Principles of Modern Radar - Volume 2 1891121537

542 CHAPTER 12 Electronic Protection12.2.3.2 Coherent Masking TechniquesA coherent jammer such as the one represented in Figure 12-3 can potentially createmasking that is much more focused and more insidious than the noncoherent jammertechniques. Coherent masking is sometimes referred to as coherent noise, a term thatannoys some purists but is actually fairly descriptive of what occurs.One form of coherent masking is Doppler noise (DN), also referred to as narrowbandDoppler noise (NBDN) or narrowband repeater noise (NBRN) [6]. A coherent repeaterachieves DN by applying a PM (or, less commonly, AM) to the received signal. Insteadof the return focusing up in the Doppler filter corresponding to the jamming platform,it is spread across multiple Doppler filters. Typical DN bandwidths range from a fewkHz to several tens of kHz—orders of magnitude smaller than RSN-type bandwidths. Ananalogous approach in the range dimension uses a DRFM or other coherent memory tocapture and continuously replay the received radar signal across multiple range bins orthe entire PRI in a CW range denial mode. This can be combined with DN to produceextended range DN.Range bin masking and velocity bin masking, or Doppler bin masking, are similar tothe preceding techniques but are distinguished from them in this chapter in that bin maskingis restricted to a smaller region in the range and Doppler domains, with a specific objectiveof attacking the radar constant false alarm rate (CFAR) detector. (This distinction is notuniversal.) The jammer attempts to place coherent returns in range or Doppler bins that areused by the radar to form its noise estimate for setting the CFAR detection threshold. TheEA biases the noise estimate higher, potentially causing the threshold to be raised enoughto prevent the target from being detected. The range or velocity bin masking energy istypically spread over a contiguous range or Doppler extent; however, similar effects canbe achieved if the energy is focused up in distinct range-Doppler bins, such as would occurfor multiple targets returns.Another form of coherent masking is automatic gain control (AGC) capture in whichthe jammer exploits the radar AGC to reduce sensitivity, as described in Section 12.8.3.12.2.4 Deception EA TechniquesDeception EA techniques inject target-like returns in the radar that contain false information.The primary classes of techniques are false targets and track deception/denial.Noncoherent jammers were once used to generate deception techniques against older,pulsed radar systems; however, against modern, coherent radars, this role has been almostexclusively fulfilled by the coherent jammer.12.2.4.1 False TargetsFalse targets are used to create distraction or confusion, primarily against search andacquisition modes, and can be generated in range, velocity, or angle. Range false targets(RFTs) typically require use of a DRFM or other coherent RF memory to create rangedelayed,coherent copies of the radar pulse. Against pulse-Doppler radar, a DRFM can beused to exploit the range ambiguity of the radar waveform and make the false target appearto be in front of the actual jammer position in range. This is accomplished by delayingthe received signal by one PRI minus the desired time advance. The jammer continuallymonitors the PRI using an ES function referred to as a PRI tracker and updates this delayas needed. Velocity false targets (VFT), or Doppler false targets, can be superimposedon RFTs or generated independently through a straight-through repeater by AM or PM