Silvilaser 2019 - Poster Presentations »
The Science of SAFE, a Smallsat Complement to SBG
The Surface Biology and Geology (SBG) mission recommended by the 2017 Decadal Survey for Earth Science and Applications from Space from the U.S. National Academies will be welcomed by the scientific community for its ability to measure vegetation functional properties such as light use efficiency, plant pigment concentrations, water content, etc. at the time of overpass. However, those measurements are dependent on diurnal effects such as vegetation stress and scene shadows, which we demonstrate using diurnal G-LiHT (Goddard's LiDAR, Hyperspectral & Thermal Imager) data acquired over eddy flux sites in North Carolina, USA. We show that the typical mid-morning crossing times used for imaging satellites in sun-synchronous near-polar orbits (such as that proposed for SBG) do not capture periods of stress that occur later in the day. The photochemical reflectance index (difference in surface reflectances between 531 nm and 570 nm divided by their sum) overestimates light use efficiency if shaded pixels are not identified. The latest National Academies recommendations also call for specific measurements for vegetation structure and function and dynamics of ecosystems. We describe a concept for measuring diurnal Structure and Function of Ecosystems (SAFE) that complements and enhances SBG vegetation science. The SAFE concept utilizes high-resolution pan band and VSWIR multispectral instruments to measure structure and function, respectively, on a smallsat bus. With respect to structure, a panchromatic visible imager will acquire stereo triplet images to achieve 1 m canopy height models. With respect to function, a multispectral radiometer will acquire spectral radiance in prescribed very narrow wavelength bands needed for functional parameters (light use efficiency, pigments, nutrients), clouds and atmospheric effects. Two to three smallsats can be placed in orbits that would enable measurements at the location of the SBG overpass at 1030 local time, but also at later times (i.e., 1330 and 1630). The results could then be used with SBG data to find markers (in the morning overpass) of the diurnal signal for plant stress. In addition the 3D models developed from SAFE could be used to estimate the shadow fraction at the time of the SBG overpass. Overall, SAFE is individually responsive to the Decadal Survey as a standalone competed mission and also as an important precursor mission to SGB, enabling maximization of SGB’s science impact.