JAMMINGIMMUNITY RESEARCH OF A RADAR OPERATING WITH CHIRP SIGNAL

Pulse compressed signal enhances the jamming immunity of a radar operating with such a signal. The matched filter is one of the most important elements in the pulse compression technique. A model of a matched filter for a chirp signal was synthesized using the Simulink tool of the Matlab software. Interference signals were feed to the input of the matched filter and output signals were measured. The matched filter’s degree of suppression of these interference signals was assessed. Conclusions were made about the jamming immunity of radar operating with chirp signal.


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frequency modulated signals were synthesized as jamming signals. These signals were feed to the input of the matched filter and output signals were measured. The matched filter's degree of suppression of these interference signals was assessed.
The study of the jamming immunity of a filter matched to chirp signal of the radar to jamming chirp signals was done in the following order: 1. A model of optimal chirp filter matched with the radar's signal was synthesized, and the parameters of the signals in the radar receiver were determined; 2. Jamming chirp signals have been selected, and the limits of their changes have been determined; 3. Jamming chirp signals have been feed to the input of the matched filter and output signals have been measured; 4. The obtained results have been evaluated and conclusions have been made; 5. The functional diagram of the simulation model of the process has shown in Fig. 2. A 7-unit matched filter for a linear frequency modulated signal was synthesized according to the block diagram shown in Fig. 3. The filter has a bandwidth B = 57÷63 kHz, and its parameters have given in Table 2.
The level at the output of the receiving system or the input of the ADC is about 20÷30 mV for radars operating with compressed signal (TBP = B.τ >> 1). These radars usually have the bandwidth at intermediate frequency B ≈ (8÷10) MHz or more, and a noise factor of the receiver's input stages about 3÷6 dB (Traveling Wave Tubes, parametric amplifiers, microwave solid-state receivers).
A level of 60÷90 mV has obtained, by taking into account the signal-to-noise ratio 3:1 of the automatic detector's threshold. Therefore, the threshold for comparison and evaluation is fixed at 100 mV.
In the time domain, the evaluation rate is τ = 200 ± 100 ns, which is a typical duration of the compressed pulse or the single element of the compressed signal code (determined by the distance resolution requirements).
The levels, thresholds, and time values of the parameters of typical radar operating with compressed signal determine the value of the respective values in the model as follows: 1. In terms of energy, the model's threshold level is 100 mV (for the input power of the tested filters 2.10-4 W accepted in the experiments -normalized amplitude 0.1V on a load of 50 Ω). The amplitude of the jamming chirp signal is normalized to 0.1V at the input of the filter at all of the experiments; 2. A scale of 1000:1 between the real system and the model has been chosen at the time domain, due to a limitation in the software product's speed. The ratio between the values of the parameters and their tolerances in the radar and the model is strictly preserved. The modeling has been performed at the ratio of the parameters between the real radar and the model by Table 1.

Results and Discussions:-
A 7-element filter was synthesized by parallel connection of standard bandpass filters, delay line in each unit, and adders. The bandwidth and delay time in each unit are selected, optimized, and adjusted according to the general characteristics of the filter (given in Table 2) and the theoretical requirements defined by (Barton2004) and (Korobko2003). The chirp filter is a dispersion type filter with a lower cutoff frequency of 57 kHz, a center frequency of 60 kHz, and an upper cutoff frequency of 63 kHz. The matched filter's parameters are selected by the filter's parameters of typical radar operating with a chirp signal.
Each simulation experiment has figures with obtained results. The oscillograms and spectrograms of the studies are shown from Fig. 4 to Fig. 7. In each figure a) is the input jamming chirp signal, b) is the response of the filter at the time domain, and c) is the response of the filter at the frequency domain.
A jamming chirp signal was applied to the filter input with a frequency deviation B = 54÷66 kHz, which is twice large as that of the filter. Figure 4 a) shows the input jamming chirp signal; figures4 b) and 4 c) show the output signal of a 7-element chirp filter. The level of the output signal is 0,16V. The output signal exceeds the detector threshold 1,6 times (4 dBU). The filter accumulates 1,6 times jamming chirp signal instead of suppressing it.
A second experiment was made. A jamming chirp signal was applied to the filter input with a frequency deviation B = 57÷63 kHz, which is the same as that of the filter. The third experiment included a jamming chirp signal, applied to the filter input with a frequency deviation B = 58÷62 kHz, which is 1,5 times smaller than that of the filter. Figure 6  A fourth experiment was made. A jamming chirp signal, applied to the filter input with a frequency deviation B = 59÷61kHz, which is 3 times smaller than that of the filter.

Conclusion:-
The following conclusions have drawn from the research of the suppression degree of the chirp filter jammed by chirp signal: 1. The filter, matched to the radar's linear frequency modulated signal does not suppress the input jamming chirp signal but also accumulates it. The degree of accumulation is proportional to the coincidence of the deviation of the jamming signal with the complex amplitude-frequency characteristic of the filter; 2. The output signal of the chirp filter is accumulated from 1.6 to 5.6 times (4÷15 dBU), which exceeds the threshold of the automatic detector several times. The amplitude of the output signal depends on the deviation value of the jamming signal; 3. Jamming signal with frequency deviation three times smaller than the bandwidth of the receiver's chirp filter causes 1.125 times higher level of its output compared to jamming signal with frequency deviation two times larger than the frequency band of the filter; 4. Counteraction by jamming chirp signals is more effective than counteraction by noise signals from an energy point of view. This is a result of the longer time for matching the spectra and phase ratios of jamming chirp signals with the frequency band of the optimal filter of the receiver and depends on the structure of the jamming signal and the complex characteristic of the filter.
The obtained results could be used to design technical devices to improve the jamming against military surveillance radars operating with chirp signals. These devices could degrade the performance of the radar, which would significantly affect its jamming immunity.