Shuttle Laser Altimeter II

SLA-02
Shuttle Laser Altimeter II

STS-85 launched August 7 and landed August 18, 1997

SLA-02 Data Products

First Science Results

Download SLA-02 Ground Track Data

Individual Observation Period Maps

Global Cloud Cover Movies

SLA-02 emits infrared laser pulses and measures their echoes from the Earth to determine the shape of land surfaces and vegetation canopies.

SLA Logo(47k)
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 Rabine.

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 Earth.

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.

Science Objectives

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 of latitude.

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 member Frawley.

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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Responsible NASA officials: James Garvin, David Harding
Web Curator: Jim Roark (SSAI)
Email roark@denali.gsfc.nasa.gov with comments or suggestions
Last modified December 13, 1999

SLA-02Shuttle Laser Altimeter II