Addendum: GNSS-R (R=Reflectrometry) is also established science.

This way you can - for example - track water height of a river next to a station. You basically receive the direct GNSS signal and the reflected one. Then do magic math.

Due to the low orbit of the GPS/Galileo/etc. satellites the reflection is basically "scanning" over the area of interest with an ultra precise signal.

It doesn't require super-cooled radio stuff and can be done with off-the-shelf-gear.