6-5] FM/CW RADAR SYSTEMS 319 



where P = transmitted average CW power 

 B = doppler filter bandwidth. 



The choice of a detection bandwidth B is governed by a number of 

 considerations derived from the tactical problem and from the realities of 

 radar design practice. 



The spectral composition of a modern-type aircraft radar reflection is sel- 

 dom more than a few cycles wide when it is caused by target characteristics 

 alone. A CW radar, transmitting a truly unmodulated wave, produces the 

 most elementary moving target spectrum. The possibility exists, therefore, 

 of detecting a radar signal as much as 190 db below a watt in about a second, 

 using simple, very narrow band filters matched to the signal waveform. 

 However, a number of practical and tactical considerations usually limit 

 the full exploitation of this potential. 



To search the frequency spectrum of expected dopplers requires a series 

 of adjacent filters or one or more individual filters scanning the spectrum. 

 The minimum allowable time on target is that time required for energy to 

 build up in one of a fixed bank of filters; scanning filters increases this 

 minimum by the ratio of scanned to actual bandwidths. Prior information 

 as to target velocity or bearing can reduce the spectrum or area to be 

 searched and thus increase the probability of detection in a given situation. 



One practical constraint on exploiting very narrow bandwidths is the 

 shift in the doppler spectrum caused by target maneuvers. This effect can 

 cause the signal to be greatly attenuated if the target doppler transits the 

 filter bandpass range before the filter has time to build up. To avoid this 

 situation, the filter bandpass must satisfy the relation 



B>^ (6-29) 



A 



where a = acceleration in ft/sec^ 

 X = RF wavelength in ft. 



Another usual requirement in a radar system is to yield additional radar 

 target parameters. Information theory advises that to process more data, 

 more bandwidth and /or time is required. To resolve target range by FM 

 on CW or to resolve bearing by AM will in general increase the minimum 

 radar bandwidth requirements. For example, angular scanning will broaden 

 the return spectrum bandwidth by the scanning frequency modulation, 

 which may be of the order of 50 cycles for a typical airborne radar. Alter- 

 natively, the use of very low information rates demands an amount of time 

 that may be unacceptable. 



An economically attractive technique is to employ one sweeping filter of 

 a bandwidth such as to satisfy all system requirements. Conventional AFC 



