -^

42.50

i

25

3.25

-F

12.46

7.59

Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Fig. 73. A schematic of the EM25 mast showing it in the collapsed (stowed) configuration, which has an

overall height of a little more than 6 ft, from two different angles. The small pipe pointing to the left and

protruding from the bottom of the lowest stage associated with the telescoping mast unit (right schematic)

is for pumping hot air into the mast in the event it gets frozen into place as a result of very cold and wet

conditions. All dimensions are given in inches.

70
2.50

J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Fig. 74. A schematic of an FM50 mast (with an electronic winch for the extension and retraction functions)

showing it in the stowed configuration (left) and the deployed, but collapsed, position (right.). As with the

EM25 mast, the small pipe pointing to the left and protruding from the bottom of the lowest stage associated

with the telescoping mast unit (right schematic) is for pumping hot air into the mast in the event it gets

frozen into place as a result of very cold and wet conditions. All dimensions are given in inches.

small boats involved could not be modified in any way

without extensive recertification by the appropriate agencies.

The large masts have two extra hand-operated winches

associated with the base unit to deploy or stow the collapsed mast into, or out of, the cradle. The primary difference between the FM50 and FM60 models is the addition

of a ladder for the FM60; otherwise, the two masts are

deployed and stowed in the same fashion. Once a mast is

vertically oriented, it is raised and lowered with the handoperated winch attached to the immovable lowest stage of

mast. Cable guides are attached to the sides of the various telescoping stages to ensure the data telemetry cable

is properly restrained.

7.5 Summary

At-sea deployments of the FM50, FM60, and EM25

masts during the Malina and ICESCAPE field campaigns

established the robustness of the basic design. Throughout both campaigns, there was only one occasion when the

combination of relative wind and ship headway was expected to produce winds in excess of the design limit, and

the mast was lowered. There were no failures of any part of

either system and all deployments resulted in the collection

of excellent solar irradiance data. For the large icebreakers,

vertical tilts on-station were almost always less than 2.5°,

and for the small boats—which are livelier platforms—no

solar irradiance data was outside the expected thresholds

and all data were usable.

Chaffing of the cable from wind luffiing was anticipated

in the larger masts, which were left extended for significant

periods of time. Split tubing with an inner diameter close

to the outer diameter of the cable was used to protect the

cable from rubbing against the cable guards mounted on

each telescoping stage. In one instance, the tubing slipped

below the cable guard and the outer braid of the cable

was worn through over the course of many days of wear;

the next layer of insulation was not degraded. This event

showed the importance of properly applying chaffing protectors on the cabling.

71
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

Chapter 8

The Cable Hauler for Optical In Situ Technologies (C-HOIST)

STANFORD B. HOOKER

NASA Goddard Space Flight Center

Greenbelt, Maryland

JOHN H. MORROW AND RANDALL N. LIND

Biospherical Instruments, Inc.

San Diego, California

ABSTRACT

Because of their large size and weight, IOP instruments are usually mounted inside a large metallic frame and

require significant resources to deploy over the side of a ship, e.g., a hydraulic winch and an A-frame. In

comparison, modern free-fall AOP profilers, like C-OPS, are sufficiently small in size and weight to be deployed

by hand. For small boat operations, AOP instruments are easily accommodated, but IOP sensors usually

are not. Most small boats are equipped with a davit for deploying small packages into the water, which,

when combined with a COTS system used by the commercial fishing industry, provides a solution to the IOP

instrument frame problem. The capabilities of this new deployment system, called the Cable Hauler for Optical

In Situ Technologies (C-HOIST), is presented along with results from the field commissioning of the prototype.

The significant advantages of C-HOIST are as follows: a) the power head uses 12 VDC power, which is available

on most small boats; b) there is no need for hydraulics; c) the payload can be raised or lowered very slowly; and

d) it can be used with standard synthetic line of any length—there is no need for an integral drum of cable.

8.1 Introduction

For most field campaigns, it is desirable to collect contemporaneous IOP measurements during, or in close temporal proximity to, AOP observations (many U.S. research

vessels do not permit more than one ivire over the side

at once). For small-boat operations, like those described

earlier during Malina and ICESCAPE (Sect. 7.2), the difficulty is in dealing with the large size and weight of IOP

instruments in comparison to the smaller and lighter modern AOP profilers. This distinction is best summarized

by the fact that IOP instruments usually require a winch

and A-frame for deployment, whereas AOP instruments

are routinely deployed by hand.

IOP sensors are usually mounted inside a large frame,

and even if the frame is built from lightweight materials,

the total weight of all the equipment needed for IOP observations can be on the order of 40 kg or more. It is not unusual for IOP systems to be so heavy—wheels are attached

to the bottom of the frame to make it easier to move around

on deck. Part of the reason for the large size and weight

is the larger diversity in IOP variables (and, thus, instruments) than AOP light-field components, coupled with the

desire to have more information about water column properties to interpret those variables, but there are also simply

more components for each instrument (some, for example,

require external pumps).

The net effect of the rather large and heavy instruments mounted inside a bulky frame is a requirement for

some significant ship resources for safe deployment, that

is, a hydraulic winch and an A-frame are needed. This is

a rather difficult requirement for many small boats. The

vessels launched from the icebreakers used during the Malina and ICESCAPE field campaigns, for example, did not

have A-frames and they did not have hydraulic winches.

Instead, they had small davits (or J-frames) and electrical winches with small-diameter stainless steel cables that

were not very long in length (40 m or less).

The small-diameter cable is a concern, because it is

hard to handle by hand, and many aspects of deploying or

recovering a frame from a small boat require handling by

the scientists or crew. A more desirable situation is to be

able to use standard synthetic line with an outer diameter

more in keeping with normal at-sea practices (e.g., 3/8 or

1/2 in). For a normal electromechanical winch, the larger

the diameter of the line, however, the larger the size of the

winch drum and, thus, the size of the winch. The amount

of line that can be wound on the winch decreases as the

line diameter increases, so a limiting size quickly emerges

if the winch size is going to be kept small.

72
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Adding to the difficulty of obtaining a small winch is

the desire to run the winch on 12 VDC, because it is the

most practical power source on small boats. There is also

the problem of being able to lead the line from the winch

through a pulley on the davit. In most situations for deploying with a davit, the davit is rotated to facilitate deploying and recovering the instrument package over the

side of the boat. Any needed movement of the davit adds

to the complexity of leading the line from the winch to

the davit; therefore, a solution that easily accommodates

this requirement is substantially more useful than one that

does not.

8.2 Description

For small research vessels that do not have a winch and

an A-frame that can lift and deploy a heavy instrument

frame, but do have a davit (which is a common piece of

equipment on small boats), the only practical solution is

to either a) create two or more smaller subunits that can

be deployed by hand, or b) bring a device that can fulfill

the task. With the former, simultaneity of observations

is lost, plus it is not easy to smoothly control the descent

and ascent of the package by hand, except for very small

instruments, so the vertical resolution of the sampling is

not very uniform.

The solution adopted by the CVO was to work with

BSI to modify an existing capability to solve the IOP deployment problem. The solution is based on what is called

a pot puller, which is used by the commercial fishing industry for raising and lowering shrimp pots, or lobster and

crab traps, over the side of small boats. Any of these can

be quite heavy, so pot pullers with a 300 lb capacity are

common.

Fishermen want to get their work done quickly, because

every hour at sea is costly and reduces profitability, so pot

pullers are designed to pull line in at a high velocity. Scientists making IOP measurements, on the other hand, prefer

slower line speeds, so the vertical sampling resolution can

be as high as possible—particularly in shallow, optically

complex waters, like the coastal zone. Consequently, part

of the design problem for using a commercial pot puller

as the basis for C-HOIST was producing a speed control

capability.

8.3 Design

The pot puller is a light-duty COTS model that is used

extensively by commercial fisherman. It is equipped with

a stainless steel self-grip sheave, 2.1 HP 12 VDC electric

power head, 8 AWG wiring harness, 80 A circuit breaker,

and has a 300 lb capacity. The gearbox assembly, which

is part of the power head unit, is a differential planetary

design that is an extremely strong and reliable unit. The

gear ratio is 159:1, which means the motor turns 159 revolutions for every revolution of the sheave. The reduction

unit is sealed with a life-time lubrication grease (a high

pressure, EP-2 rated, lit.hiuun-based grease).

The motor assembly is O-ring sealed to prevent saltwater intrusion from spray or wash down after use at sea.

Metal components are either stainless steel or powder

coated for corrosion resistance. The electric motor draws

30-80A during normal operations. If operated using a

90 A hr lead-acid (sealed) battery, approximately 25 casts

lasting 6 min each can be performed before the battery

needs recharging.

The sheave design eliminates the need to thread the line

through complicated idler wheels and pulleys. The line is

simply laid into the sheave and around the idler wheels.

A light downward pull seats the line between the rubber

lined sheave, which grips the line for both paying out and

hauling in line. The sheave line capacity is 0.25-0.75 in.

Because the line is not wound onto a drum by the power

head, the line can be as long or as short as desired. There

is no line length limit, and the pot puller will pull line as

long as the power source is available. For the ICESCAPE

campaign, the pot puller was wired into the small boat

12 VDC power system.

To provide a range of slower deployment options, the

STP-2100 was mated to a deck box control unit built by

BSI. The principal purpose of the deck box was to provide

a speed control capability and modular power connection,

that is, power from the small boat was the input to the

deck box and the output was the regulated power for the

pot puller. For making IOP casts, slow ascents and slow

descents are equally important. In the Arctic, melt water

and riverine sources can produce a thin surface layer of

water with significantly different optical properties than

the water below. Consequently, there was also the need to

be able to sample the near surface layer(s) very slowly and

then to speed up to save time.

The variable speed control of the deck box was based

on a regenerative DC-to-DC drive. Most nonregenerative

variable-speed DC drives control current flow to a motor in

only one direction. The direction of the current flow is the

same direction as motor rotation (if a motor is reversible,

there are two control options). Regenerative drives can

also provide motor torque in the opposite direction of motor rotation. This allows a regenerative drive to reverse

motor direction without contacts or switches to control an

overhauling load, and to decelerate a load faster than it

would take to coast to a lower speed.

The specific regenerative drive used with C-HOIST accepts an input voltage of 10-32 VDC, provides speed regulation to within 1%, and has a maximum (peak) armature

current of 120 A (250 A). The electronic control board is

sufficiently compact, about 7 x 5 in 2, to fit inside the saune

small waterproof case used by BSI for their standard instrument deck boxes. Speed adjustment for paying out or

hauling in line is set using a potentiometer, which is manipulated using a knob mounted on the top of the deck

box.

73
Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

There are three other switches on the deck box, which 8.5 Testing

provide the rest of the operational controls:

• The Hoist Enable switch turns the power to the

power head on and off.

• The Brake switch enables motor torque to keep the

sheave stationery.

• The Direction switch toggles the direction the motor is turning.

The power head is connected to the davit with a swivel, so

the direction for paying out or hauling in line is arbitrary

and is set by the orientation of the entire assembly on

the davit. Consequently, direction control simply reverses

whatever is the present setting (which can be initially set

or changed before line is threaded through the sheave by

rotating the entire pot puller).

8.4 Operation

A picture of the prototype C-HOIST unit being used

during an ICESCAPE station is shown in Fig. 75. The

pot puller is hanging immediately below the end of the

curved davit. The large silver disc is the self-grip sheave.

To the left of the sheave is the black power head, and

below the sheave are the two black idler wheels. The line

attached to the IOP frame threads to the left of the idler

wheel immediately below the sheave, around the sheave,

and then to the right of the idler wheel below and to the

left of the sheave. Not shown in the picture is the person

operating the deck box to control the ascent and descent

of the frame.

s ^

.00

ld^

Fig. 75. The prototype C-HOIST unit being used

during the ICESCAPE campaign from the davit on

the small boat launched from the icebreaker USCGC

Healy.

The C-HOIST power head is a COTS device for lightduty commercial crab fishing, but the motor drive and

deck box are assembled by BSI. The 80 A system is rated

for 300 lb dead-weight lifting. The revised cruise schedule

for the ICESCAPE field campaign did not permit extensive testing or local field deployment of the prototype. Inhouse testing consisted of attaching the power head to an

overhead steel beam and testing the main functions when

powered by a 12 VDC lead-acid marine battery. This testing included retention of a large (100kg) weight in both

unpowered and dynamic hold (bra ke) states, very slow

and maximum retrieve speed testing with a large (100 kg)

weight, and idle testing with the deck box power turned

on for hours. The temperature of the drive controller and

head was monitored during all testing, and overheating

was never observed under any conditions.

Field commissioning of the prototype C-HOIST during

the ICESCAPE cruise consisted of more than 30 profiles of

the NASA IOP frame in a variety of sea states. Unfortunately, the prototype suffered an unknown failure during

the recovery of the frame on cast number 33, under lightto-moderate conditions. Disassembly of the power head

aboard ship did not reveal the cause of failure and field

testing was terminated.

A post-mortem conducted by BSI on the deck box confirmed it was functioning correctly and had no fault conditions. The power head was returned to the manufacturer

where no mechanical or motor failure was discerned. Given

the information available, the most likely candidate for the

field failure is the system circuit breaker, which consisted

of two 40 A thermal breakers operating in parallel. The

breakers are of the type featuring automatic reset and no

trip indicator. The circuit breakers have been respecified

for single 80 A operation with trip indication and manual

reset.

8.6 Summary

The C-HOIST device is a COTS electrical fishing winch

with custom speed and direction control that can be operated from a small davit to aid deployment and retrieval

of lightweight (less than 100 kg) instrument packages from

almost any size boat. The system is designed to be transportable, and can be operated from any 12 VDC source

with sufficiently high current capacity, including a single

marine battery. The sheave on C-HOIST is designed to

accept line or cable diameters up to 0.5 in, but unlike a

winch, the line or cable used with C-HOIST is not spooled

on the assembly, and any convenient length can be used

(assuming appropriate caution is used when the remaining line on deck is reduced to a short length). The system

requires two operators; one to operate the power head controls attached to the deck box, and one to control and feed

the line out to the instrument package.

74
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

ACKNOWLEDGMENTS

The success of the data acquisition and data processing capabilities presented here would not have been possible without

funding support from the NASA OBB Program Office. The

microradiometer, aggregator, and associated systems were developed by Biospherical Instruments, Inc., with funding from

the NASA SBIR Program under contract NNG06CA03C entitled "In Situ Radiometers: Smaller, Faster, and Scalable to

Hyperspectral." The preparation of the manuscript and its final

level of completeness benefitted significantly from the technical

editing expertise of Elaine Firestone whose attentiveness to detail, and good cheer, are greatly appreciated.

GLOSSARY

3GμR Three-Gain Microradiometer

A/D Analog-to-Digital

ADC Analog-to-Digital Converter

AERONET Aerosol Robotic Network

AOI Angle of Incidence

AOPs Apparent Optical Properties

ASCII American Standard Code for Information Interchange

AWG American Wire Gauge

BATS Bermuda Atlantic Time Series

BioGPS Biospherical Global Positioning System

BioOPS Biospherical Optical Profiling System

BioPRO Biospherical Profiler

BioSHADE Biospherical Shadowband Accessory for Diffuse Irradiance

BioSOPE Biogeochemistry and Optics South Pacific Experiment

BioSORS Biospherical Surface Ocean Reflectance System

BOUSSOLE Bougie Pour Pacquisition de Series Optiques a

Long Terme (literally translated from French

as the "buoy for the acquisition of a long-term

optical series.")

CCGS Canadian Coast Guard Ship

CCW Counterclockwise

CDR Climate-quality Data Record

CERBERUS Compact Environmental Radiometer Buoyancy Enhancements for Rate-Adjusted Underwater Sampling

C-HOIST Cable Hauler for Optical In Situ Technologies

C-OPS Compact-Optical Profiling System

COTS Commercial Off-The-Shelf

CSTARS Center For Southeastern Tropical Advanced

Remote Sensing

CVO Calibration and Validation Office

CW Clockwise

DARR-94 The first SeaWiFS Data Analysis Round

Robin

DARR-00 The second SeaWiFS Data Analysis Round

Robin

EOS Earth Observing System

EPIC Enhanced Performance Instrument Class

FAFOV Full-Angle Field of View

FEL Not an acronym, but a lamp designator.

FPA Filter-Photodetector Assemblies

FOV Field of View

FWHM Full Width at Half Maximum

CPS Global Positioning System

GSFC Goddard Space Flight Center

HD Housing Diameter

ICESCAPE Impacts of Climate on Ecosystems and Chemistry of the Arctic Pacific Environment

IOPs Inherent Optical Properties

IR Infrared

LCD Liquid Crystal Display

LED Light Emitting Diode

LoCNESS Low-Cost NASA Environmental Sampling

System

LOV Laboratoi.re d'Oceanographie de Villefranche

MERIS Medium Resolution Imaging Spectrometer

microSAS micro-Surface Acquisition System

microSD Microsecure Digital (card)

microNESS micro-NASA Environmental Sampling System

miniNESS miniature NASA Environmental Sampling

System

MMS Multiple Microradiometer System

MOBY Marine Optical Buoy

MODIS Moderate Resolution Imaging Spectroradiometer

MODIS-A Moderate Resolution Imaging Spectroradiometer-Aqua

MODIS-T Moderate Resolution Imaging Spectroradiometer-Terra

NASA National Aeronautics and Space Administration

NEI Noise Equivalent Irradiance

NER Noise Equivalent Radiance

NIR Near Infrared

NIST National Institute of Standards and Technology

NMEA National Marine Electronics Association

NPT National Pipe Tapered

OBB Ocean Biology and Biogeochemistry

OCTS Ocean Color and Temperature Scanner

OSPREy Optical Sensors for Planetary Radiant Energy

OXR OSPREy Transfer Radiometer

PAR Photosynthetically Available Radiation

PCA Printed Circuit Assembly

PE Polyethylene

PGA Programmable Gate Array

POLDER Polarization and Directionality of the Earth's

Reflectance

PP Polypropylene

PRR Profiling Reflectance Radiometer

psia Pressure per Square Inch Absolute

PU Polyurethane

PURLS Portable Universal Radiometer Light Source

QA Quality Assurance

RPD Relative Percent Difference

RSMAS Rosenstiel School of Marine and Atmospheric

Science

RTD Resistance Temperature Detector

R/V Research Vessel

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Advances in Measuring the Apparent Optical Properties (AOPs) of Optically Complex Waters

SAS Surface Acquisition System

SBIR Small Business Innovation Research

SeaBASS SeaWiFS Bio-optical Archive and Storage System

SeaFALLS SeaWiFS Free-Falling Advanced Light Level

Sensors

SeaPRISM SeaWiFS Photometer Revision for IncidentSurface Measurements

SeaSAS SeaWiFS Surface Acquisition System

SeaWiFS Sea-viewing Wide Field-of-view Sensor

SHALLO Scalable Hydro-optical Applications for LightLimited Oceanography

SIRREX SeaWiFS Intercalibration Round-Robin

Experiment

SPI Serial Peripheral Interface

SPMR SeaWiFS Profiling Multichannel Radiometer

SQM SeaWiFS Quality Monitor

SS Stainless Steel

STAR Standardized Technologies for Applied Radimetry

SuBOPS Submersible Biospherical Optical Profiling

System

SUnSAS SeaWiFS Underway Surface Acquisition System

SWIR Short-Wave Infrared

SZA Solar Zenith Angle

T-MAST Telescoping Mount for Advanced Solar Technologies

UAV Unmanned Aerial Vehicle

UPD Unbiased Percent Difference

USB Universal Serial Bus

USCGC United States Coast Guard Cutter

UTC Universal Time Coordinated

UV Ultraviolet

XTRA Expandable Technologies for Radiometric Applications

SYMBOLS

Ca, Chlorophyll a concentration.

cb(A) The angular response error of the solar reference.

C^(A) The spectral calibration coefficient.

cd (A) The angular response error of the solar reference

when measuring global irradiance.

cz(A) The angular response error of the solar reference

when exposed to isotropic radiation.

d The distance between the lamp and the diffuser

faceplate.

D(A) The average bias or dark voltage.

E(A) Spectral irradiance.

E(z, A) Spectral irradiance at a depth z.

E(0+ , A) The in-air spectral irradiance just above the sea surface.

E(0- , A) The in-water spectral irradiance at null depth (z =

0-).

Eo(A) The direct-normal spectral irradiance outside the

Earth's atmosphere (irradiance on a plane perpendicular to the detector—Sun direction).

E,,(0+ , A) The spectral irradiance at the solar reference when

the centers of the solar disk, shadowband, and diffuser are aligned and direct sunlight is completely

occluded (at time t„).

Eb(0+, A) The direct-horizontal spectral irradiance (irradiance

on a horizontal plane from direct solar illumination).

E., (A, t2 ) The spectral calibrated irradiance.

Ed (z, A) The in-water spectral downward irradiance profile.

Ed (0+ , A) The spectral global solar irradiance (from the Sun

and sky on a horizontal plane).

Ed (0+ , A) The global solar irradiance measured by a bow sensor.

Ed (0', A) The global solar irradiance measured by a stern sensor.

Ez(0+A) The spectral diffuse (sky) irradiance (irradiance

from the sky on a horizontal plane).

Ew(0+ , A) The hypothetical spectral irradiance at the solar reference for the segment of the sky that is shaded by

the shadowband when the band is at time t„ and

the shadowband is at angle v.

K, (0', A) The direct-normal spectral irradiance (irradiance on

a plane perpendicular to the detector—Sun direction).

E, (0', A) The spectral irradiance at the solar reference at time

t. when the band is at shadowband angle v and not

blocking direct sunlight.

E'P , (0+ , A) An extrapolated spectral irradiance (at the solar reference) at time tm using an interval denoted B.

EpE (0+ , A) An extrapolated spectral irradiance (at the solar reference) at time to using an interval denoted E.

E3 A solar reference sensor.

If (A) The spectral immersion factor.

K(A) The spectral diffuse attenuation coefficient.

Kd (A) The spectral diffuse attenuation coefficient computed from Ed (z, A).

Li (0', A) The spectral indirect (or sky) radiance reaching the

sea surface.

L,(0+ , A) The radiance of the plaque.

LT (0', A) The (total) radiance above the sea surface.

L..(A) The upwelled spectral radiance.

L,,(z,A) The upwelled spectral radiance at depth z.

Lw(A) The spectral radiance leaving the sea surface from

below (the water-leaving radiance).

L w (A) The spectral water-leaving radiance derived from an

above-water sampling method.

Lw (A) The spectral water-leaving radiance derived from an

in-water sampling method.

[Lw (A)] N The spectral normalized water-leaving radiance.

NI The point (in time) when the centers of the Sun,

shadowband, and collector are all aligned.

m(0) The relative optical airmass.

Np The number of photodetectors.

nw(A) The spectral refractive index of water, which is also

a function of S and T.

93 The in-water radiometric quantities in physical units

(L^, Ed , or Ems).

Pe The packing efficiency of microradiometers into a

cylinder.

T (z, A, to) A radiometric parameter (L., Ed , or E.) as it would

have been recorded at all depths z at the same time

to.

q3(0- , A) A subsurface radiometric quantity (L.., Ed , or E,,,)

at null depth z = 0-.

Q, Nadir-viewing measurements.

76
J.H. Morrow, S.B. Hooker, C.R. Booth, G. Bernhard, R.N. Lind, and J.W. Brown

Rd Radius of the diffuser.

Rrs Remote sensing reflectance.

R The effects of reflection and refraction.

Ro The R term evaluated at nadir, i.e., 0' = 0

S Salinity.

t Time.

T Water temperature.

to A reference time (generally chosen to coincide with

the start of a measurement sequence).

ti A specific time.

TS (A)The spectral transmittance of the water surface to

downward irradiance.

t„ The time when the shadowband is at angle v.

V (A, ti) Spectral digitized voltages (in counts).

W Wind speed.

x The horizontal axis (abscissa).

X An arbitrary reference measurement.

Y An arbitrary measurement to be investigated.

z The vertical (depth) coordinate, where the depth is

the height of water above the cosine collectors.

z. The critical depth.

0 Solar zenith angle.

0' The above-water viewing angle (V) refracted by the

air-sea interface.

The radiometer pointing angle with respect to the

vertical axis, z.

V' The angle V measured with respect to the zenith.

.\ Wavelength.

p The surface reflectance factor.

T(.\) The spectral optical depths of all scatters and absorbers in the atmosphere.

r4 (A) The aerosol optical depth.

TR (1) The Rayleigh optical depth.

TX (,\) Other scatters and absorbers at optical depth.

¢ The solar azimuth angle.

cp The perturbations (or tilts) in vertical alignment,

which can change the pointing angles.

¢' An angle away from the Sun (here either 90° or

135°).

¢- An angle 90° counterclockwise away from the Sun.

(P+ An angle 90 0 clockwise away from the Sun.

V) The RPD value.

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Advances in Measuring the Apparent Optical Properties (AOPs) of Optically

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13. SUPPLEMENTARY NOTES

John H. Morrow, Charles R. Booth, Gennar Bernhard, and Randall N. Lind; Biospherical Instruments Inc., San Diego, CA; and

James W. Brown: CSTARS University of Miami, Miami, FL

14. ABSTRACT

This report documents new technology used to measure the apparent optical properties (AOPs) of optically complex waters. The principal

objective is to be prepared for the launch of next-generation ocean color satellites with the most capable commercial off-the-shelf (COTS)

instrumentation. An enhanced COTS radiometer was the starting point for designing and testing the new sensors. The follow-on steps were to

apply the lessons learned towards a new in-water profiler based on a kite-shaped backplane for mounting the light sensors. The next level of

sophistication involved evaluating new radiometers emerging from a development activity based on so-called tnicroradiometers. The exploitation

of tnicroradiometers resulted in an in-water profiling system, which includes a sensor networking capability to control ancillary sensors like a

shadowband or global positioning system (GPS) device. A principal advantage of microradioneters is their flexibility in producing,

interconnecting, and maintaining instruments. The full problem set for collecting sea-truth data—whether in coastal waters or the open ocean—

involves other aspects of data collection that were improved for instruments measuring both AOPs and inherent optical properties (IOPs), if the

uncertainty budget is to be minimized. New capabilities associated with deploying solar references were developed as well as a compact solution

for recovering in-water instrument systems from small boats.

15. SUBJECT TERMS

SeaW1FS, apparent optical properties, AOP, light sensor, radiometer, microradiometer, solar reference, optically complex water

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