# Experimental Seaborne Passive Radar
## 2 Materials and Methods
### Compensation of Phase Fluctuations Caused by Ship's Motion  
We must note that, during the storm, the ship swayed intensely in the rough waves and changed, to a small extent, its orientation relative to the illuminating transmitter. As  
Figure 6: Four-channel receiver with the training signal generator and the directional couplers.  
Figure 7: Simplified block diagram of the PCL (passive coherent location) processing algorithm.  
the reference antenna was located at the front and the surveillance antennas were placed on the port side of the ship (see Figure 3), the distance between these antennas was several meters. As a result, both the phase and amplitude of the direct signal in the reference and surveillance antennas depended on the vessel's pitch and roll, and the phase offset between the reference signal and its crosstalk in the measurement signal was changing during the integration time. On the other hand, the clutter removal filter requires a constant phase relationship between these signals during a single block's coherent processing, such as in stationary PCL installation. Figure 8 shows the example plots of the discussed phase offset, estimated with 2 ms time steps during one integration period--in the cases of calm seas and the storm.  
The phase difference between the reference signal and its crosstalk fluctuated in both cases due to the ship's movement. However, during the storm, these fluctuations were found to be much more intense. Our experiments with PCL signal processing have shown that these fluctuations severely degrade the clutter removal filter's efficiency in the case of the long integration period. In such a case, because of the loss of coherency, it is almost impossible to filter out the direct path interference component and obtain clear PCL images. To address this problem, a more advanced signal processing algorithm has been developed with the structure presented in Figure 9.  
Firstly, the phase offset between the reference signal and its crosstalk is estimated by computing a cross-correlation function on subsequent parts of the integration period (block size was 4000 samples, which corresponds to 2 ms). According to the obtained estimates, the phase offset of the measurement signal is then corrected in a phase equalizer, with spline interpolation on the entire integration period. After that, both signals are divided into 200 overlapping blocks of equal length. To improve immunity against time-variant signal properties, each of these blocks is processed in a separate instance of the clutter removal filter (described previously). Finally, the filters' output signals are merged into the entire integration period and processed in a cross-ambiguity function calculator. This approach allows for practically removing the direct path interference component in severe time-varying phase offsets resulting from the rapid movement of the PCL platform. We employed such a modified PCL processing algorithm to obtain clutter images shown in the following section.  
Figure 8: Surveillance/reference signals’ phase difference during calm seas (**a**) and the storm (**b**).  
Figure 9: PCL processing algorithm modified for long integration periods.