## 3 Instantaneous Multi-Constellation Relative Positioning: Methodology and Performance
### Performance Assessment of Precise Positioning with Focus on E14 and E18 Galileo Satellites
#### 3.2.3 Performance Assessment of Instantaneous Positioning  
We start the evaluation of the processing results with the ambiguity resolution domain. A GPS-based instantaneous solution results in a very high level of the ambiguity resolution success rate, from 97% to 99%. Therefore, no significant improvement triggered by the addition of the Galileo signals should be expected. The solution based on the Galileo-only constellation experienced a significantly lower success rate. ASR in this case did not exceed 33%, which may be attributed to the current Galileo constellation, and consequently, the number of tracked satellites resulting in high PDOP. A similar effect on short-baseline Galileo processing was found in other studies [27]. In that case, researchers indicated the important impact of the multipath effect and, therefore, applying multipath corrections resulted in a significant enhancement of the ambiguity fixing reliability. According to our results, we did not, in turn, notice a significant influence on the ambiguity resolution success rate caused by the application or elimination of two Galileo satellites with incorrect orbits. On the first day of the experiment we obtained the same level of this percentage (96%) for both multi-constellation solutions (scenarios #3 and #4). On the next days, this discrepancy was at an insignificant level (1-2%). Comparing the ambiguity success rate from the coupled solution including Galileo satellites with incorrect orbits (#3) with that from the solution excluding them (#4), we can judge that the signals from these satellites did not noticeably influence the quality of the ambiguity resolution process. In the following, the positioning results in coordinate domain are discussed. Table 5 summarises the standard deviations of fixed solution derived for consecutive scenarios, whereas Figure 8 shows the scatter plots of single-epoch coordinate residuals for a sample day (3 October 2017). The highest precision of coordinates was obtained with the use of a sole GPS constellation. In this case the standard deviation reached 0.9-1.2 cm, 0.6 cm, and 2.0-2.8 cm for the N, E and U components, respectively (Table 5). The results are in agreement with previous studies where researchers indicated the higher accuracy of a single-constellation GPS-based solution than for combined ones using Galileo or BDS [61; 62; 63]. Looking at the Galileo-based solution, we can clearly see the deterioration of the precision of height component, since the _std_ reached 5.5-6.6 cm on 3-4 October 2017 and 12.8 cm on the last day of the experiment. On the other hand, plane coordinates were determined with only a slightly lower precision with respect to the GPS-based solution. In these cases, the _std_ of N, E components was in the range of 1.5-1.7 cm and 1.2-1.4 cm, respectively. Since higher deterioration was observed in the height component, this may indicate that poor satellite geometry is the main source of this phenomenon. Figure 7: Sky plots of Galileo (dark yellow), Galileo E14 & E18 (dark blue) and GPS (azure) satellites with relation to its altitude over OLSZ station during the test period (3, 4, 5 October 2017, from left to right). Looking at the precision of the solutions based on coupled GPS + Galileo signals, again we cannot detect any significant impact on the coordinate domain caused by signals from a pair of Galileo FOC satellites with incorrect orbits. The discrepancies between _std_ of fixed coordinates did not exceed 1 mm for horizontal components, whereas in the case of height this difference reached 6 mm on 3 October (Table 4). We, however, do not link this with the quality of E14 & E18 satellite signals, but with unfavourable Galileo satellites geometry. Having investigated the results in the ambiguity resolution and coordinate domains, the following analysis was based on the observation residuals. Here we focus on the residuals corresponding to dual-frequency phase signals. Close attention was paid to the observations of the Galileo E14 and E18 satellites.