There is a lot of talk of military uses here, but this technique is also used for environmental monitoring.

GNSS interferometric reflectometry (GNSS-IR) uses navigation satellites as the transmitter for a bi-static radar. The measurement device is any GNSS receiver (even your phone).

The technique can estimate environmental parameters like sea level, soil moisture, snow depth, and vegetation water content from systematic changes in the the multi-path around the antenna.

There is an open source Python package for this technique: https://github.com/kristinemlarson/gnssrefl

An introductory paper "The Accidental Tide Gauge": https://www.kristinelarson.net/wp-content/uploads/2015/10/La...