NISTIR 7637

Best Practice Guidelines for Pre-Launch 

Characterization and Calibration of Instruments 

for Passive Optical Remote Sensing

(Report to Global Space-based Inter-Calibration System (GSICS) 

Executive Panel, NOAA/NESDIS, World Weather Building, 

Camp Springs, Maryland  20746)

R. U. Datla, J. P. Rice, K. Lykke and B. C. Johnson 

NIST Optical technology Division

J.J. Butler and X. Xiong

NASA Goddard Space Flight Center 

September 2009

U.S. DEPARTMENT OF COMMERCE

Gary Locke, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

Patrick D. Gallagher, Director

Best Practice-cover.indd 2-3 12/1/09 2:38 PM
Table of Contents

ABSTRACT ................................................................................................................................................3

EXECUTIVE SUMMARY .........................................................................................................................5

1. INTRODUCTION ...............................................................................................................................9

2. SI TRACEABILITY AND BEST PRACTICE ....................................................................................15

2. 1 Best Practice for Characterization/Calibration of Satellite Optical Instruments – 3 Step Process ........16

2. 2 Pre-launch Preparation for Post-Launch Sensor Performance Assessments .........................................19

2.2.1. Plan for component performance reassessments – more on step 2 ..................................................19

2.2.2. Post-launch and pre-launch validation and SI traceability – more on step 3 ....................................20

2.2.3. On-orbit inter-comparisons and SI traceability ................................................................................22

3. SECONDARY LABORATORIES FOR SI TRACEABILITY AND ROLE OF NIST .........................23

4. UV, VIS/NIR and IR STANDARDS AT NIST FOR SI TRACEABILITY .........................................25

4. 1 Primary optical watt radiometer (POWR) ..........................................................................................25

4. 2 Irradiance and Radiance Scales ...........................................................................................................25

4.2.1. Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) ......25

4.2.2. Facility for automated spectral irradiance and radiance calibrations (FASCAL) ...............................27

4.2.3. Advanced infrared radiometry and imaging (AIRI) facility ...............................................................29

4.2.4. Synchrotron/D2 lamps ....................................................................................................................29

5. TRANSFER STANDARD RADIOMETRS AT NIST .......................................................................31

5. 1 Transfer Radiometers for the Reflected Solar Spectral Region ............................................................31

5.1.1 Ultraviolet (UV) ...............................................................................................................................31

5.1.2 Visible and near infrared (Vis/NIR) ..................................................................................................31

5.1.3 Short-wave infrared (SWIR) .............................................................................................................32

5. 2 Transfer Radiometers for the IR .........................................................................................................32

5.2.1. TXR ................................................................................................................................................33

5.2.2. FTXR and MDXR .........................................................................................................................33

6. EXAMPLES OF BEST PRACTICE ....................................................................................................35

6. 1 NASA/GSFC Intercomparison of VIS/NIR Transfer Radiometers .....................................................35

6. 2 Terra and Acqua MODIS VIS/NIR SI Traceability ...........................................................................36

6. 3 Miami Workshop 2001 .....................................................................................................................38

6. 4 TXR Deployment to ITT for GOES Imager .....................................................................................39

7. CONCLUDING REMARKS ..............................................................................................................43

8. ACKNOWLEDGEMENTS ..................................................................................................................45

3

ABSTRACT

The pre-launch characterization and calibration of remote sensing instruments should be planned 

and carried out in conjunction with their design and development to meet the mission requirements. 

The onboard calibrators such as blackbodies and the sensors such as spectral radiometers should be 

characterized and calibrated using SI traceable standards. In the case of earth remote sensing, this allows 

intercomparison and intercalibration of different sensors in space to create global time series of climate 

records of high accuracy where some inevitable data gaps can be easily bridged. The recommended best 

practice guidelines for this pre-launch effort is presented based on experience gained at National Institute 

of Standards and Technology (NIST), National Aeronautics and Space Administration (NASA) and 

National Oceanic and Atmospheric Administration (NOAA) programs over the past two decades. The 

currently available radiometric standards and calibration facilities at NIST serving the remote sensing 

community are described. Examples of best practice calibrations and intercomparisons to build SI 

(international System of Units) traceable uncertainty budget in the instrumentation used for preflight 

satellite sensor calibration and validation are presented.

Keywords: Remote sensing; SI traceability; Radiometric Calibrations; Best Practice guidelines 

5

EXECUTIVE SUMMARY

Satellite remote sensing has the potential to deliver the high accuracy data required to identify the small 

signals of climate change in a long time series record. There has been a consensus through workshops 

[1,2] in the last ten years that the best approach to achieve high accuracy and uniformity across all space 

observations for climate is to have pre-launch SI (International System of Units) traceable calibrations 

of sensor performance and post launch validations. The word traceability does not imply that the 

metrological reference necessarily should be an SI standard [3]. However, for optical sensors it is feasible 

to have SI standards for calibration and so SI traceability is recommended. In this report, a set of best 

practice guidelines for achieving SI traceability is described as a 3-step process as shown in the Figure 1. 

Figure. 1. Summary steps of best practice in pre-launch calibration.

Active Interaction with NMI/NIST or Other 

Professionals in the Field of SI Calibrations

Calibration Planning - Develop Calibration 

Requirements

& Calibration Approach

Develop & Characterize SI Traceable Onboard Calibration Set-ups 

Perform System Level End-to-End SI 

Traceable Calibration

Step 1: Mission Requirements

and

Instrument Requirements

Cost & Schedule

Develop Sensor Design

& Radiometric Model

(Measurement Equation)

Step 2: Characterize Subsystems and Components 

and 

Model Radiometric Sensor Performance 

Step 3: Compare Model Predictions and Validate System Level 

Calibration Measurements

Establish Pre-launch Radiometric Sensor Measurement Uncertainty

Calibration Process Mission Process
6

Step1: The mission and calibration requirements are to be determined first. In most cases they are 

determined by the project scientists for the type of measurements to be made. The project scientists, 

together with their contractors for instrument building, will participate in establishing the instrument 

requirements for the mission and developing the sensor design. It is ideal to have radiometric experts from 

National Metrology Institutes (NMIs) such as National Institute of Standards and Technology (NIST) 

or other professionals active in the field of SI calibrations involved in the deliberations on radiometric 

accuracy requirements and choice of SI standards for calibrations. The best practice is to have the project 

scientists, calibration experts and instrument vendors work as a team in developing the sensor design, 

meeting the mission requirements with a plan for calibration and SI traceability within the cost and 

schedule considerations. The benefits of following this best practice guideline are discussed at length in 

the body of this report. 

Step 2: Component/subsystem characterization and sensor performance modeling is the next step of the 

best practice. As discussed in Reference 4, characterization involves determining the component- and 

subsystem-level instrumentation responses for various operating and viewing conditions on orbit, through 

laboratory emulation. The sensor performance is modeled based on the sensor measurement equation. This 

equation describes all the influencing parameters on the sensor responsivity. The influencing parameters 

can be broadly categorized as radiometric, spectral and spatial in nature. The radiometric detector 

characteristics include linearity, stability, and cross talk; spectral characteristics include responsivity, 

stability and accuracy; and spatial characteristics include pointing, spatial responsivity and angular 

responsivity. It is best to follow the axiom “Test as you fly”. That means it is important to have these 

characterizations performed at the same environmental conditions, such as temperature and vacuum, 

which will be experienced on orbit. However, cost and schedule are to be evaluated and characterizations 

are to be planned accordingly to meet the requirements. Often NMIs like NIST are well-equipped to 

perform critical component evaluations and subsystem testing independently to confirm the sensor 

model, corrections and uncertainties. It is highly recommended to take advantage of such capabilities and 

expertise to get critical measurements done and gain a high degree of confidence in building the sensor 

model. There are standard measurement equations that are given in Reference 5 for the measurement 

of radiance, irradiance, or Bi-directional Distribution Function (BRDF). The measurement equation is 

discussed in detail in the body of this report.

As part of this step, the report recommends the following;

1. It is very valuable to procure extra samples of components and duplicates of filters with the flight 

hardware and keep in the inventory for reexamination, so data discrepancies can be sorted out 

once the mission is in operation.

2. In the manufacturing of key instrument subsystems (in particular filters and other optical 

components) it is best to have mission representatives closely monitor the component testing and 

assure that the duplicates supplied are authentic replicas of flight hardware. 
7

Step 3: Comparison of the modeled sensor spectral responsivity predicted from component-level 

characterization with the system-level end-to-end testing to establish pre-launch radiometric sensor 

measurement uncertainty is the last step of the best practice. The comparison using the model can 

validate the system-level testing and bring to light unanticipated effects that could be mitigated by taking 

appropriate measures and optimizing the sensor performance to meet the requirements. The SI traceable 

standards such as blackbodies, integrating spheres and transfer standard radiometers are to be used for 

the system-level testing. A “minimally acceptable” criterion should be developed and followed to test the 

stability of the instrument under flight conditions. Also, in order to ensure SI traceability on orbit the 

following pre-launch and post-launch activity is recommended as part of Step 3 for SI traceability on orbit.

1. The onboard calibrators are to be characterized and calibrated pre-launch using SI traceable 

standards.

2. New technologies, e.g. Digital Micromirror Devices (DMD) and Liquid Crystal Displays (LCD), 

that can project radiometrically calibrated scenes for validating sensor performance will be very 

valuable to pursue as a pre-launch activity once those technologies mature. 

3. Observation of well-characterized ground sites for validating sensor performance on orbit is an 

important activity to be pursued.

4. The use of the moon as a on-orbit stability monitor is highly recommended for sensors in the 

Visible/NIR range and periodic sensor viewing of the moon is highly recommended for ensuring 

sensor stability.

5. The technique of Simultaneous Nadir Observations (SNO) to compare sensors in different satellites 

by observing the same footprint at the same time is very useful to validate sensor performance and 

assess sensor SI traceability. This type of intercomparison using high altitude aerial platforms with 

radiometrically calibrated sensors is also very useful to validate satellite sensor performance.

The rest of the report is devoted to describing the capabilities at NIST to provide SI traceable calibrations. 

Four examples of best practice to build SI traceability for instrumentation in remote sensing programs 

where NIST was involved are presented. The first example is the intercomparison of the SI traceable 

VIS/NIR transfer radiometers at the Earth Observing System (EOS) facility at Goddard Space Flight 

Center (GSFC) at NASA to assess the current state-of-the-art radiance and irradiance uncertainty 

across the available instrumentation. The second is the historical and current account of achieving 

SI traceability for the VIS/NIR bands of the Terra and Aqua satellite Moderate Resolution Imaging 

Spectroradiometer (MODIS) sensors from pre-flight to current operational scenarios. The third is 

the laboratory intercomparison of infrared radiometers funded byNational Environmental Satellite, 

Data and Information Service (NESDIS) of NOAA, European Organization for the Exploitation of 

Metereological Satellites (EUMETSAT), European nSpace Agency (ESA) and NASA in 2001 at the 

University of Miami’s Rosenstiel School of Marine and Atmospheric Science (RSMAS). The fourth one 

is the TXR deployment for the calibration of the blackbody in the GOES imager test chamber at ITT, 

Ft. Wayne, Indiana in 2001. 
8

In summary, the general guidelines of best practice for building SI traceability for optical remote sensing 

are presented. The work done in the last two decades at NIST, NASA and NOAA to provide SI traceability 

for optical sensor calibrations is reviewed. 
9

1. INTRODUCTION

Satellite remote sensing provides continuous coverage and has the potential to allow observation of 

climate variables through long time periods. Climate modelers require continuous data over long time 

periods to test their models and predict global climate variability. However, the data has to be accurate 

and the measurement uncertainties well understood to be of value to the modelers. Two workshops were 

held to identify the accuracy requirements for radiometric measurements and identify ways to achieve 

those goals [1, 2]. Table 1 shows the required accuracies and stabilities for climate variable data sets and 

Table 2 shows the corresponding radiometric accuracies and stabilities of satellite instruments to meet 

those requirements, based upon the workshops [1]. The requirements are very demanding and the golden 

rule for achieving the needed accuracy is to make measurements traceable to international standards (SI) 

[2]. The result of a measurement can be traceable to international standards through an unbroken chain 

of comparisons all having stated uncertainties [3]. The recommended practice to evaluate and document 

the stated uncertainties is to follow the International Organization for Standardization (ISO) Guide to 

expression of uncertainty in measurement [4]. This process allows uniformity and intercomparability 

of measurements on different satellite platforms in space taken simultaneously as well as at different 

times potentially spanning decades as needed for climate observations. In Section 2, we will discuss SI 

traceability and best practice for pre-launch characterization and calibration of sensors for achieving the 

measurement accuracy goals on-orbit. In Section 3, the absolute standards for radiometric scales at NIST 

are presented. In Section 4, the transfer radiometers at NIST and those built by NIST for NASA for SI 

traceability are discussed. In Section 5, illustrations of best practice for calibrations and intercomparisons 

to build SI traceable uncertainty budgets for instrumentation used for sensor calibrations for customers 

are discussed. Concluding remarks are given in Section 6.
10

Table 1. Required accuracies and stabilities for climate variable data sets. Column labeled signal indicates 

the type of climate signal used to determine the measurement requirements.

Signal Accuracy Stability(per decade)

SOLAR IRRADIANCE, 

EARTH RADIATION 

BUDGET, AND CLOUD 

VARIABLES

Solar irradiance Forcing 1.5 W/m2 0.3 W/m2

Surface albedo Forcing 0.01 0.002

Downward longwave flux: 

Surface Feedback 1 W/m2 0.2 W/m2

Downward shortwave 

radiation: Surface Feedback 1 W/m2 0.3 W/m2

Net solar radiation: Top of 

atmosphere Feedback 1 W/m2 0.3 W/m2

Outgoing longwave 

radiation: Top of 

atmosphere

Feedback 1 W/m2 0.2 W/m2

Cloud base height Feedback 0.5 km 0.1 km

Cloud cover (Fraction of sky 

covered) Feedback 0.01 0.003

Cloud particle size 

distribution Feedback TBD* TBD*

Cloud effective particle size Forcing: Water 

Feedback: Ice 

Water: 10 % Ice: 

20 % 

Water: 2 % Ice: 

4 %

Cloud ice water path Feedback 25 % 5 %

Cloud liquid water path Feedback 0.025 mm 0.005 mm

Cloud optical thickness Feedback 10 % 2 %

Cloud top height Feedback 150 m 30 m

Cloud top pressure Feedback 15 hPa 3 hPa

Cloud top temperature Feedback 1 K/cloud 

emissivity 

0.2 K/cloud 

emissivity

Spectrally resolved thermal 

radiance Forcing/ climate change 0.1 K 0.04 K

ATMOSPHERIC 

VARIABLES 

Temperature

Troposphere Climate change 0.5 K 0.04 K

Stratosphere Climate change 0.5 K 0.08 K
11

Signal Accuracy Stability(per decade)

Water-vapor Climate change 5 % 0.26 %

Ozone

Total column Expected trend 3 % 0.2 %

Stratosphere Expected trend 5 % 0.6 %

Troposphere Expected trend 10 % 1.0 %

Aerosols

Optical depth (troposphere/ 

stratosphere) Forcing 0.01/0.01 0.005/ 0.005

Single scatter albedo 

(troposphere) Forcing 0.03 0.015

Effective radius (troposphere 

/stratosphere) Forcing

greater of 0.1 μm or 

10 % of particle size 

/ 0.1 μm

greater of 0.05 μm or 

5 % of particle size / 

0.05 μm

Precipitation 0.125 mm/h 0.003 mm/h

Carbon dioxide Forcing/ Sources-sinks

0.001 % by volume 

/0.001 % by 

volume

0.00028 % by 

volume/0.0001 % by 

volume

SURFACE VARIABLES 

Ocean color 5 %** 1 %

Sea surface temperature Climate change 0.1 K 0.04 K

Sea ice area Forcing 5 % 4 %

Snow cover Forcing 5 % 4 %

Vegetation Past trend 3 % 1 %

*

 To be determined, ** The 5% accuracy requirement for Ocean color is for the at-satellite component of the water leaving 

radiance. The accuracy of an ocean color radiometer must be 0.5% or better to meet this goal.

Table 2. Required accuracies and stabilities of satellite instruments to meet requirements of Table 

1.The instrument column indicates the type of instrument used to make the measurement.

Instrument Accuracy Stability (per decade)

SOLAR IRRADIANCE, 

EARTH RADIATION 

BUDGET, AND CLOUD 

VARIABLES

Solar irradiance Radiometer 1.5 W/m2 0.3 W/m2

Surface albedo Vis radiometer 5 % 1 %
12

Instrument Accuracy Stability (per decade)

Downward longwave 

flux: Surface

IR spectrometer and 

Vis/IR radiometer

See tropospheric 

temperature, water-vapor, 

cloud base height, and 

cloud cover

See tropospheric 

temperature, water-vapor, 

cloud base height, and 

cloud cover

Downward shortwave 

radiation: Surface

Broad band 

solar and Vis/IR 

radiometer

See net solar radiation: 

TOA, cloud particle 

effective size, cloud 

optical depth, cloud top 

height, and water-vapor

See net solar radiation: 

TOA, cloud particle 

effective size, cloud 

optical depth, cloud top 

height, and water-vapor

Net solar radiation: Top 

of atmosphere Broad band solar 1 W/m2 0.3 W/m2

Outgoing longwave 

radiation: Top of 

atmosphere

Broad band IR 1 W/m2 0.2 W/m2

Cloud base height Vis/IR radiometer 1 K 0.2 K

Cloud cover (Fraction 

of sky covered) Vis/IR radiometer

See cloud optical 

thickness and cloud to 

temperature

See cloud optical 

thickness and cloud to 

temperature

Cloud particle size 

distribution Vis/IR radiometer TBD* TBD*

Cloud effective particle 

size Vis/IR radiometer

3.7 μm: Water, 5 %; Ice, 

10 %

1.6μm: Water, 2.5 %; Ice, 

5 %

3.7 μm: Water, 1 %; Ice, 

2 %

1.6μm: Water, 0.5 %; Ice, 

1 %

Cloud ice water path Vis/IR radiometer TBD* TBD*

Cloud liquid water path Microwave and Vis/

IR radiometer

Microwave: 0.3 K

Vis/IR: see cloud optical 

thickness and cloud top 

height

Microwave: 0.1 K

Vis/IR: see cloud optical 

thickness and cloud top 

height

Cloud optical thickness Vis radiometer 5 % 1 %

Cloud top height IR radiometer 1 K 0.2 K

Cloud top pressure IR radiometer 1 K 0.2 K

Cloud top temperature IR radiometer 1 K 0.2 K

Spectrally resolved 

thermal radiance

IR 

spectroradiometer 0.1 K 0.04 K

ATMOSPHERIC 

VARIABLES 

Temperature

Troposphere MW or IR 

radiometer 0.5 K 0.04 K
13

Instrument Accuracy Stability (per decade)

Stratosphere MW or IR 

radiometer 1 K 0.08 K

Water-vapor MW radiometer

IR radiometer

1.0 K

1.0 K

0.08 K

0.03 K

Ozone

Total column UV/VIS 

spectrometer

2 % (λ independent), 1 

% (λ dependent) 0.2 %

 Stratosphere UV/VIS 

spectrometer

3 % 0.6 %

Troposphere UV/VIS 

spectrometer

3 % 0.1 %

Aerosols VIS polarimeter Radiometric: 3 %

Polarimetric: 0.5 %

Radiometric: 1.5 %

Polarimetric: 0.25 %

Precipitation MW radiometer 1.25 K 0.03 K

Carbon dioxide IR radiometer 3 % Forcing: 1 %; 

Sources/ sinks: 0.25 %

SURFACE VARIABLES 

Ocean color VIS radiometer 5 % 1 %

Sea surface temperature IR radiometer 0.1 K 0.01 K

MW radiometer 0.03 K 0.01 K

Sea ice area VIS radiometer 12 % 10 %

Snow cover VIS radiometer 12 % 10 %

Vegetation VIS radiometer 2 % 0.80 %

*

 To be determined

15

2. SI TRACEABILITY AND BEST PRACTICE

A question often raised is, what is the difference between having SI traceability as a requirement versus 

not having that stated in the requirements? The difference is that such a requirement specifically mandates 

that the characterizations and calibrations are to be performed against standards traceable to the SI. 

Also, the uncertainties are to be carefully evaluated, tabulated component by component, and the total 

uncertainty budget is to be made transparent for peer review and independent critical analysis. There are 

two kinds of uncertainties to be evaluated according to the ISO Guide [4] called Type A and Type B. 

Type A uncertainties are basically the random type and represent the uncertainty in the repeatability of 

measurements. In general, because of good environmental control on the instrumentation and computer 

acquisition and analysis of the data at a fast rate, the random uncertainties can be made very small in the 

pre-launch phase. However, these uncertainties have to be re-characterized post launch and periodically 

re-assessed on orbit using space view of the sensor. While on orbit there may be good repeatability within 

a short measurement time interval, in a long time series of measurements the sensor may have a drift due 

to its degradation in the space environment. This is a systematic effect which could be corrected if it could 

be measured or scientifically estimated. The systematic effect or its correction will have an uncertainty that 

must be estimated based on the ISO guide. The systematic uncertainties evaluated in the characterization 

of various parts of the sensor system are called Type B uncertainties and they are also to be evaluated in 

the pre-launch and post-launch mission phases. The square root of the sum of the squares (RSS) of these 

two types of uncertainties gives the combined standard uncertainty, uc and an expanded uncertainty

U

p

 = k

p

 uc where k

p

 is called the coverage factor. For a normal distribution, the level of confidence

p for k

p =1 corresponds to 68.27 %. For example, in Table 1, the accuracy and stability requirements are 

quoted for k

p =1. (In current usage of this coverage factor kp

 is abbreviated as k). In the remote sensing 

terminology the ability of the sensor to maintain its repeatability over a period of time is called the 

stability of the sensor. Stability is measured by the maximum excursion (drift) of the short-term average 

measured value of a variable after appropriate corrections through on orbit calibrations under identical 

conditions over a decade. The accuracy is a measure of the closeness of the result of measurement and the 

true value. It is measured as the standard uncertainty of the combined result of all measurements. Both 

these quantities are prone to have Type A and Type B uncertainties.In practice, based on the standard 

uncertainties in Table 1, the expanded uncertainties are to be used with appropriate coverage factors to 

define the interval for expressing the level of confidence for the measured value to lie [1, 2].
16

2. 1 Best Practice for Characterization/Calibration of Satellite Optical Instruments – 3 Step Process

The best practice to achieve the stability and accuracy requirements for satellite optical sensors is presented 

as a 3 step process. Figure 1 shows the three step process.

Step 1: The first step is to determine the mission and calibration requirements. The mission requirements 

are determined by the project scientists for the type of measurements to be made. It is ideal to have 

radiometric experts from National Metrology Institutes (NMIs) such as NIST or other professionals 

active in the field for SI calibrations involved in the deliberations on radiometric accuracy requirements 

and availability of SI standards for calibrations.

For example, the variables in Tables 1 and 2 are linked in their role in the energetics of the climate system. If 

Active Interaction with NMI/NIST Step 1: Mission Requirements or other professionals in the field for 

SI Calibrations

Instrument Requirements

Calibration Planning - Develop Calibration 

Requirements

& Calibration Approach Cost & Schedule

Develop Sensor Design

& Radiometric Model

(Measurement Equation)

Develop & Characterize SI traceable 

on-board Calibration Set-ups 

Step 2: Perform Subsystem / Component Characterization 

and 

Model Radiometric Sensor Performance 

Step 3: Compare Model Predictions and Validate 

System Level Calibration Measurements

Establish Pre-launch Radiometric Sensor 

Measurement Uncertainty

Perform System level end to end 

SI traceable calibration

Figure. 1. Summary steps of best practice in pre-launch calibration.
17

we consider one specific requirement, accurate measurements of solar irradiance are important to defining 

climate radiative forcing, and its accuracy requirements are specified in those tables in that context. 

Deliberations at the workshop in November 2002 [1] between climate modelers, calibration experts, and 

principal investigators of various satellite missions resulted in development of those requirements. Stringent 

requirements for climate demand improvement of capabilities at the NMIs to provide SI traceable standards, 

component characterization and system level performance validation tasks to meet those requirements for 

pre-launch calibrations. The mission requirements are generally specified at the product level, and the 

development of instrument design and radiometric models with predictions of uncertainties are left to the 

contractors who compete to fulfill the requirements of the mission. Again, the involvement of experts from 

NMIs in the calibration planning with mission scientists will help to specify calibration requirements and 

approaches for testing SI traceability in the requisition for proposals. Interaction between NIST radiometric 

experts and NASA project scientists took place (although not as ideally as suggested here) for the Earth 

Observing System (EOS) instruments in various platforms and provided rich experience with lessons 

learned for dealing with future missions. Currently similar interaction is being actively pursued with the 

Visible/Infrared Imager Radiometer Suite (VIIRS) and Cross-track Infrared Sounder (CrIS) instruments 

for the National Polar-orbiting Operational Environmental Satellite System (NPOESS) Preparatory 

Project (NPP). Also, interaction with NIST for the Advanced Baseline Imager (ABI) instrument in the 

Geostationary Operational Environmental Satellite-R Series (GOES-R) program of NOAA and NASA 

is being established. An active interaction has just been initiated with NIST for the incubator projects for 

the future Climate Absolute Radiance and Refractivity Observations (CLARREO) mission at NASA.

The selection of SI traceable transfer radiometers from NIST depends upon the accuracy requirements of 

the mission. Careful attention is to be paid seeking advice from experts in Radiometry at NMIs or elsewhere 

to develop sensor specifications based on the accuracy requirements for the mission. Often this had not 

been the case and the sensor specifications were vague. For example, let us examine a sensor specification 

like “absolute radiance accuracy < 5 % required”. Such a statement without giving a coverage factor will be 

very vague and prone to different interpretations. In other words, is this <5 % at coverage factor, k=1, k=2 

or k=3 level? The choice of the SI traceable transfer standard will be different for different interpretations 

of the uncertainty requirement for the sensor. For example, if the required level of confidence is 68.26 % 

corresponding to the interpretation of k=1coverage factor for the accuracy requirement of 5 % for prelaunch calibration, the requirement on the choice of the transfer standard is not very stringent. There is 

more flexibility in the distribution of the uncertainty in the error budget for the choice of the transfer 

standard and satisfy the accuracy requirement. Generally transfer standards having standard uncertainties 

(k =1) above 2 % to 3 % are available to the industry. If the interpretation of the 5 % accuracy requirement 

is for coverage factor of k=2 or above, standards with uncertainties below 1 % level (k =1) will be required 

for calibration. Acquiring standards with uncertainties of 1 % or below (k =1) is challenging and they 

may be only available at NMIs. The Step 1 shown in Fig. 1 recommends to draw expertise from NMIs 

and professionals in the field to develop the sensor specifications and calibration strategies that satisfy the 

mission requirements set by the project scientists. 
18

Step 2: The component and subsystem characterization and modeling the sensor performance is the 

next step. As discussed in Ref. 5, characterization involves determining the component and sub-system 

level instrumentation responses for various operating and viewing conditions on orbit emulated in the 

laboratory. The sensor performance is modeled based on the sensor measurement equation. It describes 

all the influencing parameters on the sensor responsivity. The influencing parameters are of broadly 

radiometric, spectral and spatial categories..The radiometric detector characteristics, like linearity, stability, 

and cross talk; spectral characteristics such as responsivity, stability and accuracy; and spatial characteristics 

such as pointing, spatial and angular responsivity etc. are to be characterized. The spectral transmission 

of filters if used is very important to be characterized at operating temperatures. The mirror reflectivity 

and its angular dependence is also very important to be characterized. It is best to follow the axiom “Test 

as you fly”. That means it is important to have these characterizations performed at the environmental 

conditions such as temperature and vacuum as will be on orbit. However, cost and schedule are to be 

evaluated and characterizations are to be planned accordingly to meet the requirements. Often NMIs like 

NIST are well equipped to perform critical component evaluations and subsystem testing independently 

to confirm the sensor model, corrections and uncertainties. It is highly recommended to take advantage 

of such capabilities and expertise to get critical measurements done and gain high degree of confidence 

in building the sensor model. There are standard measurement equations that are given in Ref. 5 for the 

measurement of radiance, irradiance, or BRDF. As an example the measurement equation of a sensor 

measuring radiance in digital units can be written in a simplified equation 

(1)

where DNi, j is the digital number output by instrument detector i in band j, G is the instrument detector 

plus digitization gain, L(λ) is the spectral radiance at the instrument entrance aperture, Ai,j is the area of 

detector i in band j, Ω is the instrument acceptance solid angle, Δλ is the bandwidth, η is the detector 

quantum efficiency in electrons per incident photon, t is the integration time, τ is the instrument optical 

transmission. Instrument response non-linearity, background, focal plane temperature effects, and 

response versus scan angle effects are not shown in Eq. 1. These quantities are determined in pre-launch 

instrument characterization tests and are incorporated in instrument radiometric models and in the 

production of measured radiances.

Eq. 1. can be re-written as

 (2)

where

 (3)

is the inverse of the product of the instrument responsivity and gain. 

€ 

DNi, j = G ⋅ Ai, j ⋅ Ω⋅ L(λ)⋅ Δλ ⋅η⋅ t ⋅ τ

€ 

L(λ) = DNi, j ⋅ m

€ 

m = 1

G ⋅ Ai, j ⋅ Ω⋅ Δλ ⋅η⋅ t ⋅ τ
19

Step 3: To compare model predictions and validate system level calibration measurements, m is determined 

pre-launch for an end-to-end remote sensing instrument by viewing uniform sources of known radiance, 

such as well-characterized and calibrated integrating sphere sources and blackbodies. The characterization 

of integrating spheres and blackbodies using SI traceable standards at NIST has been the hallmark of 

interaction between NIST and NASA for many of the EOS instruments including the Sea-viewing Wide 

Field-of-view Sensor (SeaWiFS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) prelaunch sensor level calibrations. Such interactions also took place between NIST and NOAA in the past 

and lessons learned will be discussed in later sections of this report. 

The quantity, m, can also be determined pre-launch through component and subsystem characterization 

measurements of quantities such as mirror reflectance, polarization responsivity, spectral radiance 

responsivity. These subsystem level characterization measurements are used as input to instrument 

radiometric sensor models used to validate the system level pre-launch calibration and in the calculation 

of instrument measurement uncertainty as shown as the final result of the best practice.

The quantity, m, in Eq. 3 is monitored on-orbit using stable, uniform on-board sources of known radiance. 

Again, on-board blackbody sources or artifacts like solar diffusers for BRDF measurements are to be 

developed and characterized as SI traceable standards using the expertise at NMIs like NIST as identified 

in Fig. 1 in Steps 2 and 3 of the best practice, which are further elaborated in section 2.2 below. 

2. 2 Pre-launch Preparation for Post-Launch Sensor Performance Assessments

Preparation for post launch assessments of measurements and uncertainties is part of the best practice that 

is to be simultaneously undertaken during pre-launch preparations. A real important aspect of pre-launch 

testing is determining how to prove instrument stability under on-orbit conditions. Stability is often specified 

for long periods (Tables 1 and 2) such as mission lifetimes. It is not possible to test that long. So a “minimally 

acceptable” criterion should be developed during calibration planning and sufficient time should be allocated 

for this pre-launch activity to take place during the final phase of system level end to end calibration.

2.2.1. Plan for component performance reassessments – more on step 2 .

One of the lessons learned at NIST in previous interactions with NASA and NOAA is that some of 

the sensor data problems on orbit could not be isolated fully because no duplicates or even samples of 

components were available for reexamination. Duplicates of filters, apertures, mirror samples, diffusers 

etc. are very valuable to have for reexamination at the metrology laboratories where high accuracy data 

can be obtained simulating the space environment and conditions of on-orbit operation to sort out data 

discrepancies. For example, the band edge wavelength of filter transmission is temperature dependent and 

it could be re-measured to understand on-orbit data. At NOAA, in the case of both GOES sounder on 

GOES – N and High Resolution Infrared Sounder (HIRS) on Polar Operational Environmental Satellites 

(POES) NOAA –N programs, a large discrepancy - as high as 6 K - was observed between the measured 
20

radiance of an on-orbit blackbody and that calculated using the pre-launch vendor-supplied spectral 

response function (SRF) of the sensor. This affected the on-orbit product retrieval and assimilation of 

Numerical Weather Prediction Models because the atmospheric quantity of interest is determined by 

varying it to make calculated radiances match with observed atmospheric radiances. The calculated 

radiance is essentially a convolution of the SRF with the monochromatic radiances from radiative transfer 

computation. Therefore, as a first step NOAA employed NIST to make independent measurements of 

SRFs of witness samples of filters of on-orbit GOES sounders. In the affected channels of GOES -8 

and GOES-10 sounders, NIST measurements done at the on-orbit operational temperature conditions 

disagreed with SRFs in use by NOAA and also were found to be more consistent with on-orbit radiance 

observations at known blackbody temperatures, thus explaining the possible discrepancy [6]. However, 

the NIST measurements on witness filters were so different compared to those used at NOAA, the vendor 

expressed doubts on the witness samples as being authentic. A similar investigation was carried out on 

HIRS filters to compare vendor measurements and NIST measurements. Again, there were noticeable 

discrepancies and NOAA analysis showed such discrepancies affect product retrievals and their inferences 

on weather prediction models. As a lesson learned from this interaction, it is essential to have SRFs 

measured at simulated on-orbit operating conditions and they should be independently verified with 

authentic witness samples. In another program at NASA, the only best representative apertures of a sensor 

on orbit were lost in the shipment to NIST, compromising the results of a comparison of aperture area 

determinations among similar sensors on orbit. So one simple best practice based on all these lessons 

learned is that each satellite mission at least should require duplicates of critical components of their 

radiometric instruments for future on-orbit data reassessments. Furthermore, this best practice guideline 

could be extended to the manufacturing of the key instrument subsystems (in particular filters), to have 

instrument representatives in the manufacturing facility to closely monitor the component testing and 

acquire authentic duplicates of space hardware. 

2.2.2. Post-launch and pre-launch validation and SI traceability – more on step 3.

Post-launch: Part of the overarching principles advocated by the workshop report [1] for high quality 

climate observations is to arrange for production and analysis of each Climate Data Record (CDR) 

independently by at least two sources. It goes on to say “Not only instruments, but also analysis algorithms 

and code require validation and independent confirmation.” This is because that confidence in the 

quantitative value of a geophysical parameter will be achieved only when different systems, different 

techniques produce the same value (within their combined measurement uncertainties). Comparisons 

of the results by different sources should reveal the flaws in particular sources to be corrected by their 

advocates for improving the confidence on their systems and techniques to produce high quality data. This 

process is broadly called as validation and is defined by the Working Group on Calibration and Validation 

(WGCV) of the Committee on Earth Observing Satellites (CEOS). The definition goes as “the process of 

assessing, by independent means the quality of the data products derived from the system outputs [7].” 

Traditionally it is carried out post launch through ground “truth” campaigns where different satellite 

systems compared their measurements on ground “truth” sites. This process of validation requires that the 
21

ground truth system is proven to be capable of credible and well proven data set on the ground “truth” 

site. So post launch validation should be planned using land sites of known radiometric characterization. 

For instruments like the Advanced Very High Resolution Radiometer (AVHRR) the ground “truth” sites 

essentially provide what is called vicarious calibration. For satellite sensor like the SeaWIFS the instrument 

calibration that began in the laboratory is continuing through vicarious calibration by comparison of data 

retrievals to in-water, ship, and airborne sensors to adjust instrument gains. The WGCV of CEOS is 

identifying suitable sites and their characteristics for on orbit sensor validations and vicarious calibrations 

for sensors across the world [7]. CEOS WGCV members are working with NMIs like NIST in U.S.A. and 

the National Physical Laboratory (NPL) in Britain in this effort. One of such sites selected is the moon 

as an on-orbit stability monitor for the Visible and near IR spectral region up to 2.5 micrometers. The 

SeaWIFS and MODIS sensors currently on-orbit have been successfully viewing the moon as a stability 

monitor. One of the recommendations of the ASIC3 workshop is that necessary lunar observations are 

to be carried out to make the moon an SI traceable absolute source for on-orbit satellite calibrations [2]. 

There are programs at NASA and NOAA to provide high altitude aerial platforms with radiometrically calibrated 

sensors for validation of satellite sensor data by simultaneously observing the satellite sensor footprint of earth’s 

atmosphere. The University of Wisconsin Scanning Hyperspectral Imaging Spectrometer (HIS) is an example [8].

 

Pre-launch: In following the various steps of best practice in Fig. 1, one should always be looking into new 

research methods and other advancements of technology to help improve the uncertainties.. Although 

not yet proven for satellite sensor prelaunch validation activity, it is becoming possible in the laboratory 

to project special scenes that are radiometrically calibrated [9]. It is achieved by using a light source and 

a Digital Micromirror Device (DMD) to project a scene of interest as shown in Figure. 2. The spectral 

engine and the spatial engine are two DMD projectors independently illuminated by light sources and 

controlled to project appropriate combination of the spectral features and the spatial features. There are 

algorithms available now in the literature to develop appropriate combination of basis spectra to project 

the real scene of interest.

Spectral

Engine

DMD1

DMD2

Spatial

Engine

Unit

Under Test

Reference 

Instrument

Figure. 2. Digital Micromirror Device (DMD) projector technology to project appropriate scenes that are radiometrically 

calibrated using NIST standards.
22

High quality image data could be projected to the sensor and thus preflight validation could be emulated 

with on-orbit sensor data samples. As the accuracies of this validation equipment improve, such an exercise 

could help evaluate the sensor performance more realistically [10]. 

2.2.3. On-orbit inter-comparisons and SI traceability.

It is best to have inter-comparisons of similar sensors on orbit to assess consistency in data products and 

sensor performance. Such inter-comparisons are possible when both sensors being intercompared are 

SI traceable on orbit. Intercomparison of on-orbit sensors has become possible with the technique of 

Simultaneous Nadir Observations (SNO) when both satellites observe the same foot-print at nearly the 

same time they cross each other in their orbits [11]. In order to bring self-consistency and intercalibration of 

sensors across the world, a group called Global Space-based Inter-Calibration System (GSICS) formed and 

is actively pursuing SI traceability for intercalibrations working with NMIs like NIST. Intercomparisons 

may show good agreement or disagreement between sensors in their radiance measurements. In either case, 

lessons will be learned on possible systematic effects that are currently ignored or neglected. As the true 

value of the measurand on orbit will always be an unknown quantity, the accuracy of the measurement 

can best be assessed by combining the results from different sensors and calculating the uncertainty of that 

Combined Reference Value (CRV) based on the individual sensor data [12]. The CRV and the estimate of 

its uncertainty in the time series will allow scientists to look into methods to minimize uncertainties and 

achieve the stated accuracy requirements by using the lessons learned through intercomparisons. 
23

3. SECONDARY LABORATORIES FOR SI TRACEABILITY 

AND ROLE OF NIST

For many remote sensing projects, the extensive, direct incorporation of NMI calibration experts and 

equipment is impractical. In some cases and depending on target calibration and characterization 

uncertainties, these projects are served by NMI-provided calibration standards, by the project’s internal 

calibration and characterization programs, and by other industrial, academic, or government secondary 

standards facilities. For many manufacturers of satellite instruments this is the case. In these cases, the 

satellite manufacturer, industrial, academic, and government facilities are secondary standards laboratories 

themselves, establishing or providing SI traceability through NMI-provided calibration artifacts coupled 

with sound measurement methodologies and techniques. For these laboratories, the development and 

maintenance of an internal calibration program require a substantial capital investment. This is particularly 

true for manufacturers of smaller remote sensing instruments used in field validation work and for projects 

that provide remote sensing measurements in the field. In the case of satellite remote sensing programs, 

independent field measurements are a critical source of calibration and validation measurements, that is, 

of ground truth for the post launch operation of the satellites. Here the SI traceability of the secondary 

standards laboratories used by validation projects complements the SI traceability of the pre-launch 

calibration of the satellite sensors. Both are necessary for long-term, satellite-based, climate-quality 

remote sensing programs.

25

4. UV, VIS/NIR and IR STANDARDS AT NIST FOR SI 

TRACEABILITY

The optical radiation measurements are generally referenced to one of two SI scales: optical power in 

watts or thermodynamic temperature in kelvin. The SI unit, electrical watt is tied to the optical watt via 

electrical substitution achieved by the use of cryogenic radiometers. The temperature scale is derived from 

the triple point of water. The SI unit kelvin is defined as 1/273.16 of the thermodynamic temperature 

of the triple point of water. The temperature at that point is defined as 0.01 0C in the commonly used 

Celsius scale. 

4. 1 Primary optical watt radiometer (POWR)

At NIST, the Optical Technology Division realizes and maintains the unit of optical power (watt) using a 

custom built state of the art electrical substitution radiometer, called the Primary Optical Watt Radiometer 

(POWR), as shown schematically in Fig. 3. It is operated at 2 K or 4.2 K to minimize background effects. 

Its dynamic range is 1 μW to 1 mW in measuring optical power from intensity stabilized lasers. The 

standard uncertainty achieved in POWR measurements is at 0.02 % level [13]. Silicon trap detectors 

which have absolute quantum efficiency close to unity are used to transfer the power scale from POWR 

to other cryogenic radiometers or detectors. The standard uncertainty for such transfers is 0.02 % to

0.04 % [13]. 

4. 2 Irradiance and Radiance Scales

The irradiance, E (W/cm2) and the radiance L (W/cm2 sr) can be derived from the measurement of 

radiant power or from the Planck’s law for the radiance of a perfect blackbody. The former method of 

scale realization is called as detector based and the later is called as source based. NIST realizes these scales 

both ways and the following sections describe the corresponding facilities at NIST. 

4.2.1. Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS).

In order to calibrate detectors and radiometers for spectral irradiance responsivity and spectral radiance 

responsivity from the UV to the IR, a facility called SIRCUS is available at NIST and is shown in 

Fig. 4. The light from a tunable laser is made to illuminate the integrating sphere. The output of the 

integrating sphere through a precision aperture is used for irradiance and radiance calibrations. In this 

facility, tunable lasers covering the wavelength range from 210 nm to 5.3 μm are coupled to integrating 

spheres to produce either uniform irradiance at a reference plane or uniform radiance within the sphere 

exit port at high levels. Detectors are calibrated directly against reference standards such as the trap 

detectors referenced earlier. Lasers ultimately determine the spectral coverage available at SIRCUS, while 

the uncertainties achievable are in the 0.1 % level [14] for the visible and at few percent levels for the 
26

infrared detector calibrations [15].The SIRCUS covers the UV-Vis-NIR spectral region with one set of 

lasers and an integrating sphere with Spectralon coating, and the other set of tunable lasers for the IR from 

700 nm to 5.3 μm using a diffuse gold coated integrating sphere. Also discrete lasers extend the spectral 

coverage to 10 μm. A portable, table-top, tunable laser system, complete with integrating spheres and 

transfer standard detectors, called Traveling SIRCUS, to cover UV-Vis-NIR region is available at NIST to 

visit customer facilities and provide on-site calibrations. It has been to many customer facilities, including 

of NASA, NOAA, and United States Geological Survey (USGS) sites to characterize instruments.

Figure 3. Schematic of NIST primary standard for optical power measurements, Primary Optical Watt Radiometer, POWR.
27

Figure 4. Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) Facility at NIST

4.2.2. Facility for automated spectral irradiance and radiance calibrations (FASCAL).

The FASCAL facility derives and maintains its radiance scale from the absolute radiance of a gold fixedpoint blackbody whose temperature is held at the freezing point of gold (1064.18 °C) and whose radiance 

follows Planck Radiation Law. The gold fixed-point blackbody at NIST is used to realize and disseminate 

the 1990 NIST Radiance Temperature Scale (1990 NIST). Experiments at NIST established that the 

1990 NIST Spectral Radiance Scale derived from the gold fixed-point blackbody compared well with the 

radiance scale derived from optical watt [16]. The FASCAL facility uses a CARY 14 spectroradiometer for 

radiance calibrations with respect to a working standard variable-temperature blackbody (VTBB) whose 

radiance temperature is derived from the gold fixed–point blackbody. The spectroradiometer covers the 

wavelength range from 225 nm to 2500 nm with suitable detectors. A S-20 response photomultiplier 

tube is used to cover 200 nm to 900 nm and a 4 stage TE cooled InGaAs photodiode is used to cover 800 

nm to 2500 nm.
28

For remote sensing applications in the reflected solar spectral region, an integrating sphere, the the NIST 

Portable Radiance (NPR) source, was developed by NIST as part of the EOS collaboration [17]. The 

NPR is 30 cm in diameter with a 10 cm diameter exit aperture. It is used during EOS comparison 

activities to verify the stability of the NIST absolute radiometers and to provide a calibration of the NIST 

transfer radiometers. There are four baffled lamps arranged symmetrically about the inner lip of the exit 

aperture, and different radiance levels are achieved by operating with one to four lamps illuminated. The 

NPR is calibrated on FASCAL; the relative expanded uncertainties (k = 2) in spectral radiance for the 

NPR are 0.5 % in the visible, increasing to 1.0 % at 300 nm and 2.5 % at 2400 nm (all k=1). Because it 

is a stable source with two monitor photodiodes and the radiance assignment is made directly using the 

national radiance standards maintained at NIST, the NPR can also be used to assess the accuracy of the 

radiance calibration of participants’ radiometers. Such an assessment is important because the common 

method of establishing spectral radiance traceability to NIST involves the spectral irradiance standards 

(i.e. the FEL lamps) and reflectance standards. Thus the NPR provides a direct comparison to NIST’s 

spectral radiance scale. 

Figure 5. NPR: The NPR and the GSFC scanning monochromator during the EOS intercomparison at NASA’s Ames 

facility in 1999 [18]. The NPR system is packaged in two shipping containers, the sphere is shock-mounted in the upper blue 

case, and the electronics in the lower white case.
29

4.2.3. Advanced infrared radiometry and imaging (AIRI) facility. 

The kelvin thermodynamic temperature scale is realized through melting points of various pure metals 

as defined in the International Temperature Scale of 1990 (ITS-90) [19]. At NIST these blackbodies 

are used to calibrate standard platinum resistance thermometers (SPRTs) to cover the temperature range 

from 15 0C to 170 0C and standard gold-platinum thermocouples to cover the range from 400 0C to 

900 0C. These temperature sensors are used in the ideal blackbodies such as the water bath, cesium heat 

pipe, and others to provide SI traceable calibrations of radiance temperature and radiance using a transfer 

standard spectroradiometer. These heat pipe blackbodies and various fixed point blackbodies for radiance 

temperature measurements and radiometers and pyrometers at the Advanced Infrared Radiometry and 

Imaging (AIRI) Facility at NIST are shown in Figs. 6a. and 6b. The AIRI facility allows realization of 

uncertainties in the 50 mK range in the radiance temperature calibrations. 

 

 

 Fig. 6a Fig. 6b

Figure 6a. Variable temperature heat pipe Blackbodies (BB) at the AIRI Facility: 1. Controlled Background Plate for 

Unit Under Test; 2. Ammonia BB (-50 °C to 50 °C), 3. Water Bath BB (15 °C to 75 °C), 4.water heat pipe BB (60 °C to

250 °C), 5.Cs heat pipe BB (300 °C to 650 °C) and 6. Na heat pipe BB (500 °C to 1100 °C), 7. spectral comparator (3μm 

to 14.8 μm) 

Figure 6b. Fixed Point Blackbody (BB) Bench of the AIRI Facility: 1.Ga BB; 2. High T Furnace #1 (Al, Ag and Au), 3. Low 

T Furnace (In, Sn and Zn), 4.High T Furnace #2 (Al and Ag), 5.Out-of-Field Scatter Tool, 6. NIST Transfer Standard 

Pyrometer RT1550L (150 °C to 1064 °C), 7. NIST Transfer Standard Pyrometer RT900 (600 °C and higher), 8. 

Transfer Standard Pyrometer TRT (- 50 °C to 300 °C)

4.2.4. Synchrotron/D2 lamps.

Synchrotron radiation has been recognized as an absolute radiation as its characteristics are calculable by 

the Schwinger equation just as the blackbody radiation is calculable by using Planck equation. In the UV 

and shorter wavelength regions, synchrotron radiation from a storage ring has the highest accuracy in 

calculable irradiance among all currently available source standards. NIST has a synchrotron storage ring, 

the Synchrotron Ultraviolet Radiation Facility III (SURF III), with two beam lines, Beam 3 and Beam 4, 
30

dedicated to calibrations in the UV. Beam 4 provides calibrations of UV detectors. Beam 3 provides source 

based radiometry and is called the Facility for Irradiance Calibration Using Synchrotron, FICUS. It provides 

calibrations from 200 nm to 400 nm [20]. The combined relative uncertainty for FICUS is 0.7% (k=2). 

Standard light sources such as D2 lamps used in remote sensing laboratories and else where are calibrated for 

irradiance at FICUS. The relative combined uncertainty for D2 lamp calibrations is 1.2% (k=2). 
31

5. TRANSFER STANDARD RADIOMETRS AT NIST

Radiometers have been built at NIST and calibration protocols have been established for deployment at 

customer sites to provide SI traceable calibrations or validations. Here we describe transfer radiometers for 

the reflected solar, and the thermal IR spectral regions. 

5. 1 Transfer Radiometers for the Reflected Solar Spectral Region

5.1.1 Ultraviolet (UV).

The UV spectral region is significant to the ozone and downwelling solar surface irradiance communities. 

Two types of instruments have been used over the years – a portable scanning monochromator and 

a UV filter radiometer. The NIST scanning monochromator is a 0.125 m f/4.7 Fastie-Ebert double 

monochromator with 0.02 nm wavelength accuracy [21]. The configuration is versatile, allowing the 

instrument to be used in comparisons of spectral irradiance scales within NIST or between NIST and 

other NMIs [22], participation in field comparisons of UV downwelling surface spectral irradiance [23], in 

EOS comparisons of spectral radiance standards [24], and for calibrating sources at vendor facilities [25]. 

The NIST UV spectroradiometer (UVSR) is operated as a transfer radiometer in the strictest sense, that is, 

the reference source and the unknown source are compared sequentially during the measurement activity. 

For field work, this means a stable, calibrated reference source such as the NPR must be available. The 

core elements of the UV filter radiometer are a silicon trap detector, a five position temperature stabilized 

filter wheel, and a pair of precision apertures separated by a fixed distance to define the throughput [26]. 

The narrow bandwidth filters were selected to coincide with ozone retrieval algorithms. The UVFR 

has been used to calibrate and monitor sources at vendor facilities [27] and to validate spectral radiance 

scales in the UV [23, 27]. To date, the spectral transmittance data is based on values supplied by the 

manufacturer, which increases the uncertainty in the validation exercises.

5.1.2 Visible and near infrared (Vis/NIR).

The Vis/NIR spectral region is the basis of many retrieval algorithms that are used to derive ocean, land, 

and atmosphere products such as chlorophyll concentration, leaf area index, aerosol optical thickness, and 

so on. In support of the SeaWiFS Project, NIST designed and delivered a six channel filter radiometer, the 

SeaWiFS Transfer Radiometer (SXR) [28]. In subsequent years, the mechanical and optical configuration 

was improved and produced commercially by Reyer Corporation, Frederick, Maryland. Three copies 

were made: the Visible Transfer Radiometer (VXR) [29], the Landsat Transfer Radiometer (LXR) [30], 

and the SXR II [31]. The VXR is currently maintained and operated by NIST for NASA/GSFC, with 

original funding provided by the EOS Project Science Office. The LXR was funded by Landsat 7 and is 

now part of the cal/val activities for the Operational Land Imager (OLI) Project; the SXR and SXR II were 

funded by the SeaWiFS Project Office. 
32

These four filter radiometers are essentially the same – each has six narrowband channels between 400 nm 

and 900 nm defined using interference filters and six silicon photodiode detectors. The instruments 

measure radiance. A camera lens is the foreoptic, producing an image at the field stop. Behind the field 

stop, six different wedge-shaped mirrors with spherical curvature focus the field stop at six image locations, 

where the six filter/detector elements are located. The temperature of the field stop, filters, and detectors is 

maintained at a constant value using thermoelectric elements. An on-axis optical system is used to align 

and focus these radiometers. The field-of-view is about 2.5°, and the minimum object distance is 85 cm. 

Two methods have been used to calibrate these filter radiometers. The first is traceable to the spectral 

radiance values of a reference source such as the NPR, with the necessary relative spectral responsivity 

functions for the channels derived from independent, system-level measurements. The second is traceable 

to POWR using SIRCUS. Here, the absolute spectral radiance responsivity is determined with subnanometer spacing for the in-band region of the filter transmittance response and coarser spacing over the 

out-of-band regions by comparison to a reference detector. The relative combined standard uncertainty 

of the VXR’s calibration coefficients is about 0.74% for the NPR calibration, and 0.25% for the SIRCUS 

calibration [31].

5.1.3 Short-wave infrared (SWIR).

The spectral region from around 800 nm to 2500 nm has important applications such as surface soil 

and mineral mapping, cirrus cloud detection, and land and cloud properties. In collaboration with 

the EOS Project Office, NIST developed the EOS Shortwave infrared radiometer (SWIXR) [32] for 

use as a transfer radiometer in this spectral region. The EOS SWIXR is a scanning, double-grating 

monochromator-based instrument equipped with all-reflective input optics and a liquid-nitrogen-cooled 

indium antimonide (InSb) detector. The full-angle field-of-view is 5.2o and a cold filter in front of the 

InSb detector reduces the amount of stray light and thermal infrared background radiation incident on 

the detector. Order sorting filters further reduce the stray light in the system, resulting in a stray light 

rejection of 10-7. The wavelength reproducibility is 0.1 nm, and the corrected wavelength uncertainty 

is 0.2 nm. A chopper, three-stage trans-impedance amplifier, and a lock-in amplifier comprise the InSb 

detection system. The SWIXR is used as a transfer radiometer, with calibration in the field using the 

NPR. The relative combined standard uncertainty in the SWIXR responsivity is approximately 2 % from 

1000 nm to 2200 nm, and increases slightly to 2.5 % at 2400 nm.

A summary of intercomparisons in the Vis/NIR/SWIR is given in references 33 and 34. The participants 

included NIST, NASA/GSFC, the University of Arizona (UA), and Japan’s National Research Laboratory 

of Metrology (NRLM). Between February 1995 and April 2001, 10 intercomparions were held utilizing 

11 different transfer radiometers [33]. The reported level of agreement was ±1.80 % at 411 nm, ±1.31 % 

at 552.5 nm, ±1.32 % at 868.0 nm, ±2.54 % at 1622 nm, and ±2.81 % at 2200 nm.
33

5. 2 Transfer Radiometers for the IR

5.2.1. TXR. 

NIST developed the TXR in support of NASA’s Earth Observing System (EOS) program [35] and its 

deployment at the customer site is in line with the best practice advocated earlier in Section 2.1. The 

TXR measures radiance scales in the thermal-infrared spectral region for satellite sensors calibrated by 

extended area blackbody sources at customer sites [36]. The TXR is a portable two channel radiometer, 

with one channel at 5 μm using a photovoltaic InSb detector and the other at 10 μm using a photovoltaic 

MCT detector. It has a self-contained vacuum jacket and liquid nitrogen (LN2) reservoir. It can be used 

for radiance scale verifications of blackbody sources either in cryogenic vacuum chambers or in ambient 

conditions. The standard uncertainty for radiance measurements using the TXR is of the order of 0.2 % 

(k=1) or better. In terms of radiance temperature deduced from TXR measurements for blackbodies 

operated between 200 K to 350 K, the uncertainty is in the 50 mK (k=1) range. The TXR can be used 

to measure emitted radiance as well as, with special setups, the reflected radiances from the blackbody. 

From such measurements the blackbody emissivity can be deduced. In a deployment at Raytheon Santa 

Barbara Remote Sensing, the TXR characterized the blackbody calibration source (BCS) that was used 

to calibrate NASA’s MODIS sensor. The measurements verified the emissivity to the 0.001 % level and 

the emitted radiance scale was found to be in agreement with the NIST scale with no corrections needed. 

The TXR calibration at NIST is carried out by using several methods. One method uses at the system 

level the ambient background water bath blackbody or a Large Area Black Body (LABB) for the absolute 

calibration. Another method still under development is a system-level approach using a laser-illuminated 

integrating sphere at the NIST SIRCUS described in Section 3. The TXR has been successfully deployed 

about six times during the past several years to several different aerospace calibration facilities for in-situ 

measurements of various sources in space-simulating chambers. These measurements were used to verify 

the infrared radiance scales currently used by several NASA, NOAA, DOE, and DOD satellite programs. 

The results of a deployment of TXR to the GOES calibration chamber at the contractor (ITT) site in Ft. 

Wayne, IN in 2001 is reviewed in Section 4 as an illustration of best practice. 

5.2.2. FTXR and MDXR. 

The Fourier-transform Thermal-infrared Transfer Radiometer (FTXR) is a spectroradiometer system 

designed to measure spectral radiance in the infrared. The original motivation for its use at NIST was 

to improve upon the spectral coverage of the TXR for comparisons of extended-area blackbodies such as 

those used to calibrate Earth-observing satellite and validation instruments. The spectral coverage of the 

FTXR for use in viewing such blackbodies is 800 cm-1 to 12000 cm-1 using both an MCT and an InSb 

detector. Both of these detectors are used at the same time, since the FTXR has two detector ports that 

share a common input port, and their spectra are concatenated to provide the full spectral range. It is 

based on a four port Michelson interferometer. It has corner cubes and flexture mounts, for a spectral 
34

resolution of roughly one cm-1. The scale of the FTXR can in principle be derived from the NIST Water 

Bath Blackbody (WBBB) rather than the on-board blackbodies. The FTXR is designed to operate in 

ambient environments, so it needs to look through a window to view blackbodies in vacuum chambers. 

In such an arrangement one has to limit observations to atmospheric transmission regions of the IR 

spectrum unless a very good purge arrangement is available. Also the window transmission, reflection, 

and stray light introduce extra systematic uncertainties. Therefore, a new radiometer called the Missile 

Defense Transfer Radiometer (MDXR) is under construction to mitigate these problems. The MDXR will 

have the capabilities of the TXR and also is equipped with a cryogenic Fourier transform spectrometer to 

cover the wavelength of interest, a cryogenic radiometer traceable to POWR, and a blackbody calibrated 

at the NIST Low Background Infrared (LBIR) facility. A vacuum compatible fluid bath blackbody is also 

under construction to provide kelvin scale calibration. The MDXR will also be self-contained to serve 

user facilities in both radiance and irradiance modes at the power levels that are of interest.
35

6. EXAMPLES OF BEST PRACTICE 

Four examples of best practice to build SI traceability for instrumentation in remote sensing programs 

where NIST was involved are described below. The first example was the intercomparison of the SI 

traceable VIS/NIR transfer radiometers at the EOS facility at Goddard Space Flight Center (GSFC) 

at NASA to assess the current state of the art radiance and irradiance uncertainty across the available 

instrumentation. The second was the historical and current account of achieving the SI traceability for 

the VIS/NIR bands of the Terra and Aqua satellite MODIS sensors from preflight to current operational 

scenarios. The third was the laboratory intercomparison of infrared radiometers funded by NOAA/

NESDIS, EMETSAT, ESA and NASA in 2001 at the University of Miami’s Rosenstiel School of Marine 

and Atmospheric Science (RSMAS). The fourth one is the TXR deployment for the calibration of the 

blackbody in the GOES imager test chamber at ITT, Ft. Wayne, Indiana in 2001. 

6. 1 NASA/GSFC Intercomparison of VIS/NIR Transfer Radiometers

For more than two decades, NASA GSFC’s Radiometric and Reflectance Calibration Facilities (RRCF) 

have provided calibration and characterization services to a large number of ground-based, airborne, and 

space-borne Earth viewing sensors. Radiance and irradiance calibrations in the RRCF are performed in 

the Radiometric Calibration Laboratory (RCL) and the Calibration Development Laboratory (CDL). 

The primary artifacts used in these laboratories for the radiance calibration of these sensors in the UV to 

the SWIR, e.g., 230 nm to 2500 nm, are lamp illuminated integrating sphere sources. In April 2001, 

a measurement comparison was held at the RRCF to validate the radiance scales assigned to the RCL 

180 cm and CDL 50.8 cm spheres, to critically examine sphere operation, sphere radiance repeatability 

and stability within the RCL and CDL, and to validate the radiance measurements of a number of 

radiometers used in the vicarious calibration of fundamental NASA Earth viewing satellite instrument 

measurements. In addition, a Spectralon reflectance panel illuminated by lamps calibrated against the 

NIST 1990/1992 and NIST 2000 irradiance scales was measured by several radiometers to examine 

the effect of scale differences on their radiance measurements. This comparison was one of a series of 

validation activities overseen by the EOS Calibration Program to ensure consistency in the radiometric 

calibration accuracy of sensors used in long-term, global, climate remote-sensing.

In this comparison, the radiance scale on the RRCF spheres was realized using two GSFC transfer 

radiometers: the SSBUV and the 746/ISIC instruments. Both are scanning monochromators with 

radiance responsivities that are traceable to the NIST standard irradiance lamps. A number of transfer 

radiometers were used in an independent determination of these radiance scales: the UVSR, UVFR, 

VXR, SWIXR from NIST; the UA VNIR and UA SWIR from the University of Arizona; the LXR and 

the SXR II from GSFC; three ASD’s, one from SDSU, one from NASA Ames, and one from GSFC; 

and the Research Support Instrument (RSI) Calibration Transfer Standard Radiometer System (CTSS) 

Multifilter Spectroradiometer (MFS). During the comparison, the radiometers made measurements 

over the spectral range from 300  nm to 2400  nm, spanning the UV to SWIR wavelength regions.
36

Four radiance sources were measured: the NPR, the GSFC Code 614.4 180 cm diameter integrating 

sphere, the GSFC Code 613.3 50.8 cm diameter integrating sphere, and a Spectralon panel illuminated 

by two NIST irradiance standard lamps, type FEL, designated F-496 and F-512, respectively.

The primary goal of the comparison was to assess the uncertainties of the spectral radiance assignments 

of the two GSFC spheres. A second goal was to characterize the source stability and repeatability using a 

newly implemented monitoring detector system on one of the GSFC spheres. The results were analyzed 

by using the radiance scale realized by the GSFC 746/ISIC and SSBUV spectroradiometers and the 

individual spectral responsivity functions for the transfer radiometers to calculate the band-averaged 

spectral radiance; these results were then compared to the spectral radiances measured by the instruments. 

The preliminary results showed that the 180 cm sphere was not stable in time for wavelengths below 

550 nm but the 50.8 cm sphere was stable in this spectral region [37]. The higher than expected results 

for the 746/ISIC below 450 nm were later identified to be the result of uncorrected stray light in this 

single grating monochromator [24], and it was noted that this effect is likely affecting the results with the 

ASDs. Another bias that was observed was suggested to be understood in terms of the irradiance scale 

assigned to the FEL lamps – the NIST 2000 was not implemented on all the FEL lamps consistently, 

so the known differences between that scale assignment and the previous, NIST 1990/1992 assignment, 

would be evident. The overall results were consistent within the k=2 uncertainties. One exception was 

the NIST SWIXR and the UA SWIR compared to the GSFC 746/ISIC on the180 cm and 50.8 cm 

spheres – beyond 2000 nm the results were outside these expanded uncertainty values for the small, but 

not the large sphere. Also identified were likely calibration errors in some of the CTSS channels and the 

SDSU ASD. The results of the lamp/plaque radiance comparison were reproducible and also indicated 

the presence of the bias caused by using two different NIST irradiance scales [38]. This reinforced the 

need for irradiance standard lamps to be calibrated against the NIST 2000 detector-based irradiance 

scale, resulting in improved accuracy and reduced uncertainties, especially in the SWIR. 

6. 2 Terra and Acqua MODIS VIS/NIR SI Traceability

MODIS is a key instrument for the NASA’s EOS missions, currently operated onboard the Terra and 

Aqua spacecrafts, launched in December 1999 and May 2002, respectively. MODIS is a cross-track 

scanning radiometer. It collects data in 36 spectral bands, from VIS to LWIR, and at three nadir spatial 

resolutions: 0.25 km, 0.5 km, and 1 km. Improved over its heritage sensors, such as AVHRR and Landsat, 

MODIS was designed with stringent on-orbit calibration requirements: 2 % for VIS/NIR reflectance 

and 5 % for the radiances ( all k=1). Because of this, it was built with a set of state of the art on-board 

calibrators, including a solar diffuser (SD), a solar diffuser stability monitor (SDSM), a blackbody (BB), 

a spectroradiometric calibration assembly (SRCA), and a space view (SV) port. The SD and SDSM are 

used together for the VIS/NIR and SWIR calibration and BB for the thermal infrared (TIR) calibration. 

The SRCA is primarily used for the sensor spectral (VIS/NIR) and spatial characterization [39].
37

Both Terra and Aqua MODIS went through extensive preflight calibration and characterization activities. 

The VIS/NIR preflight radiometric calibration was made in the thermal vacuum (TV) using a spectral 

integrating sphere (SIS) operated at multiple radiance levels (or SIS lamp combinations). The SIS was 

characterized using SI traceable standards at NIST. To simulate sensor on-orbit operational conditions, 

the thermal vacuum was operated at three different instrument temperatures: cold, nominal, and hot, and 

the sensor characterized with its primary and redundant electronics. Key parameters examined during 

MODIS preflight TV calibration and characterization included VIS/NIR detector response (gain), 

signal-to-noise ratio (SNR), nonlinearity, dynamic range, and detector gain’s sensitivity to the instrument 

temperatures. Other system level calibrations for the VIS/NIR spectral bands were made at ambient 

environment, including sensor relative spectral response (RSR), polarization sensitivity, band-to-band 

registration, and response versus scan angle [40].

MODIS VIS/NIR on-orbit calibration is reflectance based with reference to the bi-directional reflectance 

factor (BRF) of its on-board solar diffuser (SD), as shown in Figure 7. The SD BRF characterization was 

made pre-launch by the instrument vendor using reference samples traceable directly to NIST. Its on-orbit 

degradation is tracked by the on-board SDSM at 9 wavelengths, closely matched to some of the MODIS 

VIS/NIR spectral bands. The SDSM functions as a radiometer that ratios alternate signals, making 

alternate observations of direct sunlight and the sunlight diffusely reflected from the SD. The SDSM is 

operated during each scheduled SD calibration event. Figure 8 illustrates the on-orbit degradation of the 

SD on Aqua MODIS at 0.41mm, 0.46mm, and 0.53mm. The SD degradation is much smaller at other 

longer wavelengths, 0.64 mm and 0.86 mm. In addition to SD/SDSM calibrations, both Terra and Aqua 

MODIS use monthly lunar observations to monitor the VIS/NIR radiometric calibration stability [41].

Figure 7: MODIS solar diffuser (SD) panel

Figure 8. Aqua MODIS solar diffuser degradation (2003 to 2008).
38

6. 3 Miami Workshop 2001 

The intercomparison workshop that NIST participated in at the University of Miami in 2001 dealt with 

the intercomparison of blackbodies used to calibrate radiometers that are deployed on ships to measure 

sea surface temperature [42]. As NIST employed the TXR for this purpose, this process provided an 

independent experimental check of the SI traceability of sea surface temperature measurements. Such 

intercomparisons are highly recommended in this paper as a best practice for SI traceability. 

The NIST TXR described earlier 

in sec. 4.2.1. was employed 

[36] in reasonably controlled 

laboratory conditions to view 

several cavity blackbodies 

and measure the brightness 

temperature of each. The 

laboratory blackbodies were 

five in total, one a reference 

blackbody, the NIST water bath 

blackbody (WBBB), and four 

other participating blackbodies 

(BB): The RSMAS BB, the Jet 

Propulsion Laboratory (JPL) 

BB, the Combined Action for 

the Study of the Ocean Thermal 

Skin (CASOTS) Rutherford 

Appleton Laboratory (RAL) 

BB, and the CASOTS 

Southampton Oceanography 

Centre (SOC) BB. All of these 

were operated independently 

of each other in the same 

laboratory at RSMAS during 

the workshop. Each BB consisted of a conical metal cavity with a black coating on the inside and each was 

surrounded on the outside by its own stirred fluid bath to improve temperature uniformity. They each 

had a calibrated thermometer located in the stirred bath, which was used to determine the temperature 

of the cavity. All cavity exit apertures were of the order of 10 cm to 11 cm in diameter, and all BBs were 

designed to be horizontally emitting. Beyond these general similarities, these five BBs can be classified 

into two groups depending on whether the bath temperature has active control or not. The NIST water 

bath blackbody (WBBB) and the RSMAS BB have active temperature control of the bath and essentially 

follow a design described previously [43]. The JPL BB, the CASOTS RAL BB, and the CASOTS SOC 

Figure 9. Comparisons are brightness temperature. The symbols are from the 

mean values of data points averaged over the last 100 seconds of each plateau 

of the temperature setting for each blackbody. The error bars are the standard 

deviation of the values over this time interval.
39

BB do not have active temperature control and follow another general design described previously [44]. 

The blackbodies intercompared were designed so that the emissivity is as high as possible even with a 

relatively large aperture diameter of about 10 cm to 11 cm. The TXR target spot diameter was about

3 cm, so it underviewed these apertures. The TXR was placed sufficiently close to each BB under test such 

that the TXR 30 mrad field of view was overfilled by the BB aperture. 

The results of the intercomparison are shown in Figure 9 in terms of 10-μm TXR brightness temperature 

(Tb2) derived from its calibration with the NIST waterbath blackbody minus contact temperature (Tc) for 

the test blackbody as function of test blackbody temperature for all four participating test blackbodies: 

RSMAS BB, JPL BB, CASOTS RAL BB, and CASOTS SOC BB. The Tc values for each BB are from 

the user’s choice of thermometer placed in the BB water bath. Averages over the last 200 seconds of 

each plateau interval are reported for all but the CASOTS SOC comparison, and the error bars represent 

the standard deviation of the 100 readings of this last, most stable interval on the plateau. The RSMAS 

BB error bars are much lower, down at the stability level of the TXR, since the RSMAS BB was under 

active temperature control and so its plateaus were very flat with time. For the other blackbodies, the 

lack of active temperature control over the plateau caused temperature drift to dominate the uncertainty, 

hence the larger error bars. Plotting only instantaneous points, rather than interval averages, gives similar 

results. Figure 9 also lets us infer that the RSMAS BB 10-μm brightness temperature agrees with that of 

the NIST water bath blackbody, which is the reference blackbody for TXR calibration over the entire 

range of temperatures studied, to within the ±0.05 °C (k=2) uncertainty of the TXR.

 

The CASOTS RAL and JPL blackbodies did not agree at temperatures away from ambient, although 

they did agree to within ±0.1 °C as long as they were near ambient. Effective emissivity values relative to 

the NIST water bath blackbody were near 0.991 at 10 μm for both of these blackbodies. The CASOTS 

SOC blackbody was not measured carefully enough to draw any definite conclusions. Careful use of 

these blackbody targets to calibrate ship-based radiometers used in the validation of satellite-derived 

skin sea-surface temperatures could therefore result in validation data sets that have uncertainties within 

±0.1 °C. This intercomparison also demonstrated some of the verification capabilities that are now 

available to the environmental remote sensing community with the use of the NIST TXR.

6. 4 TXR Deployment to ITT for GOES Imager

The NIST thermal-infrared transfer radiometer (TXR) was deployed in the GOES Imager calibration 

chamber in July of 2001 and performed radiometric measurements of the calibration targets used to 

calibrate GOES Imager instruments. The GOES Imager emissive band pre-flight absolute calibration 

is generically similar to most radiometric calibration exercises in that it is based on measurements of the 

instrument response to two blackbodies held at different temperatures in a space-simulating cryogenic 

vacuum chamber. Traditionally, the radiance entering the sensor aperture is modeled, starting with the 

temperature sensors in the warm blackbody, here called the Earth Calibration Target (ECT). As the 
40

measurements are performed in a significant thermal-infrared background, correction for backgroundinduced offsets is made by subtracting the response to a liquid-nitrogen temperature blackbody, here 

called the Space Clamp Target (SCT). 

It is believed that the honeycomb surface of the ECT can support substantial temperature gradients 

forced by the thermal-infrared background. The GOES calibration model makes a correction for the 

non-ideal behavior of the ECT. The TXR was deployed to provide independent quantitative verification 

of this model through careful radiance measurements from the ECT and the SCT. 

Within the GOES Imager 

chamber, the TXR was 

mounted on a platform in a 

specially-constructed GOES 

instrument simulator. The 

instrument simulator was an 

aluminum frame structure 

which supported multilayer 

insulation. The simulator 

was designed by ITT to look 

from the outside as much like 

a GOES Imager instrument 

as possible, except that it had 

the TXR inside instead of an 

actual GOES instrument. 

The instrument simulator 

included an optical port baffle, 

just as the GOES imager does 

when in the chamber. This is 

normally run at one of three 

temperatures during GOES 

instrument testing: Mission 

Low (ML), Mission Nominal 

(MN), and Mission High 

(MH). For the TXR deployment, the optical port baffle was run at only the two lowest temperatures: ML 

and MN. The radiative cooler patch panels, used with a real GOES instrument to provide cooling for the 

GOES radiative coolers, were also adjusted between ML and MN conditions to the same temperatures 

that they are set at for real GOES instrument testing. This was all done so as to simulate the radiometric 

environment existing during a typical GOES instrument calibration. 

Figure 10. The recommended ECT adjustment curves, delta T’ vs. Tc. with Tc

defined as the average of all 13 ECT sensors. Correction of the ECT to blackbody 

radiance requires only these curves and no emissivity adjustment. This is not 

to say that the emissivity of the ECT was unity. Rather, a value of unity has 

been historically used for the ECT in the GOES program, so these curves lump 

the combined effects of non-unity emissivity and temperature error into one 

temperature depended parameter in order to make the required corrections 

simpler to implement using the existing GOES calibration algorithms.
41

There were two main types of tests: TXR response measurements of its internal calibration source (CS), 

and TXR response measurements to the ECT (or SCT). The response data were collected and analyzed as 

band-integrated radiances and compared to that expected for an ideal blackbody. An analysis procedure 

was developed that enabled parameterization of the results in terms of a non-unity emissivity and a 

temperature gradient in the GOES Earth Calibration Target (ECT). The model was used to compute 

the correction to be made to the ECT temperature sensor readings so that the measured results are in 

agreement with the NIST radiometric scale. Values for the true temperature and ECT emissivity at 

standard uncertainty levels below 0.1 K and below 0.2%, respectively, were obtained. Recommended 

values for ECT temperature correction as shown in Figure 10, were computed based upon a fit of the 

model to the data. The data agreed qualitatively with the expectations. These data, in the form of an 

electronic text file, have been provided to ITT to enable the recommended ECT correction to be made. 

Use of this recommended correction curve will enable GOES calibration model to be more directly 

traceable to NIST.

43

7. CONCLUDING REMARKS

Satellite remote sensing has the potential to deliver the high accuracy data required to identify the small 

signals of climate change in a long time series record. The hallmark for accuracy is pre- launch SI traceable 

calibrations of sensor performance and post launch validations. The steps to be taken in pre-launch as a 

best practice are to plan and implement calibration activity from the beginning of the mission, allocating 

resources as necessary to achieve SI traceability. In this regard it is best to have calibration experts from 

NMIs involved from the mission start and through the entire life of the mission in various steps as 

recommended in this paper. Several research and development plans involving NIST are underway to 

improve accuracy for space based radiometry for climate data records. Ideally, the celestial bodies such as 

sun, moon and stars can be made as calibration transfer sources if their radiances are absolutely calibrated 

and made SI traceable. Both the workshops held in recent years for improving accuracy in remote sensing 

radiometry [1,2] highly recommended this task. Current knowledge of the lunar spectral irradiance is 

based on the work of Kieffer and Stone [45] who developed the RObotic Lunar Observatory based in 

Flagstaff, Arizona. They developed a model (ROLO model) to predict lunar irradiance across numerous 

spectral bands of interest in the VIS/NIR spectral region. The ROLO model corrects for phase and 

libration and provides a time series prediction for satellite sensors to use for calibration. The estimated 

absolute uncertainty in their temporal model predictions is around 3% (k=1) where as the relative 

uncertainty is estimated better than 1% (k=1). A preliminary study at NIST showed that an absolute 

uncertainty of 1% (k=1) or less is achievable in calibrating the moon as a transfer standard in VIS/NIR 

spectral region by using suitable spectral radiometer observations from high mountain tops to avoid 

atmosphere effects and coupling with such radiometer observations from high altitude balloon flights [2]. 

However, based on the observations of SeaWiFS, MODIS and other satellites, the use of the moon as 

a stability monitor for the sensor on orbit for the VIS/NIR spectral region is found to be very effective. 

and to do so is a sound practice as the moon is a very stable reflector of sun’s radiation in that spectral 

region. There is considerable effort from the Laboratory of Astronomy and Space Physics (LASP) of 

the University of Colorado, Boulder to calibrate total solar irradiance measurement (TIM) and spectral 

irradiance measurement (SIM) instrumentation directly with NIST standards to establish SI traceability 

for their measurements from space. Also NIST has initiated a project to improve the calibration of stars 

which can be used as transfer standards by working with astronomy community. In the thermal and long 

wavelength infrared spectral region, the CLARREO incubator project plans to use fixed point blackbodies 

and monitor the cavity emissivity while on orbit and is working with NIST to implement and improve 

the uncertainty budget and build SI traceability.

45

8. ACKNOWLEDGEMENTS

This report is based on the experiences of the past and present staff at NIST Optical Technology Division 

and NASA Goddard Space Flight Center who worked on NASA/ EOS and experts at NOAA on 

radiometric calibrations for remote sensing for the last two decades. Therefore, the information provided 

and discussed happened to be specific to these organizations. However, the best practice guidelines are 

generally applicable for all organizations towards climate quality data from satellite sensors. 

Certain commercial companies are identified in this paper to foster understanding. Such identification 

does not imply recommendation or endorsement by the National Institute of Standards and technology 

(NIST), nor does it imply that they are the best available for the purpose. DMD and DLP are trademarks 

of Texas Instruments, Inc.

REFERENCES

1. G. Ohring, B. Weilicki, R. Spencer, W. Emery and R. Datla, eds. “Satellite instrument calibration for 

measuring global climate change,” National Institute of Standards and Technology, Publication NISTIR 

7047, Gaithersburg, MD, 101 pp. (2002).

2. G.Ohring, ed. “Achieving satellite instrument calibration for climate change,” Eos, Transactions, American 

Geophysical Union 88, 136 (2007).

3. http://ts.nist.gov/Traceability/SupplMatls/suppl_matls_for_nist_policy_rev.cfm 

4.	 ISO, Guide to the Expression of Uncertainty in Measurement, 2nd ed., International Organization for 

Standardization (1995). 

5. J. J. Butler, B. C. Johnson, and R.A. Barnes, “The calibration and characterization of Earth remote sensing 

environmental monitoring instruments,” in Optical Radiometry, Experimental Methods in the Physical 

Sciences Vol. 41, A. C. Parr, R. U. Datla, and J. L. Gardner, eds., Elsevier Academic Press, Chapter 10 (2005) .

6. C. Cao, M. Weinreb, and S. Kaplan, “Verification of the HIRS spectral response functions for more accurate 

atmospheric sounding” Presentation at the CALCON Meeting, Logan, Utah (2004). 

7. http://wgcv.ceos.org/wgcv/wgcv.htm

8. D. C. Tobin, H. E. Revercomb, R. O. Knuteson, F. A. Best, W. L. Smith, N. N. Ciganovich, R. G. Dedecker, 

Dutcher, Steven; Ellington, Scott D.; Garcia, Raymond K.; Howell, H. Benjamin; LaPorte, Daniel D.; Mango, 

Stephen A.; Pagano, Thomas S.; Taylor, Joe K.; van Delst, Paul; Vinson, Kenneth H. and Werner, Mark W. 

“Radiometric and spectral validation of Atmospheric Infrared Sounder observations with the aircraft-based 

Scanning High-Resolution Interferometer Sounder,” J. Geophys. Res., 111, D9 (2006).

9. S.W. Brown, J.P. Rice, J.E. Neira, R. Bousquet, and B.C. Johnson, “Hyperspectral image projector for 

advanced sensor characterization,” Proc. SPIE 6296, 629602 (2006). 

10. S. W. Brown, B. Myers, R. A. Barnes, and J. P. Rice, “Characterization of earth observing satellite instruments 

for response to spectrally and spatially variable scenes,” Proc. SPIE 6677, 667705 (2007). 

11. C. Cao and A. Heidinger, “Inter-comparison of thye longwave infrared channels of MODIS and 

AVHRR/NOAA-16 using simultaneous nadir observations at orbit intersections, Proc. SPIE 4814, 

306-316 (2002).
46

12. R. U. Datla, R. Kessel, A. W. Smith, R. N. Kacker and D. B. Pollock, “Uncertainty Analysis of remote sensing 

optical sensor data – guiding principles to achieve metrological consistency,” Int. J. of Remote Sensing in Print 

(2009).

13. J.M. Houston and J.P. Rice, “NIST reference cryogenic radiometer designed for versatile performance,” 

Metrologia 43, S31-S35 (2006).

14. S. W. Brown, G. P. Eppeldauer, J. P. Rice, J. Zhang, and K. R. Lykke, “Spectral Irradiance and Radiance 

Responsivity Calibrations using Uniform Sources (SIRCUS) facility at NIST,” Proc. SPIE 5542, 363-374 

(2004).

15. G.P. Eppeldauer, J.P. Rice, J. Zhang, and K.R. Lykke, “Spectral irradiance responsivity measurements between 

1 μm and 5 μm,” Proc. SPIE 5543, 248-257 (2004).

16. H. W. Yoon, D. W. Allen, C. E. Gibson, M. Litorja, R. D. Saunders, S. W. Brown, G. P. Eppeldauer, K. 

R. Lykke, “Thermodynamic-temperature Determinations of the Ag and Au Freezing Temperatures using a 

Detector-based Radiation Thermometer,” Appl. Opt. 46, 2870-2880 (2007).

17. S. W. Brown and B. C. Johnson, “Development of a portable integrating sphere source for the Earth Observing 

System’s calibration validation programme,” Int. J. Remote Sensing 24, 215-224 (2003).

18. S. W. Brown, B. C. Johnson, S. F. Biggar, E. F. Zalewski, J. Cooper, P. Hajek, E. Hildum, P. Grant, R. 

A. Barnes, and J. J. Butler, “Radiometric validation of NASA’s Ames Research Center’s Sensor Calibration 

Laboratory,” Appl. Opt. 44, 6426-6443 (2005).

19. P. Shaw, U. Arp, R. D. Saunders, D. Shin, H. W. Yoon, C. E. Gibson, Z. Li, A. C. Parr, and K. R. Lykke, 

“Synchrotron radiation – based irradiance calibration from 200 to 400 nm at the synchrotron ultraviolet 

radiation facility III,”Appl. Opt. 46, 25-35 (2007).

20. B. W. Mangum, G. T. Furukawa, K. G. Kreider, C. W. Meyer, D. C. Ripple, G. F. Strouse, L. Tew, M. R. 

Moldover, B. C. Johnson, H. W. Yoon, C. E. Gibson, and R. D. Saunders, “The Kelvin and Temperature 

Measurements,” J. Res. Natl. Inst. Stand. Technol. 106 (1), 105-149 (2001).

21. A. Thompson and E. A. Early, “Portable high-accuracy double spectrometer for ultraviolet spectral irradiance 

measurements,” Proc. SPIE 3818, 46-55 (1999).

22. A. Thompson, E. A. Early, and T. R. O’Brian, “Ultraviolet spectral irradiance scale comparison: 210 nm to 

300 nm,” J. Res. Natl. Inst. Stand. Technol. 103, 1-13 (1998).

23. K. Lantz, P. Disterhoft, E. Early, A. Thompson, J. DeLuisi, J. Berndt, L. Harison, P. Kiedron, J. Ehramjian, 

G. Bernhard, L. Cabasug, J. Robertson, W. Mou, T. Taylor, J. Slusser, D. Bigelow, B. Durham, G. Janson, 

D. Hayes, M. Beaubien, A. Beaubien, “The 1997 North American interagency intercomparison of ultraviolet 

spectroradiometers including narrowband filter radiometers,” J. Res. Natl. Inst. Stand. Technol. 107, 19-62 

(2002).

24. J. J. Butler, B. C. Johnson, S. W. Brown, D. W. Allen, E. A. Early, S. F. Biggar, E. F. Zalewski, B. L.Markham, 

E. Kaita, J. W. Cooper, S. Schiller, G. Meister, D. f. Heath, P. Hajek, J. Marketon, G. Smith, and R. A. Barnes, 

“Radiometric measurement comparisons at NASA’s Goddard Space Flight Center,” in preparation.

25. E. A. Early, B. C. Bush, S. W. Brown, D. W. Allen, B. C. Johnson, “Radiometric calibration of the Scripps 

Earth Polychromatic Imaging Camera,” Proc. SPIE 4483, 77-84 (2002).

26. D. W. Allen, G. P. Eppeldauer, S. W. Brown, E. A. Early, B. C. Johnson, and K. R. Lykke, “Calibration and 

characterization of trap detector filter radiometers,” Proc. SPIE 5151, 471-479 (2003).

27. J. J. Butler, S. J. Janz, B. C. Johnson, R. D. Saunders, J. W. Cooper, M. G. Kowalewski, and R. Barnes, 

“Calibration of a radiance standard for the NPP/OMPS instrument,” Proc. SPIE 7106, 71060Z-1 to 71060Z10 (2008).

28. B. C. Johnson, J. B. Fowler, and C. L. Cromer, The SeaWiFS Transfer Radiometer (SXR), NASA Tech. 

Memo. 1998-206892, Vol. 1, S. B. Hooker and E. R. Firestone, Eds., NASA Goddard Space Flight Center, 

Greenbelt, Maryland, 58 pp.
47

29. B. C. Johnson, S. W. Brown, G. P. Eppeldauer, and K. R. Lykke, “System-level calibration of a transfer 

radiometer used to validate EOS radiance scales,” Int. J. Remote Sensing 24, 339-356 (2003).

30. B. L. Markham, J. S. Schafer, F. M. Wood, P. W. Dabney, and J. J. Butler, “Monitoring large-aperture spherical 

integrating sources with a portable radiometer during satellite instrument calibration, Metrologia 35, 643-

648 (1998).

31. G. Meister, P. Abel, R. Barnes, J. Cooper, C. Davis, G. Fargion, R. Frouin, M. Godin, D. Korwan, R. 

Maffione, C. McClain, S. McLean, D. Menzies, A. Poteau, J. Robertson, and J. Sherman, “Comparison 

of spectral radiance calibrations at oceanographic and atmospheric research laboratories,” Metrologia 40, 

S93-S96 (2003).

32. S. W. Brown, B. C. Johnson, and H. W. Yoon, “Description of a portable spectroradiometer to validate EOS 

radiance scales in the shortwave infrared,” The Earth Observer 10 (3), 43-48 (1998).

33. J. J. Butler and R. A. Barnes, “The use of transfer radiometers in validating the visible to shortwave infrared 

calibrations of radiance sources used by instruments in NASA’s Earth Observing System,” Metrologia 40, 

S70-S77 (2003).

34. J. J. Butler, B. C. Johnson, S. W. Brown, H. W. Yoon, R. A. Barnes, B. L. Markham, S. F. Biggar, E. F. Zalewski, 

P. R. Spyak, J. W. Cooper, F. Sakuma, “Radiometric measurement comparisons using transfer radiometers in 

support of the calibration of NASA’s Earth Observing System (EOS) sensors,” Proc. SPIE. 3870, 180-192 

(1999).

35. J. P. Rice, and B. C. Johnson, “The NIST EOS thermal-infrared transfer radiometer,” Metrologia 35, 505-509 

(1998).

36. J. P. Rice, S. C. Bender, W.H. Atkins, and F.J. Lovas “Deployment test of the NIST EOS thermal-infrared 

transfer radiometer,” Int. J. Remote Sensing 24, 367-388 (2003).

37. J. J. Butler, B. C. Johnson, and R. A. Barnes, “Radiometric measurement comparisons at NASA’s Goddard 

Space Flight Center: Part I. The GSFC sphere sources,” The Earth Observer 14 (3), 3-8 (2002).

38. J. J. Butler, B. C. Johnson, and R. A. Barnes, “Radiometric measurement comparisons at NASA;s Goddard 

Space Flight Center: Part II. Irradiance lamp comparisons and the NIST sphere source,” The Earth Observer 

14 (4), 25-29 (2002).

39. Xiong, X., K. Chiang, J. Esposito, B. Guenther, and W. Barnes, “MODIS On-Orbit Calibration and 

Characterization,” Metrologia 40(1), 89-92, (2003).

40. Xiong, X., and W.L. Barnes, “An Overview of MODIS radiometric calibration and characterization,” Adv. 

Atmos. Sci. 23(1), 69-79, (2006).

41. Xiong, X., J. Sun, and W. Barnes, “Inter-comparison of On-orbit Calibration Consistency Between Terra 

and Aqua MODIS Reflective Solar Bands Using the Moon,” IEEE Geosci. Remote Sens. Let. 5(4), 778-782, 

(2008).

42. J. P. Rice, J. J. Butler, B. C. Johnson, P. J. Minnett, K. A. Mailet, T. J. Nightingale, S. J. Hook, A. Abtahi, C. 

J. Donlon, and I. J. Barton, “ The Miami 2001 Infrared Radiometer Calibration and Intercomparison. Part 

I: Laboratory Characterization of Blackbody Targets,” J. Atmos. Oceanic Technol. 21, 258 – 267 (2001).

43. J. B. Fowler, “A third generation water bath based blackbody source,” J. Res. Natl. Inst. Stand. Technol. 100, 

591-599 (1995).

44. C. J. Donlon, T. Nightingale, L. Fiedler, G. Fisher, D. Baldwin, and I. S. Robinson, “The calibration and 

intercalibration of sea-going infrared radiometer systems using a low cost blackbody cavity,” J. Atmos. 

Oceanic Technol. 16, 1183-1197 (1999). 

45. H. H. Kieffer and T. C. Stone, “The spectral irradiance of the moon,” Astronom. J. 129, 2887-2901 (2005).