The purpose of this work was to develop and demonstrate technologies for a next-generation, efficient, swath-mapping space laser altimeter. The Lidar Surface Topography (LIST) mission concept allows simultaneous measurements of 5-meter-spatial-resolution topography and vegetation vertical structure with decimeter vertical precision in an elevation imaging swath several kilometers wide from a 400-km-altitude Earth orbit. To advance and demonstrate needed technologies for the LIST mission, the Airborne LIST Simulator (ALISTS) pathfinder instrument was developed. ALISTS is a micropulse, single photon-sensitive waveform recording system based on a new and highly efficient laser measurement approach utilizing emerging laser transmitter and detector technologies.

One of the goals is to show the path for realization of a system capable of generating 1,000 laser beams and photodetector channels efficiently. To fulfill LIST measurement requirements from space, a swath of 5 km can be generated using 1,000 beams, each having a 5-meter footprint. According to link analysis, with the current photon counting detector sensitivity, the energy requirement per channel is ≈50 μJ at 10 kHz, and

ALISTS is a 16-beam, arranged in a 4×4 grid pattern, instrument providing a swath of ~80 meters. The instrument was installed on a LearJet. Backscatter from the surface is collected with a telescope, and the spots from the swath are imaged onto a 4×4 single photon-sensitive detector array. The output from each detector element is histogrammed and analyzed to determine ranges to the surface and derive echo waveforms that characterize the vertical structure of the surface. This signal processing technique allows for through-foliage interrogation in order to observe the ground surface beneath vegetation cover and to characterize vegetation vertical structure. The most challenging measurement goal for LIST is detection of the ground surface beneath vegetation cover with 98% closure at 5-meter spatial resolution. The measurement approach of ALISTS is designed to meet that requirement.

The optical transceiver for ALISTS consists of a laser transmitter and a receive telescope mounted on opposite sides of an optical bench. The laser is based on a high-repetition-rate Yb:YAG microchip laser with a Cr4+:YAG saturable absorber and a volume Bragg grating as output coupler for wavelength stabilization. The output energy of the microchip laser is ≈100 μJ at 10 kHz, with a 960-ps pulse width and split into 16 beams. Each of the 16 beams has an energy of ~5 μJ, a divergence of 0.5 mrad with center-to-center beam separation of 2 mrad. At 10-km altitude, the individual beam spot on the ground will be approximately 5 m.

This work was done by Anthony W. Yu (PI), David J. Harding (Science PI), Michael A. Krainak, James B. Abshire, Xiaoli Sun, Luis A. Ramos- Izquierdo, John F. Cavanaugh, Susan Valett, Thomas K. Winkert, Michael E. Plants, and Cynthia A. Kirchner of Goddard Space Flight Center; Peter Dogoda, Brian Kamamia, and R. Faulkner of Sigma Space Corporation; and Alexander Betin of Raytheon. GSC-17029-1

Photonics Tech Briefs Magazine

This article first appeared in the November, 2015 issue of Photonics Tech Briefs Magazine.

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