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International Civil Aviation Organization (ICAO)

What is the code?

For conventional SSR (i.e. not mode-S) the choice of questions is very simple. The controller wants to know the identity of the aircraft („Who are you?”). The Radar gives a 2 dimensional position fix of the aircraft but air traffic control is very much a 3 dimensional process, so „What height are you?” completes the positional fix. These different questions determine the MODE of operation. The aircrafts transponder reply with a CODE.

The problem now is that we need to have full co-operation between the radar and the transponder on the aircraft. Every parameter must be agreed. In the military environment this is not difficult, for a given system the same manufacturer can make the radar and the transponder. But to establish an international standard that works throughout the world, with radars and transponders being made by various manufactures requires standardisation. The classical SSR standard is called as the MK X - A („Mark ten A”) standard.

The Standard: ICAO Annex 10

is the document laying down the standards of civil aviation technical infrastructure.
VOLUME IV is specifically for SSR systems.
There is an ICAO web site form which http://www.icao.int/

Manual on Mode S Specific Services ( Doc 9688)
1st edition, 1997. 88 pp.
English, French, Russian, Spanish
Order No. 9688

Manual of the Secondary Surveillance Radar (SSR) Systems. (Doc 9684)
1st edition, 1997, 193 pp
English, French, Russian, Spanish
Order No. 9684

Volume IV (Surveillance Radar and Collision Avoidance Systems)
2nd edition, incorporating amendments 1-73, 204pp
English, French, Russian, Spanish
Order No. AN10-4

Use of SSR equipment became widespread. However, as the air traffic volume increased, some limitations became apparent and a further enhancement of the SSR system, known as Mode S (or Mode Select) was developed as the Mark Ⅻ („Mark twelve”) standard. The SSR Mode S address shall be one of 16,777,214 twenty-four-bit aircraft addresses allocated by ICAO or the State of Registry or common mark registering authority. The allocation of Mode S addresses is described in ANNEX 10 Volume III Chapter 9 (from Amendment 71 onwards).


A similar document to Annex 10 is STANAG 4193 which is published by NATO. This document covers additional specifications concerned with the characteristics of the military Modes 1, 2 and 3, as well as Mode 4.