Reflected GPS signals have a distinct signature or set of characteristics that are different from directly received signals. Some of the most important are:
By comparing the ray-path lengths of the reflected and direct radio-links, the vertical distance between the receiving platform and the surface level can be measured. This is the altimetric application. On the other hand, when the distortion of the reflected signal is analyzed, some geophysical parameters that characterize the reflecting surface can be estimated, such as it roughness and dielectric properties. The roughness essentially acts spreading the signal through the glistening zone, reducing the peak power of the reflected waveform and adding contributions at longer delays. These longer delays are the result of signal ray-paths that have been reflected in areas of glistening zone farther away from the specular - the link transmitter-specular-receiver has, by definition, the shortest ray-path (see Figure 1).
- Polarization. The GPS signal is right-hand circular polarized. When it is reflected by the surface, the signal might change into left-hand polarization. So, a left-hand circular polarization (LHCP) antenna is used to collect reflected data.
- Code delay and Doppler shift. The reflected signal has to propagate an extra path segment compared with the direct one, thus causing an additional time delay (the equation 2h∙,sin-θ.= c∙τ is used to estimate the extra delay of the specular point.
- Correlation. The correlation characteristic can be expressed by the integral function of correlation. [DONGKAI YANG, YANAN ZHOU, AND YAN WANG BEIHANG, 2009]
Furthermore, generating a waveform by cross-correlating signal and replica during an integration time Ti, filters out any signal arriving with frequencies beyond +- 1/Ti Hz from the central one. Because different surface patches induce different Doppler shifts, only certain areas of the glistening zone will contribute to the cross-correlation. This allows to define Doppler stripes on the surface (areas from which the reflected signal will have the same Doppler shift +- 1/Ti). When the reflected signal is correlated along the central frequency solely, it is called Delay Map (DM). Complementary, the correlation can be repeated for different frequencies, mapping different Doppler stripes. The latter measurement is called Delay-Doppler-Map (DDM). [Institut de Ciències de l'Espai]