# Multi-Frequency Target Detection Techniques for DVB-T Based Passive Radar Sensors
## 6 Experimental Results for a Maritime Surveillance Application
Therefore it is likely that a combination of the effects discussed in Section 3.1.2 occurred in this test campaign. Obviously when such misalignment is present, the performance of the MF techniques rapidly degrades, especially when a decentralized detection scheme is applied as it operates after a binary decision separately performed on each channel. In contrast, the centralized MF approach is able to mitigate this effect since it benefits from the integration at intensity level so that even the tails of the target response could contribute to the enhancement of the detection capability. Nevertheless, in the final integrated map, the target could be smeared in the range dimension thus yielding a reduced multiple detections and/or a reduced accuracy. Some examples along this line are reported in Figure 10 for the same target considered in Figure 9. In this case the raw detection results are shown as a function of the scan number by reporting the range coordinates of the obtained plots. Particularly, Figure 10a refers to the separate exploitation of SF channels and clearly shows the range misalignment of the resulting plots across consecutive observations. Consequently, Figure 10b reports analogous results after the application of different MF strategies. This figure highlights the detection loss (i.e., scan 42, 44 and 45) experienced by the decentralized approach when a unitary window is adopted in the range dimension. In contrast, the centralized approach is shown to be less sensitive to the observed misalignment that, in this specific case, is contained within a few range cells. Nevertheless, we observe that the scan 44, 45 and 46 yield two detections. Finally, the decentralized approach applied against a range window of three consecutive cells (i.e., \(W=3\)), is even able to provide additional correct detections with respect to the best performing single channel. According to the analysis above, the exploitation of a decentralized MF scheme operating over widened range windows allows one to partially recover the detection loss for targets affected by the discussed limitations. This is clearly apparent also from Table 3, where we observe that the DEC-MF-2/4 with \(W=1\) is not able to improve the performance with respect to the best performing single channel. In contrast, by using a window size \(W=3\), we obtain a number of correct detections slightly higher than the best channel and almost tripled with respect to the worst performing channel. Finally we notice that further increasing the window size W yields a corresponding degradation of the final performance due to the increase in the detection threshold needed to control the false alarm rate. Figure 9: Range response at scan 45 of the target labeled “MSC La Spezia” when observed at two different DVB-T channels (enlarged view around the target bistatic range of the sections of the CAF extracted at the target velocity). To complete the picture, Figure 11 reports the detection results of the best centralized and decentralized MF approaches for the same case of Figure 8. Specifically the CEN-MF-3 and the DEC-MF-2/4 with \(W\) = 3 are considered in Figure 11a,b, respectively. Figure 11: _Cont._  
Figure 10: Detection results at few consecutive scans against the target labeled ”MSC La Spezia” when (**a**) using SF channels; (**b**) when exploiting MF integration strategies. First of all we observe that a remarkable improvement is obtained with both the considered MF approach in term of false alarms control with respect to the results in Figure 8. We recall that, in this case, the detection threshold was set in order to obtain a nominal \(P_{\beta t}=10^{-8}\) after the 2-out-of 2 criterion on the available surveillance channels. However, the actual false alarm rate obtained in Figure 8 appears to be much higher. The CEN-MF-3 approach guarantees a higher continuity in target detection performance since longer and much denser plot sequences are obtained against the targets of opportunity.