Microlight » Flight Radio » SSR Transponders

Transponders

A transponder is an aircraft electronic device used to assist Air Traffic Control (ATC) surveillance and aircraft identification.

With traditional Secondary Surveillance Radar (SSR):

• a ground station sends an interrogation signal;
• the aircraft transponder receives the interrogation;
• the transponder automatically sends a coded reply.

Depending on the equipment fitted, the information transmitted may include:

• the aircraft's SQUAWK code;
• pressure altitude;
• aircraft identification; and
• other information when using Mode S or ADS-B equipment.

Transponder Codes

A four-digit transponder code is commonly referred to as a SQUAWK code.

Unless ATC assigns a specific code, common New Zealand VFR codes include:

• 1200 — VFR aeroplanes
• 1500 — VFR helicopters

ATC may allocate an aircraft a different code to allow it to be individually identified.

Transponder Controls

A typical older transponder may have four selectors used to enter the required four-digit SQUAWK code.

Each digit may be selected from 0 to 7.

IDENT

The IDENT button is used when instructed by ATC.

When pressed:

• the transponder sends an identification signal;
• the aircraft's surveillance target is highlighted to the controller;
• this assists ATC in positively identifying the aircraft.