An imaging lidar system is being developed for use in navigation, relative to the local terrain. This technology will potentially be used for future spacecraft landing on the Moon. Systems like this one could also be used on Earth for diverse purposes, including mapping terrain, navigating aircraft with respect to terrain and military applications. The system has been field-tested aboard a helicopter in the Mojave Desert.

The use of imaging lidar systems to generate digital data sets equivalent to topographical maps is well established. Such systems are commercially available and often denoted simply as topographical or topographic lidar systems. As in other imaging lidar systems, a gimballed, actuated mirror is used to raster-scan a narrow laser beam across a field of view, the laser beam is emitted in short laser pulses, the pulses are reflected from the terrain, and the distance to the terrain in a given direction is determined from the total time of flight from the emission of the outgoing pulse to the receipt of the reflected pulse. Then the combination of direction (azimuth and elevation angles associated with the mirror orientation) and the range (distance) for each such direction constitute raw data that can be used to generate a topographical map of the terrain.

When this system was designed, digitizers with sufficient sampling rate (2 GHz) were only available with very limited memory. Also, it was desirable to limit the amount of data to be transferred between the digitizer and the mass storage between individual frames. One of the novelty design features of this system was to design the system around the limited amount of memory of the digitizer. The system is required to operate over an altitude (distance) range from a few meters to ≈1 km, but for each scan across the full field of view, the digitizer memory is only able to hold data for an altitude range no more than 100 m. Therefore, the acquisition of data is limited to an altitude range 100 m wide in the following way: Initially a pulse is emitted and digitized over an altitude range of 5 km. This process is repeated four more times, and the median time of the first return pulse of all five measurements is computed as the distance from which to expect future laser pulse to be reflected. A distance of 50 m is subtracted from the expected distance and the resulting distance is fed as a programming input to a programmable-delay pulse generator, which is triggered by the outgoing laser pulse and which, in turn, turns on the digitizer after the programmed delay. Thus, the digitizer is started at 50 m before the expected receipt of the return pulse. The digitizer then operates over an altitude interval of 100 m; it is stopped at 50 m after the expected return of the receipt of the return pulse.

This work was done by Carl Christian Liebe, Gary Spiers, Randy Bartman, Raymond Lam, James Alexander, James Montgomery, Hannah Goldberg, Andrew Johnson, Patrick Meras, and Peter Palacios of Caltech for NASA's Jet Propulsion Laboratory.


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A Topographical Lidar System for Terrain-Relative Navigation

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This article first appeared in the August, 2008 issue of NASA Tech Briefs Magazine.

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