GPS interferometric reflectometry: applications to surface soil moisture, snow depth, and vegetation water content in the western United States

Abstract

GPS interferometric reflectometry is a new environmental sensing technique that can be used to measure near-surface soil moisture, snow depth, and vegetation water content variations. The spatial scale of this technique,~1000 m 2 , is intermediate to that of other in situ sensors (<1 m 2 ) and satellites (>100 km 2 ). Soil moisture and snow depth retrievals have accuracies of 0.04 m 3 /m 3 and 0.04 m, respectively. These accuracies are sufficient for many hydrologic applications. Fortuitously, GPS interferometric reflectometry can be used with consumergrade off the shelf GPS instruments that are operated by the geodetic, geophysical, and surveying communities. This means that GPS data from thousands of sites are potentially available for environmental scientists seeking new in situ data for soil moisture, snow depth, and vegetation water content. The technique can be applied to data from existing archives or for new sites. Although the accuracy of the technique has only been evaluated for the GPS constellation, the technique can also be used for other navigation constellations such as GLONASS, Galileo, and Beidou.

Decadal changes of surface elevation over permafrost area estimated using reflected GPS signals

“…Larson (2016) provides an overview of the GPS-IR technique. Here we only describe the method of using GPS-IR to measure the reflector height, which refers to the height of the GPS receiver antenna phase center above the reflecting surface.…”

“…The strength of the interference, quantified by the SNR of the received power, oscillates with the elevation angle (e). For a horizontal planar reflector, such as the flat surface surrounding SG27, the SNR oscillation is characterized by a dependency on sine of the elevation angle (Larson, 2016):…”

GPS Interferometric Reflectometry Reveals Cyclic Elevation Changes in Thaw and Freezing Seasons in a Permafrost Area (Barrow, Alaska)

“…GPS interferometric reflectometry (GPS‐IR) is a new technique that uses the interference of direct and reflected GPS signals for environmental studies (Larson, ). The GPS‐IR technique has been successfully applied to estimate snow depth (Larson & Nievinski, ), sea level changes (Larson, Ray, et al, ), soil moisture (Chew et al, ), and vegetation water content (Chew et al, ).…”

Deriving surface soil moisture from reflected GNSS signal observations from a grassland site in southwestern France

“…The GNSS-IR technique allows relating the reflected signal to the characteristics of the reflecting surface and to retrieve geophysical variables. Over land, variables such as soil moisture, snow depth and vegetation parameters can be observed using this technique (Larson et al, 2008;Small et al, 2010;Larson and Nievinski, 2013;Wan et al, 2015;Larson, 2016;Roussel et al, 2016;Zhang et al, 2017). GNSS satellites emit active Lband microwave signals (between 1.2 and 1.6 GHz).…”

“…Such antennas have an 15 antenna gain pattern optimised for Right Hand Circular Polarization (RHCP) and minimized for Left Hand Circular Polarization (LHCP). A GNSS network called Plate Boundary Observatory (PBO) H2O with geodetic-quality antennas on ground in western USA is currently used to monitor VSM (Larson et al, 2013;Larson, 2016;Chew et al, 2016) and snow depth (Larson et al, 2009). The basic observation used in this technique is the signal-to-noise ratio (SNR) which is related to temporal changes in the interference between the direct and the reflected GNSS signals.…”