The GPS satellites are constantly broadcasting radio signals to the Earth. Part of the signals is reflected back from the Earth's surface. The delay of the GPS reflected signal on the differential paths between direct and reflected signals could provide information about the surface.
The observables of GNSS-R are the waveforms, the cross-correlation between the signals and replicas (models) of them. Only replicas modelled appropriately (with the GNSS Pseudo-Random-Noise - PRN, the range and frequency corresponding to the incoming signal) will generate waveforms above the noise. Therefore, tuning the replicas to the right parameters permits to know the range and frequency -changes in the range- associated to a given satellite-receiver radio-link.
Another approach is to cross-correlating the reflected signals against the direct radio-link ones. This technique has the advantage of using the entire bandwidth of the signal, including the encrypted codes. This technique provides altimetric estimates of at least twice precision than the clean-replica approach. [Cardellach et al., 2013]. Together with information on the antenna position, the delay measurements associated with the properties of the reflecting surface can be used to produce the surface roughness. For example, the measurements of GNSS reflected signals from the ocean surface could retrieve the ocean surface height, wind speed and wind direction. The key issue is to extract information from the GPS reflected signal. The primary measurement is the received power from the GPS reflected signal for a variety of delays and Doppler values in a glistening zone surrounding a nominal specular reflection point (Figure right) [Garrison et al., 1997; Clifford et al., 1998].