Range resolution improvement in passive coherent location radar systems using multiple FM radio channels

Abstract

With the improvement in digital signal processor technology, the topic of Passive Coherent Location (PCL) radar systems that exploit 'illuminators of opportunity' became a popular area of research. Comparative studies of Ambiguity function properties of broadcast signals such as FM radio, analogue television, cellular phone signals and digital audio broadcast (DAB) revealed that FM radio broadcast signals, providing up to 30dB peak to side lobe levels in range, and up to 40dB peak to side lobe levels in Doppler, are well suited for passive radar purposes.However, due to low modulation bandwidth in FM, which never reaches the allowable channel bandwidth of 150 kHz, the range resolution is severely limited. While 150 kHz modulation bandwidth can provide lkm range resolution, at best, 30 kHz modulation bandwidth is achieved, which results in 5km range resolution. Furthermore, the modulation bandwidth is strongly dependent on the content that is being broadcasted.An improvement in range resolution is obtained by using multiple adjacent FM channels, emitted from co sited transmitters, which is often the case in large towns in countries, where the FM channel allocations are weakly regulated. A number of channels can be used independently or together. When channels are used together the range resolution is dominated by the total channel bandwidth. The auto ambiguity function of multiple channel FM signals shows high side lobes. Once the range is estimated using a single FM signal, it can be refined by using more FM channels subsequently. The number and locations of the FM channels that will be used in the PCL process can either be predetermined or chosen at each recording separately, taking into account the power and modulation bandwidth of each channel.A new scheme is proposed, which takes multiple FM radio channels emitted from co sited radio transmitters.Again two receiver antennas are used: One for direct signal reception and one for the target scattered signal reception. The direct signal is used in the computation of the autocorrelation function of the FM waveform. The signals from both antennas are used in the computation of the cross ambiguity function. The cross ambiguity function is compared to the autocorrelation function. The range information is obtained by the delay between the cross ambiguity function and the auto ambiguity function. Simulations show 24 that down to -30dB signal to noise ratio (SNR) the autocorrelation function of 7 adjacent FM channels with random content can be successfully extracted from the cross ambiguity function. Hence, the range resolution is significantly improved. Best results are achieved when all channels have equal power. For sufficiently large SNR, the cross ambiguity function for multi FM channel waveform has an envelope similar to the cross ambiguity function a singular FM channel waveform. Range resolution can be improved down to 180 meters when 7 adjacent FM channels are used together.

Range resolution improvement in passive bistatic radars using nested FM channels and least squares approach

“…We are able to surpass the performance of other side-lobe suppression method presented in Ref. 6 in all the examples that we tried. Our deconvolution algorithm effectively increases the overall detection performance of the matched filter.…”

“…In Ref. 6, a band wider than one FM channel is transferred to baseband as if it is a single FM channel and then fed to the matched filter as input. Thus multiple consecutive FM channels are present in the signal and higher range resolution is achieved.…”

“…6,7 In Ref. 6, it is shown that concatenated multiple FM channels in frequency domain improves range resolution. However, side-lobes next to target peaks are also generated at the output of the standard matched filter.…”

“…In Ref. 6, a frequency domain method is developed as a solution to the side-lobe problem. The frequency domain method is based on division and it requires a low-pass filter to prevent division by zero and noise amplification.…”

Range resolution improvement in FM-based passive radars using deconvolution

“…Multiple FM signals at different channels are used at the same time to overcome the low range resolution problem of FM-based PBR systems [2,17]. In [2,17], FM radio channels (and later DAB and DVB channels) are concatenated in the frequency domain to obtain a wide bandwidth illumination signal, the so-called multichannel FM signal. This approach increases the range resolution of a PBR system, but additional high amplitude peaks at the vicinity of target peaks are also generated as a side effect of matched filter processing.…”