NASA/TMm2000-209891, Vol. 93

Technical Report Series on the

Boreal Ecosystem-Atmosphere Study (BOREAS)

Forrest G. Hall and Jaime Nickeson, Editors

Volume 93

SLICER Airborne Laser Altimeter Characterization of Canopy Structure and

Sub-canopy Topography for the BOREAS Northern and Southern Study Regions:

Instrument and Data Product Description

D.J. Harding and J.B. Blair, NASA Goddard Space Flight Center

D.L. Rabine and K.L. Still, SSAI

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

September 2000

https://ntrs.nasa.gov/search.jsp?R=20010021953 2018-05-15T06:33:54+00:00Z
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NASA/TMm2000-209891, Vol. 93

Technical Report Series on the

Boreal Ecosystem-Atmosphere Study (BOREAS)

Forrest G. Hall and Jaime Nickeson, Editors

Volume 93

SLICER Airborne Laser Altimeter Characterization of Canopy Structure and

Sub-canopy Topography for the BOREAS Northern and Southern Study Regions:

Instrument and Data Product Description

D.J. Harding and J.B. Blair, NASA Goddard Space Flight Center

D.L. Rabine and K.L. Still, SSAI

National Aeronautics and

Space Administration

Goddard Space Flight Center

Greenbelt, Maryland 20771

September 2000
NASA Center for AeroSpace Information

7121 Standard Drive

Hanover, MD 21076-1320

Price Code: A17

Available from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Price Code: A10
SLICER Airborne Laser Altimeter Characterization of Canopy Structure

and Sub-canopy Topography for the BOREAS Northern and Southern

Study Regions: Instrument and Data Product Description

D.J. Harding, J.B. Blair, D.L. Rabine, K.L. Still

Summary

SLICER data were acquired in support of BOREAS at all of the TF sites in the SSA and NSA, and

along transects between the study areas. Data were acquired on 5 days between 18-Jul and

30-Ju1-1996. Each coverage of a tower site is typically 40 km in length, with a minimum of 3 and a

maximum of 10 lines across each tower oriented in a variety of azimuths. The SLICER data were

acquired simultaneously with ASAS hyperspectral, multiview angle images. The SLICER Level 3

products consist of binary files for each flight line with a data record for each laser shot composed of

13 parameters and a 600-byte waveform that is the raw record of the backscatter laser energy reflected

from Earth's surface. The SLICER data are stored in a combination of ASCII and binary data files.

Note that the SLICER data are not contained on the BOREAS CD-ROM set. An inventory listing

file is supplied on the CD-ROM to inform users of the data that were collected. See Sections 15 and 16

for information about how to acquire the data.

Table of Contents

• Data Set Overview

• Investigator(s)

• Theory of Measurements

• Equipment

• Data Acquisition Methods

• Observations

• Data Description

• Data Organization

• Data Manipulations

• Errors

• Notes

• Application of the Data Set

• Future Modifications and Plans

• Software

• Data Access

• Output Products and Availability

• References

• Glossary of Terms

• List of Acronyms

• Document Information

1. Data Set Overview

1.1 Data Set Identification

BOREAS Scanning Lidar Imager of Canopies by Echo Recovery: Level-3 Data

1.2 Data Set Introduction

Scanning Lidar Imager of Canopies by Echo Recovery (SLICER) is an airborne laser altimeter and

surface lidar instrument that acquires canopy height, vertical structure, and ground elevation data along

Page 1
transectsthatareapproximately50m wide.Thedatahaveahorizontalresolutionof approximately10

m andaverticalresolutionof approximately1m.Eachlaserfootprintis individuallygeoreferencedby

combiningthelaserrangingdatawith orientationandpositionalinformationprovidedby theInertial

NavigationSystem(INS) andGlobalPositioningSystem(GPS),respectively.SLICERwasflown

onboardaNationalAeronauticsandSpaceAdministration(NASA)C130-Qaircrafton5 daysbetween

18-Juland30-Jul-1996,acquiringdataalongtransectscrossingall theTowerFlux (TF)sitesin the

SouthernandNorthernStudyAreas(SSAandNSA,respectively)of theBOReal

Ecosystem-AtmosphereStudy(BOREAS)studyregion(Harding,1998).

1.3 Objective/Purpose

The objectives of this activity were to (1) provide the BOREAS project with measurements of

canopy structure and ground topography, which are not readily achievable by ground-based or other

remote sensing techniques, and (2) utilize BOREAS ground-based canopy measurements to assess

SLICER performance and capabilities. Canopy architecture plays a fundamental role in controlling the

exchanges of radiative energy, sensible heat, water, carbon dioxide, and trace gases between the

surface and the lower atmosphere. Canopy architecture also plays an important role in controlling

remote sensing image characteristics, such as the self-shadowing effects on at-sensor radiance

observed by passive optical imagers. However, the importance of canopy architecture has rarely been

directly quantified due to the difficulty in obtaining consistent, regional observations of

three-dimensional canopy structure. The SLICER data collection for BOREAS provides the

opportunity to incorporate such observations in an integrated study of the boreal ecosystem.

1.4 Summary of Parameters and Variables

The SLICER data provided to the BOREAS project are a Level 3 product consisting of a mixture of

raw and derived parameters for each laser shot yielding a valid return along the flight. The data record

for each laser shot consists of 13 parameters and a 600-byte waveform that is the raw record of the

backscatter laser energy reflected from Earth's surface. Level 3a consists of all shots along the flight

lines that yielded a valid reflection from Earth's surface. Automated software has been applied to

identify the ground reflection within each waveform. Level 3b consists of segments extracted from the

Level 3a data that are in the vicinity of the TF sites. The Level 3b data have been manually inspected

and edited to refine the identification of the ground reflection within the waveforms.

The 13 parameters are the laser shot number; across-track beam position; energy of the transmitted

pulse; time of pulse transmission; diameter of the laser footprint on the surface; azimuth and inclination

of the laser pointing vector; latitude and longitude of the footprint; elevation of the highest surface

within the footprint (e.g., canopy top, or ground where unvegetated); and the distance along the laser

vector from the highest surface to what is inferred to be the start, peak, and end of the laser reflection

from the ground.

1.5 Discussion

SLICER acquired data on flight lines that crossed each of the TF sites in both the SSA and the

NSA in the BOREAS study region. Data on a minimum of 3 and a maximum of 10 flight lines were

acquired for each tower site. Additional data were acquired on transect flights between study areas and

for calibration purposes. Data for each flight line, which are typically 40 km in length, are contained in

individual binary files (Level 3a). For ease of use, where the ground track is close to a BOREAS TF

site, the data were extracted from the full flight lines, edited for ground return identification, and stored

in separate files (Level 3b). Each file contains 1,001 laser shots centered near a tower, extending

approximately 1 km outward from the tower.

The BOREAS SLICER data are available for distribution on two CD-ROMs; one contains the SSA

and transect data and the other contains data for the NSA. The CD-ROMs also contain acquisition and

processing notes, aircraft flight trajectory data, summary flight line location maps for each study area

and tower site, summary plots of each flight line that include line location, aircraft attitude, surface

elevation, and return pulse width (a measure of vegetation height), and software for interactively

viewing the data in the Interactive Data Language (IDL).

Page 2
1.6 Related Data Sets

BOREAS RSS-02 Extracted Reflectance Factors Derived from ASAS Imagery

BOREAS RSS-02 Level-lb ASAS Imagery: At-sensor Radiance in BSQ Format

SLICER Level 0, 1, and 2 data are archived and distributed on CD-ROM by the Laser Altimeter

Processing Facility (LAPF) at NASA's Goddard Space Flight Center (GSFC). Level 0 consists of raw

GPS, INS, and altimetry ranging data as recorded during a flight. Level 1 consists of the aircraft

trajectory, derived by a kinematic differential solution applied to the raw GPS records, and INS and

altimetry data converted to physical units and segmented into individual flight segments. Level 2

consists of geolocation results (position and elevation) for each laser shot, derived by merging the

GPS, INS, and altimetry data, along with some engineering data not included in Level 3.

Nadir-viewing VHS video records of the SLICER flight lines are also archived by the LAPF.

The LAPF maintains a World Wide Web (WWW) site at http://denali.gsfc.nasa.gov/lapf/that

shows location maps and profiles for each flight line. Refer to

http://denali.gsfc.nasa.gov/lapf/slicer/slicer.html for SLICER information. The latest software version

of a browser for viewing and editing the Level 3 data within IDL is also distributed at the WWW site,

along with a sample Level 3 data set for the NSA Old Jack Pine (OJP) site.

2. Investigator(s)

2.1 Investigator(s) Name and Title

David J. Harding, Principal Investigator

2.2 Title of Investigation

The SLICER data acquisition was not a formal part of BOREAS Experiment Plan. It was approved

as a no-cost, add-on experiment entitled Airborne Laser Altimeter Characterization of Canopy Structure

and Sub-canopy Topography at BOREAS Tower Flux Sites.

2.3 Contact Information

Contact 1:

David J. Harding

NASA GSFC

Code 921

Greenbelt, MD 20771

(301) 614-6503

(301) 614-6522 (fax)

David.J.Harding@ gsfc.nasa.gov

Contact 2:

Kathy L. Still

NASA GSFC

Code 921

Greenbelt, MD 20771

(301) 614-6458

(301) 614-6522 (fax)

Kathy.L.Still@ gsfc.nasa.gov

Page 3
Contact 3:

Jaime E. Nickeson

Raytheon ITSS

NASA GSFC

Code 923.4

Greenbelt, MD 20771

(301) 286-3373

(301) 286-0239 (fax)

Jaime.E.Nickeson@ gsfc.nasa.gov

3. Theory of Measurements

The principles of laser altimetry are rather simple and are based on precise timing of the round-trip

travel time of a short duration laser pulse (Bufton, 1989). By combining the range distance, derived

from the travel time, with knowledge of the laser pointing vector and the platform position, the

horizontal and vertical position of the reflecting target is established in an absolute reference frame

(Figure 1). Topographic profiles or images can then be constructed showing the elevation of the

reflecting target.

AIRBORNE ALTIMETRY VERTICAL REFERENCE SURFACES

lsser

._--ranging

distanoe

p oi nti ng labe or

angle ocean_rfa_

off nadir parallelb geoid

GP8

Base antenna geodetic or

s'_ton height ellipsoidal /ant, nnae _.r.t.

_n_m_rh geoidal elevati on

elevation ground w.r.t.

ground w.r.t, ellipsoid

geoid model

benchm ark orthometric geoid

/ elevation elevation _ height

b_nchrnarh wx.t ground w.r.t, geoid w_.t

antennae ellipsoid local dalum ellipsoid elevation

anlBn nae wI.t

ellipsoid

local datum

dfset

datum w.r.t, geoid

Figure 1: Airborne Altimetry Vertical Reference Surfaces

AIRCRAFT

KINEMATIC GPS

TRAJECTORY

aircraft altitude

aircraft G PS an_nnae

w.r.t, elli pe_id

TOPOGRAPHY

WGS 84

ELLIPSOID

GEOID M ODEL

NG8 GE OID93 (U.8.)

EGM96 (global)

LOCAL VERTICAL

DATUM

e.g. mean sea level

approxirnalJon of geoid

Page 4
LASER ALTIMETRY ECHO RECOVERY

®I

i

E

<

__=

'_.

i

Transmitted

Pulse Backscatter Return

Canopy

Optical _d A Sloped Ground

'L -3/e-c-_ rJ::-_lic-H°i--se =DZ_!isZ-Iresh°l_, , , , ,_,,= _.,--_

--__ ..... ,l, i, wD--,w--w -- _,_,m'i,,iW--'_ll'_il i i i i i i i i i i i i i i i i i ,i ,i i i i i i i i i i_ll_ rmi 1l¢

J Time

Time-Interval-Unit Range _l Waveform Digitizer Resolution

FOOTPRINT PLAN VIEW

contours ct laser

irradianoe high- albedo

( Gaussian _.r"_'tree crown

s pati al _

distribution) _ surface

/N elev ion

contours

Iow-albedo

sloping ground

Figure 2: Laser Altimetry Echo Recovery

The backscatter waveform is a

record c)fthe vertical distribution

of intercepted, nadir-prqected

surface area weighted by the

reflectivity of the surfaces at the

monochromatic laser wavelength and the spatial distribution

of laser energy across the

fodprint.

The SLICER instrument extends this altimeter capability by recording a lidar waveform. The lidar

waveform is a record of the amplitude of backscattered laser energy received as a function of time

(Figure 2). Unlike traditional lidar, where returns from the entire atmospheric column are recorded at

low vertical sampling, SLICER records reflections only from Earth's surface (in the absence of

clouds). The return signal is recorded at very high vertical sampling, thus providing a finely sampled

measure of the vertical distribution of illuminated surface area within the footprint, including plant area

throughout the vegetation canopy (Figure 3). Where laser energy penetrates to the canopy floor and is

reflected back to the receiver, a measure of canopy height is obtained for that laser pulse from the

travel-time between canopy top and ground reflections. The waveform amplitude decreases with depth

through the canopy due to occlusion (reflection and absorption) of the transmitted laser energy; less

laser energy per unit area penetrates into the canopy with increasing depth. The amount of occlusion at

a specific depth into the canopy is dependent on the amount of canopy area encountered higher in the

canopy.

The intensity of the received backscatter return at a given depth in the canopy depends on the

amount of laser illumination penetrating to that depth, and on the reflectivity of the intercepted surfaces

at the wavelength of the laser. Also, the spatial distribution of laser energy is not constant across a

laser footprint, but instead has a circular, Gaussian distribution. Therefore, the horizontal organization

of reflecting surfaces with respect to the laser energy spatial distribution affects the intensity of the

return. In summary, the time history of backscatter energy is a measure of the vertical distribution of

illuminated surface area, projected in the direction of the laser vector, weighted by the reflectance of the

surfaces at the monochromatic laser wavelength and the spatial distribution of laser energy across the

footprint. The vertical resolution of the backscatter waveform is a function of the duration (i.e., pulse

width) of the transmitted laser pulse and the bandwidth (i.e., response time) of the detector used to

convert the backscatter energy into an analog signal. The vertical sampling of the backscatter waveform

is a function of the sampling rate of the digitizer used to convert the analog signal to a digital record.

Page 5
BACKSCATIEN RETURN WAVEFORM

Normalized Cumulative Return Energy

(canopy "cover" fraction)

O 0.2 0.4 0.6 0.8 1.0

Transmit Pul se

_-, 0

o

50

5

i0

15

0 40 80 120 160 200

Return Energy (Digitizer Counts)

Figure 3: Backscatter Return Waveform

The received laser energy reflected from the surface consists of returns due to both single and

multiple-scattering events. Single-scattering events consist of photons that encounter only one surface

and are reflected directly back to the receiver at 0 ° phase angle (parallel illumination and view angles;

i.e., hot spot orientation). Multiple-scattering events consist of photons that encounter more than one

surface before being reflected back to the receiver, as can be the case for laser energy that is transmitted

through foliage and is subsequently reflected from another surface. Laser energy reflected from the

ground consists of single-scattered photons, where a gap extends through the entire canopy to the

ground in the direction of the transmit pulse, as well as some fraction of multiple-scattered photons.

The path for multiple-scattered photons is longer than the straight-line path between instrument and

target (the single-scattering path) and thus those photons appear delayed in the waveform compared to

single- scattered photons.

The relative strength of the canopy and ground returns provides information on canopy closure.

The term canopy closure is used here to mean the fraction of plant area per unit area, projected along

the direction of the transmitted laser pulse. It is equal to one minus the gap fraction. The cumulative

height distribution of canopy energy, normalized by the total return energy, is a relative measure of

canopy closure as a function of height (Figure 3). The measure is relative because the reflectance of the

surfaces encountered at 0 ° phase angle, at the wavelength of the transmitted laser pulse, determines the

return signal strength from each surface. Independent knowledge of the average reflectance of the

canopy and ground surfaces within the laser footprint is necessary to convert the cumulative

distribution to an absolute measure of canopy closure. The cumulative distribution also makes the

simplifying assumption that only single-scattering events contribute to the return signal. Furthermore,

the closure measure depends on the spatial organization of plant area and gaps with respect to the

Gaussian energy distribution across the footprint. For an equal amount of plant area within a footprint,

the plant area concentrated in the center of the footprint will yield a higher measure of closure as

compared to that concentrated at the outer edges of the footprint (Figure 4).

Page 6
k,.

e--

c"

8-

time

8-

¢-

time

_ low albedo

 ,iiOil/OO

tree crown

low apparent

canopy closu re

high apparent

canopy closure

Figure 4: Canopy Closure

See Harding et al. (in review) for a more complete discussion of the measurement theory of surface

lidar in the presence of vegetation.

4. Equipment

4.1 Sensor/Instrument Description

The SLICER airborne lidar altimeter system consists of a ranging component and ancillary

instrumentation for geolocation. The components of the SLICER system are schematically illustrated in

Figure 5. The ranging component consists of a laser transmitter, scan mechanism, receiver telescope,

detector, timing electronics, waveform digitizer, and an instrument control and data collection system.

The SLICER laser transmitter (Coyle and Blair, 1995; Coyle et al., 1995) is a cavity-dumped,

Q-switched, diode-pumped, Nd:YAG oscillator-amplifier. The laser outputs a short duration pulse (4

nsec full-width at half-max pulse width) with a beam divergence of approximately 2 mrad at a

wavelength of 1064 nm (near-infrared) and at pulse rates up to 80 Hz. A high-speed oscilloscope is

used to visually monitor the temporal character and amplitude of the transmitted and received pulses.

The scan mechanism consists of optional lenses for changing the beam divergence, a fixed turning

mirror and an oscillating mirror that is rotated by means of a computer-controlled galvanometer. The

Page 7
receiver is a 200-mm aperture refracting telescope with a focal length of 245 mm with aft optics that

increase the f-number to 3.65. The detector is an EG&G C30955E silicon avalanche photodiode

(Si:APD) that converts optical energy into an output analog voltage with a bandwidth of 50 MHz. The

timing electronics consist of a Lecroy 4204 time interval unit (TIU) that has a 156.25-picosec

resolution (equivalent to 2.3-cm ranging resolution in a one-way travel sense). The waveform digitizer

is a Lecroy 6880B analog-to-digital converter capable of sampling at a rate up to 1.348 giga samples

per second. The TIU and digitizer modules are housed in a Camac crate. The instrument control and

data collection system is based on a 486DX PC-compatible mother board with the altimetry data stored

to hard disk and backed up on 8-mm tape.

SLICER Components and Data Flow

Figure 5: SLICER Components and Data Flow

. backscatter

1rsrnit relurn Irar

guise

The ranging instrumentation is augmented by an INS for precise determination of laser beam

pointing, GPS receivers for differential, kinematic determination of aircraft position and for use as a

time source, and video equipment for image documentation of the ground track. The INS for BOREAS

was a Litton LTN-92 laser ring gyro system. Gyroscopically determined accelerations are converted to

roll, pitch, and yaw by the INS and reported at a rate of 64 Hz. The INS data stream is sampled by the

altimetry data system at the time of each laser pulse fire, and the roll, pitch, and yaw data are recorded

as a part of the altimetry data record for each laser pulse. For position and time data, Ashtech Z-12

dual-frequency GPS receivers were utilized at BOREAS, with the GPS data recorded to laptop

computer hard disks. For redundancy, two receivers were deployed both on the aircraft and at the

ground base station. The dual aircraft receivers were connected via a signal splitter to a single Ashtech

avionics GPS antenna mounted on top of the aircraft fuselage on the airframe centerline. Similarly, the

Page 8
dual ground receivers were connected to a single Ashtech ground plane antenna mounted on a

surveyor's tripod above a geodetic benchmark. One of the onboard Ashtech receivers provides

time-referencing data for the altimetry data stream. Absolute time is output from the receiver as a GGA

message page that is received by a clock box and reformatted for recording by the altimetry data

system. In addition, a 1 pulse per second time mark from the GPS receiver is used as a 1-second

counter that, in conjunction with a PC timer card containing a GHz oscillator time source, yields

0.016-msec resolution on the laser pulse transmit time. The second in-flight GPS receiver provides

real-time position data that are utilized by flight manager software running on a separate laptop

computer. The flight manager displays navigation information to the pilot on a color monitor. Color

video imagery, acquired using a video camera pointing at nadir, was recorded on to VHS tapes with

time annotated on each video frame using an external time-stamp generator that was manually synched

to GPS time.

4.1.1 Collection Environment

Data were collected with the SLICER instrument mounted in the interior of a pressurized aircraft.

Temperature, although controlled, did vary within the aircraft interior during the course of a flight

mission. The temperature variations were not monitored. Although the laser ranging function of the

SLICER instrument is not temperature sensitive, the recorded amplitude of the received backscatter

laser energy is potentially affected by temperature variations (see Section 11.1).

4.1.2 Source/Platform

The aircraft used for BOREAS was a C130-Q turboprop stationed at NASA GSFC's Wallops

Flight Facility (WFF). The aircraft was based at the Prince Albert Airport during the BOREAS

deployment. SLICER acquisitions in Iceland, the Azores, and Florida also used the WFF C130-Q

aircraft. Remaining SLICER acquisitions utilized a Sabreliner T-39 twin engine jet, also stationed at

WFF.

4.1.3 Source/Platform Mission Objectives

The primary mission objective at BOREAS was acquisition of Advanced Solid-state Array

Spectroradiometer (ASAS) multilook angle, hyperspectral images of the TF sites. The aircraft was

flown on specific headings across the towers in order to acquire ASAS images parallel, perpendicular,

and oblique to the solar principal plane. SLICER was operated at nadir on each of the flight lines as

ASAS images were acquired. SLICER data were also acquired during transect flights between the SSA

and NSA while ASAS acquired nadir view-zenith-angle images. Additionally, specific flight lines were

flown to acquire SLICER calibration data.

4.1.4 Key Variables

The principal quantities measured by SLICER are:

• The round-trip travel time of a short-duration 1064-nm pulse of laser light, from the aircraft

platform to the first surface that reflects sufficient laser light above a detection threshold at the

receiver.

• The intensity of the backscatter return reflected from the surface as a function of time.

• The orientation of the laser pointing vector, obtained from the orientation of the laser scan

mirror and an INS measurement of instrument attitude based on gyroscope-determined

accelerations.

• The trajectory of the aircraft, obtained from a kinematic differential solution of dual-frequency

GPS observations.

• Observation time for each of the above quantities.

4.1.5 Principles of Operation

A short-duration, 1064-nm laser pulse (4 nsec full width at half the maximum peak amplitude) is

generated by the laser transmitter at rates up to 80 Hz. The laser transmitter is commanded to fire at a

specified repetition rate by computer control using an interrupt-driven fire command for each pulse.

The transmit time of each laser pulse is established using two time sources. Absolute time is reported

Page 9
by anonboardGPSreceiverat2 Hzandrecordedby thealtimetrydatasystem.A relativetransmittime

is measuredwith 0.016-msecresolutionby usingatimecounterwhichis basedonaPCtimercard

containingaGHz oscillatorthatis synchedusinga 1pulseper secondtimemarkprovidedby theGPS

receiver.Thelasertransmitterincludesaphotodiodedetectorthatsamplesa smallfractionof theoutput

pulseenergyatapointwithin thetransmitbeam.Thevoltagesignalfrom thephotodiodedetectoris

accumulatedin anintegrator,yieldingameasureof outputpulseenergyasa singlebytevalue.After

exitingthelasertransmitter,theintensityof thetransmitpulseis modulatedby theuseof filtersin

orderto achieveabackscatterenergybelowthesaturationlimit of thereceiverdetector.Filtersare

manuallyinsertedinto thetransmitbeampathby theinstrumentoperatorin orderto optimizethereturn

signallevel.Thefilter usedisnotrecordedin thedatastructure.

Outputpulsesaredirecteddownwardandscannedacrosstheaircraftflight directionby meansof a

galvanometer-drivenscanassembly.Thecomputer-controlledgalvanometerrotatesa smallmirrorto a

specifiedangularpositionprior to initiationof eachlaserpulsefire command.Laserpulseenergythat

is backscatteredfrom areflectingsurfaceat0° phaseangle(i.e.,thehotspotorientation)iscollectedby

areceivertelescopeandfocusedontoa50-MHzSi:APDthatconvertsopticalenergyintoanoutput

voltage.

Theoutputvoltagefrom thedetectoris splitbetweenaTIU andananalog-to-digitaldigitizer.The

TIU preciselymeasuresthetimeintervalbetweenthetransmittedlaserpulseandthereceived

backscatteredpulsebasedonaleading-edge,threshold-detectionscheme.TheTIU containsa

high-frequencyinternaloscillatorthatyields156.25picosecondtimingresolutionof thepulse

round-triptraveltime,equivalento aone-wayrangeresolutionof 2.3cm.A start-pulsefiberoptic

transfersasmallportionof thetransmitlaserpulseenergyintothereceivertelescope,whereit is

focusedontotheSi:APDdetector.TheTIU startswhentheleadingedgeof thetransmitpulserises

aboveastart-pulsethresholdvalue,whichis establishedbytheinstrumentoperator.TheTIU stops

whentheoutputvoltageof thedetectorisesaboveastop-pulsedetectionthreshold,whichis also

establishedby theinstrumentoperator.Thedetectoroutputvoltagecanexceedthestop-pulsedetection

thresholddueto avalid returnfrom areflectingsurfaceor dueto instrumentelectronicnoiseand/or

backgroundsolarillumination.A rangegateisusedto definearangingtimeoverwhichthestop-pulse

detectionthresholdwill beapplied.A signalgreaterthanthedetectionthresholdoccurringbeforeor

aftertherangegatedoesnotstoptheTIU. Therangegateisestablishedby theoperatortobracketthe

distanceto theanticipatedtarget(e.g.,landsurface),giventheknownflight altitudeabovetheground.

In somecases,nodetectoroutputvoltageabovethethresholdoccurswithin therangegate,yieldinga

no-rangeresult.Thestop-pulsedetectionthresholdandrangegateareestablishedto maximizethe

frequencyof rangingto validreturnsandminimizethenumberof noiseandno-rangeresults.Valid

returnsarereceivedfrom thefirst encounteredsurfacethatbackscatterssufficienttransmittedlaser

energyto exceedthestop-pulsedetectionthreshold.Thiscanbeanopticallydensecloudlayer,a

vegetationcanopytop,baregroundin unvegetatedareas,or aniceorwatersurface.

Upondetectionof a signalabovethestop-pulsethreshold,theTIU measureof thestartto stoptime

intervalis recorded,whichcorrespondsto theround-triptraveltimeof thetransmittedlaserpulse.A

valueof zeroisrecordedfor no-rangeresults.In additionto TIU rangingto thefirst detectedreturn,

SLICERimplementsasurfacelidarfunctionby digitizingthetimehistoryof theenergybackscattered

from thesurface.Upondetectionof thestoppulse,aconstantnumberof binsarereadfrom the

digitizer,yieldingawaveformfor thatlasershotthatrecordsreturnamplitudeasafunctionof time.

Thedigitizercontinuouslysamplesthedetectoroutputandscalesthevoltageto an8-bitvalue.A

constantoffsetis addedafterscalingto displacethedetectormeanbackgroundnoiselevel,whichis

zero-referenced, to yieldall positivevalues.Thescalefactorin theanalog-to-digitalconversionandthe

offsetappliedtypicallyremainconstantduringaSLICERflight mission,butcouldvarybetweenflight

missions.Upondetectionof theTIU stopevent,a specifiednumberof binsarereadfrom thedigitizer

andstored,yieldingawaveformthatrecordsthereturnamplitudeasafunctionof time.Formost

SLICERacquisitions,includingBOREAS,600digitizerbinswerestored.

Theverticalresolutionof thebackscatterwaveformis afunctionof theduration(i.e.,pulsewidth)

of thetransmittedlaserpulseandthebandwidth(i.e.,responsetime)of thedetectorusedto convertthe

backscatterenergyinto ananalogsignal.Verticalresolutionrefersto theseparationrequiredbetween

surfacesin orderto distinguishbackscatterenergyreflectedfrom eachof thesurfaces.Theconvolution

Page10
of thetransmitpulseshapein timewith thebroadeningcausedby thedetectoresponse(definedby its

bandwidth)yieldsanimpulseresponse,theoreticallythenarrowestpossiblereturnsignalforreflection

from aperfectlysmooth,flat surface.The4-nsec(full-width athalf-max)SLICERpulsewidth and

50-MHzdetectorbandwidthcombineto yield animpulseresponsepulsewidth of approximately6

nsec(full-width athalf-max),equivalentto awidth of 1.8m.Note,however,thatthewidth of the

impulseresponseincreaseswith increasingopticalenergyreceived(i.e.,asthepeakamplitude

increases).Theverticalresolutionof thesystemis one-halftheimpulseresponse(0.9m), becausethe

traveldistanceof thelaserpulsedown-and-backbetweentwo surfacesis twicetheirvertical

separation.

Theverticalsamplingof thebackscatterwaveformis afunctionof thesamplingrateof thedigitizer

usedtoconverttheanalogsignalto adigitalrecord.Thedigitizersamplesthedetectoroutputvoltageat

arateof 1.348GHz,with aresultingtemporalsamplingof 0.742nsec.Becausethedigitizeris

recordingthetwo-waytransittimeof light (e.g.,downthroughacanopyandbackout),thevertical

samplingdistanceishalf thetemporalsampling(i.e.,0.371nsec),equivalento 0.1112m per

waveformbin.Thewaveformisnormally storedat thefull digitizerbin resolution(0.1112m). In

somecases,digitizerbinsareaveragedtogetherbeforestoringin thewaveformarray,yieldinga

waveformsamplingdistanceincreasedby anintegeraveragingfactor.No digitizeraveragingwas

appliedto theBOREASdata.

Thebinsreadfrom thedigitizercorrespondin timeto arangeof binsreferencedto theTIU stop

event,with thestopeventassignedin theacquisitionsoftwareto a specificwaveformbin position.For

BOREAS,thatpositionis bin 28,usinganarray-labelingconventionwheretheinitial positionisbin 0.

Bins0 to 27of thewaveformcorrespondto thedetectoroutputvoltagerecordedprior to thestopevent

(i.e.,backgroundnoisebeforethestartof thereturn).Theremainingbinsfrom theTIU stopeventto

theendof thewaveform(bins28to 599)correspondto aheightrangeof 63.6064m (572binsx

0.1112m/bin),whichis significantlygreaterthantheheightof thecanopiesstudied.Thus,the

waveformextendsbeyondthedistanceatwhichthegroundreturnis expected.Thelastportionof the

waveform,therefore,correspondsto backgroundnoiseoccurringafteranygroundreflection.The

digitizerimplementationin SLICERis flexible,permittingadjustmentin thenumberof totalbins,

distanceresolutionperbin,andbin positionof theTIU stopevent.Thevaluesreportedhereare

specificto theBOREASdataset.Appropriatedigitizerfactorsfor otherdatasetsarecontainedin the

headerecordof eachLevel3 datafile.

Thelocationof thelaserfootprint,referencedto thefirstdetectedreflection,is determinedby

combiningtheTIU rangingdatawithknowledgeof thelaserpointingangleandtheabsoluteposition

of theaircraftderivedfrom aGPStrajectory.Thelaserpointingangleis determinedby combiningthe

scanmirror anglewith ameasurementof instrumentorientation,obtainedusingaLitton LTN-92INS.

TheINSreportsroll, pitch,andyawparameterscomputedfrom gyroscope-determinedaccelerationsat

arateof 64HzTheINSdatastreamis sampledby thealtimetrydatasystematthetimeof eachlaser

pulsefire, andtheroll, pitch,andyawdataarerecordedasapartof thealtimetrydatarecordfor each

shot.Theaircrafttrajectoryisdeterminedby postflightprocessingof 2-HzdifferentialkinematicGPS

dataemployingGPSreceiversontheaircraftandatafixedbasestation.AshtechZ-12dual-frequency

receiverswereusedfor this study.

4.1.6 Sensor/Instrument Measurement Geometry

The GPS avionics antenna is mounted on top of the aircraft fuselage on the a_'rame centerline. The

x, y, and z offsets from the GPS antenna to the laser altimeter receiver are established with a

measuring tape. For BOREAS, the base station GPS receiver was installed over a permanent survey

mark selected by the GPS technician (Earl Frederick, NASA GSFC EG&G contractor) near the Prince

Albert Airport runway. The following published position for the survey mark, in WGS84 coordinates,

was used to compute the trajectories: 53 ° 12" 59.160480' N, 105 ° 39" 37.141840', elevation

402.6360 meters. The GPS tripod and antenna were reinstalled over the survey mark for each flight,

and the height of the antennae above the mark was measured on each flight day.

The laser transmitter, receiver, and INS are all mounted on an optical bench in order to maintain

alignment between these devices. The optical bench is mounted inside the aircraft above an

optical-quality, fused-silica window that is 17 inches in diameter. The laser transmitter, mounted on

Page 11
topof theopticalbench,generatesahorizontallytransmittedlaserpulse.Spatiallyeachlaserpulseis

circularin cross-sectionwith aGaussiandistributionof energy.Thedivergenceof thelaserbeam

rangedfrom approximately2 mrad,definedastheangleatwhichthepulseenergyhasfallenoff to

l/e2 times(13.5%)thatatthebeamcenter.Temporally,thelaserpulsehasaRaleighdistributionwith

afastleading-edgerisetime,ontheorderof 1.5nsec,andatrailing-edgethatfallsoff moreslowly

asymptotically.

SLICER 5-beam Swath Geometry

l

rfov Sa

d = footprint diameter= tan(laser divergence) x altitude above ground

W = swath width = cross-track spacing x cross-track footprints

S c = cross-track spacing = tan(scanner step angle _ aircraft roll) x altitude above ground

Sa= along-track spacing = ground speed/(laser rep rate / cross-track footprints)

r = roll-induced swath displacement = tan(arcraft roll) x altitude above ground

11:0",/= receiver ground field-of-qew = tan(telescope angular fov) x altitude above ground

Figure 6: SLICER 5-beam Swath Geometry

The transmit beam scan assembly is attached to the laser transmitter box at the exit aperture of the

laser beam. Lenses can be installed in a mount in the scan assembly that causes the laser beam to

diverge more than the nominal 2-mrad transmit divergence. Lenses producing beam divergence of

approximately 3 mrad, 8 mrad, and 10 mrad are available. All but one flight line at BOREAS were

acquired using the standard divergence of approximately 2 mrad. One flight line flown at a low altitude

(line 8 on 29-Jul) was acquired using the lens yielding a divergence of approximately 10 mrad. The

horizontally transmitted laser pulses are reflected downward in the scan assembly and pass through a

tube that extends from the scan assembly to the window. The laser pulses are scanned in a direction

perpendicular to the aircraft flight direction by means of a small mirror that is rapidly rotated to fixed

angular positions using a galvanometer. The laser transmitter is triggered by computer control to fire a

single laser pulse after the mirror is rotated to a specified position. For BOREAS, five laser pulses

were typically fired for each cross-track scan (Figure 6), although for two flight lines the galvanometer

was held fixed at nadir, yielding a single-beam profile (lines 6 and 8 on 29-Jul). The resulting pattern

of laser footprints on the ground depends on their size and along-track and cross-track spacing (Figure

6). The footprint size depends on the laser divergence and aircraft altitude above ground. The nominal

Page 12
flight altitudeabovegroundatBOREASwas4500m, yieldingfootprintsnominally9m in diameter.

Cross-trackfootprint spacingis determinedby theangularseparationbetweensuccessivetransmitted

pulsesandaircraftaltitudeaboveground.Theangularseparationisdeterminedby theprogrammed

angularpositionsof themirror andaircraftroll. Forthiswork,thegalvanometerwascontrolledto

yield footprintsthatarecontiguousacrossthetrackduringlevelflight (i.e.,2-mradcross-trackangle

yielding9-mcross-trackspacing).However,low-frequencyroll excursionsof severaldegreesduring

dataacquisition,combinedwiththehigh-frequencygalvanometerscanning,makethecross-track

patternof footprintsnon-uniform.Along-trackspacingisdeterminedby laserpulserepetitionrate,

numberof cross-trackfootprints,aircraftgroundspeed,andchangesinpitch attitude.Changesin pitch

weresmallandthushaveaninsignificanteffectonalong-trackspacing.With five cross-track

footprintsandalaserpulserepetitionrateof 80Hz,thetypicalgroundspeedresultedin anominal

along-trackspacingof approximately10m.However,wind speedanddirectionattheaircraftaltitude

affectsgroundspeedandtheresultingalong-trackspacing.

Laserilluminationbackscatteredfromreflectingsurfaceswithin eachfootprintiscollectedin a

receivingtelescopethathasanaperturediameterof 20cm anda 10mradfield-of-view(FOV).The

telescopehousingextendsdownwardfrom opticalbenchtojust abovethewindow.Theamountof

backscatterenergycollectedby thetelescopedependsonits apertureandthedistanceto thereflecting

target,decreasingwith thesquareof thedistanceassumingLambertianscatteringfrom thetarget.The

five contiguous,cross-trackfootprints(eachhaving2-mraddivergence)werealignedto fit acrossthe

10-mradreceiverFOV.Thereceivingtelescope'sopticalefficiencyis uniformacrossmostof theFOV

butdeclinesattheedgesby approximately15%,sothebackscatterenergycollectedfortheouterscan

footprintsis slightlylowerthanfor thecentralfootprints.

Thebackscatteredlaserlight andanybackgroundreflectedsolarilluminationcollectedby the

receivertelescopeisfocusedthroughabandpassfilter ontoa 1-mm-diameterSi:APD.Thebandpass

filter significantlyreducesbackgroundsolarilluminationby onlypermittingtransmissionof light in a

narrow,2-nm-widewavelengthrangecenteredatthelaserwavelength.TheSi:APDdetectorconverts

input opticalenergyintoanoutputvoltage.Thehighbandwidthof thedetector(50MHz) achieves

temporalresolutionof thebackscatteredlaserenergythatis nearlyequalto thedurationof the

transmittedpulse.Thecombinationof narrowlaserpulsewidth andhigh detectorbandwidthyieldsa

verticalresolutionof approximately0.9m,whereverticalresolutionrefersto theseparationrequired

betweensurfacesin orderto distinguishbackscatterenergyreflectedfrom each.

4.1.7 Manufacturer of Sensor/Instrument

The SLICER system was designed and integrated by personnel at NASA GSFC. Bryan Blair

(Code 924, (301) 614-6741, bryan@avalon.gsfc.nasa.gov) led the development of the SLICER

system, adapting the Airborne Topographic Laser Altimeter System (ATLAS), a profiling laser

altimeter, developed by Jack Bufton (Code 920, (301) 286-8591, jbufton@pop900.gsfc.nasa.gov).

ATLAS was first modified by the addition of a new laser transmitter and the waveform acquisition

capability (Blair et al. 1994), and subsequently by the addition of the scanning capability.

The manufacturers of the principal components in SLICER are:

Laser transmitter:

NASA GSFC, Barry Coyle, lead

Code 920, (301) 286-0411, bcoyle@pop900.gsfc.nasa.gov

Scan mechanism:

NASA GSFC, Bryan Blair, lead

Code 924, (301) 614-6741, bryan@avalon.gsfc.nasa.gov

Galvanometer in scan mechanism:

Cambridge Technology, Inc.

109 Smith Place

Cambridge, MA 02138

(617) 441-0600

Page 13
Receivertelescope:

NASA GSFC,JackBufton, lead

Code920,(301)286-8591,jbufton@pop900.gsfc.nasa.gov

Detector:

EG&G, Inc.

45William Street

Wellesley,MA 02481

(781)237-5100

Timingelectronicsandwaveformdigitizer:

LeCroyCorporation

700ChestnutRidgeRd

ChestnutRidge,NY 10977

(800)553-2769

InertialNavigationSystem:

Litton Industries,Inc.

21240BurbankBoulevard

WoodlandHills, CA 91367-6675

(818)598-5000

GPSReceivers:

MagellanCorp./AshtechPrecisionProducts

471E1CaminoReal

SantaClara,CA 95050-4300

(408)615-5100

GPSto altimeterclockbox:

NASA GSFC,JohnWard,lead

Code571,(757)824-1010

4.2 Calibration

Calibration of the SLICER system includes measurement of laser pulse divergence and temporal

quality, time and angle biases between the INS unit and the laser transmitter, a system delay range

bias, and laser pulse transmit energy.

Precise measurement of far-field laser beam divergence is difficult. For the SLICER laser

transmitter, divergence was measured in the laboratory for the as-transmitted divergence

(approximately 2 mrad), and with lenses in place that are used to increase the divergence (to

approximately 3, 8, and 10 mrad). Several standard divergence measurement methods were used,

yielding inconsistent results; the reported divergence values are accurate only at the 10% level.

Temporal beam quality (pulse shape and duration) is calibrated by measurement in the laboratory

using high-speed detectors and an oscilloscope. Components of the laser transmitter are individually

aligned in order to minimize the pulse width (at approximately 4 nsec full-width at half-max) and to

yield a Raleigh distribution of energy with time having a minimal 'back porch' effect (see Section

11.2).

INS-to-altimeter time and attitude biases are calibrated using roll and pitch maneuver data (typically

+/- 5 degrees for each axis) acquired in-flight over water, which provides a fiat, level reference

surface. Small lakes in or near the SSA and NSA were utilized in the case of BOREAS. The roll and

pitch maneuvers cause systematic variations in the ranging distance to the water surface. Biases are

found by iteratively varying each until the resulting elevation for the water derived from the geolocation

process is level (exclusive of wave structure). The degree to which the water surface is level in the

geolocation results is qualitatively assessed by visually examining elevation profiles. Time delays

between firing of a laser pulse and the recorded roll, pitch, and yaw parameters are established first.

Page 14
Thetimedelaysfor eachof theattitudeparametersconsistsof two components,anINSinstrument

reportingdelayandasynch-updelaybetweentheconstantINSdatarateandthesoftware-controlled

laserfire rate.TheINSinstrumentreportingdelayis constantfor eachparameterandis determinedby

applyingatimeoffsetthatyieldslevelwater.Thesynch-updelayschangebetweeneachdatafile and

aredeterminedby aniterativeshiftingandinterpolationof therecordedattitudeparameterswithrespect

to thelaserfire timesthatis doneautomaticallyasapartof thegeolocationprocessing.Roll andpitch

alignmentbiasesarealsoestablishedby iterativelyvaryingeachuntil asufficientlylevelwatersurface

results.Thegeolocationresultsareinsensitiveto smallerrorsin yaw,sono yawbiasis establishedor

appliedto theattitudedata.

TheTIU rangedataarecorrectedfor arangebiasdueto asystemtiming delaycausedby optical

andelectronicsignaldelays,whichvarybetweeninstallationsof theinstrument.Thesystemdelayis

foundby solvingfor theelevationof asurfaceataknowndistance.Typically,dataacquiredin aflight

acrossasitewith anestablishedgeodeticbenchmarkareused,suchasthelocationof theGPSbase

station,yieldinganend-to-endsystemdelaymeasurement.Thesystemdelayestablishedfor the

BOREASinstallationwas15.4m.

Thetransmitenergyperpulsewasmeasuredin alaboratoryprior to installationontheaircraft

usinganexternalenergymeter.Thetransmitenergyperpulseisnominally0.7mJ.However,the

transmitenergyjitters from shotto shotby about10%dueto randomeffects.Duringflight, thelaser

outputenergypershotis monitoredby aninternalphotodiodedetectorandrecordedin thedata

structure,but thenoiselevelonthismonitoris veryhigh(asmuchas50%of signal),greatly

exceedingtheactualenergyvariationdueto shot-to-shotjitter.

In additionto randomshot-to-shotjitter, laboratoryresultsdocumenthatthelaseroutputenergy

variessignificantlyasafunctionof lasertemperature.Thetransmitteroperatesmostefficientlywhen

thelaserdiodesareat30°C;belowandabovethistemperature,thetransmitterislessefficientand

pulseenergyis reduced.Excessheatis generatedduringoperationof thetransmitter,soathermal

managementsystem(thermalelectriccoolers,copperheatstrappingto aheatsink,andfans)is usedto

removetheexcessheat,maintainingthediodesat ornear30°C.Theshotenergymonitordoesshow

long-termvariationsduringthecourseof aflight lineacquisitionthatislikely dueto changesin laser

transmittertemperature.A fairly rapidincreasein laserenergyoverseveralminutesatthestartof an

acquisitionis likely dueto thediodetemperatureincreasingfromambientconditionsupto 30°C.A

gradualdecreasein shotenergythenoftenoccurs,probablyasthediodetemperatureslowlyexceeds

30°C.However,theobservedvariationsmayalsobeartifactsdueto thermallyinducedmechanical

flexureof thephotodiodedetectormount.In addition,mechanicalinstabilityof thepulseenergy

monitorhasbeenobserved,whereaircraftattitudevariations(especiallyduringpitchcalibration

maneuvers)causesignificantvariationin therecordedpulseenergylevels.Also,in someinstancesthe

long-termtrendin lasertransmitenergyis abruptlyoffsetin amplitude.Theamplitudeoffsetis

constantandappliesto large,continuoussegmentsof lasershots(100'sto 1000'sof shots).The

abruptoffsetsareprobablynotactualchangesin transmitenergy.Thecauseof theoffsetis notknown

butis likely dueto againorbiasshiftin thestartpulseenergydetectionelectronics.

4.2.1 Specifications

Thewaveformdigitizer samplingrate,definingthe0.1112 m vertical sampling per waveform bin,

has not been independently calibrated. Factory specification of the digitizer is relied on for this value.

Factory specifications for the LTN92 INS unit are a sampling resolution of 0.1 mrad and a drift

rate of 0.17 mrad per axis per hour during operation.

4.2.1.1 Tolerance

Due to problems detailed above, the laser pulse energy measure contained within the Level 3 data

structure should be considered unreliable. Furthermore, the intensity of the transmit pulse is modulated

by the use of filters in order to achieve a backscatter energy below the saturation limit of the receiver

detector. Filters are manually inserted into the transmit beam path by the instrument operator in order to

optimize the return signal level. The filter is inserted in the beam path after the position of the internal

pulse energy monitor, and the filter used is not recorded in the data structure. Therefore, the pulse

energy measure in the data structure is both unreliable and incomplete.

Page 15
4.2.2 Frequency of Calibration

Beam divergence has been measured only one time for the SLICER instrument, in 1995, due to the

difficulty of the measurement. Beam temporal quality and transmit energy are typically measured

immediately prior to installation of the instrument in the aircraft, as was done for BOREAS.

Roll and pitch maneuvers over water are normally conducted at least once on each flight day. The

INS reporting delay remained constant as expected during the BOREAS deployment. Ideally,

alignment biases should also remain constant for the duration of an installation. However, small

alignment bias variations have been observed from day to day, and even during a single flight, so roll

and pitch biases specific for each flight day are used in the geolocation processing. The cause of the

alignment bias variations is unknown, but is likely related to the unexplained drift between the transmit

pulses and receiver FOV (see Section 11.1) and/or to temporal drifts in the INS attitude solution.

Flights across the GPS base station to establish the range bias due to the system delay were

conducted on several days at BOREAS, but only one flight yielded a SLICER ground track close

enough to the base station to be used to establish the bias.

4.2.3 Other Calibration Information

The LTN92 INS unit is maintained by the Aircraft Operations Branch at WFF. It is periodically

returned to the manufacturer for recalibration on a routine maintenance schedule.

5. Data Acquisition Methods

A SLICER flight mission typically consists of the following steps:

• Collect GPS data at both the fixed base station and onboard the aircraft while the aircraft is not

moving for a period of approximately 30 minutes, in order to provide static differential data to

initialize the forward kinematic solution.

• Initialize the INS unit while the aircraft is not moving.

• After takeoff, fly over a water surface at approximately 10,000 ft in level flight to align the

laser footprints within the receiver FOV by manually adjusting mirror mounts to maximize the

return energy for each footprint (returns off of a water surface are normally used as it provides

a flat reference target with uniform albedo).

• Acquire data over a water surface at approximately 10,000 ft, conducting a sequence of roll

maneuvers and pitch maneuvers (typically +/- 5 degrees for each axis) to be used in data

postprocessing to establish time and angle biases between the laser altimeter and INS unit.

• Acquire data across the target sites, typically at an altitude of 17,500 ft (approx. 5,000 m)

above mean sea level, using a flight management system that displays aircraft position

(obtained from a GPS receiver) relative to a programmed flight line (at BOREAS the desired

flight line was defined by preprogrammed coordinates for the tower positions and a flight

orientation through a tower site established in-flight by ASAS requirements for a specified

azimuth with respect to the solar principal plane); data acquisition is typically initiated several

minutes prior to reaching the target location and terminated several minutes after passing the

target (flight time to a target location is reported by the flight management system).

• Prior to landing, acquire data across the GPS base station location to be used in data

postprocessing to establish the system range bias.

• After landing, collect GPS data at both the fixed base station and onboard the aircraft while the

aircraft is not moving for a period of approximately 30 minutes, in order to provide static

differential data to initialize the backward kinematic solution.

Due to time constraints imposed by ASAS acquisition requirements, steps 3 through 6 were not

necessarily completed in that order nor were all those steps always completed for every flight mission.

Page 16
6. Observations

6.1 Data Notes

Data processing notes for production of the Level 3 data for each flight line are included on the

Level 3 CD-ROMs in day-specific subdirectories in the Procdata directory.

The file fluxtowr.sum in the Documnts directory on the SLICER Level 3 CD-ROMs contains a

summary that lists, for each SLICER flight line, the BOREAS TF site(s) crossed by that flight line,

and the approximate laser shot number in the data record closest to the tower.

6.2 Field Notes

Instrument operator log notes for each flight day are included on the SLICER Level 3 CD-ROMs in

the Documnts directory.

7. Data Description

7.1 Spatial Characteristics

7.1.1 Spatial Coverage

A SLICER flight line typically consists of a 45-m-wide swath composed of five cross-track laser

footprints (Figure 6). For BOREAS acquisitions, the length of each flight line varies, but is typically

about 40 kin. In a few cases, profile data were acquired (no cross-track scanning of the footprints).

Flight lines across all TF sites in the SSA and NSA were acquired, with a minimum of 3 lines and a

maximum of 10 lines per site. Data were also acquired on transect lines between the study areas, and

for calibration purposes. The North American Datum of 1983 (NAD83) coordinates for the tower sites

are:

Site Longitude Latitude

NSA-BVP 98

NSA-FEN 98

NSA-OBS 98

NSA-OJP 98

NSA-YJP 98

SSA-FEN 104

SSA-OBS 105

SSA-OJP 104

SSA-YJP 104

SSA-9OA 106

SSA-9YA 105

.02747 ° W

42072 ° W

48139 ° W

62396 ° W

28706 ° W

61798 ° W

11779 ° W

69203 ° W

64529 ° W

19779 ° W

32314 ° W

55 84225 °

55 91481 °

55 88007 °

55 92842 °

55 89575 °

53 80206 °

53 98717 °

53 91634 °

53 87581 °

53 62889 °

53 65601 °

7.1.2 Spatial Coverage Map

The SLICER Level 3 CD-ROMs contain summary flight line location maps for each study area and

detailed maps for each tower site, as well as summary plots of each flight line that include a location

map.

7.1.3 Spatial Resolution

Each illuminated laser footprint is circular and is nominally 9 m in diameter. The cross-track

spacing between footprints is variable, but is nominally 9 m. The along-track spacing is also variable,

but is nominally 10 m. See Section 4.1.6 for a discussion of parameters that influence footprint

diameter and spacing.

Page 17
7.1.4 Projection

Eachlaserfootprintis individuallygeoreferencedwith latitude,longitude,andelevationto the

World GeodeticSystemof 1984(WGS84)ellipsoid.Latitudeandlongitudein theWGS84ellipsoid

referenceframeareessentiallyidenticalto thosereferencedtotheNAD83ellipsoid.Thetwocanbe

consideredto beidenticalfor all practicalpurposes.

7.1.5 Grid Description

Not applicable.

7.2 Temporal Characteristics

7.2.1 Temporal Coverage

SLICER data were acquired on five flight days between 18-Jul and 30-Jul-1996.

7.2.2 Temporal Coverage Map

Data were acquired for NSA sites on 18-Jul and 24-Jul; for the SSA sites on 20-Jul, 29-Jul, and

30-Jul; and on transects between the study areas on 18-Jul and 24-Jul-1996.

7.2.3 Temporal Resolution

In SLICER's typical operating mode, the laser pulse repetition rate is 80 pulses per second. Each

laser pulse yields a unique observation. The time between flight lines across a tower site on a flight day

varied between tens of minutes and several hours.

7.3 Data Characteristics

7.3.1 Parameter/Variable

The parameters in the SLICER data files (.dat and .edt extensions) in the Procdata and Sitedata

directories on the SLICER CD-ROMs are:

header record

TIU BIN

DIG2WF

WVFM BINS

NUMSHOTS

(one per file):

data record

SHOTNUM

BEAM

STARTEN

GPSTIME

DIAMETER

AZIMUTH

INCLINATION

LATITUDE

LONGITUDE

ELEVATION

GRNDSTART

GRNDPEAK

GRNDEND

WAVEFORM

(one per laser pulse):

Page 18
Theparametersin theaircrafttrajectoryfiles (.trj extension)in theGps_miscdirectoryontheSLICER

CD-ROMsare:

header record (one per file) :

OBSERVATIONS

data record (one per GPS epoch at a rate of two per second, i.e. 2 Hz) :

GMTTIME

LATITUDE

LONGITUDE

ALTITUDE

SERVICES

PDOP

RMS

FLAG

The BOREAS CD-ROM set does not include SLICER data, due to space limitations, but instead

contains inventory files that document the SLICER coverage. The parameters contained in the SLICER

inventory files on the BOREAS CD-ROM set are:

Column Name

SPATIAL COVERAGE

DATE OBS

START TIME

END TIME

RUN START LAT

RUN START LONG

RUN END LAT

RUN END LONG

NUM SHOTS

CRTFCN CODE

7.3.2 Variable Description/Definition

The definitions of the parameters in the SLICER data files (.dat and .edt extensions) in the Procdata

and Sitedata directories on the SLICER CD-ROMs are:

header record:

TIU BIN waveform bin corresponding to TIU stop-trigger event using

an array-labeling convention where the initial bin is 0

number of digitizer bins averaged together to form a waveform bin

number of bins per waveform

number of data records in the file data record:

laser shot number from the Level 2 .geo source file

cross-track beam position:

1 (left) to 3 or 5 (right) in scan mode; 0 in profile mode

energy of transmitted laser pulse

time of laser pulse transmission past GPS midnight

i/e2 diameter of footprint : range x laser divergence

azimuth of the laser pointing vector from north; 0 to 360

clockwise

INCLINATION inclination of the laser pointing vector from horizontal;

90 : vertical

LATITUDE** latitude of the footprint at highest detected surface;

DIG2WF

WVFM BINS

NUMSHOTS

SHOTNUM

BEAM

STARTEN

GPSTIME

DIAMETER*

AZIMUTH

Page 19
LONGITUDE**

ELEVATION

GRNDSTART

GRNDPEAK

GRNDEND

WAVEFORM

degrees north in WGS84 ellipsoid

longitude of the footprint at highest detected surface;

degrees east in WGS84 ellipsoid

elevation of the highest detected surface within laser footprint

with respect to WGS84 ellipsoid

distance along laser vector from elevation to ground return start

distance along laser vector from elevation to ground return peak

distance along laser vector from elevation to ground return end

laser pulse backscatter return amplitude per waveform bin;

a bin corresponds to a distance of 0.1112 m x DIG2WF

* NOTE: The laser footprint diameter is computed as the tangent of the laser

divergence times the ranging distance to the target. A divergence of 2

mrad was inadvertently used for all files, causing an error for those flight

lines where a greater divergence was used. See Section 11.2 for a list of

those files and the appropriate correction that must be applied.

** NOTE: Latitude and Longitude in the WGS84 ellipsoid reference frame are

essentially identical to those referenced to NAD83. The two can be considered

to be identical for all practical purposes.

The definitions of the parameters in the aircraft trajectory files (.trj extension) in the Gps_misc

directory on the SLICER CD-ROMs are:

header record:

OBSERVATIONS number of data records in the file

data record:

GMTTIME

LATITUDE

LONGITUDE

ALTITUDE

SERVICES

PDOP

RMS

FLAG

time of the observation past Greenwich Mean Time (GMT) midnight

latitude of the GPS antenna phase center; degrees north in

WGS84 ellipsoid

longitude of the GPS antenna phase center; degrees east in

WGS84 ellipsoid

altitude of the GPS antenna phase center with respect to WGS84

ellipsoid

number of services (GPS satellites) observed

position dilution of precision, a figure of merit for the

angular distribution of GPS satellites (low values are better)

one sigma position error

flag for solution acceptability

The descriptions of the parameters contained in the SLICER inventory files on the BOREAS CD-ROM

ore:

Column Name Description

SPATIAL COVERAGE

DATE OBS

START TIME

END TIME

RUN START LiT

The general term used to denote the spatial area over

which the data were collected.

The date on which the data were collected.

The starting GPS time for the data collected

(see Section 11.3 for GPS to GMT difference

The ending GPS Time for the data collected

(see Section 11.3 for GPS to GMT difference

The WGS84 latitude coordinate at the beginning of the

data line.

Page 20
RUN START LONG

RUN END LAT

RUN END LONG

NUM SHOTS

CRTFCN CODE

The WGS84 longitude coordinate at the beginning of the

data line.

The WGS84 latitude coordinate at the end of the data line.

The WGS84 longitude coordinate at the end of the data

line.

The number of laser pulses for the data line.

The BOREAS certification level of the data. Examples are

CPI (Checked by PI), CGR (Certified by Group), PRE

(Preliminary), and CPI-??? (CPI but questionable) .

7.3.3 Unit of Measurement

The parameters in the SLICER data files (.dat and .edt extensions) in the Procdata and Sitedata

directories on the SLICER CD-ROMs are all stored as signed long integer or byte values. After

appropriate scaling (see Section 8.2) the units of measurement are:

header record:

TIU BIN unitless

DIG2WF unitless

WVFM BINS unitless

NUMSHOTS unitless

data record:

SHOTNUM

BEAM

STARTEN

GPSTIME

DIAMETER

AZIMUTH

INCLINATION

LATITUDE

LONGITUDE

ELEVATION

GRNDSTART

GRNDPEAK

GRNDEND

WAVEFORM

unitless

unitless

uncalibrated integrator counts

seconds

meters

degrees

degrees

decimal degrees

decimal degrees

meters

meters

meters

meters

uncalibrated digitizer counts

The unit of measurement for the parameters in the aircraft trajectory files (.trj extension) in the

Gps_misc directory on the Level 3 CD-ROMs are:

header record:

OBSERVATIONS unitless

data record:

GMTTIME seconds

LATITUDE decimal degrees

LONGITUDE decimal degrees

ALTITUDE meters

SERVICES unitless

PDOP unitless

RMS meters

FLAG unitless

Page 21
The measurement units for the parameters contained in the SLICER inventory files on the BOREAS

CD-ROM are:

Column Name Units

SPATIAL COVERAGE [none]

DATE OBS [DD-MON-YY]

START TIME [HHMM GMT]

END TIME [HHMM GMT]

RUN START LAT [decimal degrees]

RUN START LONG [decimal degrees]

RUN END LAT [decimal degrees]

RUN END LONG [decimal degrees]

NUM SHOTS [count]

CRTFCN CODE [none]

7.3.4 Data Source

The SLICER instrument is the source of all of the parameters in the data files (.dat and .edt

extensions) in the Procdata and Sitedata directories on the SLICER CD-ROM.

Output from Ashtech Precise Differential GPS Navigation and Surveying software (PNAV),

applied to the GPS differential data, is the source of the parameters in the aircraft trajectory file (.trj

extension) in the Gps_misc directory on the SLICER CD-ROM.

Other than the CRTFCN_CODE parameter, the source of the parameters contained in the SLICER

inventory files on the BOREAS CD-ROM is the SLICER CD-ROMs supplied by the LAPF. The

CRTFCN_CODE was assigned by the BOREAS Information System (BORIS).

7.3.5 Data Range

The range of valid values in the SLICER data files (.dat and .edt extensions) in the Procdata and

Sitedata directories on the SLICER CD-ROMs are:

header record:

TIU BIN always 28 for BOREAS; up to 200 for all SLICER data

DIG2WF always 1 for BOREAS; 1 or 2 for all SLICER data

WVFM BINS always 600 for BOREAS; up to 1200 for all SLICER data

NUMSHOTS 0 to 50,000 for BOREAS; up to 150,000 for all SLICER data

data record:

SHOTNUM 0 to 50,000 for BOREAS; up to 150,000 for all SLICER data

BEAM 0 to 5

STARTEN 0 to 255

GPSTIME 0 to 86,400

DIAMETER 0 to 12 for BOREAS; up to 90 for all SLICER data

AZIMUTH 0 to 360

INCLINATION 0 to 90

LATITUDE -90 to 90

LONGITUDE 0 to 360

ELEVATION 0 to 700 for BOREAS; -105 to 4,500 for all SLICER data

GRNDSTART 0 to 63.7 for BOREAS; 0 to 132.4 for all SLICER data

GRNDPEAK 0 to 63.7 for BOREAS; 0 to 132.4 for all SLICER data

GRNDEND 0 to 63.7 for BOREAS; 0 to 132.4 for all SLICER data

WAVEFORM 0 to 254

The above are the range of physically valid numbers, not necessarily the range of numbers that actually

appear in the data set.

Page 22
The actual range of values in the aircraft trajectory files (.trj extension) in the Gps_misc directory on

the SLICER CD-ROM are:

header record:

OBSERVATIONS 36,743 to 71,175

data record:

GMTTIME

LATITUDE

LONGITUDE

ALTITUDE

SERVICES

PDOP

RMS

FLAG

0.0 to 86,399.50

52.993773 to 56.443220

253.35387 to 262.35518

181.180 to 5245.6430

0 to 9

0 to 99.9

0.02 to 14.31

1 to 1

The following table gives information about the parameter values in the SLICER inventory files on the

BOREAS CD-ROM:

Minimum Maximum Missng Unrel Below Data

Data Data Data Data Detect Not

Column Name Value Value Value Value Limit Cllctd

SPATIAL COVERAGE N/A N/A None None None None

DATE OBS 18-JUL-96 30-JUL-96 None None None None

START TIME 1533 2153 None None None None

END TIME 1541 2228 None None None None

RUN START LAT 53.07728 56.22971 None None None None

RUN START LONG -106.56928 -98.01645 None None None None

RUN END LAT 53.10713 56.06671 None None None None

RUN END LONG -106.31541 -98.15718 None None None None

NUM SHOTS i001 49992 None None None None

CRTFCN CODE CPI CPI None None None None

Minimum Data Value -- The minimum value found in the column.

Maximum Data Value -- The maximum value found in the column.

Missing Data Value -- The value that indicates missing data. This is used to

indicate that an attempt was made to determine the

parameter value, but the attempt was unsuccessful.

Unrel Data Value -- The value that indicates unreliable data. This is used

to indicate an attempt was made to determine the

parameter value, but the value was deemed to be

unreliable by the analysis personnel.

Below Detect Limit -- The value that indicates parameter values below the

instruments detection limits. This is used to

indicate that an attempt was made to determine the

parameter value, but the analysis personnel determined

that the parameter value was below the detection

limit of the instrumentation.

Data Not Cllctd -- This value indicates that no attempt was made to

determine the parameter value. This usually

indicates that BORIS combined several similar but

not identical data sets into the same data base table

but this particular science team did not

measure that parameter.

Page 23
Blank -- Indicates that blank spaces are used to denote that type of value.

N/A -- Indicates that the value is not applicable to the respective column.

None -- Indicates that no values of that sort were found in the column.

7.4 Sample Data Record

The data files (.dat and .edt extensions) in the Procdata and Sitedata directories on the SLICER

CD-ROM are binary files consisting of scaled integers or bytes. After applying appropriate scale

factors (see Section 8.2) an American Standard Code for Information Interchange (ASCII)

representation of a record is:

header record:

28 1 600 25679

data record:

211 2 207 63099.437 8.94060 21.8098 84.4478

54.164695 254.90834 590.359 12.3432 13.3440 15.3456 17

18 18 19 19 17 21 17 18 16 19 17 18 18 17 18 19 17 19 17 20 18

20 19 22 21 23 22 23 23 25 25 26 29 32 36 36 37 38 41 44 46 47

48 52 53 54 55 57 58 60 59 61 62 64 60 60 61 58 58 59 58 57 54

52 51 51 52 52 50 48 47 46 43 45 42 45 41 42 41 39 38 40 35 38

36 37 35 38 34 33 31 29 28 30 29 29 27 29 28 26 27 23 24 22 22

26 21 22 21 21 22 25 21 22 19 22 19 20 20 19 21 22 20 21 23 21

21 22 22 23 22 27 26 31 36 46 60 76 91 117 128 152 154 183 192 204 199

207 183 162 127 113 87 77 68 61 56 50 41 40 32 35 27 28 26 22 16

19 18 20 14 13 8 ii i0 16 8 15 i0 13 13 15 12 14 13 16 ii 15

12 14 13 16 13 18 15 19 18 17 17 19 21 18 19 18 17 19 18 19 18

18 17 18 17 18 17 19 17 19 19 19 18 18 19 20 17 19 17 20 17 19

18 20 19 23 16 18 19 17 18 19 16 18 16 18 19 18 16 19 19 19 17

19 18 19 18 14 17 16 18 18 19 17 16 18 17 19 17 17 19 18 17 18

17 17 18 16 18 17 16 16 16 19 18 16 18 18 18 17 16 18 17 21 17

17 17 16 18 17 17 15 18 19 17 18 16 17 16 19 16 19 16 18 17 19

17 18 18 18 19 16 16 17 17 19 18 17 18 16 18 19 17 17 18 17 20

17 16 17 16 19 17 17 16 17 18 16 17 16 18 18 18 17 17 18 17 19

16 18 17 19 17 18 16 19 18 17 16 17 16 17 14 17 17 18 16 17 18

19 17 20 18 19 17 17 14 18 17 19 18 18 17 17 16 18 17 16 16 17

16 17 16 18 17 18 17 18 16 17 18 17 17 17 16 20 17 18 16 18 18

19 19 16 18 18 18 19 18 19 18 19 16 17 17 17 17 18 16 18 15 17

18 18 18 17 18 18 18 17 18 18 18 20 17 17 16 16 16 16 17 17 16

18 19 19 16 17 16 17 18 17 16 16 18 18 18 16 17 18 18 18 18 19

17 19 16 15 17 17 17 18 18 20 18 17 16 19 16 17 17 17 16 19

18 18 18 17 19 17 18 18 17 14 18 16 19 17 17 17 17 17 18 17 17

15 17 19 18 18 17 17 17 16 18 16 14 18 19 18 18 17 17 16 15 17

16 18 21 17 16 17 17 17 19 17 18 18 18 17 17 16 16 16 16 17 18

16 17 18 19 17 18 17 18 17 14 17 17 17 19 17 16 17 16 16 19 20

17 16 17 18 20 16 18 17 19 17 18 16

Page 24
Samplerecordsfor theASCII aircrafttrajectoryfile (.trj extension)in theGps_miscdirectoryonthe

SLICERCD-ROMare:

header record:

36743

data record:

55443.00 53.21413777 105.67980529 407.9080 7 1.5 0.03 1

The following is a wrapped version of a data record from a sample inventory file on the BOREAS

CD-ROM.

SPATIAL COVERAGE,DATE OBS,START TIME,END TIME,RUN START LAT,RUN START LONG,

RUN END LAT,RUN END LONG,NUM SHOTS,CRTFCN CODE

'NSA',I8-JUL-96,1641,1649,56.05955,-98.99594,55.71156,-98.36916,38212, 'CPI'

8. Data Organization

8.1 Data Granularity

The smallest unit of SLICER data that can be ordered is a CD-ROM of SLICER data and

associated ancillary files. On the CD-ROMs the smallest unit of data is the Level 3b flight line, which

is binary data files corresponding to data near flux towers. These files each contain 1,001 laser shots

centered near a tower, extending approximately 1 km outward from the tower. The Level 3a binary

files, containing all data along a flight line, contain tens of thousands of laser shots typically extending

for 40 kin.

8.2 Data Format(s)

The BOREAS CD-ROM inventory listing file consists of numerical and character fields of varying

length separated by commas. The character fields are enclosed with single apostrophe marks. There are

no spaces between the fields.

The GPS aircraft trajectory files are stored as ASCII data. A trajectory file consists of ASCII data

composed of one header record and then a data record for each GPS epoch. The header record consists

of one parameter and a data record consists of eight parameters. The naming convention for these files

consists of an eight-character name and a .trj suffix. The eight characters consist of YY_MM_DD,

where YY is the last two digits of the year in which the data were collected, MM is the month of the

year, and DD is the day of the month. The trajectory files are located in the Gps_misc directory on the

SLICER CD-ROMs.

The SLICER data files are stored as binary data. A SLICER data file consists of binary data

composed of one header record and then a data record for each laser shot. The header record consists

of 4 signed long integers and a data record consists of 13 signed long integers and a byte array

composed of 600 elements. The long integer byte order follows the Motorola (UNIX or Macintosh)

convention. The bytes must be swapped for use on systems following the Intel convention (PC or

VAX). Some of the parameters are stored as scaled integers and must be divided by a scale factor per

the following table in order to be converted to the units as defined in Section 7.3.3. Note that scale

factors of sufficient precision must be used to achieve the necessary precision for the output results.

Page 25
Parameter Stored As Bytes Scale Factor

Divisor

header record:

tiu bin long 4 none

dig2wf long 4 none

wvfm bins long 4 none

numshots long 4 none

Scale Factor

Precision

Output As

long 4

long 4

long 4

long 4

Bytes

data records:

shotnum long 4

beam long 4

starten long 4

gpstime long 4

diameter long 4

azimuth long 4

inclination long 4

latitude long 4

longitude long 4

elevation* long 4

grndstart long 4

grndpeak long 4

grndend long 4

waveform byte array 600

none

none

none

1.00E+04

1 00E+06

1 00E+06

1 00E+06

1 00E+06

1 00E+06

1 00E+06or04*

1 00E+06

1 00E+06

1 00E+06

none

- long 4

- long 4

- long 4

double flt double flt 8

float float 4

float float 4

float float 4

double flt double flt 8

double flt double flt 8

float float 4

float float 4

float float 4

float float 4

- byte array 600

Data Record Total: 652 664

* NOTE: The elevation scale factor of 1.00E+6 limits the highest elevation that can be stored as a

signed long integer to be 2,147.48 m (i.e., [2^32]/2 * 1E-6). This is an adequate maximum elevation

for the BOREAS data, but is inadequate for sites where the topography exceeds this limit. SLICER

Level 3 CD-ROMs for non-BOREAS sites where the topography exceeds the limit along any data line

for that site were created using a scale factor of 1.00E+04, yielding a maximum permissible elevation

of 214,748 m. Flight days using the 1.00E+04 elevation scale factor are 19-Sep, 20-Sep, 24-Sep,

28-Sep, and 30-Sep in 1995, and 21-Feb-1997. Also, all flight days in 1993 use the 1.00E+04 factor.

The latest version of the SLICER browser software available from

http://denali.gsfc.nasa.gov/lapf/slicer/browser.html automatically applies the appropriate elevation

scale factor when reading the data into IDL.

SLICER data files consists of two types. Level 3a data consist of all valid laser returns along a flight

line. The location of the ground return in these files has been identified by automated signal processing

software. The naming convention for these files consists of an eight-character name and a .dat suffix.

The eight characters consist of YYMMDDLL, where YY is the last two digits of the year in which the

data were collected, MM is the month of the year, DD is the day of the month, and LL is the flight line

number on that day. Flight lines are numbered in sequence in the order in which they were collected.

Some line numbers refer to instrument calibrations, either done on the ground (pre- or postflight) or

in-flight (fiber calibrations), during which no surface returns are collected. These calibration lines have

not been included in the SLICER data, so flight line numbers can be skipped. In some cases, flight

lines are broken into segments for processing purposes, designated by a and b as part of the LL label.

The Level 3a files (.dat extensions) are located in flight-day specific subdirectories within the Procdata

directory on the SLICER CD-ROMs.

Level 3b data consist of segments of laser shots copied from the Level 3a flight lines. These files

each contain 1,001 laser shots centered near a BOREAS flux tower, extending approximately 1 km

outward from the tower. The automated identification of the ground return for each laser shot has been

visually inspected on a plot of the backscatter waveform; errors in ground identification have been

corrected by interactive selection of the ground position on the plot (using the IDL data browser

Page 26
routinesincludedontheCD-ROM).Thefile formatis thesameasthatof theLevel3adata.The

namingconventionischanged,consistingof aneight-characternameandan.edtsuffix to indicatethe

datawasedited.Theeightcharactersconsistof afour-letterdesignationof thetowersitefollowedby

DDLL (dayandlinenumber).Thefour-letterdesignationfollowstheBOREASsiteabbreviation

conventions(i.e.,NOBS,SOBS,SOJP).TheLevel3bfiles arelocatedin theTowr_edtsubdirectory

within theSitedatadirectoryontheSLICERCD-ROMs.

TheTowr_datsubdirectorywithintheSitedatadirectoryontheSLICERCD-ROMsconsistsof

exactcopiesof theLevel3adatafor the1,001shotsegmentsnearthetowers.Theautomated

identificationof thegroundreturnlocationwithin thewaveformhasbeenretained;nomanualediting

of thegroundreturnlocationwasdone.Thenamingconventionfollowsthatof theLevel3bdata,

exceptthatthe.datsuffix ispreservedto indicatenoeditingwasdone.

9. Data Manipulations

9.1 Formulae

Not provided. Contact D. Harding for information on specific formulae used in data processing.

9.1.1 Derivation Techniques and Algorithms

Derivations in the SLICER Level 3 data products consist of three types: geolocation of the laser

footprint, determination of the laser pointing vector orientation, and identification of the ground return

in the backscatter waveform.

The laser footprint latitude, longitude, and elevation are derived using geolocation methods adapted

by Bryan Blair from those described in Vaughn et al. (1996). Postflight processing software uses

rotation matrices to compute the laser footprint in coordinates referenced to the WGS84 ellipsoid, by

combining the absolute position of the aircraft platform, the distance to the surface, and the laser

pointing vector (Figure 1).

The laser vector inclination and azimuth are computed based on a Euler angle rotational

transformation from the roll, pitch, and yaw parameters obtained from the INS.

A sequence of processing steps establishes the ground return position in the backscatter waveform.

First, to improve the signal-to-noise ratio of the distribution, the raw digitizer count amplitudes are

summed by accumulating the signal in adjacent waveform bins. Typically, four adjacent bins are

summed when the data have a bin resolution of 0.1112 m, yielding a 0.44-m vertical sampling, which

is approximately one-half the vertical resolution defined by the laser pulse width and detector response.

Note that the 6-nsec-wide (1.8-m) impulse response is expressed in the digitized waveform as 0.9 m

wide because time in the waveform is divided by two to convert two-way, down-and-back pulse travel

to a one-way distance. Next, the mean of the background noise is established using a portion of the

waveform not expected to have a signal. Due to the short stature of much of the vegetation in the

BOREAS region, the final 10% of the waveform is used beyond any potential last ground return. For

other SLICER data sets, where tall canopies can cause a signal return to the end of the waveform, the

portion of the waveform prior to the TIU_BIN is used if a statistically sufficient number of bins occur

before TIU_BIN. The mean background noise is subtracted from the summed distribution, yielding a

signal above the noise level. Negative results, where the variance in the noise causes the waveform

signal to be less than the mean noise, are set to zero. The ground reflection in the signal is identified by

assuming that it is the last return above noise. The end of the last return is defined as the latest signal

above a threshold that is a multiple of the maximum noise in the portion of the waveform used to

define the mean noise level. The multiplication factor is adjusted depending on desired detection

sensitivity (a higher factor causes fewer false triggers of noise but misses more low ground returns,

whereas a smaller factor causes more false triggers of noise after the true ground return but misses

fewer low ground returns). The appropriate multiplication factor depends on signal-to-noise and is

determined by visual examination of ground profile results for a selected set of test cases. The peak of

the last return is defined to be the first inflection in signal strength prior to the end of the last return,

identified using its first derivative. The start of the last return cannot be uniquely identified from the

raw distribution because backscatter return from low vegetation could be convolved in time with the

Page 27
groundreturn.Therefore,thestartof thelastreturnis identifiedbasedonthewidthcharacteristicsof

theasymmetric,Raleigh-shapedsystemimpulseresponse.Theimpulseresponseisthetheoretical

signalrecordedfrom asmoothandflat surfaceanddependsontheconvolvedeffectsof pulsewidth

anddetectorresponse.TheSLICERimpulseresponseis establishedfrom minimum-widthreturns

from watersurfaces.A ratioisdeterminedfor theimpulseresponsebetweenthewidth from thesignal

endto peakcomparedto thewidth from peakto start.Theobservedend-to-peakwidth of thelast

returnis scaledby thisratioin orderto definethestartpositionof thelastreturn.Thismethodaccounts

for anypulsebroadeningof thelastreturndueto slopeor roughnessof thegroundwithin the

footprint.

Thedistancesto thegroundreturnstart,peak,andendarereferencedto theTIU_BIN waveform

position.Thisis thewaveformbin correspondingto thefirstreturnreceivedabovetheinstrument

detectionthresholdthatcausestheTIU stopevent(i.e.,canopytopin thepresenceof vegetation).The

latitude,longitude,andelevationparametersdefiningthelocationof thelaserreturnarespecificto this

TIU_BIN positionin thewaveform.TheTIU stop-pulsebin positionis assignedin softwareandis

constantfor adatafile, butit canvary significantlybetweenfilesif instrumentoperationalparameters

arechanged.Thewaveformbin positioncorrespondingto theTIU stop-pulsetimeis notrecordedin

thedatasystem;instead,it is establishedfor adatafile by visuallyassessingmultiple,overplotted

waveformrecords.Typically,waterreturnsareusedto determinein whichbin thesignallevelfirst

consistentlyincreasesabovethebackgroundnoise.Thisobservedbin positionis thevaluerecordedin

theTIU_BIN parameterof theLevel3 dataheader.

9.2 Data Processing Sequence

9.2.1 Processing Steps

The SLICER data processing sequence is diagrammed and described in Figure 7.

SLICER Data Processing Flow

SLICER

722 

cnaPC

ra# alUrne_'y & INS

data stored in-flight

to PC_ligqt

computer hard disk

t4a sector IocaUcn

copy

Ashtech GPS

typcally 2

and 2on

ra# dual-frequency

data stored in-flight to

la_ophard risks:

e & b file for each

receiter

8ORWAVE8

IDL b'o wser

for interac'_ _ data

disFlay and

ediUng

Level 3b

.edt bnary file

subset of .daf 111e

wilh corrected

ground rebrns

/

image of hard diskl

secfors backed u __

cn 8 mm exabyte I copy

Level 0

.Idn 111e(on a PC

cr SUN)

which is a secfor

image, tylacall y

for a single light

data

extraction &

con version

PNAV IIkinema'ac aircraft I convert

pcJI aj ct y I

or "1 1as AscII 111efora

I Kraldllll recater par I '03SUN_

£itar / (c file from PNAV) I

.trj frajecfor y

as ASCII 1ile

for use in IDL

IDL Focedures

] subsetof .dat 1ile I Level 3a 1 foriden'_1ng

I for sp_cillc I _ goundrebrn,

I_. targels I .dat Icinary file

, _ , adh canopy top comlsu'dng I:eam

I and manual I elevation, distance orienta'dcn, and

edUng of ground I to ffound, & beam data editing

J return position I oriaqtaUon (_d range and cloud removal)

.idl Idnary

1il8 for a

SlCecillc

li_qt

segment

Level 1

IDL p'ocedures

for data mergng,

Idas determinations,

fotal pulse _i_h,

and g_loca'don \;

Level 2

.geo Wnary 111e

#ith geoloca_cn

data for each

lase" shot

Figure 7: SLICER Data Processing Flow

Page 28
An editingstepis appliedin convertingLevel2 productsto Level3 in orderto deletelasershots

thatdid notyieldvalid returns.Invalidlasershotsthatareexcludedconsistof: (1)no-rangedata,(2)

cloudsandnoiseaboveanelevationthresholdsetabovethehighestexpectedground,(3)noisebelow

anelevationthresholdsetbelowthelowestexpectedground,(4) andafewpercentof shotswith

anomalouslylow startpulseenergy(describedin Section11.2).No-rangedataconsistof lasershots

for whichnobackscattersignalwasreceivedabovetheinstrumentdetectionthreshold.Forthe

BOREASdata,thehighandlow elevationthresholdsto removecloudreturnsandnoisewere700and

0 m,respectively.

9.2.2 Processing Changes

None.

9.3 Calculations

9.3.1 Special Corrections/Adjustments

In some instances of very low return amplitudes throughout the waveform, the algorithm described

in Section 9.1.1 finds no signal above the ground end detection threshold. In those instances, the

ground start, peak, and end positions are assigned to the last three bins at the end of the waveform as a

flag to indicate failure to detect any waveform signal above threshold. In other SLICER data sets with

tall canopies, the positions are set equal to the TIU_BIN to flag no signal detection.

The standard determination of the ground return position fails for saturated ground returns (see

Section 11.2 for a description of saturated returns). In saturated cases, the threshold determination of

the end of the ground return often corresponds to a small detector ringing peak after the return.

Software checks are included to minimize detection of these false peaks. Also, because the saturated

peak does not have a well-defined inflection point, the ground peak is simply set to the mid-point of

the saturated portion of the waveform. Thus, the peak to end half-width does not have physical

significance. Lacking a valid half-width, the ground start is defined to be the point prior to the peak at

which the slope of the waveform record decreases below a constant, but arbitrary, magnitude. The

cutoff slope magnitude is selected so that the resulting ground start positions look qualitatively correct.

The determination of the ground end, peak, and start is noisier and thus less accurate when the ground

return is saturated. Also, the ground start technique for saturated returns will not effectively separate

returns from vegetation directly above the ground. However, in most cases saturation of the ground

return occurs where the surface is unvegetated and the intensity of the transmit pulse reaching the

ground is not diminished by any overlying vegetation.

9.3.2 Calculated Variables

The distance from the instrument to the reflecting target is computed from the TIU round-trip travel

time by multiplying by the speed of light through an atmosphere of standard temperature and pressure.

The laser footprint latitude, longitude, and elevation are derived using geolocation methods adapted

from those described in Vaughn et al. (1996). Postflight processing software uses rotation matrices to

compute the laser footprint in coordinates referenced to the WGS84 ellipsoid, by combining the

absolute position of the aircraft platform, the distance to the surface, and the laser pointing vector

(Figure 1). The offset between the laser instrument and the GPS antenna (the reference point for the

aircraft trajectory) is accounted for in the calculation.

The laser footprint diameter is computed as the tangent of the laser divergence times the ranging

distance to the target. A divergence of 2 mrad was inadvertently used for all files, causing an error for

those flight lines where a greater divergence was used. See Section 11.2 for a list of those files and the

appropriate correction that must be applied.

The laser vector inclination and azimuth are computed based on a rotational transformation from the

roll, pitch, and yaw parameters obtained from the INS.

The distance from the first detected return in the backscatter signal to the start, peak, and end of the

ground return is derived by multiplying the number of waveform bins times the one-way travel

distance represented by each waveform bin. The distance per bin is a function of the sampling rate of

the waveform digitizer and any digitizer averaging applied. No averaging was used for the BOREAS

Page 29
datasothebin resolutionis always0.1112m.If digitizeraveragingis appliedit isreportedin the

headerecordof eachdatafile.

9.4 Graphs and Plots

The SLICER CD-ROMs include several types of plots as PostScript (.ps) files. For each flight

day, plots located in the Gps_misc directory show attributes of the aircraft trajectory as a function of

GPS time (elevation, elevation difference between successive elevations, quality metrics for the

kinematic solution). For each flight line, plots located in the day-specific subdirectories in the Procdata

directory show a regional location map and profiles of aircraft attitude, elevation of the laser return,

and a preliminary version of the backscatter pulse width (an approximation of vegetation height).

These flight line plots are also stored as GIF format files (.gift in the same subdirectories. Regional

summary maps showing the SLICER ground tracks for all flight lines at the SSA and NSA are located

in the Tower ps subdirectory in the Sitedata directory.

Detailed flight line maps showing the location of the laser footprint ground tracks in the vicinity of

each TF site are also located in the Tower ps subdirectory in the Sitedata directory. On these maps,

small circles indicate the location of individual laser footprints. Four-digit numbers adjacent to ground

tracks indicate the acquisition day and flight line number for that track. Plus signs at the center of the

maps indicate the location of the tower for that site. Narrow lines extending from the towers indicate

the location of ground transects along which canopy properties where measured by Chen (BOREAS

Remote Sensing Science (RSS)-07 team). Open squares on these ground transects indicate the location

of plots where stem and hemispherical photo data were measured by Rich (BOREAS Terrestrial

Ecology (TE)-23 team). The open triangle indicates the location of the target coordinate that was

entered into the flight navigation computer to guide acquisition of the data (the target coordinates were

approximations of the tower locations and thus not exactly coincident with the towers).

10. Errors

10.1 Sources of Error

The accuracy of the footprint geolocation results (latitude, longitude, and elevation) depends on the

accuracy of the ranging data; pointing knowledge; platform trajectory; and determination of the system,

angle, and time biases.

The accuracy of the distance measurements from the first return (e.g., canopy top) to the start,

peak, and end of the ground return depends on the signal-to-noise ratio in the backscatter waveform,

the implementation of automated signal processing routines to identify the ground return, and the

resolution and accuracy of the digitizer sampling rate.

The accuracy of the transmit pulse energy measurement depends on instrumental characteristics of

the start pulse energy monitor.

10.2 Quality Assessment

10.2.1 Data Validation by Source

SLICER geolocation and elevation results were assessed by differencing the profiles with respect

to a Digital Elevation Model (DEM) for the SSA. The National Imaging and Mapping Agency (NIMA)

Digital Terrain Elevation Data (DTED) Level 1 DEM product was used for the comparison. DTED

Level 1 has a 30 arc second spatial resolution (approximately 90 m). SLICER profiles were shifted

with respect to the DTED grid in order to establish the best-fit location for the SLICER profiles based

on a minimization of the root mean square (RMS) difference between the SLICER ground elevations

and the DTED elevations. Unlike similar comparisons to 3 arc second United States Geological Survey

(USGS) DEMs for the conterminous U.S. (Section 10.2.2), very inconsistent results were obtained

using DTED, probably due to the low DTED spatial resolution, its relatively poor height accuracy, and

uncertainty about what, if any, correction for vegetation height was applied to the DTED elevations.

The relatively poor and uncertain quality of the DTED, combined with the extremely low relief of the

BOREAS landscape, cause the DTED to be a poor representation of the topography in the BOREAS

Page 30
region.TheSLICERto DTEDresultswere,therefore,judgedto beunreliableandnotauseful

measureofSLICERgeolocationaccuracy.NosuchcomparisonwasdonefortheNSAbecauseDTED

dataarenotavailableforthatregion.

10.2.2 Confidence Level/Accuracy Judgment

Because SLICER employs a threshold detection method for ranging, the range data are affected by

an error source known as range walk. Low amplitude returns typically have slower rise times on the

leading edge of the return than do high amplitude returns (Figure 8). Thus, for targets at equal distance

from the laser transmitter, the detection threshold is crossed slightly later in time for low amplitude

returns as compared to high amplitude returns. As a result, the target with a low amplitude return will

appear to be at a slightly greater ranging distance, and thus, lower in elevation. A range walk

calibration curve is measured during each SLICER flight by transmitting laser pulses of varying

intensity through a fiber optic delay cable of fixed length (fiber calibrations). However, range walk

corrections can be effectively applied only to returns from discrete, continuous surfaces, such as bare

ground or water, that yield a well-behaved waveform leading edge. Diffuse, vertically distributed

surfaces, such as an open vegetation canopy, result in broadened returns with complex leading edges

that are not easily corrected for range walk. Therefore, SLICER elevation data in the Level 3 data sets

are not corrected for range walk. The resulting uncorrected ranging error is typically in the 10's of

centimeters, but can be at the meter level for the greatest excursions of return pulse amplitude.

Amplitude Dependent Range Walk

,i

l

r-..,

high am plitude

ret u rn

<E

,=

"_ Iow am p litude

return

Amplitude Effects:

tem p-variable transm it energy

atm ecpheric transmissi vity

surfaoe reflectivity

off- nadir pointing

specular reflection

pulse spreading

reoeiver FOV alignment

Detection T hreshold

ground Noise

T1 T2 Time

Figure 8: Aplitude Dependent Range Walk

I!

For the BOREAS aircraft trajectories, forward and backward kinematic solutions were computed

and combined in order to optimize the resulting accuracy. For short (< 10 km) separations between the

receivers, PNAV yields trajectories with cm-level accuracy. However, for the baseline length used at

BOREAS (up to 700 kin), the trajectory accuracy is probably at the meter-level (horizontal and vertical)

Page 31
basedonunpublishedcomparisonsof trajectoriesderivedfor commondatasetsby PNAV and

softwareoptimizedfor longbaselines(GITARtrajectoriesprovidedby B. Krabill, GSFCCode972).

ThePNAV solutionisreliableonlywith aminimumof five satellitesobservedandaPositionDilution

of Precision(PDOP)of lessthan4. TheRMSpositionerroris onlyvalid in anabsolutesensewhen

certainconditionsaremet(whichisprobablynotthecasefor theselongbaselineflights),butit givesa

relativesenseof wherethesolutionis of pooreraccuracy.

Theaccuracyof thefootprintgeolocationresultsdependsontheaccuracyof therangingdata;

pointingknowledge;platformtrajectory;anddeterminationof thesystem,angle,andtimebiases.For

thehigh altitudestypicalof theBOREASacquisitions(nominally4.5km aboveground),pointing

errorsarethelargestsourceof geolocationerrors.Basedonthespecifiedaccuracyof theINSandthe

magnitudeof theunmonitoredalignmentdrifts,thepointingknowledgeaccuracyshouldbeonthe

order2mrad,yieldingpositionaccuraciesequivalento thelaserfootprintdiameterin thenominal

five-beammode.Geolocationaccuraciesof thisorderhavebeenverifiedfor non-BOREASSLICER

datasetsby comparingSLICERelevationprofilesto correspondingimagefeatures(e.g.,edgesof

building,forestclearcutboundaries)in severalindependentlygeoreferencedhigh-resolutionimages

andto topographyin independentlygeoreferencedDEMs.ManySLICERlinesin theconterminous

U.S.havebeencomparedto USGS7.5-minutequadrangleDEMs,whichhaveahorizontalresolution

of 3 arcsecond(approximately30m). In mostcases,theSLICERelevationprofilesmatchtheDEMs,

indicatingthattheSLICERgeolocationis accurateatleastattheDEM pixel scale.In afew cases,

unexplainedhorizontaloffsetsof theSLICERprofilesareobserved(seeSection11.2for alist of flight

lineswithknowngeolocationproblems).

Theprecisionandaccuracyof theidentificationof thegroundreturnin thebackscatterwaveform

dependonthealgorithmemployed(seeSection9.1.1).Theprecisionof thegroundstart,peak,and

enddistancesis reducedfrom theintrinsicwaveformbin resolution(0.1112m for BOREAS)by the

summationof adjacentbinsto improvesignal-to-noise.Fouradjacentbinsaretypically summed,

yieldingaprecisionof 0.44m forthegrounddistanceparameters.Also,thehalf-widthdetermination

of thestartof thegroundreturnis noisy,atthesubmeterlevel,dueto thesensitivityof thegroundend

thresholddetermination.Thegroundendthresholdtechniqueis sensitiveto theasymptoticfall-off in

energyof thetransmit-pulsetrailingedgeandto thepotentialpresenceof avariableamplitudeback

porcheffectduringperiodsof non-optimalbeamquality.A peakfitting approachto selectionof thelast

returnlikely wouldachieveamoreaccurateidentificationof thegroundsignal.

Theaccuracywith whichtheTIU_BIN positionin thewaveformcanbeestablisheddependsonthe

visualinspectionof multiple,overlaidwaveformplots(seeSection9.1.1).Theappropriatebin

positioncanonly bedeterminedwiththismethodto within severalwaveformbinsdueto binjitter

from shotto shot,andthevariablerange-walkcharacterof thereturnpulseleadingedgeasafunction

of surfaceconditions.TIU_BIN jitter is causedby inexacttiming synchronizationbetweentheTIU

oscillatorandthewaveformdigitizeroscillator.

10.2.3 Measurement Error for Parameters

None available.

10.2.4 Additional Quality Assessments

Qualitative assessments of data consistency were made by constructing a variety of plots for

SLICER data in the vicinity of each flux tower. Plots included three-dimensional perspective views of

canopy top and ground elevation profiles, contoured transect plots of the height distribution of canopy

surface area and closure, and plots of individual and averaged canopy height profiles derived from the

backscatter waveforms (examples shown in Harding, 1998). No anomalous data characteristics were

observed in these plots.

10.2.5 Data Verification by Data Center

Verification by BOREAS staff" was minimal. The data header files were read and inventory

information was extracted (see Section 7.3). These metadata were loaded in the BORIS data base, and

files containing the metadata were extracted for inclusion on the CD-ROM.

The IDL software provided by SLICER personnel was used to view some of the .dat files.

Page 32
1 1. Notes

11.1 Limitations of the Data

The amplitude of the backscatter signal in the raw waveform is in units of digitizer counts (0-255).

These amplitude counts are not calibrated, nor are they consistent as a function of beam position or

time. Thus, the waveform cannot be used as an absolute measure of received backscatter energy.

Causes of variation in receive energy per digitizer count include:

• Detector efficiency and gain.

• Radial fall-off in the efficiency of the receiver telescope.

• Scanner-to-receiver alignment.

• Digitizer offset and gain.

A significant reason for lack of calibration of the digitizer counts is varying efficiency of the Si:APD

detector. The detector efficiency in converting input energy to output voltage is temperature dependent;

the temperature dependence of the SLICER detector has not been calibrated, nor is the detector

temperature controlled. It instead varies with ambient and operating conditions. Therefore, the

amplitude of the output voltage is not an absolute measure and can vary with time as the temperature of

the detector varies. Furthermore, the gain level of the detector, defining the conversion of input energy

to volts, could be varied. Detector gain would typically not be varied during a deployment of the

instrument, but could be changed between deployments.

In addition, the receiving telescope's optical efficiency is uniform across most of the FOV but

declines at the edges by approximately 15%. Therefore, the backscatter energy is less efficiently

collected for the outer scan positions as compared to the central footprints (Figure 9). Furthermore,

although the laser scan pattern is aligned to fit within the receiver FOV in-flight, typically at the start of

a flight mission, the alignment has been observed to drift during flight due to unknown causes. The

alignment drift causes unmonitored variations in return signal strength, especially for the two outer

footprints in the five footprint configuration as they can fall partially to wholly outside the receiver

FOV (Figure 9).

A) proper alignment

L

10 mrad reoeiver field-of-view

Figure 9: SLICER Alignment

B) misalignment

idealized contours of reoeiver sensitivity

wit h ~15% radial fal I-off tow ard c_te r m argi n

Page 33
Finally, the analog-to-digital conversion applied by the waveform digitizer depends on offset and

gain factors. These factors are not normally altered during a deployment of SLICER, but can vary

from deployment to deployment.

For backscatter waveforms with low amplitude ground returns and/or poor signal-to-noise

performance, the threshold method used to identify the ground return (see Section 9.1.1) can miss the

ground return. Instead, a higher canopy return is identified as the ground return. In some rare cases,

the ground end position is properly identified, but the ground peak position misses the maximum

ground return and instead identifies a maximum return from a higher canopy layer. The ground start

position is then placed higher up into the canopy by the half-width method. A final error is the

identification of waveform noise occurring after, and thus below the ground. These various anomalous

ground return detections are usually readily identified by examining a profile plot of the canopy top and

ground start, peak, and end elevations.

11.2 Known Problems with the Data

The theoretical shape of the transmit pulse in time is Raleigh, with a sharp leading edge rising to a

peak and a slower, asymptotic decrease in energy after the peak (Figure 10). However, with

non-optimal alignment of laser transmitter elements, the asymptotic fall-off of laser energy can at times

be corrupted, yielding a pronounced step-like "back porch" on the pulse trailing edge (Figure 10). The

back porch effect can cause problems in properly identifying the end of the ground return and the pulse

width. For BOREAS, the laser transmitter elements were well aligned, and the back porch effect is

negligible. For some other SLICER missions, the back porch effect can be prominent and a significant

source of error in identifying the ground return. The best characterization of pulse shape, and the

magnitude of any back porch effect, is achieved by examining the digitized backscatter pulses reflected

from flat surfaces such as water bodies. Note that the 6-nsec-wide (1.8-m) impulse response is

expressed in the digitized waveform as 0.9 m wide because time in the waveform is divided by two to

convert from two-way (down-and-back) pulse travel to one-way distance.

Characteristic Backscatter Wavelorms lrom a Flat, Smooth Surlace

A) Impulse Response forRalei_q

Tran smit PuIse

03

.,=q

detect ce _ thresh old ab3ve

_ "_ ]]me

-121.. o "0

19) Impulse Response forRalei_ C)

Transmit Pulse w/'Sack Porch'

03

.O,ere- c.,t _, _ thresh old a b3ve

I ----.wavebrmndse

$ -nine

"O

03

.,=q

Pespmse for Return bat

Saturated 1he Detector

s-'-'-'.

c_t_on I lhreshold at:o_

i

) ]]me

'1_

"121

Figure 10: Characteristic Backscatter Waveforms from a Flat, Smooth Surface

Received backscatter energy can exceed the detection limit of the Si:APD detector, causing

saturation of the amplitude signal. Detector saturation is prevented by reducing the transmit pulse

energy. This is accomplished by manually inserting neutral density absorbing filters into the transmit

laser pulse path prior to the galvanometer scan assembly. An appropriate filter is selected in-flight by

the instrument operator (as a function of flight altitude, atmospheric transmissivity, target reflectivity,

and pulse broadening due to within-footprint surface height variability) in order to reduce the

maximum received backscatter laser energy below the saturation level of the detector. In some cases,

Page 34
the selected filter is insufficient to prevent detector saturation; for example, when changing from a

low-amplitude, broadened return from vegetation to a high-amplitude, narrow return from bare

ground.

Detector saturation typically causes specific waveform characteristics (Figure 10). The

leading-edge component of the signal, up to the point saturation is entered, accurately preserves the

backscatter signal input to the detector. Thus, the TIU range to the first detected surface is correct.

When saturation occurs, the detector output voltage remains at a high, nearly constant level (but with

detector electronic noise still superimposed on the signal). Due to the detector bandwidth, the saturated

output voltage continues later in time than the end of the actual received optical energy reflected from

the target. The higher the input signal exceeds the saturation limit, the longer the detector stays in a

saturated state. After exiting the saturated state, there is no longer any optical input and as a result, the

detector output voltage abruptly drops in amplitude. The detector voltage typically overshoots the

background mean noise level, creating an anomalous low, and then enters a 'ringing' phase with

anomalous amplitude oscillations about the mean noise level damping out with time. This oscillation

causes small false peaks above the noise following the saturated return.

Identification of the ground return start, peak, and end in the waveform is less accurate for

saturated returns, and the ground start identification will not properly differentiate return energy from

the ground versus vegetation directly above the ground (see Section 9.3.1). However, saturated

returns most likely occur when the ground is unvegetated.

In some cases, the entire recorded waveform can be extremely noisy, for either saturated or

non-saturated returns. The source, or sources, of the noise is uncertain. One source may be radio

frequency interference from pilot transmissions. Often the noise is present as alternating high and low

amplitudes between even and odd digitizer bin positions. When no averaging of digitizer bins is

applied in recording the waveform, as was the case for BOREAS, this noise between even and odd

bins appears in the waveform record. Averaging of adjacent waveform bins can be applied by the data

user to greatly reduce this noise. The latest version of the SLICER data browser available from

http://denali.gsfc.nasa.gov/lapf/slicer/browser.html always applies averaging of the even and odd bins

when plotting the waveform, in order to reduce this noise and improve the waveform appearance.

Due to problems detailed in Section 4.2, the laser pulse energy measure contained within the Level

3 data structure should be considered to be unreliable and incomplete.

In rare instances (a few percent), individual laser shots have markedly lower recorded output

energy values than the majority of the other shots. The cause of this reduced laser shot energy is not

known, but is possibly due to Q-switch mistiming in the transmitter. These single shots do appear to

indeed have significantly lower transmit energies because the ranges for those shots are anomalous,

being slightly shortened as compared to adjacent shots, probably due to range walk (see Section

10.2.2). Range walk on these rare low-energy transmit pulses causes the TIU start to occur late, and

the resulting range measurement is slightly shorter than normal. The derived elevations are slightly

higher (10's of cm) than would be determined with a transmit pulse of normal output energy. Because

of this ranging error effect, these low-transmit energy pulses have been deleted from the SLICER

BOREAS data products. Although present in the BOREAS data, this effect has never been observed in

other SLICER data sets.

Misalignment between the transmitted laser scan pattern and the receiver FOV causes a reduction in

receive energy for footprints falling partially outside the receiver FOV. The laser scan pattern is aligned

to the receiver FOV in-flight, typically at the start of a flight mission, by manually adjusting mirror

mounts to maximize the return energy of each footprint. Laser-to-receiver alignment drift of unknown

cause can occur during a flight (Figure 9), causing varying, unmonitored return signal strength,

especially of the two outer footprints in the five footprint configuration (as they can fall partially to

wholly outside the receiver FOV). For footprints falling partially outside the receiver FOV, the

resulting waveform can have an anomalous, low signal-to-noise ratio. The low signal of the return, as

compared to the center footprints, also causes greater range walk, and thus an anomalously long TIU

range and a resulting low elevation.

The quality of the GPS trajectory for aircraft position depends on the number and angular

distribution of observed GPS satellites. The GPS trajectories for each flight day at BOREAS show a

large PDOP and RMS excursion near 17 hrs GMT, due to a sub-optimal satellite geometry. The

Page 35
incrementaldifferenceof theaircraftaltitude,from onealtitudemeasurementto thenext,in theGPS

trajectoryshowslittle (lessthan1m) tonoinstantaneouserrorassociatedwith theonsetof these

events.However,short-term(secondsto minutes)trajectoryerrorsatthemeter-levelcouldbe

associatedwith theseeventsdueto theKalmanfiltering utilizedby thePNAV software.

Thelaserfootprintdiameteris computedasthetangentof thelaserdivergencetimestheranging

distanceto thetarget.A divergenceof 2 mradwasinadvertentlyusedin thecomputationfor all files,

causinganerrorfor thoseflight lineswhereagreaterdivergencewasusedto acquirethedata.This

erroris presentin oneBOREASfile (96072908)andanumberof files from othermissions.The

followingfiles musthavethefootprintdiametervaluesin theLevel3 filesmultipliedby thecorrection

factorindicatedbelowto yieldtheproperdiameter.Thelatestversionof theSLICERdatabrowser

obtainablefromtheLAPFWebsiteautomaticallycorrectsthefootprintdiameterswhenopeningthese

erroneousfiles.

YYMMDDLL Mission Location Correction

97021912 Puerto Rico x5

96072908 BOREAS x5

96072909 BOREAS x5

95090718 SERC, MD x5

95092102 Canopy Crane, WA x5

95092412 Canopy Crane, WA x5

95092408 Mount Rainier, WA xl.5

95092410 Canopy Crane, WA x5

95093004to08 Death Valley, CA x5

95111603 Orlando, FL xl.5

95111611to22 Orlando, FL x5

95111624 Orlando, FL x5

94082414to17 Performance, MD xl.5

94082418 Performance, MD x4

94082419 Performance, MD xl.5

94100503to07 Duke Forest, NC xl.5

Many SLICER lines in the conterminous U.S. have been compared to USGS 7.5-minute quadrangle

DEMs, which have a horizontal resolution of 3 arc seconds. SLICER lines across the principal target

of interest have been compared to the DEMs; lines acquired during transects and calibrations have not

been compared. SLICER profiles were shifted with respect to the DEM grid in order to establish the

best-fit location for the SLICER profiles based on a minimization of the RMS difference between the

SLICER ground elevations and the DEM elevations. SLICER elevations were converted from the

WGS84 ellipsoid datum to a mean sea level datum by subtracting the ellipsoid to geoid difference at the

footprint location obtained from the Earth Geoid Model 1996 (EGM96). EGM96 was jointly

developed by GSFC and the Defense Mapping Agency. In most cases, the position of the best fit

match is within +/- one DEM pixel north-south and east-west of the Level 3 geolocation for the

SLICER profile. The distance represented by one 3 arc sec DEM pixel is 31 m north-south and is

variable east-west, as a function of latitude, but is approximately 24 m. Due to ambiguities in the

matching method, one-pixel offsets are probably not significant and do not necessarily imply

mislocation of the SLICER data. SLICER data outside the conterminous U.S. have not been assessed

in this way due to lack of suitable DEMs. Also, the DEM-matching technique is inaccurate for areas of

extremely low relief lacking unique topographic features, such as the BOREAS study areas. In the

U.S., low-relief areas where the DEM-matching method failed are the Roanoke River and Performance

missions, both acquired on 24-Aug-1994. For the other missions in the conterminous U.S., SLICER

lines that matched the DEMs to within +/- one pixel are:

Page 36
YYMMDD Mission

941005 Duke Forest, NC

941006 Mahantango, PA

950907 SERC, MD

950919 Mt. St. Helens, WA

950920 Mt. St. Helens, WA

950924 Mt. St. Helens, WA

950920 Mount Rainier, WA

950924 Mount Rainier, WA

950921 Canopy Crane, WA

950924 Canopy Crane, WA

950923 Coast Range, OR

950928 Long Valley, CA

950930 Long Valley, CA

950903 Death Valley, CA

951116* Orlando, FL

FI

4

3

4

4

3

3

5

5

4

2

3

2

2

8

7

ight Lines

6, 8

4, 5, 6 7, 8

5, 7, 8 9, i0,

5, 6, 7 8, i0

6, 7, 8 i0, ii,

6, 7, 8

5, 6, 7 8, 9,

4, i0

4, 6, 7 8, ii,

3, 4a, 5, 6, 7,

3, 4

9

9, i0, ii, 12,

ii, 12

12, 13

i0, ii

12

8, 9, i0, 12, 13

14, 16, 17, 18, 19, 20, 21

* NOTE: SLICER ground elevations for the Orlando mission are consistently below the USGS DEM

elevations by an average of 3.0 m. The source of this bias is not known; the most likely sources are

use of an incorrect system delay in the SLICER processing and/or use of an incorrect offset between

the GPS antenna and SLICER instrument.

In a few cases, horizontal offsets of more than one pixel are observed when comparing the SLICER

profiles to the DEMs, indicating unexplained errors in the geolocation results contained in the Level 3

data files. Flight lines known to have errors in geolocation are listed below. The line number, mission,

and the offsets between the geolocation position and the best fit to DEM position are given. A positive

offset indicates the best-fit match is shifted north and/or east with respect to the geolocation result. If

no well-defined minimization of the RMS difference (i.e., convergence) was obtained, 'no

convergence' is listed rather than the offsets. Lack of convergence probably indicates that the

mislocation offset was larger than the search area over which the SLICER profile was shifted (+/- 30

arc seconds north-south and east-west).

YYMMDDLL Mission

94100505 Duke Forest, NC

94100507 Duke Forest, NC

95091912* Mt. St. Helens, WA

95092401* Mt. St. Helens, WA

95092112 Canopy Crane, WA

95092309 Coast Range, OR

9509284b Long Valley, CA

95092811 Long Valley, CA

95093007 Death Valley, CA

95093010 Death Valley, CA

95111606 Orlando, FL

95111608 Orlando, FL

95111613 Orlando, FL

NS offset

0 00 m

-30 89 m

0 00 m

0 00 m

92 67 m

0 00 m

-24 46 m

30

-123

-61

30

no

no

89

56

78

89

EW offset

-49 95 m

-49 93 m

64 ii m

-i06 99 m

-128 96 m

-43 94 m

154 44 m

convergence

convergence

m 49. 68 m

m -27. i0 m

m 245.29 m

m 54.26 m

* NOTE: The caldera dome at Mt. St. Helens experienced significant (100's of meters) growth

between the time the acquisition of stereo photos used to construct the DEM and the SLICER mission.

This dome change may be biasing the matching results, causing an erroneous offset.

The elevation scale factor of 1.00E+06 (used to convert the signed integer elevation values stored in

the Level 3 files to meters) limits the highest elevation that can be stored to 2,147.48 m (i.e.,

[232]/2* 1E-6). This is an adequate maximum elevation for the BOREAS data, but is inadequate for

Page 37
siteswherethetopographyexceedsthislimit. SLICERCD-ROMsfor non-BOREASsiteswherethe

topographyexceedsthislimit alonganydataline forthatsitewerecreatedusinga scalefactorof

1.00E+04,yieldingamaximumpermissibleelevationof 214,748m.The 1.00E+04elevationscale

factorwasusedon 19-Sep,20-Sep,24-Sep,28-Sep,and30-Sep-1995,and21-Feb-1997.Also, all

flight daysin 1993usethe1.00E+04factor.Thelatestversionof theSLICERbrowsersoftware

availablefrom http://denali.gsfc.nasa.gov/lapf/slicer/browser.html automaticallyappliestheappropriate

elevationscalefactorwhenreadingthedataintoIDL.

Twoflight linesaremislabeledonthemapshowingthelocationof groundtracksin thevicinity of

theSSA-OJPtowerlocatedin theTower pssubdirectoryin theSitedatadirectory.Thelabelsforlines

96072909and9607296bareinadvertentlyswitchedonthemap.Also, theflight lognotesfor

96072909shouldindicatethat,like 96072908,it wasacquiredat approximately2,400feetabove

groundlevelusinga 10-mradbeamdivergencein profiling mode.

11.3 Usage Guidance

In orderto find SLICERdatafor aspecificBOREAStower site,referto thefile fluxtowr.sumin

theDocumntsdirectoryontheSLICERCD-ROMs.It containsasummarythatlistsfor eachSLICER

flight linetheTF site(s)crossedby thatflight line, andtheapproximatelasershotnumberin thedata

recordclosestto thetower.Also,detailedflight line maps,asPostScriptfiles, showingthelocationof

thelaserfootprintgroundtracksin thevicinity of eachtowerarelocatedin theTower pssubdirectory

of theSitedatadirectory.Eachgroundtrackis labeledwith afour-digitnumbercorrespondingto the

flight dayof yearandlinenumber.

Dueto thenumerousinstrumentfactorsthataffectthedigitizedrecordof backscatterreturn

(describedin precedingsections),theSLICERwaveformamplitudesareuncalibratedandshouldnot

beusedasanabsolutemeasureofreturnsignalstrengthnorof surfacereflectance.Someof thefactors

affectingreturnsignalstrengthareconstant.In particular,theoutwardfall-off in receivertelescope

sensitivityisconstant,andtheresultinglowerreturnstrengthof theouterbeams,ascomparedto the

innerbeams,couldbeestablishedby acomparisonofreturnsfrom asurfaceof uniformreflectivity,

suchaswater.However,manyof therelevantinstrumentfactorsareunmonitoredandvary slowly

overtime,includingtransmitpulseenergy,thetemperature-dependent detectorsensitivity,and

alignmentdrift betweenthescannedtransmitpulsesandthereceivertelescopeFOV. Overshort

segmentsof data(secondsto minutes),theseslowlyvaryingeffectsprobablydonot significantly

changeinstrumentperformance.Thus,successiveshotsalongasinglebeampositioncanbecompared

in arelativesense.Otherunmonitoredfactorsabruptlychangedigitizedsignalstrength,including

filtersusedto attenuatethetransmitbeamstrengthandthedigitizergainsetting.Changesin these

factorsshouldappearasinstantaneous,andusuallypronounced,changesin receiveenergyfrom shot

to shot.However,theattenuationdifferencesbetweensomeof thefilters arequitesmall,andthe

resultingdifferenceinreceivedsignalmaynotbeeasilyrecognized.Becauseof thesenumerouseffects

ondigitizeramplitude,themostreliableuseof SLICERwaveformsis to treateachindependentlyand

useit onlyasarelativemeasureof theverticaldistributionofreturnsignalstrengthfor thatfootprint.

Furthermore,for casesof detectorsaturation,preventingthecorrectmeasurementof peakreturn

amplitudes,thewaveformrecordshouldnotbeusedevenin thisrelativesense.

TheLevel3aidentificationof thegroundreturnstart,peak,andendin eachwaveformisbasedon

automatedsignalprocessingsoftware.Therearemanycircumstancesin whichthegroundreturncan

bemisidentified(seeSection11.1).In thevicinity of eachflux tower,Level3bproducts(.edtfiles in

theTowr edtsubdirectoryof theSitedatadirectory)havebeenproducedwherethegroundreturnfor

eachlaserreturnhasbeenvisuallyinspectedandeditedasneeded.Usersof Level3adata,for

non-towerdata,shouldinspect,andmodify asneeded,theautomatedidentificationof theground

returns,usingtheprovidedIDL-baseddatabrowseror otherplotting software.Profileplotsof

individualcross-trackbeampositionsshowingtheelevationof thecanopytop,groundstart,ground

peak,andgroundend(seeSection12)provideavisualindicationof incorrectlyidentifiedground

returnparametersdueto inconsistenciesfrom shotto shotalongtheprofile.

In mostcases,theSLICERgeolocationaccuracyis thoughtto beatthescaleof thelaserfootprint

(i.e.,approximately10m orbetter).However,someflight linesfor non-BOREASdatasetsare

knownto bemislocatedby upto 100'sof metersbasedoncomparisonswith DEMsandgeoreferenced

Page38
images(seeSection11.2for a list of mislocatedflight linesin theU.S.).UsersofBOREASSLICER

datawhoarecomparingSLICERresultsto otherdataatspecificlocationsshouldconfirmtheaccuracy

of theSLICERgeolocationresults.Thisis bestdoneby identifyingdistinctivefeaturesin SLICER

groundandcanopytopelevationprofiles,suchasdistinctforeststandedges,andconfirmingthatthese

featuresareconsistentwith thecorrespondingfeaturesin independentlygeoreferencedhigh-resolution

images.

SLICERelevationsarereferencedto theWGS84ellipsoid.To compareSLICERelevationsto

topographicdata(mapsor DEMs)whichtypicallyuseaverticaldatumreferencedto meansealevel,an

ellipsoidto geoidcorrectionneedsto beapplied.Becausetheellipsoidto geoidseparationvaries

spatially,thecorrectionneedsto beappliedto eachlaserfootprintasafunctionof its latitudeand

longitude.TheU.S.NationalGeodeticSurveydistributessoftwarecalledGEOIDwhichcomputesthe

ellipsoidto geoidseparationwithin theconterminousU.S.asafunctionof latitudeandlongitude.

Similarsoftwareis likely availablefor theBOREASregionfrom theCanadiangeodeticsurvey.

EGM96providesaglobalrepresentationof thegeoid.EGM96wasjointly developedby GSFCand

theDefenseMappingAgency.

Latitudeandlongitudein theWGS84ellipsoidreferenceframeareessentiallyidenticalto those

referencedto NAD83.Thetwo canbeconsideredto beidenticalfor all practicalpurposes.

Timesintheaircrafttrajectoryfiles arewithrespecto GMT andtimeintheSLICERdatafilesare

with respectto GPStime. GMT timesarelessthantheGPStimesby asmall,constantnumberof

secondsdueto thedifferencebetweenGMT andGPStimein effectatthetimeof theflight. GMT

progressivelyfallsbehindGPStime asleapsecondsareaddedto GMT butnotto GPStime.TheGPS

- GMT timedifferenceduringtheBOREASacquisitionwas11seconds.Duringtheperiodof SLICER

operations,theGPS- GMT timedifferencewasasfollows afterthelisteddate:

31-DEC-1990 +7 sec

30-JUN-1992 +8 sec

30-JUN-1993 +9 sec

30-JUN-1994 +i0 sec

31-DEC-1995 +ii sec

30-JUN-1997 +12 sec

31-DEC-1998 +13 sec

11.4 Other Relevant Information

David Rabine (GSFC Code 924 and SSAI) and Sean Warren (GSFC Code 921) operated the

SLICER instrument for the BOREAS deployment. Earl Frederick (GSFC Code 972 and EG&G)

operated the GPS base station at Prince Albert Airport.

Funding for the development of SLICER was provided by the Goddard Director's Discretionary

Fund and NASA's Solid Earth and Natural Hazards Program. Acquisition of the BOREAS SLICER

data was funded by NASA's Terrestrial Ecology Program through an RTOP grant to D. Harding.

Processing and generation of BOREAS data products was conducted by the LAPF using funding

provided by NASA's Mission to Planet Earth Program Office.

12. Application of the Data Set

The data can be used to construct topographic profiles of the canopy top directly using the elevation

parameter, and of the underlying ground using the elevation minus the vegetation height. The canopy

top profile consists of the highest detected vegetation surface within each laser footprint. Vegetation

height is obtained from the ground start parameter (the distance along the laser vector from elevation to

the start of the ground return) corrected for off-nadir pointing by multiplying by the cosine of 90

degrees minus the inclination parameter. The start of the ground return should be used because the TIU

ranging component of SLICER is based on leading-edge ranging (i.e., timing is done with respect to

the start of the transmit pulse and start of the return pulse). For consistency, ranging to the ground

should therefore also be done with respect to the start of the ground return. Using the ground start

Page 39
parameter,thegroundprofileconsistsof thehighestdetectedgroundsurfacewithin eachlaser

footprint.

Thebroadeningof theobservedgroundreturnsignalascomparedto thesystemimpulseresponse

canbeusedto estimatetheheightdistributionof thegroundwithin thelaserfootprint.In thisway,

informationontheaveragewithin-footprintelevationof thegroundandtherelief,dueto groundslope

and/orroughness(Figure11),canbeinferred.Thisderivationrequiresdeconvolutionof thesystem

impulseresponsefrom theobservedgroundreturn;thedatastructureparametersof distanceto the

peakandendof thegroundsignalincludetheimpulseresponsepulsewidth.Therefore,theyshould

notbeuseddirectlyasmeasuresof thedistanceto themedianandlowestgroundsurfaceswithin the

footprint.

Pulse Spreading Due to Surface Slope and Roughness

LU

I

l

I

AT = 4 nsec

IN CID ENT PULSE

\\\\\\\\\\\\\\\\\\\":_LL:_

BACKSCATTER ECH 0

\

SIGN AL AN PLITUD E BACKSCATTER

SO URCE DISTRIBUTIO N

Figure 11: Pulse Spreading Due to Surface Slope and Roughness

The vertical distribution of canopy surface area (i.e., within-footprint canopy height profiles) and

closure can be estimated from the vertical distribution of backscatter energy recorded in the waveform.

Methods for deriving these distributions, and comparisons to ground-truth data, are described in

Harding et al. (in review), Lefsky (1997), Lefsky et al. (1998; 1999; in press), and Means et al.

(1999).

13. Future Modifications and Plans

None. SLICER was decommissioned in 1998. Experience derived from SLICER was incorporated by

Blair et al. (1999) in a completely redesigned, next-generation scanning surface lidar system referred to

as the Lidar Vegetation Imaging Sensor (LVIS), which has superseded SLICER. LVIS incorporates a

new laser transmitter with a faster pulse repetition rate and a scanned receiver FOV, thus achieving

much wider scan patterns, and calibrated measurements of transmit and receive energy. LVIS is

Page 40
primarilyusedfor algorithmdevelopmentandvalidationactivitiesin supportof theVegetationCanopy

Lidar (VCL), thefirst missionselectedby NASA'sEarthSystemSciencePathfinderProgram(ESSP).

VCL is scheduledforlaunchintoEarth-orbitin 2000.

14. Software

14.1 Software Description

Aircraft trajectories were produced using PNAV, proprietary software marketed by Magellan

Corp./Ashtech Precision Products. PNAV is a Kalman filter-based program that processes GPS

observations sequentially by applying carrier-phase processing to dual-frequency, full-wavelength

data. Cycle slips are corrected by "on-the-fly" resolution of carrier-phase integer ambiguities.

All other data processing was implemented using IDL, a product of Research Systems, Inc. IDL

procedures for processing were written by personnel at GSFC. Geolocation procedures were written

principally by Bryan Blair, with contributions from David Rabine and Michelle Hofton. Procedures for

identification of ground returns, data editing, formatting, and plotting were written principally by

David Harding, with contributions from Jim Roark. David Harding wrote the browser procedures for

interactive display and editing of Level 3 data.

14.2 Software Access

PNAV: Magellan Corp./Ashtech Precision Products, 471 E1 Camino Real, Santa Clara, CA

95050-4300, (408) 615-5100, http://www.magellangps.com/.

IDL: Research Systems, Inc., 2995 Wilderness Place, Suite 203, Boulder, CO 80301, (303)

786-9900, http://www.rsinc.com/.

IDL processing procedures for SLICER are maintained by the LAPF (contact D. Harding or K. Still;

see Section 2.3). IDL procedures constituting the SLICER Level 3 data browser are included on the

BOREAS SLICER CD-ROMs. The latest, updated version of the browser is available from

http://denali.gsfc.nasa.gov/lapf/slicer/slicer.html.

15. Data Access

The SLICER data are available from the Earth Observing System Data and Information System

(EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC).

15.1 Contact Information

For BOREAS data and documentation please contact:

ORNL DAAC User Services

Oak Ridge National Laboratory

P.O. Box 2008 MS-6407

Oak Ridge, TN 37831-6407

Phone: (423) 241-3952

Fax: (423) 574-4665

E-mail: ornldaac@ornl.gov or ornl@eos.nasa.gov

15.2 Data Center Identification

Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory

(ORNL) Distributed Active Archive Center (DAAC) for Biogeochemical Dynamics

http://www-eosdis.ornl.gov/.

Page 41
15.3 Procedures for Obtaining Data

Users may obtain data directly through the ORNL DAAC online search and order system

[http://www-eosdis.ornl.gov/] and the anonymous FTP site [ftp://www-eosdis.ornl.gov/data/] or by

contacting User Services by electronic mail, telephone, fax, letter, or personal visit using the contact

information in Section 15.1.

15.4 Data Center Status/Plans

The ORNL DAAC is the primary source for BOREAS field measurement, image, GIS, and

hardcopy data products. The BOREAS CD-ROM and data referenced or listed in inventories on the

CD-ROM are available from the ORNL DAAC.

16. Output Products and Availability

16.1 Tape Products

None.

16.2 Film Products

None.

16.3 Other Products

Although the inventory is contained on the BOREAS CD-ROM set, the actual SLICER data are

not. See Section 15 for information about how to obtain the data.

In addition to the Level 3a and 3b SLICER data files, the SLICER CD-ROMs also contain

acquisition and processing notes, aircraft flight trajectory data, summary flight line location maps for

each study area and tower site, summary plots of each flight line that include line location, aircraft

attitude, surface elevation, and return pulse width (a measure of vegetation height), and programs for

use in IDL for interactively viewing and editing the data.

In addition, the LAPF maintains a Web site at that shows location maps and profiles for each flight

line. Refer to 1 for SLICER information.

17. References

17.1 Platform/Sensor/Instrument/Data Processing Documentation

The SLICER CD-ROMs include instrument and processing documentation in the Docmnts

directory as a PostScript file and as a Microsoft Word RTF file. In addition to the material contained

herein on Level 3 products, the documentation includes information on Level 2 data products.

17.2 Journal Articles and Study Reports

Blair, J.B., D.B. Coyle, J.L. Burton, and D.J. Harding. 1994. Optimization of an airborne laser

altimeter for remote sensing of vegetation and tree canopies. Proceedings of IGARSS '94.2:939-941.

Blair, J.B., D.L. Rabine, and M.A. Hofton. 1999. The Laser Vegetation Imaging Sensor (LVIS): A

Medium-Altitude, Digitization-Only, Airborne Laser Altimeter for Mapping Vegetation and

Topography. ISPRS J. Photogram. Rem. Sens. 54:115-122.

Bufton, J.L. 1989. Laser altimetry measurements from aircraft and spacecraft. Proc. IEEE

77(3):463-477.

Coyle, D.B. and J.B. Blair. 1995. Development of a q-switched/cavity-dumped, sharp pulsed laser

transmitter (SPLT) for airborne altimetry. Proc. Adv. Solid-State Lasers 24:5-8.

Page 42
Coyle,D.B., D.V. Guerra,andR.B.Kay. 1995.An interactive numerical model of diode-pumped,

q-switched/cavity-dumped lasers. J. Phys. D: Appl. Phys. 28:452-462.

Harding, D.J. 1998. Airborne lidar observations of canopy structure at the BOREAS tower flux sites.

Proc. IGARSS '98, Seattle, WA. 1550-1552 pp.

Harding, D.J., J.B. Blair, J.B. Garvin, and W.T. Lawrence. 1994. Laser altimetry waveform

measurement of vegetation canopy structure. Proceedings of IGARSS '94 2:1251-1253.

Harding, D.J., M.A. Lefsky, G.G. Parker, and J.B. Blair. (In review.) Lidar altimeter measurements

of canopy structure: Methods and validation for closed-canopy, broadleaf forests. Rem. Sensing of

Environ.

Lefsky, M., W. Cohen, S. Acker, G. Parker, T. Spies, and D. Harding. (In press.) Lidar remote

sensing of biophysical properties and canopy structure of forests of Douglas-fir and Western

Hemlock. Rem. Sensing of Environ.

Lefsky, M., W. Cohen, S. Acker, T. Spies, G. Parker, and D. Harding. 1998. Lidar remote sensing

of forest canopy structure and related biophysical parameters at H.J. Andrews experimental forest,

Oregon, USA. Proc. IGARSS '98, Seattle, WA. 1252-1254 pp.

Lefsky, M.A. 1997. Application of lidar remote sensing to the estimation of forest canopy and stand

structure. Ph.D. thesis, Environmental Sciences, University of Virginia, Charlottesville, VA.

Lefsky, M.A., D.J. Harding, W.B. Cohen, G.G. Parker, and H.H. Shugart. 1999. Lidar remote

sensing of forest basal area and biomass: application and theory. Rem. Sensing of Environ. 67:83-98.

Means, J.E., S.A. Acker, D.J. Harding, J.B. Blair, M.J. Lefsky, W.B. Cohen, M.E. Harmon, and

W.A. McKee. 1999. Use of large-footprint scanning airborne lidar to estimate forest stand

characteristics in the western Cascades of Oregon. Rem. Sens. Environ. 67:298-308.

Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J. Nickeson,

A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. 2000. Collected Data of The

Boreal Ecosystem-Atmosphere Study. NASA. CD-ROM.

Sellers, P. and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version

1994-3.0, NASA BOREAS Report (EXPLAN 94).

Sellers, P. and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version

1996-2.0, NASA BOREAS Report (EXPLAN 96).

Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994

Operations. NASA BOREAS Report (OPS DOC 94).

Sellers, P., F. Hall, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996

Operations. NASA BOREAS Report (OPS DOC 96).

Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B.

Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal

ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year.

Bulletin of the American Meteorological Society. 76(9): 1549-1577.

Page 43
Sellers,P.J.,F.G.Hall, R.D.Kelly, A. Black, D. Baldocchi,J. Berry, M. Ryan,K.J. Ranson,P.M.

Crill, D.P.Lettenmaier,H. Margolis,J.Cihlar, J.Newcomer,D. Fitzjarrald,P.G.Jarvis,S.T.

Gower,D. Halliwell, D. Williams, B. Goodison,D.E.Wickland, andF.E.Guertin.1997.BOREAS

in 1997:ExperimentOverview,ScientificResultsandFutureDirections.Journalof Geophysical

Research102(D24):28,731-28,770.

Vaughn,C.R.,J.L.Bufton, W.B. Krabill, andD. Rabine.1996.Georeferencingof airbornelaser

altimetermeasurements.Int. J.Rein.Sens.17(11):2185-2200.

17.3 Archive/DBMS Usage Documentation

None.

None.

18. Glossary of Terms

ASAS

ASCII

ATLAS

BOREAS

BORIS

BSQ

CD-ROM

DAAC

DEM

DTED

EGM96

EOS

EOSDIS

ESSP

FOV

GIS

GMT

GPS

GSFC

IDL

INS

LAPF

LVIS

NAD83

NASA

NIMA

NSA

OJP

ORNL

PANP

PDOP

PNAV

RMS

RSS

19. List of Acronyms

- Advanced Solid-state Array Spectroradiometer

- American Standard Code for Information Interchange

- Airborne Topographic Laser Altimeter System

- BOReal Ecosystem-Atmosphere Study

- BOREAS Information System

- Band Sequential

- Compact Disk - Read-Only Memory

- Distributed Active Archive Center

- Digital Elevation Model

- Digital Terrain Elevation Data

- Earth Geoid Model 1996

- Earth Observing System

- EOS Data and Information System

- Earth System Pathfinder Program

- Field of View

- Geographic Information System

- Greenwich Mean Time

- Global Positioning System

- Goddard Space Flight Center

- Interactive Data Language

- Inertial Navigation System

- Laser Altimetry Processing Facility

- Lidar Vegetation Imaging Sensor

- North American Datum of 1983

- National Aeronautics and Space Administration

- National Imaging and Mapping Agency

- Northern Study Area

- Old Jack Pine

- Oak Ridge National Laboratory

- Prince Albert National Park

- Position Dilution of Precision

- Precise Differential GPS Navigation and Surveying

- Root Mean Square

- Remote Sensing Science

Page 44
Si:APD

SLICER

SSA

TE

TF

TIU

URL

USGS

VCL

WFF

WGS84

WWW

- Silicon Avalanche Photodiode Detector

- Scanning Lidar Imager of Canopies by Echo

- Southern Study Area

- Terrestrial Ecology

- Tower Flux

- Time Interval Unit

- Uniform Resource Locator

- United States Geological

- Vegetation Canopy Lidar

- Wallops Flight Facility

- World Geodetic System of

- World Wide Web

Survey

1984

Recovery

20. Document Information

20.1 Document Revision Date

Written: 04-Apr- 1998

Updated: 16-Nov- 1999

20.2 Document Review Date(s)

BORIS Review: 27-Aug- 1999

Science Review:

20.3 Document ID

20.4 Citation

When using these data, please include the following acknowledgment as well as citations of

relevant papers in Section 17.2:

Development of the SLICER instrument was led by Bryan Blair, and collection and production of

the BOREAS SLICER data was led by David Harding, with instrument operations by David Rabine,

Sean Warren, and Earl Frederick and data processing by Kathy Still, all at NASA's Goddard Space

Flight Center.

Document authors: D.J. Harding, J.B. Blair, D.L. Rabine, and K.L. Still

If using data from the BOREAS CD-ROM series, also reference the data as:

Harding, D.J., "Airborne Laser Altimeter Characterization of Canopy Structure and Sub-canopy

Topography at BOREAS Tower Flux Sites." In Collected Data of The Boreal Ecosystem-Atmosphere

Study. Eds. J. Newcomer, D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J.

Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers. CD-ROM. NASA,

2000.

Also, cite the BOREAS CD-ROM set as:

Newcomer, J., D. Landis, S. Conrad, S. Curd, K. Huemmrich, D. Knapp, A. Morrell, J.

Nickeson, A. Papagno, D. Rinker, R. Strub, T. Twine, F. Hall, and P. Sellers, eds. Collected Data of

The Boreal Ecosystem-Atmosphere Study. NASA. CD-ROM. NASA, 2000.

20.5 Document Curator

20.6 Document URL

Page 45
REPORT DOCUMENTATION PAGE FormApproved

OMB No. 0704-0188

Public reporting burden for this collection of informationis estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

September 2000 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Technical Report Series on the Boreal Ecosystem-Atmosphere Study (BOREAS)

SLICER Airborne Laser Altimeter Characterization of Canopy Structure

and Sub-canopy Topography .... 923

6. AUTHOR(S) RTOP: 923-462-33-01

D.J. Harding, J.B. Blair, D.L. Rabine, and K.L. Still

Forrest G. Hall and Jaime Nickeson, Editors

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS (ES)

Goddard Space Flight Center

Greenbelt, Maryland 20771

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS (ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PEFORMING ORGANIZATION

REPORT NUMBER

2000-03136-0

10. SPONSORING / MONITORING

AGENCY REPORT NUMBER

TM--2000-209891

Vol. 93

11. SUPPLEMENTARY NOTES

D.L. Rabine and K.L. Still: SSAI; J. Nickeson: Raytheon ITSS

12a. DISTRIBUTION / AVAILABILITY STATEMENT

Unclassifie_Unlimited

Subject Category: 43

Report available from the NASA Center for AeroSpace Information,

7121 Standard Drive, Hanover, MD 21076-1320. (301) 621-0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

SLICER data were acquired in support of BOREAS at all of the TF sites in the SSA and NSA, and

along transects between the study areas. Data were acquired on 5 days between 18-Jul and 30-Jul1996. Each coverage of a tower site is typically 40 km in length, with a minimum of 3 and a maximum of 10 lines across each tower oriented in a variety of azimuths. The SLICER data were acquired

simultaneously with ASAS hyperspectral, multiview angle images. The SLICER Level 3 products

consist of binary files for each flight line with a data record for each laser shot composed of 13 parameters and a 600-byte waveform that is the raw record of the backscatter laser energy reflected from

Earth's surface. The SLICER data are stored in a combination of ASCII and binary data files.

14. SUBJECT TERMS

BOREAS, remote sensing science, SLICER.

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

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Unclassified

19. SECURITY CLASSIFICATION

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15. NUMBER OF PAGES

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20. LIMITATION OF ABSTRACT

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