1-4] FUNCTIONAL CHARACTERISTICS OF RADAR SYSTEMS 11 



tions, a given area may be mapped by scanning the area and correlating 

 the signals received with the antenna position. This method — often 

 referred to as microwave thermal mapping (MTR) — is similar in concept 

 to the various forms of infrared mapping. The only difference is the 

 frequency spectrum covered. The use of microwave frequencies sometimes 

 alleviates the severe weather limitations of the much higher-frequency 

 infrared spectrum. Counterbalancing this advantage is the inherently 

 poorer resolution obtained at microwave frequencies and the vastly smaller 

 amounts of thermal radiation energy at these lower frequencies. 



Navigation. The mapping capability can be used to perform a portion 

 of the navigation function, particularly under conditions of poor visibility. 

 Prominent land masses, land-water boundaries, and objects located in a 

 relatively featureless background such as an aircraft carrier at sea are 

 usually readily distinguishable — even on a radar picture obtained from 

 a radar system not specially designed to perform the mapping function. 



By a proper choice of radar parameters, cloud formations that represent 

 a potential flight hazard can readily be detected by a radar of appropriate 

 design. Radar systems designed specially to perform this function have 

 become standard equipment on many transport and military aircraft. A 

 typical radar picture obtained from such a system is shown in Fig. 14-15. 

 Information such as this represents a valuable navigational aid. It can 

 permit the successful completion of many missions that might otherwise 

 be aborted because of weather uncertainty. Radars designed for other 

 purposes can provide this information as an auxiliary function. 



Another radar navigational aid is the radar beacon system (Fig. 1-7). 

 In this system an airborne radar transmits microwave energy at a specified 

 beacon frequency. When some of the energy is received by a beacon station 

 tuned to this frequency, this energy is, in effect, amplified greatly and 

 transmitted back to the interrogating aircraft. There is preset, fixed time 

 delay 4 between the reception and the transmission in the beacon. Thus 

 if the total time between interrogation of the beacon and the reception of 

 the beacon reply is ti /xsec, the range to the beacon is 



R = ^{ti- 4) 



R = 164(/i - 4) yards (1-12) 



where c jl = \ propagation speed of light in yd/jusec 

 ti = propagation transit time 

 4 = beacon delay time. 



