# GNSS Code Multipath Mitigation by Cascading Measurement Monitoring Techniques
## 6 Field Data Analysis
### Static Test
#### 6.1.1 Monitoring Results  
The time-averaging approach (discussed in Section 4.1) was used to estimate the mean value of the different monitoring metrics. A simple moving average was used with the length of 10 min to take into account the effect of static multipath oscillations. The 10 min interval was selected based on twice the average period observed for the quasi-periodic oscillations of the all PRNs exhibiting static multipath [32]. While similar results were observed for all PRNs, PRN 10, affected by different levels of multipath, is used to examine the sensitivity of the detection procedure. At each epoch, the CMC metric was obtained by subtracting carrier phase measurements from corresponding pseudoranges according to Equation (8). The nominal mean value was then filtered out to extract the CMC-based monitoring metric defined in Equation (9). This was done using the moving average discussed above. Since the cycle slips may result in a new unknown ambiguity in the carrier phase measurements [1,8], the moving average buffer was reset if a cycle slip was detected. The cycle slip detection procedure was performed based on the phase velocity trend method with a threshold of 1 cycle [36]. Since the pseudorange multipath error is considerably larger than that of the carrier phase, the CMC-based monitoring metric is mostly an indication of code (pseudorange) multipath as discussed in Section 3.1. For PRN 10, Figure 5 shows the L1 C/A, L2C and L5 CMC-based monitoring metrics as an indication of the corresponding code multipath errors. The corresponding Root-Mean-Square (RMS) values have also been presented where measurements are divided in three levels in terms of multipath error. As seen in Figure 4, PRN 10 travels on the southeast side of the antenna from high to low elevation and surrounding reflectors on the west side create multipath. Generally, higher multipath was observed on L2C as expected due to the relatively lower chipping rate of the latter [0.5115 MHz for L2C (M and L) compared to 1.023 MHz for L1 C/A and 10.23 MHz for L5 (I and Q)] and signaling bandwidth [20.46 MHz for L2C (M and L) compared to 24 MHz for L5 (I and Q)] [37,38]. Besides multipath characterization, the extracted CMC-based monitoring metric is used in the following subsections to correct the corresponding pseudoranges according to Equation (10). In addition to code and carrier phase measurements (and thus CMC metric), multipath affects the measured signal power and C/N\({}_{0}\) values as shown in Figure 6. The carrier and noise power and thus C/N\({}_{0}\) are estimated by the receiver, based on in-phase and out-of-phase correlator outputs [39]. Affected by the time-varying phase lag between the direct and reflected signal(s), as they add constructively or destructively with each other, the power of the combined signal and consequently the measured C/N\({}_{0}\) fluctuate with time. Since the phase lag is frequency dependent, the C/N\({}_{0}\) is affected differently on each frequency [32]. Figure 6 shows the corresponding C/N\({}_{0}\) measurements where the constructive and destructive effects of multipath are observed on each frequency with different phase lags. For each PRN, the measured C/N\({}_{0}\) will be used (in the next subsections) to perform the stochastic weight scheme defined by Equation (11). Figure 4: Multipath data collection using a Trimble R10 antenna-receiver surrounded by reflectors—Static test scenario. For detection purposes, C/N\({}_{0}\) measurements were first filtered to remove their mean values and were then normalized based on an estimation of C/N\({}_{0}\) SD previously measured by the receiver in an open sky low multipath environment. For the resulting unit-free monitoring metrics, the detection thresholds were set to \(\pm 3\) as three times the normalized nominal SD for L1, L2C and L5 frequencies. Figure 7 shows the C/N\({}_{0}\)-based monitoring metric and corresponding _M of N_ detection results.