Characterization of Signal Quality Monitoring Techniques for Multipath Detection in GNSS Applications  
[PERSON]  
1 Position, Location and Navigation (PLAN) Group, Schulich School of Engineering, University of Calgary, Calgary, AB T2N 1N4, Canada; [PERSON]DDRESS] (A.B. ); [EMAIL_ADDRESS] (G.L. )1  
[PERSON] and [PERSON]  
110 June 2017; Accepted: 3 July 201711  
diversity (e.g., [4; 5]) is another solution to reduce multipath which relies on different polarization and arrival angles of the reflected signals. These methods, however, require special hardware (antenna) considerations and modifications. Other solutions include three-dimensional building models (3 DBMs) in urban environments (e.g., [6; 7]) which also require additional geospatial information of nearby reflectors. However, in the context of reliability, the general approach to satisfy some level of navigation integrity is to detect and exclude (or de-weight) distorted measurements when exclusion does not degrade measurement geometry significantly (e.g., [7; 8; 9]). In this way, Signal Quality Monitoring (SQM) techniques have been developed to detect GNSS multipath distortions by incorporating monitoring correlators at the tracking level. References [10; 11] incorporated different early-late correlators to define symmetric and asymmetric delta and ratio metrics to detect multipath distortions in the tracking correlation peaks. Reference [12] investigated an asymmetry test statistic called the Slope Asymmetry Metric (SAM) to evaluate signal quality in the presence of multipath. SAM compares the left and right slopes of the correlation curve and provides the receiver with timely alarms when one slope is steeper than the other as a result of multipath. References [13; 14] exploited the Carrier-to-Noise-density ratio (C/N\({}_{0}\)) as a quality metric to weight GNSS measurements and improve positioning performance in multipath environments. Beside C/N\({}_{0}\), [15; 16] investigated a multipath detection approach using SAM and the DLL discriminator output to monitor the correlation peak quality. The correlator output samples are evaluated and integrity warning is set when the SQM metrics deviate from their nominal values. Based on a multi-correlator structure, [17] developed a signal quality index at the tracking level to evaluate the quality of GNSS signals and improve the performance of Receiver Autonomous Integrity Monitoring (RAIM) modules under multipath scenarios. Despite the common use of SQM metrics, their sensitivity and effectiveness in detecting multipath have not been fully characterized. It has been shown that multipath affects SQM metric statistics (e.g., mean and variance) by distorting the correlation peak but there is no analytical discussion about how sensitive different SQM metrics are under different multipath scenarios. It has also been discussed that SQM-based monitoring results can be exploited for reducing the effect of multipath through de-weighting or excluding affected measurements but there are few investigations on how effective SQM metrics can be for multipath mitigation with respect to different tracking and signaling strategies. This research carries out a characterization and performance evaluation of SQM techniques under different multipath scenarios. In Section 2, after modelling the received GNSS signals in the output of tracking correlators, monitoring correlators are defined based on their relative code delays from the reference tracking correlators. Different SQM metrics are defined in Section 3. Prior to setting an appropriate detection threshold, the statistical properties of the SQM metrics are investigated. The analytical discussion includes Binary Phase-shift keying (BPSK(1)) and Binary Offset Carrier (BOC(1,1)) modulations as the base schemes used for GPS and Galileo. For instance, BPSK(1) is the operational scheme used in GPS L1 C/A signaling. BOC(1,1) is also considered as the base scheme for Galileo and modernized GPS signals.