International Archives of Photogrammetry and Remote Sensing, Vol. 32, Part 3W14, La Jolla, CA, 9-11 Nov. 1999 
    
  
PROCESSING OF SHUTTLE LASER ALTIMETER 
RANGE AND RETURN PULSE DATA IN SUPPORT OF SLA-02 
Claudia C. Carabajal', David J. Harding?, Scott B. Luthcke?, Waipang Fong”, Shelley C. Rowton’, and J. J. Frawley® 
INVI, Inc., (0 NASA/Goddard Space Flight Center (Code 926), claudia@stokes.gsfc.nasa.gov 
*NASA/Goddard Space Flight Center (Code 921), harding@denali.gsfc.nasa.gov 
?Raytheon ITSS, (à NASA/Goddard Space Flight Center (Code 926), sluthcke@ geodesy2.gsfc.nasa.gov 
‘Raytheon ITSS, Greenbelt, MD 20770, USA, waipang@magus.stx.com 
Raytheon ITSS, Greenbelt, MD 20770, USA, srowton@magus.stx.com 
® Herring Bay Geophysics, @ NASA/Goddard Space Flight Center, hbgjjf@ltpmail.gsfc.nasa.gov 
KEY WORDS: space-borne laser altimetry, remote sensing, laser bounce-point geolocation, laser backscatter modeling. 
ABSTRACT 
The second flight of the Shuttle Laser Altimeter (SLA) flew on board the space Shuttle Discovery, during August 1997 during the 
STS-85 Mission. 
The nearly 3 million laser shots transmitted during the course of the 11 day SLA-02 mission yielded 
approximately 590,000 geolocated returns from land and more than 1,500,000 from ocean surfaces. These data were analyzed to 
produce a data set that provides laser altimetry elevations of high vertical accuracy that can be used for scientific purposes. 
Processing of the data included the geolocation of surface returns, involving precision TDRSS-tracking based Shuttle orbit 
determination and pointing bias calibration, ellipsoid to geoid reference frame transformations, conversion of engineering 
parameters to physical units, application of scaling factors to obtain a consistent measure of the backscatter energy, and 
classification of the returns based on comparisons with reference elevation data (TerrainBase Digital Elevation Model (DEM) and 
mean sea level). Additionally, the digitized laser returns were analyzed and modeled using constrained non-linear least-squares 
optimization techniques. The elevation data were compared to both high-resolution DEMs and a reference ocean surface to 
assess data accuracy. Ancillary data, such as NDVI (Normalized Digital Vegetation Index) and Land Cover classification data, 
were also included in the distributed data set. Key aspects of the data analysis are discussed. Further documentation concerning 
SLA-02 data processing procedures, problems evidenced in the data, and its distribution format is provided in the SLA-02 site 
(http://denali.gsfc.nasa.gov:8001/). 
1. INTRODUCTION 
The Shuttle Laser Altimeter (SLA) was designed as a pathfinder 
experiment to evaluate engineering and algorithm techniques to 
aid the transition of the airborne laser altimeter and lidar 
technology developed at Goddard Space Flight Center to low 
Earth orbit operational space-borne systems (Garvin et al., 
1996). Two flights of SLA have provided high-resolution, 
orbital laser altimeter observations of terrestrial surfaces that 
constitute scientific data sets of value in addressing global Earth 
System science issues. SLA also serves as a test-bed for 
upcoming orbital laser altimeters, such as the Multi-Beam Laser 
Altimeter (MBLA) (Bufton et al.,, 1999) and the Geoscience 
Laser Altimeter System (GLAS), that will be launched aboard 
the Vegetation Canopy Lidar (VCL) mission in 2000 and the 
Ice, Cloud and land Elevation Satellite (ICESat) in 2001, 
respectively. The equatorial observations provided by the first 
flight of SLA (SLA-01) were extended to 57 degrees by SLA- 
02, characterizing ocean, land, and cloud top elevations in 100 
meter diameter footprints spaced every 700 meters utilizing a 
laser transmitter firing at a rate of 10 pulses per second (Figure 
1). The SLA instrument provides the round-trip travel distance 
of short duration (1064nm wavelength) laser pulses to the first 
encountered surface, either a cloud top, vegetation canopy top, 
bare ground, or water, with a 0.75 m precision (Bufton et al., 
1995). As for SLA-01, ranging was augmented by digitizing 
the time-varying return pulse energy from surfaces distributed 
vertically within the laser footprint, enabling a measurement of 
within-footprint relief introduced by vegetation cover and 
topographic slope and roughness. Combining the laser ranging 
data with shuttle position and pointing knowledge yielded 
highly accurate surface elevation data. The processing 
procedures involved in producing the SLA-02 data set are 
summarized here. The processing procedures and data 
distribution format are fully specified in the documentation 
accompanying the data set, available for downloading at 
http://denali.gsfc.nasa.gov:8001/. The data were acquired and 
processed as ‘observations’, which represent a continuous 
period of instrument operation.