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 (eg radar , echolocation ) is active sensing ; Sensing what is already available in the surrounding environments without changing the environment to do so (eg eyesight, hearing) is passive sensing . GNSS reflectometry is passive sensing which takes advantage of and relies on separate active sources – the satellites generating the navigation signals.
The UK-DMC satellite , Reviews part of the Disaster Monitoring Constellation built by Surrey Satellite Technology Ltd , caries a secondary reflectometry payload That HAS Demonstrated the feasibility of receiving send and measuring GPS signals reflected from the surface area of the Earth’s oceans from ict 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. 
- Jump up^ S. Gleasonet al. ,IEEE Transactions on Geoscience and Remote Sensing,IEEE Transactions on Geoscience and Remote Sensing, Vol. 43, No. 6, p. 1229-1241, June 2005.
- Jump up^ MP Clariziaet al. ,Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean,Geophysical Research Letters, 29 January 2009.
- Jump up^ GPS: Got plenty of snow? , Phil Berardelli, ScienceNOW Daily News, 29 September 2009.