P-band and VHF microwave observations can penetrate dense vegetation to sense Root-Zone Soil Moisture (RZSM), a key variable in plant–soil–atmosphere interactions with applications to wildfire risk and agricultural drought forecasting. Spaceborne P-band remote sensing faces severe engineering constraints. The antenna size to meet diffraction-limited resolution scales with wavelength. The P-band/VHF spectrum is heavily allocated for communications, with limited protection for science and substantial risk of radio frequency interference (RFI). Signals of Opportunity (SoOp) exploit existing non-cooperative spaceborne transmitters as bistatic radar sources, offering an alternative. The forward-scatter geometry yields orders-of-magnitude higher signal-to-noise ratio than monostatic backscatter. At P-band, reflected signals are expected to remain coherent for rougher surfaces than at L-band. Surface resolution of a coherent reflection is set by the first Fresnel zone rather than antenna beamwidth.
SNOOPI (SigNals of Opportunity: P-band Investigation), a CubeSat mission under NASA's InVEST program, was built to demonstrate SoOp remote sensing between 230 and 380 MHz using a cross-dipole antenna capturing a combined direct and reflected signal (interference in space). Deployed from the ISS on 18 April 2024, it completed six data captures before de-orbiting on 26 September 2024. Despite a shortened mission and satellite bus anomalies, a 6 June 2024 capture off the coast of Brazil yielded 83.50 ms of ocean-reflected signal. Using autocorrelation to isolate the reflected path and matching delay-Doppler peaks to publicly available ephemerides, we identified the transmitting satellites and confirmed a coherent reflection. Retrieved surface reflectivity agreed with model predictions to within an equivalent wind speed error of 1.15 m/s, confirming coherence up to wind speeds of 5.1 m/s.
Although land remote sensing was the motivation for SNOOPI, this result demonstrates, for the first time from an orbiting receiver, the feasibility of phase altimetry of the ocean surface in VHF/P-band with implementation on a small-satellite constellation.