The MOSS VHF/UHF spaceborne SAR system testbed

Summary form only given. This paper details our recent development of a system prototype for a spaceborne sensor that addresses the current NASA science priority of measuring soil moisture "under a substantial vegetation canopy and reaching a useful depth within the uppermost soil layer". The mission is named MOSS, for the Microwave Observatory of Subcanopy and Subsurface. It will enable measurement and derivation of data products not obtained from any other current, planned, or proposed instrument, with a solution that offers high science value through a low-mass and, in the long-term, low-cost approach. The proposed system is a spaceborne synthetic aperture radar (SAR) operating at the two low frequencies of 435 MHz (UHF, P-band) and 137 MHz (VHF) to enable sensing through vegetation and down into soil. The future mission scenario is achieved from a Sun synchronous orbit of 1313 km altitude, with a swath width of 430 km, incidence angle ranges of 17-30 degrees, resolution of 1 km, and a 7-day exact repeat consistent with the temporal scale of variations of the subcanopy and subsurface soil moisture. We describe a tower-based test-bed that we have developed to validate the spaceborne measurement concept. The processing software, including polarimetric calibration aspects, is detailed, and example science products are presented. The procedures and algorithms to estimate subsurface moisture are presented in detail, and the use an iterative estimation procedure is explained. The results are validated by comparing against detailed ground truth.