EW 101 Digital Radio Frequency Memories DRFMs - Part 2 By Dave Adamy Determined by Doppler ﬁlter in which signal is received Range Cells COHERENT JAMMING The Journal of Electronic Defense | November 2013 44 One of the advantages of using a digital RF memory (DRFM) is that it can generate a coherent jamming signal. This is particularly important when jamming a pulse-Doppler (PD) radar. Figure 1 shows a range vs. velocity matrix which is part of the PD radar processing - for all signals entering the radar receiver. The velocity dimension of the matrix is generated by a bank of narrow filters, usually implemented in software. Since the transmitted signal is coherent, the legitimate return signal from a target will fall into one filter, and those filters are quite narrow. However, a noncoherent jamming signal like barrage or spot noise will enter several filters. This allows the radar to reject jamming in favor of the return of its own coherent signal. Velocity Cells Determined by pulse return time Figure 1: The processing hardware and software of a PD radar includes a matrix of time vs. radial velocity for each received pulse. Angular coverage of antenna scan INCREASED EFFECTIVE J/S Noise jamming can have its effective Jamming to Signal Ratio (J/S) against a PD radar reduced by many dB by the radar's processing gain. Consider the case of an acquisition radar that has its coherent processing interval (CPI) equal to the time that its scanning beam will illuminate a target. The radar has a circular scan with a period of 5 seconds, a beam width of 5 degrees and a pulse repetition frequency (PRF) of 10,000 pulses per second. The beam illuminates the target (a time equal to the CPI) for 69.4 milliseconds, calculated from the following formula. (See Figure 2.) Target Antenna beam width Figure 2: The amount of time a scanning radar illuminates a target depends on its beam width, scan rate and angular scan coverage.