Development of GPS as a Snow Sensor

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0948957 LarsonThis grant supports development of a technique to use GPS receiver signal to noise ratio (SNR) antenna observations of ground reflections (multipath) from multiple frequencies of GPS satellite transmissions (e.g., L1, L2, L2C) to estimate snow pack depth and perhaps density proximal to the GPS antenna (within about 50 m). Larson and colleagues have already demonstrated a tight correlation between the upper 10 cm of soil moisture as measured in situ and GPS SNR observations and have been funded by NSF/ATM and EAR for that work. Now, based on observations at the Marshall Plate Boundary Observatory (PBO) GPS station in Boulder, CO just before and just after a significant snow event, Larson and colleagues have demonstrated an obvious change in the GPS SNR frequency pattern at varying satellite elevation angles before and after a snow event that compares well with model predictions based on electrodynamic theory. Forward models of the GSP SNR frequency patterns were used to compute predicted snow depth proximal to the antenna and then compared to in situ observations with very good agreement. Support from this grant will allow for further evaluation of the technique and consideration of error sources. Specifically, the PIs will : 1) Develop an electrodynamic model of snow effects on GPS data and an algorithm to estimate snow water equivalent (SNE); 2) Evaluate the sensitivity to snow of the receivers and antennas currently used in the U.S. (e.g., PBO GPS network); 3) Develop a model to flag GPS data corrupted by snow/ice effects on top of antennas and model these effects to improve positions for volcano, tectonic and atmospheric studies; 4) Improve the utility of other hydrologic products derived from GPS, such as soil moisture and hydrologic loading; and 5) Develop a low-power and low-cost GPS system to measure SWE. The project involves collaborations between GPS geodesists, hydrologists, snow and climate scientists, and electrical engineers. Three snow calibration sites will be developed in Colorado at Marshall (NCAR), Niwot Ridge (LTER), and the USFS Fraser Experimental Forest, each with in situ snow pack observational sensors and GPS sites. The GPS deployments will involve UNAVCO Facility field engineering assistance. Should the GPS snow sensor technique prove robust for wide spread application of algorithms at existing continuously operated GPS sites the broader impacts will include improved density of snow pack observations across the continental U.S. as leveraged off existing EarthScope PBO stations with important implications to water resources management and climate studies. Observations would complement existing in situ snow pack observational programs (NOAAs SNOTEL) and would provide additional ground truth for satellite borne optical observations of snow cover. ***