Title: Airborne Lidar Simulator for the Lidar Surface Topography (LIST) Mission
Primary Author: Yu, Anthony 
Organization: NASA GSFC
Co-Author(s): Anthony W. Yu, Michael A. Krainak, David J. Harding, James B. Abshire, Xiaoli Sun,
John Cavanaugh, Susan Valett, and Luis Ramos-Izquierdo

Abstract:
In 2007, the National Research Council (NRC) completed its first decadal survey for Earth science at the request of NASA, NOAA, and USGS. The Lidar Surface Topography (LIST) mission is one of fifteen missions recommended by NRC, whose primary objectives are to map global topography and vegetation structure at 5 m spatial resolution, and to acquire global coverage within a few years.

In 2009 we started a three-year Instrument Incubator Program (IIP) project, funded by NASAís Earth Science Technology Office (ESTO), for early technology development for LIST. The purpose is to develop and demonstrate technologies for a next-generation, efficient, swath-mapping space laser altimeter.

The objective of our work is to demonstrate the key capabilities for a new highly efficient laser altimeter for the LIST mission. The LIST lidar needs to be able to generate a swath with 5 m pixels 5 km wide, image this swath onto a detector array and produce a range image with the topographic height of the sampled area. This includes measuring through foliage if covered by vegetation, and measuring the 3-D structure of the vegetation cover. Our pushbroom approach uses a swath 5 km wide composed of 1000 laser beams in a linear array is oriented in the cross-track direction. The divergence of each beam yields 5-meter diameter footprint on the ground from a 400 to 425 km orbit altitude. The spots are contiguous cross-track. At 10 kHz laser pulse rate and a nominal spacecraft ground velocity of 7 km/sec, laser footprints are spaced 0.7 m along track yielding 7 illuminating laser pulses per 5 m pixel. This over-sampling along track allows detecting ground echo pulses under realistic observing conditions, such as attenuation from thin clouds and ground obscuration by vegetation.

Our basic approach is flexible and scalable in swath width, pixel width, laser power and telescope size. For the laser, we have been developing a high-efficiency high-repetition-rate master oscillator power amplifier laser transmitter, using a diffractive optical element after the laser to produce multiple beams. The laser backscatter from the surface is collected with a diffraction-limited telescope and the spots from the swath are imaged onto a sensitive detector array. We are currently working with various vendors to develop suitable high-sensitivity, low-noise avalanche photodiode (APD) detectors that operate in a quasi-analog mode.

Our development activity is in the second year of the three-year NASA ESTO IIP program. During Year 3 we plan to perform airborne testing of the swath-mapping concept. Candidate geographic regions for the field tests will be selected to demonstrate measurements that satisfy the LIST science objectives for topographic mapping in the focus areas of cryosphere, water cycle, and vegetation structure. In this paper we will summarize some of the instrument characteristics and approaches we are using to reduce risks for LIST based on the airborne instrument development and its demonstration measurements.