Observations of the Earth's topography from the Shuttle Laser Altimeter (SLA): Laser-pulse Echo-recovery measurements of terrestrial surfaces

Abstract

Meter-precision topographic measurements of a diverse suite of terrestrial surfaces have been accomplished from Earth orbit using the Shuttle Laser Altimeter (SLA) instrument flown aboard the Space Shuttle Endeavour in January of 1996. Over three million laser pulses were directed at the Earth by the SLA system during its ∼ 80 hours of nadir-pointing operation at an orbital altitude of 305 km (+/- 10 km). Approximately 90% of these pulses resulted in valid range measurements to ocean, land, and cloud features. Of those which were fired at land targets, 57% resulted in valid surface ranges, the remainder being cloud tops, false alarms, or missed shots. The SLA incorporated an electronic echo-recovery system into a pulsed, time-of-flight laser altimeter instrument in order to capture and characterize the vertical structure within each 100 m diameter surface footprint. The echoes recorded by SLA demonstrate aspects of the vertical structure of the nearly ubiquitous vegetation cover on the planet, as well as sensitivity to local slopes, surface reflectivity, and vertical ruggedness. With a vertical resolution of 0.75 m and horizontal sampling at 0.7 km length scales, SLA provides a new form of high vertical accuracy topographic data for studying problems related to the dynamics of the Earth's surface. Assessment of the error budget associated with the SLA experiment suggests that ∼2.8 m (RMS) precision was achieved for ranging measurements to oceanic surfaces, for which there are over 700,000 examples. With the availability of a precision radial orbit and post-flight Shuttleattitude information, a mid-latitude (+ 28.5° to −28.5°), georeferenced database of topographic ground control point elevations has been achieved using SLA data, consisting of ∼ 344,000 land measurements. Each of these measurements is geolocated to within 1–2 SLA footprints (100–200 m) on the Earth's surface, with vertical errors that approach the limits of resolution (0.75 m) of the instrument in topographically benign regions. When compared to available Digital Elevation Models (DEM's) with stated vertical accuracies on the order of 10–16 m, SLA's measurements differ by no more than 11 m to 46 m RMS in rugged terrain. We have computed a total vertical roughness parameter for all multi-peaked SLA echoes using a multi-Gaussian decomposition technique and have observed a very high degree of correlation of this parameter with global landcover classes. In some cases (∼6%), SLA echoes clearly resolve both the ground surface and vegetation canopy within a single footprint, suggesting that the modal height of equatorial vegetation is ∼ 18 m. The global distribution of total vertical roughness varies from ∼ 5 m to 60 m, with a mean value of 27 m and a standard deviation of 12 m. SLA successfully served as a pathfinder for high vertical resolution orbital topographic remote sensing instrumentation, and demonstrated the first high resolution echo-recovery laser altimeter observations over land surfaces.