Observations from a submerged GNSS antenna underneath a snowpack need to be analyzed to investigate its potential for snowpack characterization. The magnitude of the main interaction processes involved in the GPS L1 signal propagation through different layers of snow, ice, or freshwater is examined theoretically in the present paper. For this purpose, the GPS signal penetration depth, attenuation, reflection, refraction as well as the excess path length are theoretically investigated. Liquid water exerts the largest influence on GPS signal propagation through a snowpack. An experiment is thus set up with a submerged geodetic GPS antenna to investigate the influence of liquid water on the GPS observations. The experimental results correspond well with theory and show that the GPS signal penetrates the liquid water up to three centimeters. The error in the height component due to the signal propagation delay in water can be corrected with a newly derived model. The water level above the submerged antenna could also be estimated.
Abstract. Global Navigation Satellite Systems (GNSS) contribute to various Earth observation applications. The present study investigates the potential and limitations of the Global Positioning System (GPS) to estimate in situ water equivalents of the snow cover (snow water equivalent, SWE) by using buried GPS antennas. GPS-derived SWE is estimated over three seasons (2015/16–2017/18) at a high Alpine test site in Switzerland. Results are validated against state-of-the-art reference sensors: snow scale, snow pillow, and manual observations. SWE is estimated with a high correspondence to the reference sensors for all three seasons. Results agree with a median relative bias below 10 % and are highly correlated to the mean of the three reference sensors. The sensitivity of the SWE quantification is assessed for different GPS ambiguity resolution techniques, as the results strongly depend on the GPS processing.
Abstract. Global Navigation Satellite Systems (GNSS) contribute to various Earth observation applications. The present study investigates the potential and limitations of the Global Positioning System (GPS) to estimate in situ water equivalents of the snow cover (snow water equivalent, SWE) by using buried GPS antennas. GPS derived SWE is estimated over three seasons (2015/16 -2017/18) at a high Alpine test site in Switzerland. Results are validated against state of the art reference sensors: snow scale, snow pillow, and manual observations. SWE is estimated with a high correspondence to the reference sensors for 5 all three seasons. Results agree with a median relative bias below 10 % and are highly correlated to the mean of the three reference sensors. The sensitivity of the SWE quantification is assessed for different GPS ambiguity resolution techniques, as the results strongly depend on the GPS processing.
Global navigation satellite system (GNSS) refractometry enables automated and continuous in situ snow water equivalent (SWE) observations. Such accurate and reliable in situ data are needed for calibration and validation of remote sensing data and could enhance snow hydrological monitoring and modeling. In contrast to previous studies which relied on post-processing with the highly sophisticated Bernese GNSS processing software, the feasibility of in situ SWE determination in post-processing and (near) real time using the open-source GNSS processing software RTKLIB and GNSS refractometry based on the biased coordinate Up component is investigated here. Available GNSS observations from a fixed, high-end GNSS refractometry snow monitoring setup in the Swiss Alps are reprocessed for the season 2016/17 to investigate the applicability of RTKLIB in post-processing. A fixed, low-cost setup provides continuous SWE estimates in near real time at a low cost for the complete 2021/22 season. Additionally, a mobile, (near) real-time and low-cost setup was designed and evaluated in March 2020. The fixed and mobile multi-frequency GNSS setups demonstrate the feasibility of (near) real-time SWE estimation using GNSS refractometry. Compared to state-of-the-art manual SWE observations, a mean relative bias below 5% is achieved for (near) real-time and post-processed SWE estimation using RTKLIB.
Global Navigation Satellite Systems (GNSS) contribute to various Earth observation applications. The present study investigates the potential and limitations of the Global Positioning System (GPS) to estimate in situ water equivalents of the snow cover (snow water equivalent, SWE) by using buried GPS antennas. GPS-derived SWE is estimated over three seasons (2015/16-2017/18) at a high Alpine test site in Switzerland. Results are validated against state-of-theart reference sensors: snow scale, snow pillow, and manual observations. SWE is estimated with a high correspondence to the reference sensors for all three seasons. Results agree with a median relative bias below 10 % and are highly correlated to the mean of the three reference sensors. The sensitivity of the SWE quantification is assessed for different GPS ambiguity resolution techniques, as the results strongly depend on the GPS processing. L. Steiner et al.: Sub-snow GPS for quantification of snow water equivalent weather conditions. Sub-snow GNSS techniques have been tested lately for SWE estimation (Limpach et al., 2013; Henkel et al., 2018), which is suggested to determine liquid water content (Koch et al., 2014) or considered for avalanche rescue (Claypool, 1997; Schleppe and Lachapelle, 2008; Olmedo et al., 2012). Most studies concentrate on the use of Global Positioning System (GPS) single-frequency signals (L 1 , with a frequency of f 1 = 1575.42 MHz). GPS antennas buried under snow were first used to evaluate the potential of GPS signal reception and positioning performance in avalanche rescue research (Claypool, 1997). Schleppe and Lachapelle (2008) experimentally analyzed the GPS tracking performance under avalanche-deposited snow at two test sites in Canada. The potential of a GPS-based rescue system for victims buried under avalanches was investigated by Olmedo et al. (2012) in the framework of the SICRA project. Steiner et al. (2018) analyzed the GPS receiver behavior from antennas submerged in water and developed a model to estimate the water depth above the submerged GPS antenna based on the path delay of the GPS L 1 signals. Another study deals with the analysis of GPS L 1 signal-to-noise ratio (SNR) for snow property estimation from a ground GPS antenna compared to a GPS reference antenna above the snow surface (Appel et al., 2014). Koch et al. (2014) continuously estimated the liquid water content (wetness) based on GPS L 1 signal strength observations for a seasonal snowpack in the Swiss Alps. Henkel et al. (2018) show exemplarily the possibility of SWE estimation using GPS L 1 carrier phase residuals for the dry snow period during winter 2015/16 at the Weissfluhjoch test site of the WSL Institute for Snow and Avalanche Research (SLF). Furthermore, GNSS stations above a snowpack are used for SWE estimation based on the interference of direct and reflected GNSS signals (GNSS reflectometry). Thereby, the SWE is estimated using GPS L 1 C/A or L 2 c SNR (e.g.,
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