438 



BELL SYSTEM TECHNICAL JOURNAL 



fense, in combat zones and in rear areas. To realize the advantages of 

 different parts of the frequency spectrum, avoid interference, and keep 

 ahead of enemy countermeasures, it has been necessary for radar to exploit 

 many different frequency bands and sub-bands. These diversities have 



Fig. 2— Early radar test set for 3000 mc range— includes signal generator, oscilloscope, 

 power meter, test antenna and auxiliary units. 



bred a multiplicity of types of radar, with a corresponding variety of testing 

 problems and requirements. 



Maintenance and testing of radars must be performed at many different 

 locations. In the Army these locations, known as echelons, include (a) 

 the operating unit, (b) central service points, either fixed or mobile, for a 

 number of operating units, and (c) large depots either in the military theatres 

 or in the United States. Na\y maintenance and testing are carried out on 

 board the fighting ships or auxiliaries, and at advance and overhaul bases. 



