Characterizing GNSS Multipath in Challenging Installation Scenarios using Combined Simulation-Measurements of Dual-Polarization Antennas

Kurzfassung

Since the launch of the first NAVSTAR-GPS Block-I spacecraft almost half a century ago, satellite navigation has been and remains an indispensable element of modern life. Today, it supports a wide array of applications in domains ranging from positioning, surveying, and of course, navigation. In recent years, advances in Global Navigation Satellite Systems (GNSS) technology have focused on a push towards higher precision, targeting sub-centimeter accuracy [1], [2]. For satellite navigation through GNSS, obtaining the position-velocity-time (PVT) information of a target requires measuring the distance from satellite(s) to the user through time differences. This implies that, not only does the feeder link quality affect communication between the two GNSS segments but it also has an intrinsic implication on the capability of measuring this distance and by extension, determining the actual PVT information itself. Common sources of satellite-user channel disruptions include ionospheric and tropospheric scintillations. To improve navigation accuracy, these errors must be mitigated as much as possible. With the use of corrective methods such as dual-frequency measurements and differential pseudorange evaluation; most of the aforementioned errors, as well as several timing-based ones, can be greatly reduced or completely eliminated. However, in the very last few meters of downlink propagation, another error source emerges — multipath. Here, incoming right-hand circularly polarized (RHCP) satellite wavefronts reflecting or diffracting off objects in the vicinity of the GNSS receiver system, reach the antenna together with the line-ofsight (LoS) signals. These signals generally have different amplitudes, delays, and polarizations. These far- and nearfield conditions of the receiver installation scene result in characteristics of the standalone ’ideal’ antenna superimposed with reflected multipath rays. Consequently, effective antenna responses such as gain, phase, and group delay, exhibit aberrations, which are reproduced in key measured observables such as carrier-phase and pseudorange. Although, a well documented phenomenon, multipath remains a major error source, whose remedy is often limited largely to the choice of installed location and design of certain antenna properties [3]. Being the first element in the GNSS receiving chain, antennas significantly influence received signal quality and ultimately, precision of navigation. Therefore, their interactions with multipath rays require adequate investigation and characterization. Antenna parameters that influence multipath susceptibility have been discussed extensively in [4] In traditional GNSS applications, RHCP antennas are designed with high cross-polar discrimination to suppress multipath effects arising from strong reflections, which are mostly left-hand circularly polarized (LHCP) or mixed-polarized, in favor of direct RHCP satellite signals. By intuition, characterization of multipath would require an LHCP antenna. However, with a ’pure’ LHCP antenna alone, LoS tracking is often lost and it becomes very difficult to resolve multipath sources fully. Accordingly, new studies have attempted to solve this problem by exploiting polarimetry for better estimation of multipath conditions [5]. To achieve higher-precision navigation, proper prior estimation of an antenna’s expected performance in situ, is of significant importance. The natural approach, of course, would be to measure the antenna electromagnetic (EM) behavior, once its set up on its final platform. However, this method can be extremely expensive, difficult, and sometimes impossible to achieve through evaluation in anechoic chambers, especially in cases where the platform/scene is very large. To address this problem, new techniques such as using drone-bound aerial EM probes are currently being developed [6]. However, this technology is still in its infancy and not fully developed. Another interesting technique, which has gained popularity in the last years, is the fully simulative analysis of installed antenna performance [7]. Although a viable solution, this method has proven to be not at all easy task: required computational cost is high and exact modeling of all relevant scene contributors is difficult. Furthermore, the approach is limited to the availability of design files of antenna to be characterized, which are quite uncommon with commercialoff-the-shelf types. In this work, we present a hybrid simulative approach to GNSS multipath characterization of challenging installation scenarios. Here, we assess the installed performance of a dual-polarization antenna, developed in-house, in test scenarios [8]. The dual-sense nature of the antenna allows for reliable capture of both LoS and reflected signals for evaluation. By combining anechoic chamber measurements of this antenna with digital-twin simulations of the anticipated surrounding physical environment, sources of multipath that the antenna will experience in situ, are accurately predicted and analyzed. Moreover, by considering actual antenna characteristics which includes as effects of manufacturing defects and material tolerances, the proposed method models the real installed performance more closely. Furthermore, we harness the extra information provided by the additional antenna output to enhance multipath estimation. It will be shown that, by evaluating the performance in this way, information about the multipath-induced GNSS errors can be reliably obtained. We shall demonstrate the efficacy of the proposed method by comparing installed performance of our dual-polarization antenna to experimental field measurements.