Lunar Terrain Surface Modeling for the ALHAT Program

Abstract

The NASA Autonomous Precision Landing and Hazard Avoidance Technology (ALHAT) program is developing an autonomous precision landing system capable of landing a spacecraft on the moon. To achieve the desired 90 m (3sigma) landing accuracy some version of a Terrain Relative Navigation (TRN) is necessary. One such algorithm is described in a companion paper. These algorithms require an accurate representation of the lunar terrain for both the on-board TRN algorithms and the ground-based software test bed (truth model) to verify the performance of the algorithms. In its terminal landing phase, the spacecraft will touchdown in a safe location using two algorithms: Hazard Detection and Avoidance (HDA) and the Hazard-Reference Navigation (HRN). To verify the performance of the HDA and HRN algorithms, ALHAT needs lunar surface models incorporating rock shape and size distribution statistics. The Johns Hopkins University Applied Physics Laboratory has been tasked by ALHAT to supply lunar terrain data for ALHAT. This includes both terrain data (elevation and surface reflectivity properties) for TRN and rock statistics for HDA and HRN. At the present time, detailed terrain data from satellite measurements is not available for all the lunar regions. In particular, data for the polar regions, which have been identified as regions of high importance, are sparse. Several missions are underway to map the lunar surface. Until data from the laser altimeters and cameras on these spacecraft are available, APL will provide representative lunar terrain data to test the ALHAT algorithms. This paper discusses APL's different approaches to generating lunar digital elevation maps for TRN. It does not discuss the generation of rock statistics for HDA and HRN.