SSR A and C uses 1030/1090 MHz for interrogate/ reply sequence. Since all a/c use the same bandwidth, there is a possibility of overlap of two or more a/c transmissions which are flying close to each other. The result is a garbled - meaning jumbled -response on the radar console. The radar controller may not be able to distinguish one a/c from another flying close. The other problem with mode A/C ( ie conventional or ATCRBS(USA) ) is FRUIT. Some books give it as short for False Replies Uncorrelated In Time). This may happen when two or more a/c are transponding within range of the same radar. The radar blip appears to have fruitlike lobes attached to it.

 

Mode S eliminates these problems by using monopulse technique.

ATCRBS=Air Traffic Control Radar Beacon System