GAST-D Integrity Risks of Snow Accumulation on GBAS Reference Antennas and Multipath Effects Due to Snow-surface Reflection

Takayuki Yoshihara, Hiroki Motoyoshi, Takeshi Sato, Satoru Yamaguchi, and Susumu Saito

Abstract:	GBAS Approach Service Type D (GAST-D) is a Ground- Based Augmentation System (GBAS) based on a differential GNSS positioning system for aircraft precision approach and landing. It has a capability to support the final approach and landing defined as category III (CAT-III) using a single frequency of GNSS L1 signal with a high level of safety requirements. Electronic Navigation Research Institute (ENRI) has been developing a GAST-D ground subsystem since March 2012 to evaluate integrity risks and to develop mitigation methods to them under operational environment in Japan. As an important subject of them, we identified deep snow as risks to be evaluated for GAST-D ground subsystem. Degradation of received GNSS signal due to snow could cause lowering integrity monitor performances and increasing of differential correction noise in GAST-D messages. In this study, we examined two effects of deep snow on GAST-D ground subsystem through degradation of received GNSS signal, which are snow accumulation (i.e. snow accretion) on GBAS reference stations and multipath reflected by snow surface as a reflector with variable height and surface conditions. To examine these effects and accomplish evaluation of the risks, we conducted a special observation that a GPS antenna was set up at a snow and weather observational site of Snow and Ice Research Center, National Institute for Earth Science and Disaster Prevention (NIED) in Japan during December 2011 and May 2012. Accumulated snow covered the ground over four months and the maximum snow depth of 209 cm was observed in February 2012. Effects of snow accumulation on the GPS antenna were investigated through snow-cap-drop events and we successfully extracted rapid changes in C/N0 of about 2dB-Hz and excess delay of about 5cm in L1 carrier phase measurement in slant direction to GPS satellite. The excess delay of 5cm due to snow accumulation on GPS antenna could remarkably degrade performances of GAST-D integrity monitors such as ionospheric gradient monitor. For scientific application such a meteorological study to investigate spatial distribution of atmospheric water vapor, such excess delay of 5cm could be also a significant error source. Another important point, which was multipath effects by snow surface reflection, was analyzed using an evaluation index based on code-carrierdivergence variation during 100 seconds. As a result, its day-to-day variation in nighttime was larger than daytime and it also tended to be lager in decrease periods of snow depth than increase. It is also important to model and simulate multipath signal reflected by snow surface in order to obtain appropriate parameters to cover a component of differential correction noise due to multipath effects.
Published in:	Proceedings of the 2013 International Technical Meeting of The Institute of Navigation January 29 - 27, 2013 Catamaran Resort Hotel San Diego, California
Pages:	112 - 120
Cite this article:	Yoshihara, Takayuki, Motoyoshi, Hiroki, Sato, Takeshi, Yamaguchi, Satoru, Saito, Susumu, "GAST-D Integrity Risks of Snow Accumulation on GBAS Reference Antennas and Multipath Effects Due to Snow-surface Reflection," Proceedings of the 2013 International Technical Meeting of The Institute of Navigation, San Diego, California, January 2013, pp. 112-120.
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