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SSR Mode Use
Mode A Elicits transponder replies for identity (using SSR code – octal 0000-
7777) and surveillance.
Mode C Elicits transponder replies for automatic pressure-altitude transmission
and surveillance.
Mode A/C/S 1 all call
(intermode)
Mode A/C only all
call (intermode)
The mode used to elicit replies for surveillance of Mode A/C transponders
and acquisition of Mode S transponders.
The mode used to elicit replies for surveillance of Mode A/C transponders
only; Mode S transponders do not reply.
Mode S only all call Only Mode S transponders respond.
Mode S only
broadcast
Mode S only
selective
interrogation
To transmit information to all Mode S transponders; no replies are
elicited.
For surveillance of, and communication with, individual Mode S
transponders. For each interrogation, a reply is elicited only from the
transponder uniquely addressed by the interrogation.
Table 3-6 – ICAO Annex 10 – Definitions of SSR Modes
3.3.3.5 Other compatible technologies
The Airborne Collision Avoidance System (ACAS) provides an airborne picture of nearby
SSR transponder equipped aircraft to the flight crew, and assists in the collision
avoidance if necessary. ACAS interrogates SSR Mode A/C and Mode S transponders in
nearby aircraft, detects the transponders’ replies, and measures the relative range and
bearing. Using these measurements and the reply data (e.g. pressure altitude), ACAS
estimates the relative position of each responding aircraft, and predicts a track for that
aircraft. The RF characteristics of all ACAS signals comply with their SSR equivalent.
Multilateration and 1090 Extended Squitter are also likely to be introduced in European
airspace in the near future (initial local implementations have already started). See
section below on new technologies in-band (3.3.7).
3.3.4 Operational Requirements
3.3.4.1 Overview of operational requirements
The operational requirements for SSR stem from the need to have an accurate method of
determining aircraft position in ‘gate-to-gate’ operations. From the airlines’ viewpoint, they
want to get from departure to destination as safely and efficiently as possible. To do this,
Air Traffic Controllers provide a dual service:
• To provide safe separations for all aircraft under their control;
• To ensure the orderly and efficient conduct of flights.
The controller must know aircraft position at all times during the flight (when in controlled
airspace), and must be able to identify uniquely an aircraft. Although PSR provides
independent radar coverage near airports, in the en-route environment its performance
does not meet requirements (i.e. inadequate positional accuracy, allied to no unique
identification methods). SSR is used to track all transponder-equipped aircraft in en-route
airspace (much of the UK’s airspace is only available to such aircraft), and allows 5nm
1
Note that Mode A/C/S is a mode which can support Mode A, Mode C and Mode S
functions.
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