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When plots of several aircraft on the ATC controller’s screen appear in a small area.
The IDENT function (also called Special Position Identification, SPI), when activated by the flight crew, highlights the aircraft plot on the ATC controller’s screen. This function is available in Mode A, C, or S.
2.1.2. MODE C
When a SSR interrogates a transponder in Mode C, the transponder replies with the aircraft barometric altitude.
2.1.3. MODE S
When a SSR interrogates a transponder in Mode S, the transponder replies with a large set of surveillance data (e.g. flight number, 24-bit address, speeds, heading, track, selected altitude, air/ground status, etc).
There are three types of interrogation:
1.
All Call interrogation: All Mode A and C transponders reply. Mode S transponders do not reply to this interrogation.
2.
Mode S All Call interrogation: It is a variant of the All Call interrogation where only Mode S transponders reply. The reply contains the 24-bit address that enables selective interrogations.
3.
Selective3 interrogation: The SSR selectively interrogates a Mode S transponder.A selective interrogation prevents multiple replies from other transponders and alleviates the occupancy of the reply frequency. In Mode S, the SSR uses the 24-bit address to interrogate a selected aircraft.
2 The SSR determines the altitude via a Mode C interrogation. The transponder includes the barometric altitude in its reply.
Refer to 2.1.3.6 – 24-bit Address or Mode S Address.
A Mode S transponder is able to respond to Modes A and C interrogations.
A Mode S SSR is either basic or capable of Elementary Surveillance (ELS)/Enhanced Surveillance (EHS):
- Basic: The Mode S SSR performs selective interrogations to a given aircraft in order to get the SQWK code via Mode A, the altitude via Mode C, and other information (e.g. Airborne/Ground status).
- Capable of ELS/EHS: In addition to the basic capability, ELS and EHS interrogations enable the collection of several data, on ground requests, such as the flight number, speeds, heading, track, selected altitude, etc.
The entry into operation of Mode S introduces new terms. Hereinafter, the Mode S lexicon describes the main terms.
2.1.3.1. MODE S DATA LINK
Thanks to the Mode S, the ground can selectively collect a large set of data. Therefore, the reader may find the designation “Mode S data link” in the literature.
2.1.3.2. ELEMENTARY SURVEILLANCE (ELS)
The Elementary surveillance refers to a set of Mode S data link messages. These messages convey parameters such as:
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The aircraft 24-bit address, -The flight number,
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