# Multi-Frequency Target Detection Techniques for DVB-T Based Passive Radar Sensors
## 3 Multi-Frequency Detection Schemes
### Modifications Required for Application Against DVB-T Signals
#### 3.1.2 Typical Causes of Target Echo Misalignment across Channels and Counteracting Strategies  
For an effective MF operation, the target echoes received at different frequency channels should be perfectly aligned on the range/velocity plane. In such condition, the proposed non-coherent integration strategies are expected to enhance the resulting SNR (centralized strategy) or, equivalently, to exploit the coincidence of detections (decentralized strategy) thus improving the target detection capability of the system. The hypothesis above typically holds in FM radio-based PR exploiting multiple frequency channels emitted by the same illuminator of opportunity [23]. Basically, in that case, a moderate diversity is experienced in term of carrier frequencies due to the limited spectral extent of the FM radio broadcast band. Moreover, even in the presence of slight deviations from the ideal assumption, the robustness to these effects is still guaranteed by the coarse range resolution (order of km). In contrast, the finer range resolution and the wider frequency separation of DVB-T channels determine an increased sensitivity to non-ideal conditions that might cause a range/velocity displacement of target echoes received at different frequency channels. Therefore the validity of the above assumption has to be carefully verified and proper strategies should be devised to make the system robust to potential limitations. Among the possible causes of target echoes misalignment across frequency channels, the exact position of the transmitting antennas plays a key-role. Basically a transmitting site broadcasting multiple DVB-T channels might be composed of a few masts with multiple transmitting antennas possibly operated and maintained by different service providers. The distance between these masts can be in the order of tens of meters or more. According to the information available online, all the broadcasted DVB-T channels are labeled with the same transmitter name even if they could be emitted by largely displaced antennas. In other words, by exploiting multiple frequency channels, a pure bistatic geometry is not guaranteed so that a given target can be observed at different bistatic ranges and, possibly, at different bistatic velocity. For typical distances between transmitting antennas, the impact on the bistatic velocity is expected to be negligible compared to the velocity resolution cell usually synthesized for detection purposes. In contrast the range displacement depends on the positions of both the target and the receiver and might range from zero to a maximum value that is exactly equal to the distance between the emitting antennas (e.g., when the transmitting antennas and the target lie on a straight-line and the receiver is equidistant from both transmitters). As is apparent, this effect can be neglected in FM-based PR since the resulting displacement would be much smaller than the radar range resolution. In contrast, it would jeopardize the effectiveness of the proposed MF approach in the DVB-T-based PR case since the same displacement would cause the target echoes to occupy different range cells. Another possible cause of range misalignment between target responses at different channels is related to the target size. In fact, if the target physical size is comparable or larger than the range resolution cell, its echo in principle might span multiple range bins. However, depending on the carrier frequency of the exploited DVB-T channel, different target portions might have different scattering characteristics. In extreme cases, the dominant target echo might be generated by distinct and largely displaced parts of the target thus yielding a range displacement of the resulting responses. This effect is obviously more evident when largely diverse carrier frequencies are employed. Finally, the presence of anomalous propagation effects has to be mentioned as possible cause of target echo misalignment of the CAF map.