Principles of Modern Radar - Volume 2 1891121537

572 CHAPTER 12 Electronic Protectionextent in contiguous subbands. The regions of overlap of these subbands may have lowerpower due to band roll-off, or there may be gaps between the subbands. Second, thejammer transmit amplifier, transmission line, and antenna may have frequency-dependentgain, phase, and voltage standing wave ratio (VSWR) properties that create an unevenpower spectrum. Third, multipath may cause constructive or destructive interference thatis frequency-dependent. Finally, the radar antenna sidelobe pattern may have frequencydependentvariations in the direction of the jammer.The ability to sense the EA interference spectrum before the next transmission issometimes referred to as a sniff mode or sniffer [2,3,6]. Radar schedulers may introducean inherent lag between the time that the environment is sensed and the time the selectedfrequency is actually used; thus, there is a risk that the assessment made during the sniffmode is stale when the least jammed frequency is selected. The sniff function is usuallyscheduled to occur at a time in the radar timeline during which there is no expected returnfrom a target or clutter in the environment. A question that may arise in implementingthis technique for ESA radars is whether to adjust the antenna beam-steering commandwith frequency as the EA spectrum is characterized. This may give a more accuraterepresentation of the likely noise level at the frequency ultimately selected, but the beamsteeringaction may prove impractical.12.9 SIGNAL PROCESSOR-BASED EPThe signal processor accepts the receiver ADC samples and performs high-speed operations,including pulse compression, temporal filtering, Doppler integration, power centroiding,track error signal derivation, and threshold detection. The signal processor EPtechniques are applied within the time scale of a CPI. Applicable EP techniques include detectionand mitigation of both noise and deceptive EA, in some cases enabling subsequentEP to be performed in the data processor.12.9.1 Total Energy DetectorThe total energy detector (TED) is a conceptually simple method for detecting the presenceof noise or coherent masking. The signal processor computes the energy in a large numberof range or Doppler cells and compares the total to some threshold representative of an EAfreeenvironment. If the total energy exceeds the threshold, then jamming is suspected.Ideally, the energy samples are taken from a range-Doppler region that excludes highclutter returns; otherwise the total energy computation may be biased by extended clutterreturns and could lead to a false declaration of jamming. The TED does not inherentlydistinguish between unintended EMI and EA. However, subsequent logic in the dataprocessor might be able to correlate high-TED occurrences with specific beam positionsor multiple observations over time to help determine whether the interference is EMI orEA. The TED detection may be used to generate jam strobes, described in Section 12.10.6.12.9.2 Alternate Constant False Alarm Rate DetectionRadars with CFAR detection employ detection thresholds that vary based on estimates ofthe interference energy (such as noise and clutter) contained in a particular detection cell,referred to as a cell under test (CUT). The CFAR detection process is described in [8].