Title of Presentation: The Swath Imaging Multi-polarization Photon-counting Lidar (SIMPL): A Technology Demonstration for Space-based Laser Altimeter Swath Mapping

Primary (Corresponding) Author: David Harding

Organization of Primary Author: NASA Goddard Space Flight Center


Abstract: The Swath Imaging Multi-polarization Photon-counting Lidar (SIMPL) is an airborne prototype in development to demonstrate laser altimetry measurement methods and components that enable efficient, high-resolution, swath mapping of topography and surface properties from space.   The main focus of this instrument development, sponsored by the NASA Earth Science and Technology Office Instrument Incubator Program, is to achieve detailed monitoring of ice sheet, sea ice and glacier change from a spacecraft in low Earth orbit.  It builds upon the successes and lessons learned from the Geoscience Laser Altimeter System (GLAS) flown aboard NASA's Ice, Cloud and land Elevation Satellite (ICESat) mission.  Although currently emphasizing polar-region cryosphere objectives, the SIMPL approach is also applicable in other scientific applications including changes in land topography, forest height and structure, and inland water and snow cover height and extent. Analysis and laboratory testing of the approach documents that accurate (cm-level) ranging is achieved with only ~10's of detected photons by (1) transmitting short (~1 nsec), low-power (1 _J), high-rep rate (300 kHz) laser pulses, (2) illuminating small footprints (~10 m) thus limiting pulse broadening due to surface relief, (3) precisely timing (0.1 nsec) the detection of single photons per pulse, (4) applying time-space correlation of transmit pulses and receive photons, and (5) accumulating surface return photons from successive pulses. Because of the efficiency of this approach, multiple, adjacent, along-track profiles distributed cross-track can be measured simultaneously, forming an elevation image with 10 m spatial resolution. Measurement of the backscatter energy at 532 nm and 1064 nm with polarization parallel and perpendicular to the laser transmit pulse provides the depolarization ratio of the surface returns. Laboratory measurements we have performed of range-resolved, laser retro-reflection at these two wavelengths documents that the depolarization ratio can be used to differentiate surface types, including water, snow and ice based on differences in their wavelength-dependent scattering properties.  A one-beam, breadboard version of SIMPL is planned for airborne testing and validation of the measurement approach in 2007.  In early 2008, a four-beam version of the SIMPL prototype will be completed and tested, and deployed on airborne missions conducted as a part of the International Polar Year.