Using GPS multipath to measure soil moisture fluctuations: initial results

Abstract

Measurements of soil moisture are important for studies of climate and weather forecasting, flood prediction, and aquifer recharge studies. Although soil moisture measurement networks exist, most are sparsely distributed and lack standardized instrumentation. Measurements of soil moisture from satellites have extremely large spatial footprints (40-60 km). A methodology is described here that uses existing networks of continuously-operating GPS receivers to measure soil moisture fluctuations. In this technique, incoming signals are reflected off and attenuated by the ground before reception by the GPS receiver. These multipath reflections directly affect signal-to-noise ratio (SNR) data routinely collected by GPS receivers, creating amplitude variations that are a function of ground reflectivity and therefore soil moisture content. After describing this technique, multipath reflection amplitudes at a GPS site in Tashkent, Uzbekistan are compared to estimates of soil moisture from the Noah land surface model. Although the GPS multipath amplitudes and the land surface model are uncalibrated, over the 70-day period studied, they both rise sharply following each rainfall event and slowly decrease over a period of *10 days.

Soil Moisture Retrieval from Active Spaceborne Microwave Observations: An Evaluation of Current Techniques

“…This is due to the fact that soil moisture is difficult to measure on a large scale in a cost-effective and routine manner. In the past both optical, microwave and even Global Positioning System (GPS) reflected signals [1][2][3][4][5] have been explored and shown to be sensitive to the soil dielectric constant. Microwave remote sensors are particularly favoured, not only due to their sensitivity to variations in certain surface parameters but also because of their ability to penetrate most cloud cover conditions and their independence of solar illumination.…”

Characterization of site‐specific GPS errors using a short‐baseline network of braced monuments at Yucca Mountain, southern Nevada

“…Multipath-induced errors can appear in many forms: The annual repeat in the GPS satellite configuration could cause an approximately annually ($350 day) repeating multipath signal [Ray et al, 2007]; unmodeled subdaily signals, including those from multipath, could propagate to longerperiod signals [Penna et al, 2007;King et al, 2008]; seasonal signals could be caused by changes in the vegetation, snow pack, and surface water of the local area around the antenna [Dong et al, 2002;Larson et al, 2008]; and changes in multipath could appear or disappear at random as the local environment changes [Dong et al, 2002]. Additional to any effects from far-field reflections, near-field signal scattering by the antenna or monument itself can also be a significant source of error [Elósegui et al, 1995].…”

Open-loop GPS signal tracking at low elevation angles from a ground-based observation site

“…It appears feasible that surface reflections induce C / N 0 fluctuations at these elevation angles (see e.g. Anderson, 2000;Larson et al, 2008), degrading the quality of the O/L model and thereby causing poor O/L performance. The panorama photograph Fig.…”

“…reflections in the vicinity surrounding the antenna, can be described using geometric optics (see e.g. Elósegui et al, 1995;Anderson, 2000;Larson et al, 2008); tropospheric multipath is a diffraction phenomenon and requires wave optical methods to analyse quantitatively (see e.g. Gorbunov, 2002a;Sokolovskiy, 2001b;Jensen et al, 2003).…”