A Study on Snow Reflection Signals Using two GNSS Antennas with Different Gain Patterns and New Application to Monitor Snow Surface Conditions

Takayuki Yoshihara, Susumu Saito, Atsushi Kezuka, Hiroki Motoyoshi, Satoru Yamaguchi

Abstract: Reflected GNSS signal is used for various scientific applications using space and ground-based GNSS receivers. Using the ground-based GNSS receiver, methodology to continuously observe snow depth has been developed and demonstrated [1]. This novel technique is based on multipath analysis of fading patterns in receiving intensity (i.e. Signal-to-Noise Ratio; SNR) produced by a direct and a reflected signal from the snow surface. Namely, a vertical distance between GNSS antenna and the snow surface is estimated from such a fading pattern, which has a typical period depending on the height difference. Currently, it has been widely used over the world using GNSS stations which primary purposes are different from snow observation. In this study, we focus on reflectance at the snow surface, which depends on the snow surface conditions. Reflection coefficient is determined by permittivity of snow accumulation and it also changes with incidence angle and polarization. Therefore, simultaneous observation by two GNSS antennas which have different gain patterns has a potential to provide distinctive reflectance variations enough to monitor snow surface conditions. Based on this idea, we investigated characteristics of snow reflection signals using a choke-ring antenna and a multipath limiting antenna (MLA), which is designed to dramatically reject multipath signal with low elevation angle [2]. In general, low-elevation GNSS signal received by the choke-ring antenna is contaminated by multipath effects in contrast to the MLA. Therefore, an observational data set with the two antennas enables us to extract a component of snow reflection signals including its dependency on satellite elevation angle. To accomplish our purpose, we performed a winter experiment from December 2014 to May 2015 at meteorological observational filed of Snow and Ice Research Center, National Research Institute for Earth Science and Disaster Prevention (NIED). Our ultimate purpose is comprehensive evaluation of reflected GNSS signals and development of estimation method of the snow surface conditions. Toward the goal, we extracted components of the snow surface reflection signals from the observational data set and investigated their temporal changes over the entire snow-covered period of four months. Namely, we quantitatively analyzed temporal variations in receiving intensity and ranging errors due to interferences by snow reflection signals using observational parameters of sigma-C/N0 and sigma-Code-Carrier-Divergence (CCD), respectively. We investigated them with distinction between daytime and nighttime because meteorological conditions such as solar radiation and air temperature are different in local time. As results, we confirmed that general tendency was the same between daytime and nighttime and results for MLAs had more numbers of spike variations in nighttime. Meteorological observation is also important for the evaluation. Introducing meteorological data, another example analysis was performed with "common-satellite" single differences for both code and carrier measurements. For the analysis, a period of rapid temporal changes in snow properties from wet to dry was selected. Through the analysis, we successfully extracted a periodical variation of carrier phase measurement due to snow reflection and confirmed that its amplitude changed depending on snow surface conditions. On the other hand, it was difficult to extract a periodical variation pattern from code results. As future work, we are going to use permittivity from weekly snowpack observation at the experimental site. The permittivity enable us not only to calculate reflective coefficient at the snow surface but also to reconstruct multipath effects enough to compare real GNSS results, which are variations in receiving intensity and a common-satellite single difference for each code and carrier phase.
Published in: Proceedings of the 2016 International Technical Meeting of The Institute of Navigation
January 25 - 28, 2016
Hyatt Regency Monterey
Monterey, California
Pages: 762 - 770
Cite this article: Yoshihara, Takayuki, Saito, Susumu, Kezuka, Atsushi, Motoyoshi, Hiroki, Yamaguchi, Satoru, "A Study on Snow Reflection Signals Using two GNSS Antennas with Different Gain Patterns and New Application to Monitor Snow Surface Conditions," Proceedings of the 2016 International Technical Meeting of The Institute of Navigation, Monterey, California, January 2016, pp. 762-770.
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