# Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results
## 2 System Overview
### LNB Synchronization  
As indicated in the previous subsection, to get a sufficient SNR of PB-GB-SAR imaging, a long coherent integration time (100 us) is needed. However, two COTS LNBs of reference and surveillance channels have different local oscillators, resulting in different frequencies and phases of the received signals. Then, if a longer integration time is used, the SNR will not be increased but instead be reduced because the two channels are no longer coherent. For a single measurement where the phase information is not important, this problem can be solved by the Fourier-transform based method to estimate the frequency difference between two LNBs [37]. However, for PB-GB-SAR imaging, in order to sum multiple short-term measurements together to improve the SNR for range compression and coherently process all the data from different antenna positions for azimuth compression, the phase difference between the two LNBs of different measurements should also be corrected. When a strong, stable, and stationary target with a known position can be found in the illuminated scene, the opportunity-target-based method proposed in [46] can be used for the phase correction. In spite of its low computational complexity and hardware cost, this method may hardly be used in the real conditions where strong and known targets are not available. Therefore, we decided to modify two COTS LNBs to make their frequency and phase synchronized. Figure 8 shows the setup of the proposed LNB synchronization approach, where only one LNB is shown in the right subfigure and the diagram of the clock distribution circuit (the left subfigure) is shown in Figure 9. Based on the unpublished application note \"X-tal driver for using multiple TFF10 xx to the same X-tal as reference\" describing the mechanism for locking multiple TFF10 xx phase locked loops (PLLs) on the same reference as desired, a dedicated Pierce oscillator was built around a 25 MHz resonator and a 74 HC04 inverter. When an external local oscillator is available, the Pierce oscillator can be replaced to provide a more stable and precise clock signal. The output of the Pierce oscillator (or the input local oscillator signal) feeds another 74 HC04 used to generate three sets of in-phase and phase-opposition (180-degree phase shift) signals feeding the PLL inputs of the LNBs (three sets of signals can be used for synchronization of three LNBs, while, in this paper, only two sets of signals are used). Care must be taken on the one hand to tune the output voltage amplitude to acceptable levels (500 mVpp) using a voltage divider bridge and, on the other hand, to include a DC-blocking capacitor (5.6 nF) between the oscillator output and the PLL to avoid DC-current leakage from the PLL, which would prevent PLL locking. Figure 7: Target SNRs versus different RCSs and target–receiver distances. With the current PB-GB-SAR system, we have focused on the targets within a 100 m range. It has been observed by experiments that, under proper conditions, frequency/phase coherence between two LNBs could be stable over tens of minutes, as needed for PB-GB-SAR imaging and displacement estimation. For example, by facing two synchronized parabolic antennas to the TV satellite, as shown on the left of Figure 10, the coherence between two channels is measured by analyzing the amplitude and phase stability of the cross-correlation peak by using all the 12 TV channels. On the right of Figure 11, the time delay between two channels is estimated to be about \(-\)0.165 m. Four smaller peaks at about \(\pm\)7.5 m and \(\pm\)15 m are generated by the amplitude-filtered spectrum and the frequency gaps of the satellite digital TV signal, which can be suppressed by some advanced methods. Figure 8: LNB synchronization setup: (**left**) clock distribution circuit with three sets of in-phase and phase-opposition signal outputs feeding the phase locked loop (PLL) inputs of LNBs and (**right**) the modified feed-horn of a 45-cm parabolic antenna receiving the in-phase and phase-opposition signals.