A New Carrier-Phase Multipath Observable for GPS Real-Time Kinematics, Based on Between Receiver Dynamics

Luis Serrano, Donghyun Kim and Richard B. Langley

Abstract:	"In GPS differential carrier-phase measure ments, multipath is still present and remains as the major contribution to the error budget, especially the low-frequency, shortdelay multipath (specular). There are various approaches to mitigate this effect. The most notable is to build a multipath profile (also called an effective reflector) and to parameterize the multipath measurements that resemble the geometry between the GPS receiver and the profile. However, problems still remain in this approach since the basic measurements used for the estimators do not entirely correspond to all the physical effects of multipath. SNR measurements are used extensively due to their high correlation with the multipath environment and the receiver-satellite geometry. Due to their low precision, however, estimators converge over a long time span. Thus, they turn out to be an impractical measurement for real-time applications. On the other hand, methods to derive and separate the multipath that filter or use the carrier phase, not only may mask out the platform dynamics (especially when higher dynamics are involved), but also may introduce other unexpected biases. These limitations lead us to derive multipath observables for multipath mitigation, merging the synergies from previous approaches. The multipath observables will carry all the multipath spectra with no limitation on the number of multipath signals (including specular reflections and diffraction), and be used as a technique that does not require a priori knowledge of the environment. Since the multipath observables will be derived from the platform dynamics (i.e., GPS carrierphase derived velocity and acceleration estimates that can be designed to be immune to multipath), they can give a good signature of the low-frequency multipath. Our approach for deriving the multipath observables 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 specify the relative dynamics between the antennas of the two receivers through a rotation motion, like that provided by a momentum wheel with a lever arm with a pre-chosen rotation speed, combined with the platform specific dynamics. This has two main advantages: - A conventional approach uses a bundle of antennas, which share common multipath effects, in order to obtain redundancy for the multipath parameterization. In our approach, the relative receiver dynamics satisfies the initial system observability, based on the signature of the lowfrequency multipath, geometry between the antennas, and the multipath profile. Thus, it reduces the number of antennas required, simplifying the system and making it more robust. - Second, the relative receiver dynamics 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 phase-center variation and phase wind-up. Therefore, even before the platform starts navigating, the problem of initial low-frequency multipath and initial integer ambiguity resolution may be handled during the calibration mode. Other initial concerns, besides the obvious validation of the observables for all possible sources of multipath spectra, are the system observability and the applicability in real-time scenarios. These are obvious problems, since the derivation of multipath observables is not straightforward, requiring some mathematical manipulations of the raw data before we can even derive the observable. Such manipulations should be performed continuously until the estimator converges towards the multipath profile (ending the calibration mode). We performed initial tests to accurately validate the mathematical and stochastic signature of the multipath observables. This was performed through tests in different scenarios using a multipath software simulator for the GPS signal, and a calibrated motion table for real data. In this paper, we describe of the results of initial tests and address issues and problems of our approach, and discuss possible applicability in real-time scenarios. We further address our future work including the multipath parameterization based on the profile and the full implementation of the estimator and thus the mitigation of multipath on carrier phase measurements. The approach will be extended to signals from pseudolites since, based-on preliminary simulations, we could see no major problem to extend it to indoor scenarios."
Published in:	Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005) June 27 - 29, 2005 Royal Sonesta Hotel Cambridge, MA
Pages:	1105 - 1115
Cite this article:	Serrano, Luis, Kim, Donghyun, Langley, Richard B., "A New Carrier-Phase Multipath Observable for GPS Real-Time Kinematics, Based on Between Receiver Dynamics," Proceedings of the 61st Annual Meeting of The Institute of Navigation (2005), Cambridge, MA, June 2005, pp. 1105-1115.
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