GNSS reflection measurements are divided into ground-based and space-based reflectometry depending on the location of the receiver. For ground-based reflection measurements, if we want to fully receive the reflected signal, we need to use a specially developed receiver that conforms to the polarization of the reflected signal. In addition, the receiver antenna should face the horizontal direction to receive the reflected signal from the ground. However, most of the GNSS monitoring networks or control networks currently use geodetic receivers, with the receiver antenna facing the zenith and mainly receiving right-hand circularly polarized signals. Although the geodetic receiver suppresses the acquisition of reflected signals, reflected signals can still be received at low satellite elevation angles and will interfere with the direct signals from the satellite, affecting GNSS measurements and causing the so-called “multipath error”. This interference delay effect caused by the propagation of signals through multiple paths is called multipath effect. For traditional GNSS receivers, the reflected signal and the direct signal are coupled together with causing the multipath effect, then the surface characteristic information carried by the reflected signal will be reflected in the multipath, so we can use the extracted GNSS multipath to invert the surface characteristics. For example, Fig. 4.1 shows the reflection process of GPS L1 and L2 signals’ RHCP and LHCP on a generally selected surface.