HR: 1330h
AN: C32A-0425    [PDF]
TI: Performance of the GLAS Onboard Surface Detection Algorithm
AU: McGarry, J F
EM: Jan.McGarry@nasa.gov
AF: NASA Goddard Space Flight Center, code 924, Greenbelt, MD 20771 United States
AU: * Saba, J L
EM: jack@icesat2.gsfc.nasa.gov
AF: Raytheon ITSS, NASA GSFC, code 971, Greenbelt, MD 20771 United States
AU: Sun, X
EM: Xiaoli.Sun@gsfc.nasa.gov
AF: NASA Goddard Space Flight Center, code 924, Greenbelt, MD 20771 United States
AU: Abshire, J B
EM: James.B.Abshire@nasa.gov
AF: NASA Goddard Space Flight Center, code 924, Greenbelt, MD 20771 United States
AU: Yi, D
EM: donghui@icesat2.gsfc.nasa.gov
AF: Raytheon ITSS, NASA GSFC, code 971, Greenbelt, MD 20771 United States
AU: Brenner, A
EM: anita@icesat2.gsfc.nasa.gov
AF: Raytheon ITSS, NASA GSFC, code 971, Greenbelt, MD 20771 United States
AB: The Geoscience Laser Altimeter System (GLAS) determines the range from the satellite to the Earth's surface from the time of flight of the instrument's 1064 nm laser pulses, which are generated at a rate of 40 Hz. The time of flight is defined as the difference between the laser transmit time and the time of return of the surface echo. The detector output is digitized with a 1 ns sampling interval, starting before the laser fires and ending well after any possible surface return, for a total of 5.4 million points. Because there is not enough downlink bandwidth for the entire waveform, the algorithm must extract both the transmit and surface echo waveforms from the 5.4 million digitized points, and pass these waveforms on to be included in the science data packets. The surface echo is not always easily discernible, especially when the beam passes through clouds. To detect the surface echo the algorithm uses the following techniques: (1) The search for the surface echo is limited to a few kilometers of the digitized waveform through the use of an onboard Digital Elevation Model. (2) The detection thresholds are adjusted to maximize the probability of signal detection while maintaining a fixed false alarm rate. (3) The signal-to-noise ratio is maximized for surface echoes of varying pulsewidths by passing the waveform through six matched digital filters. (4) A modified maximum likelihood detection process is used to determine which of the filter outputs is most likely to contain the surface echo. (5) The detector post-amplifier gain is adjusted to maintain the surface echo waveforms within the optimal dynamic range of the digitizer. Results from orbit show the algorithm to be effective at finding the surface echoes. A few features of the algorithm, however, require post-launch modification. One is that cloud cover tends to cause the algorithm to raise the gain, which then causes saturation of surface echoes from clear regions immediately following the clouds. Details of the algorithm, along with specific examples and recent modifications, will be presented.
DE: 1694 Instruments and techniques
SC: Cryosphere [C]
MN: 2003 Fall Meeting