A Slope-Based Multipath Estimation technique for mitigating short-delay multipath in GNSS receivers

Abstract

The everlasting public interest on location and positioning services has originated a demand for a high performance Global Navigation Satellite System (GNSS), such as the Global Positioning System or the future European satellite navigation system, Galileo. The performance of GNSS is subject to several errors, such as ionosphere delay, troposphere delay, receiver noise and multipath. Among all these errors, multipath is the main limiting factor in precision-oriented GNSS applications. In order to mitigate the multipath influence on navigation receivers, the multipath problem has been approached from several directions, including the development of novel signal processing techniques. Many of these techniques rely on modifying the tracking loop discriminator in order to make it resistant to multipath. These techniques have proved very efficient against multipath having a medium or large delay with respect to the Line-Of-Sight (LOS) signal. In general, the multipath errors are largely reduced for multipath delays greater than around 0.1 chips (which is about 29.3 meters for Galileo E1 Open Service (OS) signal). Theoretically, this constitutes a remarkable improvement as compared to simpler techniques such as narrow Early-Minus-Late (nEML) tracking loop. However, in practice, most of the multipath signals enter the receiver with short-delay with respect to LOS signal, making most of these mitigation techniques partially ineffective. In this paper, we propose a new multipath estimation technique, namely the Slope-Based Multipath Estimation (SBME), which is capable of mitigating the short-delay multipath (i.e., multipath delays less than 0.35 chips) quite well compared with other state-of-the-art mitigation techniques, such as the nEML and the High Resolution Correlators (HRC). The proposed SBME first derives a multipath estimation equation by utilizing the correlation shape of the ideal normalized correlation function of a Binary Phase Shift Keying (BPSK)- or Multiplexed Binary Offset Carrier (MBOC)-modulated signal, which is then used to compensate for the multipath bias of a nEML tracking loop. It is worth to mention here that the SBME requires an additional correlator at the late side of the correlation function, and it is used in-conjunction with a nEML tracking loop. The multipath performance of the above-mentioned mitigation techniques is presented for Galileo E1 OS and GPS L1 C/A signals from theoretical as well as simulation perspective.