Direct signal, clutter, and multipath echoes are received along with surveillance signal in passive bi-static radars. These signals degrade the target detection capability of the radar processing algorithm and thus require additional processing to achieve a decent performance. Different clutter and multipath cancellation algorithms are devised for removal of unwanted signals. These algorithms require different computational complexity to provide different level of clutter cancellation. This chapter reviews different clutter cancellation techniques and compares their performance based on the computational complexity. This performance comparison allows understanding the computation load put up by different clutter cancellation techniques and ultimately the response rate of the radar system while maintaining decent target detection.
TopIntroduction
Passive Bi-static radars (PBR) have received renewed interest in recent years for its application as surveillance radar. PBR are low cost radar as they do not require any spectrum allocation. These radars do not emit any signal on their own due to absence to dedicated transmitters and use existing transmitters as illuminators of opportunity. This makes PBR completely inconspicuous and covert in nature (Colone, F. et al,2009). Widely available signals like FM (frequency modulation), Digital TV (DTV), Digital Audio/Video broadcast (DAB/DVB), Global system for mobile communication (GSM) are used are illuminator of opportunity in PBR. The selection of a suitable transmitter depends on transmitter’s coverage area, transmitted power, carrier frequency and bandwidth. Broadcast transmitters like a commercial FM station make a suitable transmitter for PBR due to their high coverage and high transmitted power(Howland P.E.et al.,2005). This makes the passive radars suitable candidate for continuous surveillance of stealth targets, hence, PBR is widely used in homeland security applications (Celik, N. et al.,2011).
Figure 1. Passive Bi-static radar geometry
As oppose to active radars where time reference information is available, in case of passive radars time reference is not available. Therefore, two antennas, reference antenna and surveillance antenna are used in PBR receiver circuitry as shown in fig. [1]. The reference antenna receives the signal emitted by the already deployed transmitters and is used to provide timing reference. This signal is not controlled of the radar designer. The surveillance antenna receives the surveillance signal after reflection from the target. Both the received signals are then cross correlated and a cross ambiguity function is solved to detect the targets range and doppler.
However, surveillance antenna along with target echoes also receives multipath signals as well as reflected signal from nearby clutter like buildings, ground, and tree leaves etc. The reference signal also leaks into the surveillance channel. Thus, the surveillance antenna, along with target echo also receives clutter and multipath signal. The direct signal leaked into the surveillance antenna has a comparatively higher power of approximately 60 dB - 90 dB to those of the targets echo power, ultimately degrading radar target detection capacity. These disturbance signals are mostly concentrated near zero doppler therefore a possible solution is to reject the zero doppler result of the cross ambiguity range doppler map. However, this zero doppler signal leads to high side lobes in frequency domain due to the application of fast fourier transform (FFT). These side lobes often mask the target echoes due to their much higher power. Targets with high doppler frequency are also masked by these side lobes. Thus, these unwanted signals and non anticipatory behaviour of the transmitted signal leads to time varying side lobes in the ambiguity function. This masking of target limits the radar’s detection capability and degrades its performance.
Many physical and processing techniques are used in the past to reduce the radar performance degradation due to direct path interference (DPI). With an aim to prevent the DPI from entering the receiver antenna, the transmitter and the receiver were shielded from each other. Another important physical method used to reduce DPI is beam forming where a null is formed in the receiving antenna pattern towards the transmitted signal.
Many temporal and spatial filters are designed which aims to remove the interference and increase the dynamic range of radar. Spatial filters uses beam forming for clutter cancellation(Di Lallo, A. et al.,2008). A spatial correlation matrix is formed and the eigen structure was utilize for clutter cancellation in Tao, R. et al., 2010 while Villano, M. et al.,2013 investigates the null steering methods and non adaptive beam steering methods.