Principles of Modern Radar - Volume 2 1891121537

12.2 Electronic Attack 541variations of each—blinking spot noise (BSN) and blinking barrage noise (BBN). Thetransponder mode generates techniques such as responsive spot noise (RSN) and noisecover pulse (NCP).One method of generating noise waveforms is by directly amplifying and bandlimitingthe thermal noise occurring naturally in electronic components [2]. This producesa Gaussian distribution of the noise, which is appealing because it closely matches thethermal noise in a radar receiver. The disadvantage of the Gaussian noise is that it implicitlyrequires AM of the EA signal. This results in the average output power of thetransmitter being significantly less than its peak output power rating, a fairly expensiveimpact given the high cost of HPAs. An alternate approach is to apply either frequencymodulation (FM) or rapid PM to an RF source. This allows the amplifier to be continuouslyoperated at peak output power as the spectral spreading is achieved entirely withoutresorting to AM.Spot noise produces a narrowband noise waveform that is intended to cover the radarIF bandwidth. Typical SN bandwidths are several MHz to several tens of MHz. This allowsthe jamming energy to be concentrated at the radar frequency. Spot noise is used when theradar frequency is known in advance. Multiple spot noise is a variant in which multiplefrequencies are covered with separate SN waveforms.Barrage noise is used when the specific operating frequency of a victim radar is notknown in advance or if the jammer must simultaneously cover multiple radar emitters inthe environment distributed over a wide spectral bandwidth. The BN energy is spread overa much wider spectrum than SN, possibly hundreds of MHz, for example. As a result, thejamming energy is diluted relative to any individual radar being covered. A BN jammermay achieve its wideband coverage by rapidly sweeping or stepping a SN waveform, oreven a CW tone, across the RF band. Blinking noise—blinking spot noise or blinkingbarrage noise—is sometimes used instead of continuous noise. One reason is to deprivean RF homing missile of a continuous signal to track in a home-on-jam mode, describedin Section 12.10.6.Responsive spot noise and NCP are transponder techniques in which the jammerreceives the radar transmission and generates a response that is somehow dependent onthe input. The RSN is accomplished by the jammer determining the frequency of the radarsignal and rapidly setting-on a SN waveform at that frequency. The jammer maintainscontinuous transmission except for occasional, brief look-through interruptions, duringwhich the frequency or timing is updated as necessary. The RSN allows the jammer tocover a wide RF bandwidth more effectively than a BN approach because it concentratesits noise at the radar’s current frequency. For example, if the RSN bandwidth is 20 MHz,and the BN bandwidth is 200 MHz, the RSN jammer can achieve nominally ten timesthe in-band power of a BN jammer. Key issues for RSN implementation are frequencymeasurement, set-on accuracy, and the look-through update function.The NCP technique is primarily a SSJ technique used to deny range against pulsecompression waveforms. The jammer generates a pulse of noise that is triggered by andconcurrent with the received pulse. The noise, which may be comparable in bandwidthto SN or BN, is strong enough to mask the target return even after the radar performs itspulse compression. This creates an extended-range return spread over many range binsthat hides the true target range. Often this is referred to simply as cover pulse, but we usethe name NCP in this chapter to explicitly distinguish it from a coherent cover pulse, whichmay occupy only a single range bin or a few range bins and is not necessarily restrictedto pulse compression waveforms.