GNSS reflectometry involves making measurements from the reflections from the Earth of navigation signals from Global Navigation Satellite Systems such as GPS. It is also known as GPS reflectometry.
Deliberately bouncing signals off something to learn about it (e.g. radar, echolocation) is active sensing; sensing what is already available in the surrounding environment without changing the environment to do so (e.g. eyesight, hearing) is passive sensing. GNSS reflectometry is passive sensing that takes advantage of and relies on separate active sources - the satellites generating the navigation signals.
The UK-DMC satellite, part of the Disaster Monitoring Constellation built by Surrey Satellite Technology Ltd, carries a secondary reflectometry payload that has demonstrated the feasibility of receiving and measuring GPS signals reflected from the surface of the Earth's oceans from its track in low Earth orbit to determine wave motion and windspeed.
GPS signal reflections have also been used to measure soil moisture and snow depth.