Principles of Modern Radar - Volume 2 1891121537

540 CHAPTER 12 Electronic ProtectionEA waveform. In stored delay mode, the input signal must be detected, and all or part ofthe received pulse must first be stored in memory before any of the digitized samples canbe read out. This mode, therefore, does not lend itself to simultaneously receive (writing)and transmit (reading) operation.Coherent jammers tend to use broadband, CW amplifiers. This allows them to operateagainst multiple emitters distributed across the RF spectrum, timeshare between multipleemitters simultaneously, and better accommodate the high duty factor of coherent radarwaveforms. Common technologies are the TWT amplifier, solid-state amplifier (SSA),and the microwave power module (MPM) amplifier. The MPM uses an SSA for the initialgain stage and a TWT for the final-gain and high-power stage. The SSA offers smallersize and better noise figure, while the TWT offers higher peak power. Typical HPA powerlevels of interest for coherent EA range from a few watts to hundreds of watts, dependingon the application.The architecture in Figure 12-3 is actually a blend between a traditional coherentrepeater and what is sometimes referred to as a coherent, or quasi-coherent, transponder[1,2,4,10]. In a classic coherent repeater, the coherent delay is very minimal, achievedthrough a length of microwave transmission line for example, and the signal remainsat RF throughout its transit through the jammer. A repeater that can simultaneously receiveand transmit is sometimes referred to as a straight-through repeater or modulatedstraight-through repeater. A repeater that cannot simultaneously receive and transmit dueto isolation limitations will employ the short coherent RF delay to capture a segment ofthe incoming pulse and rapidly alternate between receive and transmit states. This type ofrepeater is referred to as a chopped repeater. The addition of a DRFM, or other coherentmemory, which can provide very long coherent delays and can continually reproduce thesampled signal for time periods well beyond the received pulse width, allows the jammerto operate in a transponder-type mode. In the diagram of Figure 12-3, bypassing the coherentdelay would allow the jammer to operate as a traditional coherent straight-throughrepeater or chopped repeater. For convenience, we will forego the previous distinction infavor of the common vernacular and simply refer to the hybrid architecture of the figureas a coherent repeater.12.2.3 Masking EA TechniquesMasking EA attempts to prevent target detection or deny measurement of target positionor velocity. Masking can be achieved through either the noncoherent or coherent jammerdesigns of Figures 12-2 and 12-3. The noncoherent jammer generates noise over a bandwidththat is comparable to or exceeds the radar waveform bandwidth. For example if theradar employs a 10 MHz pulse compression waveform, the jammer would be designed togenerate a noise spectrum that is at least 10 MHz wide against that radar. The coherentjammer can generate such noise as well but also has the ability to focus its energy muchmore efficiently in the range and Doppler domains, making it better matched to the radardetector.12.2.3.1 Noncoherent Masking TechniquesThe noncoherent jammer of Figure 12-2 can generate noise in either a transmit-only, noisegenerator mode or a responsive, noise transponder mode [1,2]. The noise generator modeis used to generate techniques such as spot noise (SN), barrage noise (BN), and blinking