This 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.
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Consistent and accurate measurements of snowpack and melt rate are essential for climate studies and management of water supply and flood control systems. Existing snowpack measurements are limited in terms of their spatial footprint or require the expenditure of significant resources. An entirely different scientific community has deployed GPS instrumentation in the United States to measure how earthquake faults and volcanoes deform the Earth. Over 1100 of these GPS sites were installed by the EarthScope Plate Boundary Observatory (http://pbo.unavco.org) with funds from the National Science Foundation. The PBO network is operated by UNAVCO and the GPS data are freely available within 24 hours. The focus of this study was to determine whether these GPS instruments could be turned into operational snow sensors. Our methodology uses GPS signals that have reflected off the snow surface. By comparing these reflections with those from bare soil, we are able to estimate a snow depth value every day with a precision of a few inches. In this study we have validated the GPS method at three sites. The first is an ephemeral snow site near Boulder, Colorado. The second site is ~30 km to the west at an elevation of over 3000 m. The GPS site is hosted by the Niwot Ridge Long-term Ecological Research station. It has successfully operated for four years. The final validation site is operated with the cooperation of Utah State University in the Daniels Forest. All have provided excellent data. We currently measure snow depth using GPS data from 125 sites in the western United States, including Alaska. These data are available at http://xenon.colorado.edu/portal. We also calculate and distribute snow water equivalent – the data of most value for water managers. We are also working with scientists at NASA and NOAA to promote the assimilation of these data into their climate models. The GPS snow method is being adapted by colleagues in both Europe and Asia. The GPS snow method takes advantage of existing instrumentation, and thus has significant broader impacts. However, GPS instruments used by geoscientists are very expensive. We are also working on developing a cheaper GPS sensor, thus providing data over larger spatial scales at significantly less cost. The CU reflections group has also developed an education and outreach resource on GPS and GPS reflections. It is located at http://xenon.colorado.edu/spotlight
