Shuttle Laser Altimeter II
STS-85 launched August 7 and landed August 18, 1997
Shuttle Laser Altimeter II
SLA-02 emits infrared laser pulses and measures their echoes from the Earth to determine the shape of land surfaces and vegetation canopies.
Chief Scientist James Garvin
The SLA-02 instrument consists of a 38 cm telescope, a diode-pumped
Nd:YAG laser transmitter (Q-switched), an altimeter receiver electronics
package, and a 250 MHz waveform digitizer coupled to the altimeter
receiver for capturing the backscattered echoes from the interaction of
short duration laser pulses (~ 8 nsec) with the Earth's surface. As such
SLA-02, as in SLA-01, is a true "surface lidar", a new breed of laser
remote sensing instrumentation intended as a tool for geodetic topographic
sampling and for direct measurement of local scale surface vertical structure,
and in particular the height of such relief elements as trees, buildings,
and geomorphic features.
The Instrument Manager, Dr. Jack Bufton, together with lead engineers
J. Bryan Blair (also SLA Instrument Scientist), David Rabine, Dan Hopf,
John Cavanaugh, and colleagues, assembled the SLA sensor from spare parts of
the original Mars Observer Laser Altimeter (MOLA-1), together with COTS
parts (flight computer, digitizer, etc.).
In order to accomodate the tremendous dynamic range of backscattered
signal strengths observed with SLA-01, the SLA-02 sensor was modified to
include an innovative Variable Gain state Amplifier or VGA system in the
altimeter receiver chain. This VGA permits the SLA operator on the ground
to control the signal intensity from the Silicon Avalanche Diode detector
to the waveform digitizer to preclude saturation. Thanks to the high
level of link margin observed especially over unvegetated targets on
SLA-01, the VGA has been successfully used to virtually eliminate the
deep levels of echo saturation that plagued SLA-01 under some conditions.
The SLA program is supported by the Mission To Planet Earth program
at NASA Headquarters and at NASA's GSFC. A schedule of 4 flights has
been planned for a continuously improving SLA sensor package. Plans are
afoot to fly a small-footprint, ultra high pulse repetition rate version
of SLA-02 (namely SLA-03) in late December 1998 as part of the STS-96
mission, perhaps in partial support of the NASA/JPL/NIMA Shuttle Radar
Topography Mission (SRTM). As SRTM is slated for spaceflight in
mid to late 1999, SLA-03 would ideally assist this important mission by
measuring up to one million land ground control points (GCP's) in
a "bare Earth" sense (i.e. by finding the ground elevation under tree
canopies in places where microwave sensors such as SRTM could
be more sensitive to canopy texture).
The SLA program also supports algorithm development for the EOS Laser
Altimeter Mission (i.e., for the Geoscience Laser Altimeter System or
GLAS), which is scheduled to reach Earth orbit in 2001. Furthermore,
what has been learned (and is being learned) from SLA-01 and SLA-02
will be used by the Univ. of Maryland's Vegetation Canopy Lidar (VCL)
mission, under the leadership of Prof. Ralph Dubayah.
Instrument and Engineering Team
The SLA-02 sensor was designed and fabricated by an engineering
team under the leadership of Dr. Jack L. Bufton (Instrument
manager, SLA), and including the SLA Instrument
scientist J. Bryan Blair, and SLA engineer David
The SLA-02 sensor consists of three major components: (1) the
laser transmitter (a Q-switched, diode pumped Nd:YAG
device build by McDonnell Douglas using flight spares from the
original Mars Observer Laser Altimeter); (2) a 38 cm diameter
telescope which serves as the lidar "antenna", and (3) the
lidar altimeter receiver and waveform digitizer, as well as the
flight computer. In order to better accomodate the tremendous
dynamic range of surface return amplitudes, a Variable Gain state
Amplifier (VGA) was incorporated to allow operator control of the
signal strength from the SLA silicon avalanche photodiode detector
to the waveform digitizer. In addition, the time interval counter
that is used to measure the round-trip travel time of the laser
pulses fired by the instrument aboard the DISCOVERY orbiter is
coupled to the waveform digitizer (in time) to better allow
recognition of the leading edge of those backscattered laser
pulses that successfully result in surface renges.
What SLA-02 does differently than all laser altimeters that have
flown in space previously is to record the within-footprint
character of each backscattered laser echo at up to 250 MHz
bandwidth and most often at 100 MHz. This allows us to achieve
within-footprint resolution of ~ 1.5 m (vertically) within
each 100 m diameter footprint on the surface of the Earth. In
those cases where unsaturated, multimodal echoes are recorded,
we can then find the perceived ground portion of the return
and separate it from the additional elements of vertical
structure (buildings, trees, sand dunes, gullies, etc.) that
may fall within the illuminated footprint. Thus, SLA-02 is
a true "surface lidar" sensor and the first to globally
sample virtually all terrain and landcover classes on planet
For further information about the engineering and operation
of SLA-02, interested readers should contact Dr. Jack Bufton
or J. Bryan Blair at NASA's GSFC.
The SLA-02 (Shuttle Laser Altimeter - 02) flight experiment
aboard the STS-85 mission (August 1997) is intended as an
incremental step in orbital echo-recovery or "surface lidar". Our goal
is to acquire a global database of laser echoes describing a wide
range of land cover classes, extending from the boreal forests of
Canada and Russia, to the grasslands of South Africa and Australia.
Although SLA-02 remains an engineering pathfinder experiment, its
scientific objectives are designed to emphasize acquisition of
non-saturated laser echoes (aka waveforms) for a diverse suite
of landscapes and land cover classes between 57 degrees north and
south latitudes. As SLA-01 experienced such high signal levels off
of vegetated and desert land surfaces, SLA-02 includes special
hardware to allow us to minimize echo saturation, and, as a consequence,
optimize retrievals of the vertical roughness of complex surfaces
involving vegetation cover.
SLA-02, as with SLA-01, was built from spare components inherited
from the Mars Observer Laser Altimeter (MOLA) instrument, as well as
new subsystems designed to capture the vertical distribution of
roughness elements within a 100 m diameter footprint on the surface
of the Earth. Thanks to the legacy of MOLA (a version of which is
about to operate in orbit around Mars as part of Mars Global Surveyor),
the Shuttle Laser Altimeter series of experiments are providing
a valuable test bed of results for both engineering and science.
Our objectives with SLA-02, therefore, are to characterize
the vertical roughness of as many differnt land cover classes
and landscapes as possible on a global basis, and to focus attention
on retrieving aspects of the relief of tree canopies under
different conditions and in a broad variety of environments. It is
our intention to measure the vertical roughness of equatorial and
high latitude arid lands (deserts) in order to better understand the
limited vertical roughness data that is to be acquired with MOLA (i.e.
MOLA does not include a waveform analyzer with which to measure each echo).
Among are key target acquisition areas are: the Amazon Basin, the
Canadian Boreal forests, the Mississippi Delta, the Ganges Delta, the Nullabor
plain of Australia, the Kamchatka peninsula, north African deserts,
the gobi deserts of Asia, the Patagonian Icefields, and key target
areas in North America for which we have aircraft surface lidar
data in hand (e.g., Pacific Northwest, etc).
Synergizing SLA-02's measurements of atmospheric phenomena
(clouds) with observations to be acquired with the ISIR sensor
located nearby on the Shuttle is another goal. ISIR is a
thermal infrared imaging system under the leadership of Dr.
James Spinherne. SLA-02 observations of the heights of
clouds are to be simultaneously imaged with ISIR, and the results
compared for a variety of cloud types and as a function
Finally, SLA-02 seeks to continue the acquisition of geodetic
quality surface elevation measurements, corrected for local
slopes and vegetation heights in order to augment the
SLA-based global database of ground control points.
Coverage maps highlight the areas where
SLA-02 coverage was obtained.
Geolocated (processed) data from the SLA-02 mission are to
be released via this web page in early 1998. If possible, all
elevations will be corrected for vegetation heights to yield
"bare earth" values.
All scientific visualizations have been conducted by SLA science
team member Jim Frawley (Herring Bay Geophysics at NASA's GSFC).
Algorithms for processing SLA waveforms so that total vertical
roughness (TVR) can be retrieved have been developed by team
SLA-02 is a Hichhiker / Technology Applications and Science (TAS-01) payload.
SLA-02 Computer Program Descriptions
Results from our earlier mission - SLA-01
Zack in the POCC.
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Last modified December 13, 1999