BOREAS Scanning Lidar Imager of Canopies by Echo Recovery:  Level-3 Data 
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-Jul-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
   1) Data Set Overview
   2) Investigator(s)
   3) Theory of Measurements
   4) Equipment
   5) Data Acquisition Methods
   6) Observations
   7) Data Description
   8) Data Organization
   9) Data Manipulations
   10) Errors
   11) Notes
   12) Application of the Data Set
   13) Future Modifications and Plans
   14) Software
   15) Data Access
   16) Output Products and Availability
   17) References
   18) Glossary of Terms
   19) List of Acronyms
   20) 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 transects that are approximately 50 m 
wide.  The data have a horizontal resolution of approximately 10 m and a 
vertical resolution of approximately 1 m.  Each laser footprint is individually 
georeferenced by combining the laser ranging data with orientation and 
positional information provided by the Inertial Navigation System (INS) and 
Global Positioning System (GPS), respectively.  SLICER was flown onboard a 
National Aeronautics and Space Administration (NASA) C130-Q aircraft on 5 days 
between 18-Jul and 30-Jul-1996, acquiring data along transects crossing all the 
Tower Flux (TF) sites in the Southern and Northern Study Areas (SSA and NSA, 
respectively) of the BOReal Ecosystem-Atmosphere Study (BOREAS) study region 
(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).
1.6 Related Data Sets
BOREAS RSS-02 Extracted Reflectance Factors Derived from ASAS Imagery
BOREAS RSS-02 Level-1b 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
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.
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.
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).
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 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.
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 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:
1) 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.
2) The intensity of the backscatter return reflected from the surface as a 
function of time.
3) 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.
4) The trajectory of the aircraft, obtained from a kinematic differential 
solution of dual-frequency GPS observations.
5) 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 by an onboard GPS receiver at 2 Hz and recorded by the 
altimetry data system.  A relative transmit time is measured with 0.016-msec 
resolution by using a time counter which is based on a PC timer card containing 
a GHz oscillator that is synched using a 1 pulse per second time mark provided 
by the GPS receiver.  The laser transmitter includes a photodiode detector that 
samples a small fraction of the output pulse energy at a point within the 
transmit beam.  The voltage signal from the photodiode detector is accumulated 
in an integrator, yielding a measure of output pulse energy as a single byte 
value.  After exiting the laser transmitter, 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 used is not recorded in the data 
structure.
Output pulses are directed downward and scanned across the aircraft flight 
direction by means of a galvanometer-driven scan assembly.  The computer-
controlled galvanometer rotates a small mirror to a specified angular position 
prior to initiation of each laser pulse fire command.  Laser pulse energy that 
is backscattered from a reflecting surface at 0_ phase angle (i.e., the hot spot 
orientation) is collected by a receiver telescope and focused onto a 50-MHz 
Si:APD that converts optical energy into an output voltage.
The output voltage from the detector is split between a TIU and an analog-to-
digital digitizer. The TIU precisely measures the time interval between the 
transmitted laser pulse and the received backscattered pulse based on a leading-
edge, threshold-detection scheme. The TIU contains a high-frequency internal 
oscillator that yields 156.25 picosecond timing resolution of the pulse round-
trip travel time, equivalent to a one-way range resolution of 2.3 cm.  A start-
pulse fiber optic transfers a small portion of the transmit laser pulse energy 
into the receiver telescope, where it is focused onto the Si:APD detector.  The 
TIU starts when the leading edge of the transmit pulse rises above a start-pulse 
threshold value, which is established by the instrument operator.  The TIU stops 
when the output voltage of the detector rises above a stop-pulse detection 
threshold, which is also established by the instrument operator.  The detector 
output voltage can exceed the stop-pulse detection threshold due to a valid 
return from a reflecting surface or due to instrument electronic noise and/or 
background solar illumination.  A range gate is used to define a ranging time 
over which the stop-pulse detection threshold will be applied.  A signal greater 
than the detection threshold occurring before or after the range gate does not 
stop the TIU.  The range gate is established by the operator to bracket the 
distance to the anticipated target (e.g., land surface), given the known flight 
altitude above the ground.  In some cases, no detector output voltage above the 
threshold occurs within the range gate, yielding a no-range result.  The stop-
pulse detection threshold and range gate are established to maximize the 
frequency of ranging to valid returns and minimize the number of noise and no-
range results.  Valid returns are received from the first encountered surface 
that backscatters sufficient transmitted laser energy to exceed the stop-pulse 
detection threshold.  This can be an optically dense cloud layer, a vegetation 
canopy top, bare ground in unvegetated areas, or an ice or water surface.
Upon detection of a signal above the stop-pulse threshold, the TIU measure of 
the start to stop time interval is recorded, which corresponds to the round-trip 
travel time of the transmitted laser pulse.  A value of zero is recorded for no-
range results.  In addition to TIU ranging to the first detected return, SLICER 
implements a surface lidar function by digitizing the time history of the energy 
backscattered from the surface.  Upon detection of the stop pulse, a constant 
number of bins are read from the digitizer, yielding a waveform for that laser 
shot that records return amplitude as a function of time.  The digitizer 
continuously samples the detector output and scales the voltage to an 8-bit 
value.  A constant offset is added after scaling to displace the detector mean 
background noise level, which is zero-referenced, to yield all positive values.  
The scale factor in the analog-to-digital conversion and the offset applied 
typically remain constant during a SLICER flight mission, but could vary between 
flight missions.  Upon detection of the TIU stop event, a specified number of 
bins are read from the digitizer and stored, yielding a waveform that records 
the return amplitude as a function of time.  For most SLICER acquisitions, 
including BOREAS, 600 digitizer bins were stored.
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.  Vertical resolution refers to the separation required 
between surfaces in order to distinguish backscatter energy reflected from each 
of the surfaces.  The convolution of the transmit pulse shape in time with the 
broadening caused by the detector response (defined by its bandwidth) yields an 
impulse response, theoretically the narrowest possible return signal for 
reflection from a perfectly smooth, flat surface.  The 4-nsec (full-width at 
half-max) SLICER pulse width and 50-MHz detector bandwidth combine to yield an 
impulse response pulse width of approximately 6 nsec (full-width at half-max), 
equivalent to a width of 1.8 m.  Note, however, that the width of the impulse 
response increases with increasing optical energy received (i.e., as the peak 
amplitude increases).  The vertical resolution of the system is one-half the 
impulse response (0.9 m), because the travel distance of the laser pulse down-
and-back between two surfaces is twice their vertical separation.
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.  
The digitizer samples the detector output voltage at a rate of 1.348 GHz, with a 
resulting temporal sampling of 0.742 nsec.  Because the digitizer is recording 
the two-way transit time of light (e.g., down through a canopy and back out), 
the vertical sampling distance is half the temporal sampling (i.e., 0.371 nsec), 
equivalent to 0.1112 m per waveform bin.  The waveform is normally stored at the 
full digitizer bin resolution (0.1112 m).  In some cases, digitizer bins are 
averaged together before storing in the waveform array, yielding a waveform 
sampling distance increased by an integer averaging factor.  No digitizer 
averaging was applied to the BOREAS data.
The bins read from the digitizer correspond in time to a range of bins 
referenced to the TIU stop event, with the stop event assigned in the 
acquisition software to a specific waveform bin position.  For BOREAS, that 
position is bin 28, using an array-labeling convention where the initial 
position is bin 0.  Bins 0 to 27 of the waveform correspond to the detector 
output voltage recorded prior to the stop event (i.e., background noise before 
the start of the return).  The remaining bins from the TIU stop event to the end 
of the waveform (bins 28 to 599) correspond to a height range of 63.6064 m (572 
bins x 0.1112 m/bin), which is significantly greater than the height of the 
canopies studied.  Thus, the waveform extends beyond the distance at which the 
ground return is expected.  The last portion of the waveform, therefore, 
corresponds to background noise occurring after any ground reflection.  The 
digitizer implementation in SLICER is flexible, permitting adjustment in the 
number of total bins, distance resolution per bin, and bin position of the TIU 
stop event.  The values reported here are specific to the BOREAS data set.  
Appropriate digitizer factors for other data sets are contained in the header 
record of each Level 3 data file.
The location of the laser footprint, referenced to the first detected 
reflection, is determined by combining the TIU ranging data with knowledge of 
the laser pointing angle and the absolute position of the aircraft derived from 
a GPS trajectory.  The laser pointing angle is determined by combining the scan 
mirror angle with a measurement of instrument orientation, obtained using a 
Litton LTN-92 INS.  The INS reports roll, pitch, and yaw parameters computed 
from gyroscope-determined accelerations 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 shot.  The aircraft trajectory is determined by postflight 
processing of 2-Hz differential kinematic GPS data employing GPS receivers on 
the aircraft and at a fixed base station.  Ashtech Z-12 dual-frequency receivers 
were used for this study.
4.1.6 Sensor/Instrument Measurement Geometry
The GPS avionics antenna is mounted on top of the aircraft fuselage on the 
airframe 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 top of the optical bench, 
generates a horizontally transmitted laser pulse. Spatially each laser pulse is 
circular in cross-section with a Gaussian distribution of energy.  The 
divergence of the laser beam ranged from approximately 2 mrad, defined as the 
angle at which the pulse energy has fallen off to 1/e2 times (13.5%) that at the 
beam center.  Temporally, the laser pulse has a Raleigh distribution with a fast 
leading-edge rise time, on the order of 1.5 nsec, and a trailing-edge that falls 
off more slowly asymptotically.
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 flight altitude above ground at 
BOREAS was 4500 m, yielding footprints nominally 9 m in diameter.  Cross-track 
footprint spacing is determined by the angular separation between successive 
transmitted pulses and aircraft altitude above ground.  The angular separation 
is determined by the programmed angular positions of the mirror and aircraft 
roll.  For this work, the galvanometer was controlled to yield footprints that 
are contiguous across the track during level flight (i.e., 2-mrad cross-track 
angle yielding 9-m cross-track spacing).  However, low-frequency roll excursions 
of several degrees during data acquisition, combined with the high-frequency 
galvanometer scanning, make the cross-track pattern of footprints non-uniform.  
Along-track spacing is determined by laser pulse repetition rate, number of 
cross-track footprints, aircraft ground speed, and changes in pitch attitude.  
Changes in pitch were small and thus have an insignificant effect on along-track 
spacing.  With five cross-track footprints and a laser pulse repetition rate of 
80 Hz, the typical ground speed resulted in a nominal along-track spacing of 
approximately 10 m.  However, wind speed and direction at the aircraft altitude 
affects ground speed and the resulting along-track spacing.
Laser illumination backscattered from reflecting surfaces within each footprint 
is collected in a receiving telescope that has an aperture diameter of 20 cm and 
a 10 mrad field-of-view (FOV).  The telescope housing extends downward from 
optical bench to just above the window.  The amount of backscatter energy 
collected by the telescope depends on its aperture and the distance to the 
reflecting target, decreasing with the square of the distance assuming 
Lambertian scattering from the target.  The five contiguous, cross-track 
footprints (each having 2-mrad divergence) were aligned to fit across the 10-
mrad receiver FOV.  The receiving telescope's optical efficiency is uniform 
across most of the FOV but declines at the edges by approximately 15%, so the 
backscatter energy collected for the outer scan footprints is slightly lower 
than for the central footprints.
The backscattered laser light and any background reflected solar illumination 
collected by the receiver telescope is focused through a bandpass filter onto a 
1-mm-diameter Si:APD.  The bandpass filter significantly reduces background 
solar illumination by only permitting transmission of light in a narrow, 2-nm-
wide wavelength range centered at the laser wavelength.  The Si:APD detector 
converts input optical energy into an output voltage.  The high bandwidth of the 
detector (50 MHz) achieves temporal resolution of the backscattered laser energy 
that is nearly equal to the duration of the transmitted pulse.  The combination 
of narrow laser pulse width and high detector bandwidth yields a vertical 
resolution of approximately 0.9 m, where vertical resolution refers to the 
separation required between surfaces in order to distinguish backscatter energy 
reflected from 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
Receiver telescope:
   NASA GSFC, Jack Bufton, lead
   Code 920, (301) 286-8591, jbufton@pop900.gsfc.nasa.gov
Detector:
   EG&G, Inc.
   45 William Street
   Wellesley, MA 02481
   (781) 237-5100
Timing electronics and waveform digitizer:
   LeCroy Corporation
   700 Chestnut Ridge Rd
   Chestnut Ridge, NY 10977   
   (800) 553-2769
Inertial Navigation System:
   Litton Industries, Inc.
   21240 Burbank Boulevard
   Woodland Hills, CA 91367-6675
   (818) 598-5000 
GPS Receivers:
   Magellan Corp./Ashtech Precision Products
   471 El Camino Real
   Santa Clara, CA 95050-4300
   (408) 615-5100
GPS to altimeter clock box:
   NASA GSFC, John Ward, lead
   Code 571, (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 flat, 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.  The time delays for each of 
the attitude parameters consists of two components, an INS instrument reporting 
delay and a synch-up delay between the constant INS data rate and the software-
controlled laser fire rate.  The INS instrument reporting delay is constant for 
each parameter and is determined by applying a time offset that yields level 
water.  The synch-up delays change between each data file and are determined by 
an iterative shifting and interpolation of the recorded attitude parameters with 
respect to the laser fire times that is done automatically as a part of the 
geolocation processing.  Roll and pitch alignment biases are also established by 
iteratively varying each until a sufficiently level water surface results.  The 
geolocation results are insensitive to small errors in yaw, so no yaw bias is 
established or applied to the attitude data.
The TIU range data are corrected for a range bias due to a system timing delay 
caused by optical and electronic signal delays, which vary between installations 
of the instrument.  The system delay is found by solving for the elevation of a 
surface at a known distance.  Typically, data acquired in a flight across a site 
with an established geodetic benchmark are used, such as the location of the GPS 
base station, yielding an end-to-end system delay measurement.  The system delay 
established for the BOREAS installation was 15.4 m.
The transmit energy per pulse was measured in a laboratory prior to installation 
on the aircraft using an external energy meter.  The transmit energy per pulse 
is nominally 0.7 mJ.  However, the transmit energy jitters from shot to shot by 
about 10% due to random effects.  During flight, the laser output energy per 
shot is monitored by an internal photodiode detector and recorded in the data 
structure, but the noise level on this monitor is very high (as much as 50% of 
signal), greatly exceeding the actual energy variation due to shot-to-shot 
jitter.
In addition to random shot-to-shot jitter, laboratory results document that the 
laser output energy varies significantly as a function of laser temperature.  
The transmitter operates most efficiently when the laser diodes are at 30 _C; 
below and above this temperature, the transmitter is less efficient and pulse 
energy is reduced.  Excess heat is generated during operation of the 
transmitter, so a thermal management system (thermal electric coolers, copper 
heat strapping to a heat sink, and fans) is used to remove the excess heat, 
maintaining the diodes at or near 30 _C. The shot energy monitor does show long-
term variations during the course of a flight line acquisition that is likely 
due to changes in laser transmitter temperature.  A fairly rapid increase in 
laser energy over several minutes at the start of an acquisition is likely due 
to the diode temperature increasing from ambient conditions up to 30 _C.  A 
gradual decrease in shot energy then often occurs, probably as the diode 
temperature slowly exceeds 30 _C.  However, the observed variations may also be 
artifacts due to thermally induced mechanical flexure of the photodiode detector 
mount.  In addition, mechanical instability of the pulse energy monitor has been 
observed, where aircraft attitude variations (especially during pitch 
calibration maneuvers) cause significant variation in the recorded pulse energy 
levels.  Also, in some instances the long-term trend in laser transmit energy is 
abruptly offset in amplitude. The amplitude offset is constant and applies to 
large, continuous segments of laser shots (100's to 1000's of shots). The abrupt 
offsets are probably not actual changes in transmit energy.  The cause of the 
offset is not known but is likely due to a gain or bias shift in the start pulse 
energy detection electronics.
4.2.1 Specifications
The waveform digitizer sampling rate, defining the 0.1112 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.
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:
1) 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.
2) Initialize the INS unit while the aircraft is not moving.
3) 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).
4) 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.
5) 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).
6) Prior to landing, acquire data across the GPS base station location to be 
used in data postprocessing to establish the system range bias.
7) 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.
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 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 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 km.  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   98.02747 W   55.84225 N 
NSA-FEN   98.42072 W   55.91481 N 
NSA-OBS   98.48139 W   55.88007 N 
NSA-OJP   98.62396 W   55.92842 N 
NSA-YJP   98.28706 W   55.89575 N 
SSA-FEN  104.61798 W   53.80206 N 
SSA-OBS  105.11779 W   53.98717 N 
SSA-OJP  104.69203 W   53.91634 N 
SSA-YJP  104.64529 W   53.87581 N 
SSA-9OA  106.19779 W   53.62889 N 
SSA-9YA  105.32314 W   53.65601 N 
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.
7.1.4 Projection
Each laser footprint is individually georeferenced with latitude, longitude, and 
elevation to the World Geodetic System of 1984 (WGS84) ellipsoid.  Latitude and 
longitude in the WGS84 ellipsoid reference frame are essentially identical to 
those referenced to the NAD83 ellipsoid.  The two can be considered to be 
identical for all practical purposes.
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
Data characteristics are defined in the companion data definition file 
(lidar3.def).
7.4 Sample Data Record
Sample data format shown in the companion data definition file (lidar3.def).
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 km.
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.
Parameter   Stored As  Bytes  Scale Factor  Scale Factor  Output As  Bytes
                              Divisor       Precision
header record:
tiu_bin     long       4      none          -             long       4
dig2wf      long       4      none          -             long       4
wvfm_bins   long       4      none          -             long       4
numshots    long       4      none          -             long       4
data records:
shotnum     long       4      none          -             long       4
beam        long       4      none          -             long       4
starten     long       4      none          -             long       4
gpstime     long       4      1.00E+04      double flt    double flt 8
diameter    long       4      1.00E+06      float         float      4
azimuth     long       4      1.00E+06      float         float      4
inclination long       4      1.00E+06      float         float      4
latitude    long       4      1.00E+06      double flt    double flt 8
longitude   long       4      1.00E+06      double flt    double flt 8
elevation*  long       4      1.00E+06or04* float         float      4
grndstart   long       4      1.00E+06      float         float      4
grndpeak    long       4      1.00E+06      float         float      4
grndend     long       4      1.00E+06      float         float      4
waveform    byte array 600    none          -             byte array 600
Data Record Total:     652                                           664
* NOTE: The elevation scale factor of 1.00E+06 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 routines included on the CD-ROM).  The file format 
is the same as that of the Level 3a data.  The naming convention is changed, 
consisting of an eight-character name and an .edt suffix to indicate the data 
was edited.  The eight characters consist of a four-letter designation of the 
tower site followed by DDLL (day and line number).  The four-letter designation 
follows the BOREAS site abbreviation conventions (i.e., NOBS, SOBS, SOJP).  The 
Level 3b files are located in the Towr_edt subdirectory within the Sitedata 
directory on the SLICER CD-ROMs.
The Towr_dat subdirectory within the Sitedata directory on the SLICER CD-ROMs 
consists of exact copies of the Level 3a data for the 1,001 shot segments near 
the towers.  The automated identification of the ground return location within 
the waveform has been retained; no manual editing of the ground return location 
was done.  The naming convention follows that of the Level 3b data, except that 
the .dat suffix is preserved to indicate no editing was done.
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 ground return.  Therefore, the 
start of the last return is identified based on the width characteristics of the 
asymmetric, Raleigh-shaped system impulse response.  The impulse response is the 
theoretical signal recorded from a smooth and flat surface and depends on the 
convolved effects of pulse width and detector response.  The SLICER impulse 
response is established from minimum-width returns from water surfaces.  A ratio 
is determined for the impulse response between the width from the signal end to 
peak compared to the width from peak to start.  The observed end-to-peak width 
of the last return is scaled by this ratio in order to define the start position 
of the last return.  This method accounts for any pulse broadening of the last 
return due to slope or roughness of the ground within the footprint.
The distances to the ground return start, peak, and end are referenced to the 
TIU_BIN waveform position.  This is the waveform bin corresponding to the first 
return received above the instrument detection threshold that causes the TIU 
stop event (i.e., canopy top in the presence of vegetation).  The latitude, 
longitude, and elevation parameters defining the location of the laser return 
are specific to this TIU_BIN position in the waveform.  The TIU stop-pulse bin 
position is assigned in software and is constant for a data file, but it can 
vary significantly between files if instrument operational parameters are 
changed.  The waveform bin position corresponding to the TIU stop-pulse time is 
not recorded in the data system; instead, it is established for a data file by 
visually assessing multiple, overplotted waveform records.  Typically, water 
returns are used to determine in which bin the signal level first consistently 
increases above the background noise.  This observed bin position is the value 
recorded in the TIU_BIN parameter of the Level 3 data header.
9.2 Data Processing Sequence
9.2.1 Processing Steps
The SLICER data processing sequence is diagrammed and described in Figure 7.
An editing step is applied in converting Level 2 products to Level 3 in order to 
delete laser shots that did not yield valid returns.  Invalid laser shots that 
are excluded consist of: (1) no-range data, (2) clouds and noise above an 
elevation threshold set above the highest expected ground, (3) noise below an 
elevation threshold set below the lowest expected ground, (4) and a few percent 
of shots with anomalously low start pulse energy (described in Section 11.2).  
No-range data consist of laser shots for which no backscatter signal was 
received above the instrument detection threshold.  For the BOREAS data, the 
high and low elevation thresholds to remove cloud returns and noise were 700 and 
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 data so the bin resolution is always 0.1112 m.  If digitizer averaging is 
applied it is reported in the header record of each data file.
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 (.gif) 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 region.  The SLICER to DTED results were, therefore, judged to be 
unreliable and not a useful measure of SLICER geolocation accuracy. No such 
comparison was done for the NSA because DTED data are not available for that 
region.
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.
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 
km), the trajectory accuracy is probably at the meter-level (horizontal and 
vertical) based on unpublished comparisons of trajectories derived for common 
data sets by PNAV and software optimized for long baselines (GITAR trajectories 
provided by B. Krabill, GSFC Code 972).  The PNAV solution is reliable only with 
a minimum of five satellites observed and a Position Dilution of Precision 
(PDOP) of less than 4.  The RMS position error is only valid in an absolute 
sense when certain conditions are met (which is probably not the case for these 
long baseline flights), but it gives a relative sense of where the solution is 
of poorer accuracy.
The accuracy of the footprint geolocation results depends on the accuracy of the 
ranging data; pointing knowledge; platform trajectory; and determination of the 
system, angle, and time biases.  For the high altitudes typical of the BOREAS 
acquisitions (nominally 4.5 km above ground), pointing errors are the largest 
source of geolocation errors.  Based on the specified accuracy of the INS and 
the magnitude of the unmonitored alignment drifts, the pointing knowledge 
accuracy should be on the order 2 mrad, yielding position accuracies equivalent 
to the laser footprint diameter in the nominal five-beam mode.  Geolocation 
accuracies of this order have been verified for non-BOREAS SLICER data sets by 
comparing SLICER elevation profiles to corresponding image features (e.g., edges 
of building, forest clear cut boundaries) in several independently georeferenced 
high-resolution images and to topography in independently georeferenced DEMs.  
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 second 
(approximately 30 m).  In most cases, the SLICER elevation profiles match the 
DEMs, indicating that the SLICER geolocation is accurate at least at the DEM 
pixel scale.  In a few cases, unexplained horizontal offsets of the SLICER 
profiles are observed (see Section 11.2 for a list of flight lines with known 
geolocation problems).
The precision and accuracy of the identification of the ground return in the 
backscatter waveform depend on the algorithm employed (see Section 9.1.1).  The 
precision of the ground start, peak, and end distances is reduced from the 
intrinsic waveform bin resolution (0.1112 m for BOREAS) by the summation of 
adjacent bins to improve signal-to-noise.  Four adjacent bins are typically 
summed, yielding a precision of 0.44 m for the ground distance parameters.  
Also, the half-width determination of the start of the ground return is noisy, 
at the submeter level, due to the sensitivity of the ground end threshold 
determination.  The ground end threshold technique is sensitive to the 
asymptotic fall-off in energy of the transmit-pulse trailing edge and to the 
potential presence of a variable amplitude back porch effect during periods of 
non-optimal beam quality.  A peak fitting approach to selection of the last 
return likely would achieve a more accurate identification of the ground signal.
The accuracy with which the TIU_BIN position in the waveform can be established 
depends on the visual inspection of multiple, overlaid waveform plots (see 
Section 9.1.1).  The appropriate bin position can only be determined with this 
method to within several waveform bins due to bin jitter from shot to shot, and 
the variable range-walk character of the return pulse leading edge as a function 
of surface conditions.  TIU_BIN jitter is caused by inexact timing 
synchronization between the TIU oscillator and the waveform digitizer 
oscillator.
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.
11. 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:
1) Detector efficiency and gain.
2) Radial fall-off in the efficiency of the receiver telescope.
3) Scanner-to-receiver alignment.
4) 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).
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.
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, 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 incremental difference of the 
aircraft altitude, from one altitude measurement to the next, in the GPS 
trajectory shows little (less than 1 m) to no instantaneous error associated 
with the onset of these events.  However, short-term (seconds to minutes) 
trajectory errors at the meter-level could be associated with these events due 
to the Kalman filtering utilized by the PNAV software.
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 in the computation for all files, causing an error for those 
flight lines where a greater divergence was used to acquire the data.  This 
error is present in one BOREAS file (96072908) and a number of files from other 
missions.  The following files must have the footprint diameter values in the 
Level 3 files multiplied by the correction factor indicated below to yield the 
proper diameter.  The latest version of the SLICER data browser obtainable from 
the LAPF Web site automatically corrects the footprint diameters when opening 
these erroneous files.
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          x1.5
95092410          Canopy Crane, WA            x5
95093004to08      Death Valley, CA            x5
95111603          Orlando, FL                x1.5
95111611to22      Orlando, FL                 x5
95111624          Orlando, FL                 x5
94082414to17      Performance, MD            x1.5
94082418          Performance, MD             x4
94082419          Performance, MD            x1.5
94100503to07      Duke Forest, NC            x1.5
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:
YYMMDD  	Mission             	Flight Lines
941005  	Duke Forest, NC     	4, 6, 8
941006  	Mahantango, PA      	3, 4, 5, 6, 7, 8
950907  	SERC, MD            	4, 5, 7, 8, 9, 10, 11, 12
950919  	Mt. St. Helens, WA  	4, 5, 6, 7, 8, 10
950920  	Mt. St. Helens, WA  	3
950924  	Mt. St. Helens, WA  	3
950920  	Mount Rainier, WA   	5, 6, 7, 8, 10, 11, 12, 13
950924  	Mount Rainier, WA   	5, 6, 7, 8
950921  	Canopy Crane, WA    	4, 5, 6, 7, 8, 9, 10, 11
950924  	Canopy Crane, WA    	2, 4, 10
950923  	Coast Range, OR     	3, 4, 6, 7, 8, 11, 12
950928  	Long Valley, CA     	2, 3, 4a, 5, 6, 7, 8, 9, 10, 12, 13
950930  	Long Valley, CA     	2, 3, 4
950903  	Death Valley, CA    	8, 9
951116* 	Orlando, FL         	7, 9, 10, 11, 12, 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               NS offset    EW offset
94100505     Duke Forest, NC         0.00 m     -49.95 m
94100507     Duke Forest, NC       -30.89 m     -49.93 m
95091912*    Mt. St. Helens, WA      0.00 m      64.11 m
95092401*    Mt. St. Helens, WA      0.00 m    -106.99 m
95092112     Canopy Crane, WA       92.67 m    -128.96 m
95092309     Coast Range, OR         0.00 m     -43.94 m
9509284b     Long Valley, CA       -24.46 m     154.44 m
95092811     Long Valley, CA           no convergence
95093007     Death Valley, CA          no convergence
95093010     Death Valley, CA       30.89 m      49.68 m
95111606     Orlando, FL          -123.56 m     -27.10 m
95111608     Orlando, FL           -61.78 m     245.29 m
95111613     Orlando, FL            30.89 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 sites 
where the topography exceeds this limit.  SLICER CD-ROMs for non-BOREAS sites 
where the topography exceeds this 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.  The 1.00E+04 elevation scale factor was used on 19-Sep, 
20-Sep, 24-Sep, 28-Sep, and 30-Sep-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.
Two flight lines are mislabeled on the map showing the location of ground tracks 
in the vicinity of the SSA-OJP tower located in the Tower_ps subdirectory in the 
Sitedata directory.  The labels for lines 96072909 and 9607296b are inadvertently 
switched on the map.  Also, the flight log notes for 96072909 should indicate 
that, like 96072908, it was acquired at approximately 2,400 feet above ground 
level using a 10-mrad beam divergence in profiling mode. 
11.3 Usage Guidance
In order to find SLICER data for a specific BOREAS tower site, refer to the file 
fluxtowr.sum in the Documnts directory on the SLICER CD-ROMs.  It contains a 
summary that lists for each SLICER flight line the TF site(s) crossed by that 
flight line, and the approximate laser shot number in the data record closest to 
the tower.  Also, detailed flight line maps, as PostScript files, showing the 
location of the laser footprint ground tracks in the vicinity of each tower are 
located in the Tower_ps subdirectory of the Sitedata directory.  Each ground 
track is labeled with a four-digit number corresponding to the flight day of 
year and line number.
Due to the numerous instrument factors that affect the digitized record of 
backscatter return (described in preceding sections), the SLICER waveform 
amplitudes are uncalibrated and should not be used as an absolute measure of 
return signal strength nor of surface reflectance.  Some of the factors 
affecting return signal strength are constant.  In particular, the outward fall-
off in receiver telescope sensitivity is constant, and the resulting lower 
return strength of the outer beams, as compared to the inner beams, could be 
established by a comparison of returns from a surface of uniform reflectivity, 
such as water.  However, many of the relevant instrument factors are unmonitored 
and vary slowly over time, including transmit pulse energy, the temperature-
dependent detector sensitivity, and alignment drift between the scanned transmit 
pulses and the receiver telescope FOV.  Over short segments of data (seconds to 
minutes), these slowly varying effects probably do not significantly change 
instrument performance.  Thus, successive shots along a single beam position can 
be compared in a relative sense.  Other unmonitored factors abruptly change 
digitized signal strength, including filters used to attenuate the transmit beam 
strength and the digitizer gain setting.  Changes in these factors should appear 
as instantaneous, and usually pronounced, changes in receive energy from shot to 
shot.  However, the attenuation differences between some of the filters are 
quite small, and the resulting difference in received signal may not be easily 
recognized.  Because of these numerous effects on digitizer amplitude, the most 
reliable use of SLICER waveforms is to treat each independently and use it only 
as a relative measure of the vertical distribution of return signal strength for 
that footprint.  Furthermore, for cases of detector saturation, preventing the 
correct measurement of peak return amplitudes, the waveform record should not be 
used even in this relative sense.
The Level 3a identification of the ground return start, peak, and end in each 
waveform is based on automated signal processing software.  There are many 
circumstances in which the ground return can be misidentified (see Section 
11.1).  In the vicinity of each flux tower, Level 3b products (.edt files in the 
Towr_edt subdirectory of the Sitedata directory) have been produced where the 
ground return for each laser return has been visually inspected and edited as 
needed.  Users of Level 3a data, for non-tower data, should inspect, and modify 
as needed, the automated identification of the ground returns, using the 
provided IDL-based data browser or other plotting software.  Profile plots of 
individual cross-track beam positions showing the elevation of the canopy top, 
ground start, ground peak, and ground end (see Section 12) provide a visual 
indication of incorrectly identified ground return parameters due to 
inconsistencies from shot to shot along the profile.
In most cases, the SLICER geolocation accuracy is thought to be at the scale of 
the laser footprint (i.e., approximately 10 m or better).  However, some flight 
lines for non-BOREAS data sets are known to be mislocated by up to 100's of 
meters based on comparisons with DEMs and georeferenced images (see Section 11.2 
for a list of mislocated flight lines in the U.S.).  Users of BOREAS SLICER data 
who are comparing SLICER results to other data at specific locations should 
confirm the accuracy of the SLICER geolocation results.  This is best done by 
identifying distinctive features in SLICER ground and canopy top elevation 
profiles, such as distinct forest stand edges, and confirming that these 
features are consistent with the corresponding features in independently 
georeferenced high-resolution images.
SLICER elevations are referenced to the WGS84 ellipsoid. To compare SLICER 
elevations to topographic data (maps or DEMs) which typically use a vertical 
datum referenced to mean sea level, an ellipsoid to geoid correction needs to be 
applied.  Because the ellipsoid to geoid separation varies spatially, the 
correction needs to be applied to each laser footprint as a function of its 
latitude and longitude.  The U.S. National Geodetic Survey distributes software 
called GEOID which computes the ellipsoid to geoid separation within the 
conterminous U.S. as a function of latitude and longitude.  Similar software is 
likely available for the BOREAS region from the Canadian geodetic survey.  EGM96 
provides a global representation of the geoid.  EGM96 was jointly developed by 
GSFC and the Defense Mapping Agency.
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.
Times in the aircraft trajectory files are with respect to GMT and time in the 
SLICER data files are with respect to GPS time.  GMT times are less than the GPS 
times by a small, constant number of seconds due to the difference between GMT 
and GPS time in effect at the time of the flight.  GMT progressively falls 
behind GPS time as leap seconds are added to GMT but not to GPS time.  The GPS  
GMT time difference during the BOREAS acquisition was 11 seconds.  During the 
period of SLICER operations, the GPS - GMT time difference was as follows after 
the listed date:
31-DEC-1990      +7 sec
30-JUN-1992      +8 sec
30-JUN-1993      +9 sec
30-JUN-1994     +10 sec
31-DEC-1995     +11 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 
parameter, the ground profile consists of the highest detected ground surface 
within each laser footprint.
The broadening of the observed ground return signal as compared to the system 
impulse response can be used to estimate the height distribution of the ground 
within the laser footprint.  In this way, information on the average within-
footprint elevation of the ground and the relief, due to ground slope and/or 
roughness (Figure 11), can be inferred.  This derivation requires deconvolution 
of the system impulse response from the observed ground return; the data 
structure parameters of distance to the peak and end of the ground signal 
include the impulse response pulse width.  Therefore, they should not be used 
directly as measures of the distance to the median and lowest ground surfaces 
within the footprint.
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 primarily used for algorithm development and validation 
activities in support of the Vegetation Canopy Lidar (VCL), the first mission 
selected by NASA's Earth System Science Pathfinder Program (ESSP).  VCL is 
scheduled for launch into Earth-orbit in 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 El 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
15.1  Contact for Data Center/Data Access Information
These BOREAS data are available from the Earth Observing System Data and 
Information System (EOS-DIS) Oak Ridge National Laboratory (ORNL) Distributed 
Active Archive Center (DAAC). The BOREAS contact at ORNL is:
ORNL DAAC User Services
Oak Ridge National Laboratory
P.O. Box 2008 MS-6407
Oak Ridge, TN 37831-6407
(865) 241-3952
ornldaac@ornl.gov
ornl@eos.nasa.gov
15.2  Procedures for Obtaining Data
BOREAS data may be obtained through the ORNL DAAC World Wide Web site at 
http://www-eosdis.ornl.gov/ or users may place requests for data by letter, 
telephone, electronic mail, or personal visit using the contact information in 
Sect. 15.1.
15.3  Output Products and Availability
Requested data can be provided electronically on the ORNL DAAC's anonymous FTP 
site or on various media including, CD-ROMs, 8-MM tapes, or diskettes.
The complete set of BOREAS data CD-ROMs, entitled "Collected Data of the Boreal 
Ecosystem-Atmosphere Study", edited by Newcomer, J., et al., NASA, 1999, are also 
available.
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 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.  
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
SLICER references:
Blair, J.B., D.B. Coyle, J.L. Bufton, 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.
Coyle, D.B., D.V. Guerra, and R.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.
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, and F.E. Guertin. 1997. BOREAS in 1997: Experiment Overview, 
Scientific Results and Future Directions. Journal of Geophysical Research 
102(D24): 28,731-28,770.
Vaughn, C.R., J.L. Bufton, W.B. Krabill, and D. Rabine. 1996. Georeferencing of 
airborne laser altimeter measurements. Int. J. Rem. Sens. 17(11):2185-2200.
17.3 Archive/DBMS Usage Documentation
None.
18. Glossary of Terms
None. 
19. List of Acronyms
ASAS    - Advanced Solid-state Array Spectroradiometer
ASCII   - American Standard Code for Information Interchange
ATLAS   - Airborne Topographic Laser Altimeter System
BOREAS  - BOReal Ecosystem-Atmosphere Study
BORIS   - BOREAS Information System
BSQ     - Band Sequential
CD-ROM  - Compact Disk - Read-Only Memory
DAAC    - Distributed Active Archive Center
DEM     - Digital Elevation Model
DTED    - Digital Terrain Elevation Data
EGM96   - Earth Geoid Model 1996
EOS     - Earth Observing System
EOSDIS  - EOS Data and Information System
ESSP    - Earth System Pathfinder Program
FOV     - Field of View
GIS     - Geographic Information System
GMT     - Greenwich Mean Time
GPS     - Global Positioning System
GSFC    - Goddard Space Flight Center
IDL     - Interactive Data Language
INS     - Inertial Navigation System
LAPF    - Laser Altimetry Processing Facility
LVIS    - Lidar Vegetation Imaging Sensor
NAD83   - North American Datum of 1983
NASA    - National Aeronautics and Space Administration
NIMA    - National Imaging and Mapping Agency 
NSA     - Northern Study Area
OJP     - Old Jack Pine
ORNL    - Oak Ridge National Laboratory
PANP    - Prince Albert National Park
PDOP    - Position Dilution of Precision
PNAV    - Precise Differential GPS Navigation and Surveying
RMS     - Root Mean Square
RSS     - Remote Sensing Science
Si:APD  - Silicon Avalanche Photodiode Detector
SLICER  - Scanning Lidar Imager of Canopies by Echo Recovery
SSA     - Southern Study Area
TE      - Terrestrial Ecology
TF      - Tower Flux
TIU     - Time Interval Unit
URL     - Uniform Resource Locator
USGS    - United States Geological Survey
VCL     - Vegetation Canopy Lidar
WFF     - Wallops Flight Facility
WGS84   - World Geodetic System of 1984
WWW     - World Wide Web
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
Keywords:
Laser altimeter
Lidar
SLICER
Vegetation height
Canopy structure 
Topography 
BOREAS
SLICER.doc
11/22/99