The Secondary Surveillance Radar (SSR) has become the principal source of information for Air Traffic Control Service, since it provides discrete code and altitude information.
However, it still has deficiences, e.g., target loss and false targets, which are mainly caused by the ground reflection due to the broad vertical radiation pattern.
Since a sharp cutoff elevation pattern is effective in coping with the problems, an integral feed antenna for the SSR collocated with the Air Route Surveillance Radar (ARSR) is devised. It is an offset type antenna which has an SSR radiator by an ARSR horn. The setting, however, causes an undesirable offset angle and beam skew, but the former is remedied by a compensator. The latter, though it deteriorates the system accuracy, and is difficult to be suppressed intrinsically, is designed to be minimized. The antenna employs an ARSR reflector so that a sharp cutoff vertical pattern can be obtaind.
The antenna was installed at Iwanuma Experimental Center and an evaluation test of the antenna was made with our own aircraft.
The result of the test shows that (1) the integral feed SSR antenna greatly decreases the frequency of target loss caused by the lobing effect, (2) it suppreses false targets caused by reflecting obstacles below the horizon.
