# Multi-Frequency Target Detection Techniques for DVB-T Based Passive Radar Sensors
## 6 Experimental Results for a Maritime Surveillance Application
However, when exploiting single DVB-T channels, the false alarm rate is quite high, the detection performance is discontinuous along the target tracks and, possibly, different target tracks sections are detected by using different frequency channels. This in a measure points out the limitations of a SF operation and, in turn, highlights the diversity of information conveyed by multiple channels. These considerations are confirmed in Table 3 where a quantitative comparison is reported in term of correct detections provided for each of the available targets of opportunity. As for the aerial surveillance scenario, by applying the proposed MF detection schemes, the results can be significantly enhanced. This is clearly apparent from the last rows of Table 3 where we reported the number of correct detections provided by centralized and decentralized MF approaches. The centralized technique is employed with \(N=2\), 3, and 4 channels while the decentralized DEC-MF-2/4 scheme is used with a range window size of \(W=1\), 3, and 5. In addition, in the last column is also reported the equivalent range resolution value (\(\delta\)r) obtained by degrading the range resolution (\(\Delta\)r) by a factor of \(W\), namely \(\delta\)r = \(W\cdot\Delta\)r.  
We observe that, among the centralized configurations, the CEN-MF-3 yields the best performance as one of the integrated channel does not provide a remarkable contribution. It is worth noticing that the improvement with respect to the best performing single channel slightly reduces compared to the experimental tests reported in Section 4. This might be obviously due to a number of factors among which the particular bistatic geometry, the exploited transmitter of opportunity, the available DVB-T channels, etc. However, based on a detailed analysis of single target tracks, we verified that a limiting factor in this particular scenario is related to possible range misalignments of the target echoes received at multiple channels as discussed in Section 3.1.2. An example of this effect is shown in Figure 9 where we report sections of the CAF maps obtained, for a given scan, at two different DVB-T channels. Specifically, the sections extracted at constant velocity of \(-11.59\) m/s are shown around the range location of the vessel labeled \"MSC La Spezia\". \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & & & \\ \hline
**SF 1** (682 MHz) & 0 & 14 & 1 & 36 & 11 & 34 & 18 & 2 & 8 & 58 & 182 & 33 m \\
**SF 2** (634 MHz) & 3 & 8 & 0 & 70 & 2 & 79 & 37 & 42 & 48 & 54 & 343 & 33 m \\
**SF 3** (626 MHz) & 1 & 22 & 5 & 58 & 5 & 89 & 33 & 36 & 55 & 56 & 360 & 33 m \\
**SF 4** (546 MHz) & 0 & 0 & 0 & 9 & 2 & 75 & 16 & 4 & 25 & 5 & 136 & 33 m \\
**DEC-MF-2/4, W = 1** & 0 & **19** & 1 & 70 & 5 & 87 & 35 & 30 & 45 & 52 & 344 & 33 m \\
**DEC-MF-2/4, W = 3** & **2** & **19** & **2** & **72** & **10** & **88** & **42** & **44** & 53 & 63 & 395 & 99 m \\
**DEC-MF-2/4, W = 5** & 1 & 18 & 1 & 72 & 7 & 88 & 40 & 41 & 50 & 63 & 381 & 165 m \\
**CEN-MF-2** (SF 2-3) & 3 & **26** & **5** & **75** & 8 & **92** & 39 & 43 & 63 & 70 & 424 & 33 m \\
**CEN-MF-3** (SF 1-3) & 5 & **28** & **5** & **74** & 9 & 92 & 39 & 44 & 61 & 81 & 438 & 33 m \\
**CEN-MF-4** & 0 & 25 & 5 & 72 & 6 & 90 & 40 & 44 & 57 & 74 & 413 & 33 m \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Detection results with the single DVB-T channels and different MF approaches in the case of a maritime surveillance scenario. As is apparent, target range responses after matched filtering are centered around different bistatic ranges with a peak displacement of about 100 m. Notice that the considered target is a cargo ship so that its size is remarkable (365.82 m long \(\times\) 51 m wide). However we point out that a similar effect was noticed also for other smaller targets when employing the same couple of channels which possibly reveals that these channels are emitted by spatially separated transmitting antennas.