# GNSS Code Multipath Mitigation by Cascading Measurement Monitoring Techniques
## 5 Combination of Monitoring Techniques for a Reliable Multipath Mitigation  
None of the techniques presented in Section 3 are completely reliable and each has its own limitations in different multipath situations. Therefore, a complementary combination of the aforementioned techniques is investigated for more reliable multipath mitigation. This is done here in two cascading approaches as follows. First CMC-based error corrections are applied to pseudoranges to alleviate multipath errors and then the GF-based detection metrics are used to exclude (or de-weight) remaining errors below a tolerable PDOP threshold. For example, consider a scenario when GPS signals are available on L1, L2C and L5. At each epoch of GPS L1 positioning (when only L1 pseudoranges are used in the position solution, but measurements on other frequencies are available for monitoring purposes), first CMC-based error correction is applied on all three frequencies. The GF detection metrics are then computed using L1-L2C and L1-L5 combinations with the (partially) corrected measurements. Figure 2: Iterative change of measurement weights (measurement de-weighting) below the tolerable PDOP threshold and limited number of iterations. Multipath is detected if at least one of the corresponding _M of N_ detection outputs is one. It is noted that the C/N0-based detection metrics cannot be used to detect multipath on previously (but partially) corrected measurements. Detection results are then exploited in exclusion or iterative de-weighting approaches depending on the multipath environment and measurement geometry (see Section 4.2). For included measurements or for those not de-weighted iteratively, an elevation or C/N0-based model is applied to mitigate the effect of stochastic errors. Figure 2(a) shows the flowchart of this procedure. In this approach, the preceding CMC-based error correction potentially reduces the number of measurements excluded or de-weighted in position solutions and thus reduces measurement geometry degradation. However, since all GPS satellites do not broadcast signals on all three frequencies at this time, the GF-based detection/exclusion (or de-weighting) performance is limited to the PRNs with signals on at least two different frequencies. For example, for GPS L2C or L5 positioning (when GPS signals are monitored on L1, L2C and L5 frequencies, but only L2C or L5 pseudoranges are respectively used for positioning), at least one of the GF-based detection metrics is configurable by involving the corresponding L1 measurements. For GPS L1 positioning, the multipath errors (not corrected by CMC) without corresponding L2C or L5 measurements will remain undetected. The second approach is first exclusion (or de-weighting) of detected measurements (based on the union of all detection metrics) and then correction of multipath errors on included (or not de-weighted) PRNs by applying CMC-based error correction. The flowchart of this procedure has been illustrated in Figure 2(b). In this approach, multipath is first detected based on the union of detection results extracted by all related C/N0, DC/N0 and GF-based detection metrics. For example, for GPS L1 positioning, the combination of detection metrics will include the single-frequency C/N0-based detection metric on L1 frequency, the DC/N0 and GF-based detection metrics on L1-L2C and L1-L5 differences. At each epoch, multipath is detected if at least one of the corresponding _M of N_ detection outputs is one. The detection results are then used to exclude or de-weight the affected ones. The CMC-based correction is applied on included (or not de-weighted) measurements followed by an elevation of C/N0-based stochastic models. In this approach, since the single-frequency C/N0-based detection is involved, besides the differential metrics, there is at least one detection metric configurable for each PRN. The drawback is the possibility of excluding (or de-weighting) measurements whose errors can be potentially corrected by a preceding CMC-based error correction.