The transponder

Introduction

The transponder was introduced in WWII by the British, to be able to identify friendly aircraft in the sky. The system was called a "parrot", which lead to terms like "squawk your parrot" and "strangle your parrot".

The system was later adopted by the US under the name "identification of friend or foe", which is still used in the US military today.

General description

Definition: An electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar.

• The transponder works by the principle of Secondary Surveillance Radar (SSR)
• There are 4096 transponder codes available.
• Some transponders are equipped with automatic altitude reporting equipment (mode C), which enables ATC to see the aircraft?s current altitude.
• ATC may ask a pilot to disable altitude reporting if the mode C-transponder reports an incorrect altitude.
• If ATC states the transponder-altitude differs, the pilot should verify the altimeter setting with ATC.
• The ATC handbook states the transponder is legal to use when it displays an altitude 300 ft. of the actual altitude.
• The transponder has an "ident" feature which the pilot can use to make their aircraft light up on the ATC's monitor. This feature must only be used when prompted by the ATC.

Regulations

A transponder is required in the following scenarios:

• Class Alpha, Bravo or Charlie airspace
• Above class Bravo and Charlie airspace
• Within 30 NM of a class B airport (the mode C veil)
• Above 10 000 ft., excluding that airspace below 2 500 ft. AGL
• Crossing an ADIZ

A transponder must be tested and inspected every 24 calendar months according to ?91.413.

Secondary Surveillance Radar (SSR)

Secondary surveillance radar is referred to as "secondary", to distinguish it from the "primary radar" that works by passively reflecting a radio signal off the skin of the aircraft.

Primary radar determines range and bearing to a target with reasonably, but it cannot determine target elevation reliably.

SSR uses an active transponder to transmit a response to an interrogation secondary radar. This response includes the aircraft's pressure altitude and a 4-digit octal identifier

How it works:

1. An SSR-station sends out an interrogation signal toward an aircraft on 1030 MHz.
2. The aircraft picks up the signal.
3. The aircraft sends a reply to the station containing altitude- and squawk information on 1090 MHz.
4. The radar blip on ATC?s monitor now contains additional information.

Mode A, C, and S

Mode A:
Only provides aircraft identification information by the use of squawking a 4-digit number.

Mode C:
Provides both aircraft identification (squawk) as well as pressure altitude information. If both aircraft are equipped with a TCAS, the transponder will provide the pilots with resolution advisories.

Mode S:
Aircraft can interrogate each other for information such as altitude, airspeed, and identification. If both aircraft are equipped with TCAS, the transponder will provide a resolution advisory that is smarter than on mode C.
Each mode S transponder comes with an unchangeable 24-bit address for identification.

Transponder codes