## 7 Experimental Results
### Analysis of the Discrepancies for the Urban Environment Case  
The performance of the positioning terminal, denoted as GBPT in Figure 7, was assessed in the previous section by exploiting the record and replay approach. The discrepancy between the recorded and the replayed trajectories in the urban environment is further investigated in this section, since the storage of the raw signal samples enables the possibility of a deeper analysis of the signals. Among the data collected during the test campaign presented in Figure 9, a dataset captured in the city center of Helsinki was chosen as a test case. It has a duration of about 600 s.  
In order to assess the fidelity of the live signal with respect to the one generated by the playback system, they were analyzed and compared by means of the IF raw samples. Essentially, the RF signal generated by the playback system was down-converted to IF and the raw samples were eventually saved. The system setup is shown in Figure 11.  
As a first analysis, the two signals were analyzed in the time-frequency domain. The spectrogram of the recorded GNSS raw samples is shown in Figure 12a, where the power spectral density is color-coded. It highlights the presence of strong interference components located at about +2 and \(-0.4\) MHz with respect to the central frequency. In order to evaluate how the recording and playback systems behaved under such strong impairments, the spectrogram of the re-recorded GNSS raw samples was computed, as shown in Figure 12b. The difference between the two spectrograms, shown  
Figure 11: Test case for the urban environment. System setup. in Figure 12c, states that the two signals had the same time-frequency components, meaning that the playback system faithfully reproduced the RF signal. This was true except for the highest frequencies attenuated by the front-end filter, as well as in the time interval from 352 to 355 s.  
In order to investigate this mismatch, a snapshot of the two signal samples was taken at three different time instants. The histogram and the PSD are shown, respectively, in the top and bottom panels of Figure 13 for the recorded (blue) and re-recorded (orange) signal samples. It is possible to distinguish three cases, as follows:  
* Case I, Figure 13a,d: nominal conditions and faithful RF signal reconstruction. Histogram (a) and PSD (d) of the GNSS raw samples were computed at second 155. We can define this situation as nominal conditions due to the absence of interference components within the signal. Therefore, in nominal conditions the histogram of the live and replayed signal samples had the same Gaussian shape. In addition, the two signals exhibited identical spectra. * Case II, Figure 13b,e: presence of RFI and faithful RF signal reconstruction. Histogram (b) and PSD (e) of the GNSS raw samples were computed at second 351. In that time instant, some interference components were present and located at approximately 2 MHz away from the central frequency. In correspondence of these points, the histogram of the signal samples did not have a Gaussian shape, as it should be in nominal conditions. However, the live and replayed histograms had the same shape. In addition, the two signals exhibited identical spectra, showing the capability of the system to collect and replay the full spectral information, even in non-nominal cases. * Case III, Figure 13c,f: presence of RFI and wrong RF signal reconstruction. Histogram (c) and PSD (f) of the GNSS raw samples were computed at second 353. In that time instant, some strong interference components, located approximately 2 MHz away from the central frequency, threatened the data collection system, since the front-end went into saturation. Figure 12: Test case for the urban environment. Spectrogram of the (**a**) recorded and (**b**) re-recorded GNSS raw samples. (**c**) Difference between the two spectrograms.