An Analysis of Multipath Mitigation Techniques Suitable for Geodetic Antennas

Robin Granger, Steve Simpson

Abstract:	Roke Manor Research has been actively involved in the development of a geodetic-grade receiver and antenna for Galileo under the “Advanced Receiver Terminal for User Services" (ARTUS) programme, supported by the European GNSS Supervisory Authority (GSA). A paper [1] was presented by Roke at ION GNSS 2006 (and was subsequently featured in GPS World magazine, Feb 2007) which described the development of a wide bandwidth antenna with good axial ratio and phase centre stability ideally suited to geodetic and precise positioning applications requiring multiple GNSS carriers. Whilst bandwidth and phase centre stability are important performance criteria, the antenna must also reject multipath; another major source of phase error. In this paper, we present an analysis of the multipath environment in perhaps the most demanding of applications, namely the GNSS Reference Station. It is assumed that reference station antennas are generally sited well clear of buildings, trees and other obstructions and so attention need only be paid to multipath originating from the surface immediately below the antenna. We explain how multipath interacts with the antenna, through the processes of diffraction and surface wave propagation, and how a local groundplane can improve the situation. We also show that the traditional approximation of the ground as an infinite, perfectly conducting smooth planar surface does not apply in many real-world situations. For example, ground moisture content and surface roughness can have a significant impact on multipath. Using proprietary simulation tools it is possible to predict the likely multipath errors for any particular antenna, groundplane and ground surface combination using antenna pattern data derived either from simulations or anechoic chamber measurements. The paper will describe how such simulations may be performed. Finally, we propose a number of techniques which may be useful in reducing the effects of multipath from the ground. Some of these, such as the choke ring groundplane, are well-known; others presented perhaps for the first time. In particular, a novel, broadband groundplane structure is described which is suitable for multi-GNSS reference station installations. This groundplane, when combined with the ARTUS antenna, has been shown in simulations to reject multipath by 20dB or more at all Galileo and GPS carriers, and with scope for further improvements.
Published in:	Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008) September 16 - 19, 2008 Savannah International Convention Center Savannah, GA
Pages:	2755 - 2765
Cite this article:	Granger, Robin, Simpson, Steve, "An Analysis of Multipath Mitigation Techniques Suitable for Geodetic Antennas," Proceedings of the 21st International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2008), Savannah, GA, September 2008, pp. 2755-2765.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In