www.radartutorial.eu Radar Basics

Fruit

Figure 1: SSR-video with a disabled defruiter

View of a ppi-scope: SSR-video with a disabled defruiter

Figure 1: SSR-video with a disabled defruiter

Fruit

All secondary surveillance radar transmissions are worldwide on the two frequencies only:

This is necessary since an aircraft passes through several radar control areas on its flight path.

One describes all asynchronous interferences as a Fruit, which arises from replies, these were triggered not by the own interrogator.

The term “Fruit” is an acronym. There are a few definitions for the acronym Fruit in common usage today, e.g:
False Replies Un-synchronised In Time or
False Replies Unsynchronised to Interrogator Transmission.

Fruit should only happen when at least one of the targets involved is in the main beam of at least two interrogators. Similar to synchronous garbling, Fruit is in fact asynchronous interference where replies overlapping in time at the receiver antenna may be lost. The key difference is that one or more of these replies is not expected and is intended for another user of the frequency.

As the population and activity of transponders (and ground stations) increases in response to increases in traffic level, the levels of Fruit increase, causing increased loss of replies and false targets.

Defruiter

If the SSR- antenna sweeps once over the targets position, the aircrafts transponder has to transmit about 10 till 30 replies. These replies are stored in the defruiter and compared with each other in the next PRT now.

delay line
subtraction circuit
video and FRUIT
delayed video
video without FRUIT
one pulse period

Figure 2: Principle of a Defruiter (Criterion: needed 2 replies from 2 PRTs)

delay line
subtraction circuit
video and FRUIT
delayed video
video without FRUIT
one pulse period

Figure 2: Principle of a Defruiter (Criterion: needed 2 replies from 2 PRTs)

Depending on size of the Fruit such a comparison can be carried out for two or more replies (PRTs). Problems appear at this method by different circumstances, though:

Only a compromise can consequently represent the solution of the problem, by choosing a criterion which as many as possible suppresses Fruit, but still has much use information passed sufficiently.