Key Techniques Of Multi-FM-Broadcasting-Based Passive Radar

FM-broadcasting-based passive radar (FPR) is a kind of bistatic/multistatic radar which employs the electromagnetic signals illuminated by third party, and it works at the very high frequency band (30-300 MHz, VHF). FPR can not only make up for the band vacancy of active VHF radar, but also has the advantages of environmental protection, strong survival ability, low cost and so on. As soon as the concept of FPR was put forward, it aroused a flurry of interest in the radar community. Commercial FPR systems became available soon, and the domestic and foreign research institutions have carried out in-depth work. On the one hand the researchers are concerned with key techniques of reference signal extraction, direct wave and multipath clutter suppression and so on, on the other hand they point out intrinsic defects of FPR, such as:the waveform of FM broadcast signal is determined by the broadcast content, the bandwidth of FM broadcast signal is small, and so on.FM broadcasting has the transmitting mode of multi-frequency, multi-polarization and multi-static, and the radar receiver can also take the receiving mode of multi-frequency, multi-polarization and multi-static. This papar names the FPR which em-ploys multi-frequency, multi-polarization and multi-static techniques as multi-FM-broa-dcasting-based passive radar (MFPR). This paper demonstrates the advantages of MFPR, and studies a number of key technologies. The main contents of this paper are as follows:(1) The waveform characteristics of FM broadcast signal are analysed. The mechanism that causes side peaks of ambiguity function is illustrated, which points out that side peaks with range and doppler symmetry appear randomly. The probability distribution of FM broadcast signal monostatic range resolution is calculated based on a large number of measured data which indicates that FM broadcast signal monostatic range resolution concentrates upon 1 m.(2) The target scattering characteristics are analysed. Taking Airbus 320 and Boe- ing 747-200 as study models, target scattering coefficients of FM broadcast band are calculated and analysed. The proper frequency range for coherent synthesis is studied, which shows that:when the frequency interval is large, the scattering co-efficients of different frequencies are independent and identically distributed; when the frequency interval is less than 3 MHz, the scattering coefficients of different frequencies change faintly; so, coherent synthesis should take place in the frequency range of 3 MHz. The wideband echo model of 88-108 MHz is studied, which shows that the main peak intensity of the scattering center is equal to the mean value of the echo intensity of the narrowband signals in the working band, and wideband target detection does not have the obvious advantage compared with the multi-frequency target detection. The target polarization characteristics are stud-ied, which shows that targets have depolarization effect and the RCSs of different polarization combination are independent.(3) Advantages of MFPR's multi-frequency working system and multi-polarization work-ing system on target dection and location are demonstrated. The performance of multi-frequency target detection is analysed, which shows that multi-frequency fu-sion can improve the detection probability by exploiting the waveform diversity gain and the frequency diversity gain. The performance of multi-polarization tar-get detection is analysed, which shows that multi-polarization fusion can improve the detection probability by exploiting the polarization diversity gain. The perfor-mance of multi-frequency target location is analysed. The antenna array error of MFPR is direction-dependent and frequency-dependent, according to this charac-teristic, a kind of antenna array calibration method is proposed. Then a kind of multifrequecny angle data fusion method is introduced. The mentioned methods can improve the accuracy of target location.(4) The method of improving the range resolution by multi-frequency synthetic is stud-ied. FM broadcasting signal is similar to the step frequency signal, but the frequency interval of FM broadcasting signal is greater than the signal bandwidth. To satisfy the basic demand of the step frequency signal that the frequency interval should be not greater than the basic signal bandwidth, the mixing frequency of different FM broadcasting signal should be different. As a result, the mixing brings in phase difference, which will destroy the effect of synthetic. Hence, this paper propose two phase compensation methods. The essence of both methods is to seach the extreme value of the object function. One is based on the criterion of minimizing the range error, and the other is based on the criterion of approximation of 1. The theoretical basis of the proposed methods is given, and the theoretical analysis is verified by the measured data.(5) The criterion of selecting receiving and transmitting polarization combination is studied. In this paper, the effect of polarization on detection performance is an-alyzed from three aspects of system noise figure, target scattering characteristics, and surveillance antenna gain, then the optimal receiving and transmitting polar-ization combination is determined. The method is analyzed and validated based on the actual scene of Wuhan. It comes to the conclusions that:system noise figure is the main factor that affects the detection performance; the depolarization effect of target is the basis to utilize different kinds of surveillance antenna polarization; surveillance antenna gain depends on its height above the ground, so the surveil-lance antenna height can be optimized based on some criterion. According to the analysis, the criterion of selecting receiving and transmitting polarization combi-nation is proposed:when most of the transmitted signals in the working band are vertically polarised, the horizontally polarised receiver can obtain better detection performance than the vertically polarised one; when most of the transmitted signals in the working band are horizontally polarised, the vertically polarised receiver can obtain better detection performance than the horizontally polarised one. Theoreti-cal results are confirmed by field experiments.