CASALS: a Lidar and Spectrometry SmallSat for a Future Polar Altimeter Mission

Abstract:

Continuation of altimetry observations of the Earth’s polar regions is crucial for monitoring and predicting the response of the cryosphere to climate change over decadal scales. We are developing a highly-efficient, adaptive lidar measurement approach, intended for a SmallSat mission later in this decade, that could be the foundation for long-term ice sheet and sea ice observing in a series of moderate-cost satellites. The approach addresses three of the Explorer-class Designated Observables recommended in the 2017 Earth Science Decadal Survey: ice elevation, snow depth and snow water equivalent, and ecosystem structure. The observing system, the Concurrent Artificially-intelligent Spectrometry and Adaptive Lidar System (CASALS), includes a lidar which can rapidly adapt the distribution of laser footprint locations across a 7km wide swath. Both the ICESat-2 and GEDI profile configurations can be emulated to provide continuity for ice elevation and ecosystem observations, respectively. In addition, the footprints can be adjacent across track in a swath, achieving 3D lidar imaging for the first time from space. CASALS combines lidar and spectral imaging in order to merge height data with information about composition and function, thereby enabling new capabilities for characterization of the physical state of the Earth’s surface and processes acting upon it. The lidar uses a high-efficiency, 1 micron, solid-state fiber laser operating at up to 120KHz. A single beam with 12m diameter footprints is rapidly scanned cross-track using novel high-speed wavelength-tuning combined with passive wavelength-to-angle pointing using a dispersive grating. The receiver, with altimeter and atmosphere profiling channels, will use a state-of-the-art, linear-mode, photon-sensitive detector array with analog waveform output that is digitized using time-domain multiplexing, thereby preventing atmospheric cloud folding. The receiver telescope and spectrometer will employ free-form optics to dramatically reduce their size and weight, in order to enable a SmallSat implementation. Work to date has focused on maturing the lidar components and demonstrating performance required for space. CASALS development is funded by the GSFC Radical Innovation Initiative and NASA’s ACT, IIP, PICASSO and SBIR programs.