Sea Surface Salinity From a Cubesat with Novel Spatial Light Modulator Imaging System

PI: Twardowski, Michael (HBOI-FAU)
Co-PI(s): Ouyang, Bing (HBOI-FAU), DelCastillo, Carlos (NASA), Gong, Cuiling (TCU), Sanborn, Graham (SSC Pacific)
Start Year: 2019 | Duration: 2 years
Partners: NASA Goddard Space Flight Center, Texas Christian University, SPAWAR Systems Center Pacific

Project Abstract:

Absorption by Colored Dissolved Organic Matter (CDOM) strongly impacts ocean color
(passive spectral reflectance Rrs), particularly in the short visible and UV. In coastal waters,
the predominant source of CDOM is regional freshwater input. Because nonconservative
processes such as photodegradation and in situ production have relatively small effects on bulk
CDOM over time periods important for coastal mixing, strong inverse linear correlations
between CDOM absorption aCDOM and salinity S have been observed in coastal waters
throughout the globe. Consequently, there have been numerous algorithms developed to derive S
from remotely measured Rrs, as well as algorithms to derive aCDOM from Rrs (reviewed and
summarized in Mannino et al. 2014; Zhu et al. 2014) and a further vast literature documenting
inverse correlations between aCDOM and S (briefly, Twardowski and Donaghay 2001, 2002;
DelCastillo and Miller 2007; reviews by Bowers and Brett 2008 and Das et al. 2017).

Hyperspectral remote sensing of ocean color imagery is typically detected with CCD or
CMOS 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 retrievals, 2) acceptable photon efficiency, 3) dense imagery transmission given severe
data downlink 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 is pushing such systems to their limit in meeting fundamental sensor specifications.

We propose a novel optical acquisition hardware architecture for a pushbroom-type CubeSat
ocean color imager based on a Digital Micromirror Device (DMD) and an improved backend
compression processing scheme to optimize information transmission given data bandwidth
restrictions. 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. A DMD can serve as a powerful optical filter for imaging littoral land-ocean
interfaces with substantial changes in brightness over small areas. A DMD can allow adaptive
optimization of spectral resolution, spatial resolution, and signal-to-noise based on the particular
scene being imaged. Active, intelligent data compression coupled with the small size of a DMD-
based imager make it ideal for CubeSat applications.

The CubeSat SLM Imager (CSI) spectral range will cover 300 to 600 nm, suitable for
deriving CDOM absorption and salinity from published algorithms. Atmospheric correction will
be carried out using a black pixel assumption in the far UV and by using reflectance data from
the near-IR collected by a companion CubeSat imager already in development. In Year 2, we
expect to employ a 2560×1600 DMD, with spatial resolutions as fine as 20 m pixel-equivalent
footprint over 50 km swath, and 1600 individual 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). Co-I Sanborn at SSC Pacific will develop our satellite operations program plan.

BAA: N00014-18-S-B007
BAA Topic: CubeSats (1)