338 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



2. A modulating frequency /r sufficient to create the desired unam- 

 biguous range interval. 



3. Some form of storage and cross correlation which attempts to 

 provide an optimum match between the received signal and the 

 effective receiver bandpass filtering characteristic. 



4. Usually, great complication in comparison with a noncoherent pulse 

 radar. 



The following systems are indicated as possibilities for high resolution 

 wide bandwidth coherent systems. 



1. Wide bandwidth FM/CW. 



2. Long-pulse, low-PRF systems where the transmitted frequency is 

 FM'd during a pulse transmission period to produce the wide 

 transmission bandwidth. This type of system is often called a 

 matched filter radar}^ 



6-8 INFRARED SYSTEMS 



A book entitled Airborne Radar may seem a strange place to find a 

 discussion of infrared techniques, but it must be remembered that the 

 applications and design principles of airborne radars and of infrared 

 detectors and weapon control systems are quite similar. Actually the only 

 real differences between a passive^^ radar and an infrared system are (1) the 

 method of detection^^ and (2) the fact that the infrared radiation emanates 

 from the target itself rather than from its radars or communications 

 equipment. The tuned circuits used in the detection of radio and radar 

 radiations cannot presently be extended to the frequencies (3 X 10^ to 

 1.5 X 10'' megacycles, or 1 to 20 microns wavelength) of that portion of the 

 infrared having practical significance here. Therefore optical detectors 

 must be used and these impose their own restrictions. 



An advantage of the short wavelength of infrared radiation is that 

 interference patterns have correspondingly small angular relationships and 

 are usually not significant in instrument design or operation. For example, 

 the diameter of an infrared collector mirror depends only on the amount of 

 radiation to be collected and not on the required angular accuracy, as in the 

 case of a radar dish. Also interference patterns such as result in radar from 

 ground reflections and cause target confusion are not encountered with 

 infrared systems. Because the target is itself a source, the signal in a passive 

 infrared system diminishes with range more slowly than in an active system. 



I'^Note that in Paragraph 5-10, the term matched has been used to describe a more general 

 class of radar systems. 



i^Only passive infrared is considered here. Active systems do exist and are used, but for 

 long-range detection and tracking no sufficiently intense sources of infrared radiation are 

 available. 



'^By detection we mean the manifestation of the presence of electromagnetic radiation. 



