Signals of Opportunity (SoOp) is an emerging field in microwave remote sensing using existing anthropogenic signals (typically from communications or navigation satellites) in a non-cooperative manner as sources of illumination for bistatic radar. SoOp observations exhibit some properties common to either active radar or passive radiometry, but also have unique features distinct from these two classical approaches.  Realizing the full potential of SoOp has required the development of new instruments, signal processing algorithms, geophysical model functions, and data assimilation methods. 

This presentation will provide a review of progress made to advance the SoOp technique. Three different examples will be presented in approximate order of technical maturity.  First, Global Navigation Satellite System Reflectometry (GNSS-R) is the most advanced SoOp technique. CYGNSS, launched in 2016, provides ocean winds observations and various land surface observations (e.g. soil moisture and flood inundation) in the tropics.  A large and growing science community has evolved around the use of CYGNSS data.  Second, P-band (<400 MHz) communication signals exist in frequencies low enough to penetrate through dense vegetation and soil.  No science allocation exists in this band.  SoOp techniques, therefore, offer a new capability for sensing Root-Zone Soil Moisture (RZSM), an essential climate variable that, at present, can only be obtained through assimilation of surface soil moisture observations. SNOOPI (SigNals Of Opportunity: P-band Investigation) is a cubesat mission launched in April, 2024 to demonstrate spaceborne P-band SoOp remote sensing. Finally, wide-band (~1GHz) communications broadcast signals in Ku-band (12-18 GHz) and higher can theoretically provide altimetry (sea surface height) at cm-level precision.  A constellation of passive SoOp receivers could be launched for a fraction of the cost of a single active radar altimeter, substantially increasing the coverage, particularly in coastal regions. 

Prof. Garrison will conclude the talk with some speculative concepts and ideas for future research directions.