A New Approach for Mitigating Low-Frequency Carrier-Phase Multipath in GPS-RTK, Based on Between-Receivers Dynamics and an Effective Reflector

Luis Serrano, Don Kim and Richard B. Langley

Abstract:	Recently, and following the constant search to improve the accuracy of the solutions coming out from the UNB GPS-RTK system [Kim and Langley, 2003], we published a study concerning the development of novel multipath observables for GPS-RTK [Serrano et al., 2005]. They were designed for kinematic applications in harsh scenarios such as urban canyons or indoor sites. This paper is an evolution of the previous one, and describes the parameterization of the observables in order to describe an effective reflector and its parameter estimation. Hence, a brief summary of our approach for deriving the multipath observables, as described in the previous paper, is given as follows. It is based on the use of two GPS receivers (a master and a slave) connected to the same clock to remove the common satellite and receiver clock biases after single differencing. It is possible to control the relative dynamics between the antennas of the two receivers through a rotational motion like that of a momentum wheel with a lever arm. Also, rotation speed can be pre-chosen, and the rotational dynamics can be combined with the platform- specific dynamics. Other kinds of antenna-relative motion may be defined, depending on the specific application and the user platform. A conventional approach uses a group of closely spaced antennas, which share common multipath effects, in order to obtain redundancy for the multipath parameterization. In our approach, the relative receiver dynamics permits the initial system observability, based on the signature of the low-frequency multipath, the geometry between the antennae, and the multipath profile. Thus, our approach reduces the number of antennas required, simplifying the system and making it more robust. Besides, it allows the calibration of the system not only for the derivation of the observables, adapting itself to the multipath environment, but also for related problems such as the ambiguity fixing, antenna phasecenter variation, and phase wind-up. However, the derivation of multipath observables is not straightforward, requiring a priori some mathematical manipulations of the raw data. The goal is to obtain a single-difference carrier-phase multipath error between the two antennas (immune to other remaining biases), becoming easier to separate the multipath component in each antenna since we know accurately their relative geometry. Thus their relative multipath phase difference (the relative multipath amplitude difference is negligible), can be obtained and hence we can recover the carrierphase multipath error at the master antenna.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	94 - 103
Cite this article:	Serrano, Luis, Kim, Don, Langley, Richard B., "A New Approach for Mitigating Low-Frequency Carrier-Phase Multipath in GPS-RTK, Based on Between-Receivers Dynamics and an Effective Reflector," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 94-103.
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