Space Borne Swath Mapping Laser Altimeters - Comparison of Measurement Approaches

Laser altimetry is an important technique for studying the surface topography of the planets and the Earth from orbit. Presently orbital laser altimeters profile surface height along a single ground track, such as the Geoscience Laser Altimeter System (GLAS) on Ice, Cloud, and land Elevation Satellite (ICESat). NASA is developing new technologies for an orbiting swath mapping laser altimeter with faster pulse rate and smaller footprint size to provide an instantaneous 3-dimentional measurement of the of icesheets, land topography and vegetation structure. The goal is to provide a greater than 200 m wide swath with 5 to 10 m diameter laser footprint from a 400 km altitude orbit. To achieve these goals, we have to use more efficient laser transmitters and more sensitive detectors to allow simultaneous multi-channel measurement with a reasonable instrument size and electrical power requirement. The measurement efficiency in terms of electrical energy needed per laser ranging measurement needs to be improved by more than an order of magnitude. Several different approaches were considered, including the use of fiber lasers, shorter laser pulse widths, lower noise analog detectors and photon counting detectors. The receiver sensitivity was further improved by averaging the results from a number of laser pulse measurements. Different laser pulse modulation formats, such as the pseudo random noise code modulation used in the Global Position System (GPS), were investigated to give more flexibility in laser selection and to further improve the ranging performance. We have analyzed and compared measurement performance for several different approaches using the receiver models that was validated with GLAS in orbit measurement data. We compared measurement performance with the traditional high-power low-pulse-rate laser transmitters to those with low-energy high-pulse-rate laser transmitters. For this work we considered laser characteristics representative of Microchip lasers at 1064 and 532 nm, and for pulsed Ytterbium fiber lasers at about 1?m wavelength. We considered Si APDs for analog detection, InGaAsP photocathode hybrid photomultiplier tubes for photon counting at 1?m, and Si APD single photon counting modules at 532 nm. For all cases we evaluated the probability of detection and the standard deviation of the ranging error as a function of the apparent surface reflectance defined as the product of the surface reflectance times the two-way atmosphere transmission. We also conducted several photon counting laser ranging experiments in the lab and in the field to validate our model and measurement techniques with various photon counting detectors. The results of the analysis and the experiments will be summarized and compared for parameters representative of future missions recommended by the National Research Council (NRC), such as ICESat II, LIST, and DESDynl.