By GNSS positioning a multipath signal is often considered an undesirable phenomenon that needs to be suppressed. A reflected GNSS signal (also known as multipath- ore scattered signal) is regarded as an error source that lower the positioning accuracy. But these scattered signals can be used in many remote sensing applications.

The term GNSS Reflectometry (GNSS-R) comprises various remote sensing techniques. GNSS-R aims to retrieve information about the Earth surface by analysing the signals emitted by GNSS-satellites and captured the reflected signal by an elevated platform. The system has the characteristics of a bi-static radar and scatterometre, at L-band (microwave). In this process, the GNSS L-band satellite acts as the transmitter and an airplane or low earth orbit (LEO) satellite, as the receiving platforms. For altimetry applications, a GNSS-R receiver can also be placed on the land.

The advantages of GNSS-R remote sensing:

1. no additional transmitter (over satellite scatterometry and radar altimetry);
   
2. highly precise, continuous: more than one hundred transmitting GNSS satellites in the near future, including GPS, Galileo, GLONASS, and Beidou/Compass,;
   
3. all-weather day and night and near-real-time remote sensing tool
   
4. use of spread-spectrum communication technology to enable the receiver to receive weak signals;
   
5. wide range of uses for such things as sea-wind retrieval, seawater salinity detection, ice-layer density measurement, humidity measurement of land, and detection of moving targets. [DONGKAI YANG, YANAN ZHOU, AND YAN WANG BEIHANG UNIVERSITY Remote Sensing with Reflected Signals, 2009]

for more information check:

1. Explanation of the concept
   
2. The Use of GNSS-R
   
3. Characteristics of GPS Reflected Signal