GNSS reflectometry (GNSS-R) for environmental observation

The foundations behind the utilization of reflected GNSS signals for environmental research were laid down in the 1990s. Several pioneering works laid the groundwork for using these “signals of opportunity.” Martin-Neira proposed the Passive Reflectometry and Interferometry System (PARIS) for using signals of opportunity for ocean altimetry (Martin-Neira, 1993). The developed methodologies can be classified into categories, based on platforms and signals used. The platform classification includes spaceborne, airborne, and ground-based techniques. Classification based on the signal property includes Doppler-delay map (DDM), signal-to-noise ratio (SNR), and carrier-phase observation techniques. In terms of receiving station's setups, there are upward-looking and side-looking single-antenna setups, as well as dual-antenna setups, i.e., one upward-looking and one side- or downward-looking antenna. Each technique is briefly described in this chapter. These techniques enable the measurement of (1) wind speed over water bodies (see “GNSS-R wind speed measurements” section), (2) water level height including sea level (see “GNSS-R water level/land height measurements” section) and land elevation, (3) soil moisture (“GNSS-R soil moisture monitoring” section), (4) vegetation, and (5) snow height observations (see Fig. 8.1).