Authors response to referee comments

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-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.,