GNSS Multipath Mitigation Using High-Frequency Antenna Motion

T. Ertan, M.L. Psiaki, B.W. O'Hanlon, R.A. Merluzzi, S.P. Powell

Abstract:	A method is developed to characterize and compensate GNSS multipath by considering signal amplitude and phase variations in response to antenna motion. This method seeks to improve multipath rejection capabilities beyond those provided by choke-ring or other antenna technologies or by multi-correlator signal processing. A known antenna motion profile can be combined with a modified multi-correlator discriminator in order to better characterize the multipath components and better isolate the direct-path component from the multipath. Antenna motion yields characteristic signal responses in the presence of multipath. Suppose that high-frequency antenna motion occurs in a way that its projection onto the direct path differs from its projections onto the directions of arrival of any significant multipath components. Suppose, also, that the magnitude of the motion is an appreciable fraction of a wavelength and that the received multipath amplitudes are significant fractions of the direct signal amplitude. Then the direct and multipath signals will beat against each other to cause noticeable amplitude and carrier-phase variations of the received signal. These variations can be used to better isolate and estimate the parameters of each component, including their carrier phases and amplitudes and even their code phases. The new technique constitutes an enhanced version of the Multipath Estimating Delay Lock Loop (MEDLL) method of Refs 1 and 2. It uses a batch filter to estimate the code phase, carrier phase, and amplitude of the direct signal along with the relative code phase, carrier phase, and amplitude of each significant multipath component. In addition, it estimates the direction of arrival of the direct and multipath components. Each of these directions is parameterized as a unit vector defined in the same body-axis coordinate system as the known antenna motions. The batch estimation procedure extends over a number of accumulation intervals, as many as are needed to span a sufficiently rich antenna motion time history. The batch estimator solves a weighted nonlinear least-squares problem that involves mathematical models for the in-phase and quadrature accumulations over a span of sample times and a range of code-phase offsets. The batch estimator includes implicit high-pass filtering so that its results are not affected by low-frequency phase variations that might be caused by satellite motion and receiver clock drift. The addition of antenna motion improves the ability of the least-squares solver to isolate the effects of the multipath components. In a standard MEDLL implementation, the multipath is evident primarily because of distortions in the cross-correlation function between the received PRN code and the receiver's replica code. In the new method, additional distinctives are provided by the antenna motion. The most important new feature of the model is that the direct and multi-path components of the cross-correlation functions rotate with respect to each other in carrier-phase space. This rotation occurs because the antenna articulations evolve in a way that causes differential range variations between the direct and multipath signals. These variations translate into differential phase rotations because of the accumulated-delta-range/carrier-phase relationship. The results of analyses and experimental tests of this new system will be presented. Analyses will evaluate the observability and theoretically expected accuracy of typical multipath scenarios. Experimental tests involving data from a hardware GNSS simulator and from a roof-mounted antenna articulation system will be presented. Performance will be explored for articulation motions on the order of 5-12 cm, i.e., 1/4 to 1/2 cycle of the GPS L1 or L2 carrier wavelengths. Another issue that will be considered is the new system's performance when using a narrow-band RF front-end that would preclude good multipath rejection for a standard MEDLL-type calculation. The impact of this system on the ability to use the GPS L1 C/A and L2C signals for differential pseudorange TEC measurement will also be considered. References: [1] Van Nee, R.D.J., Siereveld. J., Fenton, P.C., and Townsend, B.R., "The Multipath Estimating Delay Lock Loop Approaching Theoretical Accuracy Limits," Proc. IEEE Position, Location, and Navigation Symp., Las Vegas, Nevada, April 1994. [2] Townsend, B.R., Fenton, P.C., Van Dierendonck, K.J., and Van Nee, D.J., "Performance Evaluation of the Multipath Estimating Delay Lock Loop," Navigation, Vol. 42, No. 3, Fall 1995, pp. 503-514.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	154 - 175
Cite this article:	Ertan, T., Psiaki, M.L., O'Hanlon, B.W., Merluzzi, R.A., Powell, S.P., "GNSS Multipath Mitigation Using High-Frequency Antenna Motion," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 154-175.
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