Spaceborne laser altimetry

Abstract

Space-based laser altimeters are effective in providing topographic measurements critically important to the understanding of the formation and early evolution of planetary bodies. Using laser altimetry data, topographic grids can be produced that provide significant insight into the shape, internal structure and evolution of the subject body. Prime examples of space-based altimetry efforts are the Clementine and the Near-Earth Asteroid Rendezvous (NEAR) missions. Clementine spent two months sampling the Moon, and through its altimetry data, provided a glimpse of the lunar surface previously unseen. NEAR will place a laser altimeter (NLR) in orbit at the near-Earth asteroid 433 Eros for a one year observation period. Specifications for such altimeters are driven by mission requirements and host spacecraft constraints. Mission requirements usually prioritize observation objectives associated with other payload instruments, therefore, altimeter design must readily accommodate other payload instruments. Constraints placed on altimeters include mass, power, and volume; also for deep-space missions, data rates are limited and become an issue especially when imaging instruments are part of the mission. Altimeter performance specification and modeling to meet these requirements are described and approaches to verify instrument performance during pre-launch testing are provided. Lessons provided from laser altimetry missions indicate the technological progression to the next-generation laser altimeters.