Signals of Opportunity: Enabling New Science Outside of Protected Bands

Signals-of-opportunity (SoOp) is a new technique for microwave remote sensing, re-utilizing powerful satellite transmissions as illumination sources for bistatic radar. SoOp has some beneficial characteristics of both active and passive microwave techniques, as well as its own unique features. These include the high signal to noise ratio resulting from the forward-scatter configuration, the use of a direct signal for calibration, and a resolution that is usually set by the signal bandwidth, rather than antenna pattern (assuming a sufficiently wide beam antenna, similar to pulse-limited altimetry). Perhaps the most revolutionary advancement possible with SoOp, however, is the capability of making remote sensing measurements outside of spectral bands protected for science applications. In this overview, a summary of ongoing research using broadcast signals allocated for space-to-earth communications will be presented. This work spans multiple Earth science applications and frequencies ranging from P- to Ka-band.At the lower frequency end, spaceborne P-band remote sensing has been a challenge due to the large antennas required, radio frequency interference from spaceborne and terrestrial UHF communications, and competition for spectrum. P-band, due to its ability to penetrate vegetation and soil layers, would enable new hydrological science through direct measurement of snow and root-zone soil moisture.At the upper end, several commercial and government-operated satellite constellations broadcast high bandwidth (> 500 MHz) and high power (> 50 dBW) sources. Full-bandwidth coastal altimetry has been demonstrated from fixed locations, with error models predicting cm-level sea surface height precision from orbit. The distribution of these sources promises improved coastal altimetry from small satellite constellations.