S IGNALS from Global Navigation Satellite Systems (GNSS) that were initially developed for positioning, navigation, and timing applications are now operationally being used to sense the Earth surface using GNSS reflectometry (GNSS-R) techniques. With over 140 current GNSS satellites transmitting open, high-quality signals, LEO-based GNSSR can achieve global measurement coverage using passive, low-power receivers. For example, the delay-Doppler maps produced by the Cyclone Global Navigation Satellite System (CYGNSS) constellation is operationally used to retrieve ocean surface wind speeds associated with hurricanes [1]. Additionally, coherent signal reflections from CYGNSS and other low Earth-orbiting (LEO) satellites have also been used to achieve centimeter-level surface altimetry over calm ocean, inland water bodies, wetlands, or sea ice [2], [3], [4]. Compared to terrestrial-based GNSS-R, reflectometry using spaceborne platforms benefits from nearly global measurement ground-track coverage.