High Quality Littoral Ocean and Aerosol Characterization from a CubeSat With Novel Spatial Light Modulator Imaging System
Lead PI: Michael Twardowski, Florida State University
Start Year: 2018 | Duration: 2 years
Partners: Space and Naval Warfare Systems Center Pacific, University of Maryland Baltimore County
A core technology for ocean and atmospheric sensing is passive imaging of reflected solar radiation, typically detected with multispectral CCD array based systems. General challenges in adapting such imaging technology to CubeSat platforms over the littoral environment include 1) sufficient signal-to-noise for adequate algorithm retrieval of environmental properties, 2) acceptable photon efficiency, 3) efficient information transmission to the ground station that enables the rendering of high quality images given severe data transmission limitations, and 4) saturation, blooming and edge effect problems with water adjacent to bright land and clouds. Even with current state-of-the-art satellite imagers, 5 nm resolution through the visible domain is pushing CCD array based systems to their limit in meeting fundamental sensor specifications.
We propose a novel optical acquisition hardware architecture for a pushbroom-type CubeSat imager based on a Digital Micromirror Device (DMD) and an improved backend compression processing scheme to optimize information transmission given data bandwidth limits. A DMD is a Spatial Light Modulator (SLM) that modulates the intensity and phase of incoming light. It consists of millions of electrostatic-actuated micro-mirrors that can be used to control light collection dynamically and adaptively from each individual pixel equivalent. DMDs have very high contrast ratios when mirrors are “off” vs. “on” and fast switching speeds (greater than 20 kHz binary patterns per second). Therefore, a DMD is ideal to implement an optical filter for imaging littoral land-ocean interfaces with substantial changes in brightness over small areas. In our case, the DMD will reflect an image modulated with subset spatial patterns onto a single, very sensitive point detector (e.g., Photomultiplier Tube) with 2-3 orders larger dynamic range than a state-of-the-art CCD array. The image is then fully reconstructed by processing the resultant signals from these patterns. Inserting the DMD in the optical train allows optimization of information content for images requiring broad dynamic range while minimizing loading of redundant data through compressive sensing techniques. It also allows on-the-fly, adaptive optimization of spectral resolution, spatial resolution, and signal-to-noise based on the particular scene being imaged. The DMD-based sensor is thus ideal for the CubeSat imaging application.
The CubeSat SLM Imager (CSI) spectral range will cover 350 to 900 nm, capable of resolving a host of ocean and aerosol parameters including ocean turbidity, phytoplankton, productivity, and bathymetry from published algorithms. The concept is also readily adaptable to imaging in the IR to derive many cloud parameters. In Year 2, we expect to employ a 2560×1600 DMD, with spatial resolutions as fine as 20 m over a 50 km swath, and 1600 bands available in the spectral dimension. Compressive sensing algorithms will be used to optimize spatial and spectral resolution to achieve SNR thresholds, dependent on the characteristics of a given scene (e.g., relative brightness). A Synthetic Aperture Imaging technique will also be assessed, where multiple viewing angles of the same scene collected through orbit or from multiple CubeSats may significantly enhance image quality and ability to derive oceanic properties, particularly through challenging atmospheric conditions commonly found in littoral regions. Co-I Williams, Chief of the CubeSat group at SSC Pacific, will develop our satellite operations program plan.