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 moisture and snow depth.
- S. Gleason et al., Processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing, IEEE Transactions on Geoscience and Remote Sensing, Vol. 43, No. 6, pp. 1229-1241, June 2005.
- M. P. Clarizia et al., Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean, Geophysical Research Letters, 29 January 2009.
- GPS: Got plenty of snow?, Phil Berardelli, ScienceNOW Daily News, 29 September 2009.