Simulation and analysis of partial pulse remained jamming to HPRF PD radar

To solve the problem that the duty cycle of HPRF PD radar is not easy to use the general tow-and-pull jamming, a method of partial pulse remained jamming is proposed. Taking the transmitting signal of PD radar acquired by DRFM as an example, the jamming effect of the signal is simulated and analyzed. The results show that the jamming signal is modulated by the radar signal acquired by DRFM, the jamming signal generated has a strong correlation with the target Echo Signal, and it can effectively jam the HPRF PD radar with less power, which proves the effectiveness of the method.


Introduction
The high pulse repetition rate (HPRF) PD radar is mainly used to solve the problem of velocity measuring mode and frequency in x-band ranging from 100kHz to 300kHz, high duty cycle must be used [1] . Moreover, the narrow bandwidth of HPRF PD radar makes noise jamming not very effective [2] .
To solve the problem that the duty cycle of HPRF PD radar is not easy to use the general tow-and-pull jamming, a method of pulse jamming is proposed. The PD radar transmit signal is acquired by DRFM technology, and the PD radar transmit signal is transmitted, and every m pulse is discarded, so the PD pulse jamming signal is produced. This kind of jamming signal keeps the coherent characteristic of the original radar signal, makes the jamming signal entering the receiver get the same processing gain as the target echo, and abandons certain pulse, compared with PD radar echo signal, the jamming signal has a great change in frequency domain, resulting in a number of frequency-shift jamming signals.

Principle of DRFM
DRFM can accurately copy the radar signal emitted by the other side, and it is a powerful means against the current advanced radar system, PD radar is no exception [3] .  A DRFM system typically has six basic parts: the down converter, ADC or PDC, memory, DAC or DPC, up converter, and controller, as shown in figure 1 [4] . Its main working process: First, will intercept the radar signal down-conversion to if signal; then, quantitative coding, and the results will be stored in memory, the digital signal is read out from the memory at an appropriate time and decoded back to the if analog signal, which is transmitted after up-conversion to the RF signal.

Analysis of frequency spectrum characteristics of PD radar
Let the PD radar signal be ) , 0 f is the carrier frequency, τ is the pulse width, N is the number of pulses. figure 2 is a spectrum of a HPRF PD radar signal ( r f is 100 KHz, duty cycle is 40% , N=20) .

Fig. 2. Spectrum of HPRF PD radar signal.
As can be seen from figure 2, the spectrum envelope of PD Radar Echo signal is a function Sa , whose period is the reciprocal of pulse width and the interval between the spectrum lines is r f . In the process of signal processing of Reference PD radar, it can be seen that PD radar first passes through a single sideband filter and then enters a narrow band filter. The bandwidth of the single sideband filter is approximately equal to pulse repetition frequency r f , and the single sideband filter is to select the echo spectrum of the central spectral line for filtering. Therefore, only when the spectrum line of jamming signal

Principle of the method of pulse interference
The pulse interference is generated as shown in figure 3. strength. The range of sub-target group is not obvious, but the range of sub-target group is quite different from that of main target group, and the bigger the M is, the more obvious the range is, the range of the main false target is 1 times, 2 times, 4 times and 9 times than that of the secondary false target. Therefore, how to choose M is very important to the jamming effect, M is too small, the number of false targets is too small, M is too big, the range of the sub-false targets is too big relative to the main false targets group, maybe the radar receiver will treat the sub-false targets as noise, therefore, the general value of M is 3 ~ 5.

Conclusion
Through the research of this paper, the main conclusions are as follows: (1) Through the analysis of the frequency spectrum characteristics of PD radar, aiming at the characteristics of HPRF PD radar, a method of pulse jamming is put forward, and the simulation experiment is carried out.
(2) Adopting the method of round pulse jamming, by controlling the period of round pulse, the repetition period of jamming signal is changed relative to radar signal, and a number of spurious targets are produced in the bandwidth of single sideband filter of radar receiver.
(3) The radar signal obtained by DRFM and then modulated, the jamming signal generated has a strong correlation with the target echo signal, which can effectively jam the HPRF PD radar with less power, the validity of the method is verified.