In the early 1990s, and with the appearance of the GNSS (Global Navigation Satellite Signals) signals, a new concept for perform ocean altimetry was proposed. It consisted of taking the navigation signals reflected of the ocean surface as signals of opportunity for ranging. Those signals will be sensed by an airborne or spaceborne receiver in a bistatic radar geometry.
Thus, as in traditional altimeters, three important descriptors of the ocean surface can be derived, i,e the bistatic path delay (from which the ocean height can be derived), the ocean surface wind, and the ocean significant wave height.
Since then, the popularity of the GNSS-R technique for Earth Remote Sensing has increased, being proposed for many other applications, such as scatterometry (wind speed, wind direction), ice altimetry, snow monitoring, soil moisture, or biomass.
One of the main advantages that offers this technique, is the possibility to obtain many reflections, as many as visible GNSS satellites, allowing to obtain an exceptional spatial-temporal sampling of the Earth. Additionally a GNSS-R receiver is cheaper and lighter, allowing to use it onboard of small satellites, or RPAS platforms (which at local scales is of high interest for agriculture applications) or as a ground base instruments (e.g for biomass or soil moisture monitoring purposes).
Hence, GNSS-R is becoming a challenging and promising technique.