Carrier-Phase Multipath Calibration in GPS-RTK Machine-Guidance Applications

Luis Serrano, Don Kim and Richard B. Langley

Abstract:	Differential carrier-phase GPS-based attitude determination represents an attractive alternative to expensive and complex inertial measurement units (IMUs) and attitude heading reference systems (AHRS), for aeronautical, marine and machine-guidance applications. Previous work in GPS-based attitude systems, using ultra-short baselines (less than a couple of metres) between three/four antennas, have been shown to provide high accuracies, most of the time to the sub-degree level in yaw, pitch and roll. Clearly multipath is one of the most limiting factors in accuracy and reliability regarding GPS-based attitude systems, as even a small separation between the antennas causes different and highly decorrelated phase-multipath errors. This is even more problematic knowing that it is conceptually almost impossible for receiver/antennas to separate phasereflected multipath signals from the direct unipath when the reflector is at a very short distance from the antennas, regardless of the quality/cost of the hardware. Usually these reflections arise from the platform itself where the antennas are positioned. Being built most of the times with good reflector characteristics material, some structural components of these machine-guidance platforms represent solid specular reflectors. Not to mention multipath prone environments where interestingly some of the most interesting GPS-based machineguidance applications are employed (e.g. container shipyards, airports, survey sites) where it is very difficult to predict and avoid serious reflections or blockages upon a moving platform with random/variable motion. At present, there are in the market low-cost single-frequency (or dual-frequency) receivers which are relatively cheap and weigh less than a kilogram (including the antenna, engine, interface board, power supply, cables) and therefore do not represent a problem for any kinematic platform to carry just two receivers. With two antennas it is possible to determine yaw and pitch angles, which for some applications is sufficient (such as for precision agriculture), and depending on their placement in the platform body, make the determination of these two angles quite robust. At UNB we have been developing carrier-phase multipathmitigation techniques using single-difference multipath observables with a dual-antenna system. These observables are obtained from the multipath-free higher-order range-dynamic observations coming from the two antenna pseudo-random motions in kinematic applications. Their purpose is to mimic the deterministic nature of carrier-phase multipath embedded in the reflected signal. However, with multipath modelled as a quasi-random process in kinematic scenarios with variable dynamics, one also has to deal with the underlying stochastic/dynamic modelling of multipath in an adaptive approach. In this paper, we describe how a GPS-based machine guidance application using a dual antenna system can be used to calibrate carrier-phase multipath in quasi real-time. Furthermore, the intricate relationship between a platform's attitude and its dynamics, and the changing multipath from nearby reflectors are explored and modelled through the proper stochastic/dynamical model. Tests were performed using reallive signals, and we demonstrate that it is possible to estimate not only the reflected signal phase-delay but also the approximate geometric position of the strong effective reflector.
Published in:	Proceedings of IEEE/ION PLANS 2008 May 6 - 8, 2008 Hyatt Regency Hotel Monterey, CA
Pages:	479 - 488
Cite this article:	Serrano, Luis, Kim, Don, Langley, Richard B., "Carrier-Phase Multipath Calibration in GPS-RTK Machine-Guidance Applications," Proceedings of IEEE/ION PLANS 2008, Monterey, CA, May 2008, pp. 479-488. https://doi.org/10.1109/PLANS.2008.4570023
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