14 Secondary Surveillance Radar (SSR)

Mode S

Mode S is being introduced in order to overcome the limitations of the present modes A and C. ‘S’ stands for Selective addressing. The new system has to be compatible with the existing modes A and C so that it can be used to supplement the present system.

The main features of the new mode S are:

Availability of Codes

The aircraft address code will be made up of a 24 bit code. This means that the system will have over 16 700 000 discrete codes available for allocation to individual aircraft on a permanent basis. The code will be incorporated into the aircraft at manufacture and remain with it throughout its life.

Data Link

The system will be supported by a ground data network and will have the ability to handle uplink/downlink data messages over the horizon. Mode S can provide ground-to-air, air-to- ground and air-to-air data exchange using communications protocols.

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Reduction ofVoice Communications

It is intended that the majority of the present RTF messages will be exchanged via the data link. Messages to and from an aircraft will be exchanged via the aircraft’s CDU resulting in a reduction in voice communications.

Height Read-out

This will be in 25 ft increments and more data on an aircraft’s present and intended performance will be available to the ground controllers.

Interrogation Modes

Mode S operates in the following modes:

•	All Call	to elicit replies for acquisition of mode S transponders.
•	Broadcast	to transmit information to all mode S transponders (no replies are elicited).
•	Selective	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.
•	Intermode	mode A/C/S All Call would be used to elicit replies for surveillance of mode A/C
		transponders and for the acquisition of mode S transponders.

Pulses

Mode S does not transmit the P3 pulse, but has an additional P4 pulse, which can be either long or short in duration.

Intermode A/C/S All Call

Interrogation will consist of P1, P2, P3 and the long P4 pulses.

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Intermode A/C only All Call

Interrogation will consist of P1, P2, P3 and the short P4 pulses.

Benefits of Mode S

Unambiguous Aircraft Identification

This will be achieved as each aircraft will be assigned a unique address from one of almost 17 million which together with automatic flight identity reporting allows unambiguous aircraft identification. This unique address in each interrogation and reply also permits the inclusion of data link messages to or from a particular aircraft i.e. selective calling will be possible in addition to ‘All Call’ messages.

Improved Integrity of Surveillance Data

The superior resolution ability of Mode S plus selective interrogation will:

•eliminate synchronous garble.

•resolve the effects of over interrogation.

•simplify aircraft identification in the case of radar reflections.

Improved Air PictureTracking and Situation Awareness

The radar controller will be presented with a better current air picture and improved horizontal and vertical tracking due to unambiguous aircraft identification, enhanced tracking techniques and the increased downlink data from the aircraft.

Alleviation of Modes A/C Code Shortage

The current shortage of SSR codes in the EUR region will be eliminated by the unique aircraft address ability of Mode S.

Reduction of R/TWorkload

Due to the progressive introduction of Mode S, R/T between a controller and an aircraft will be reduced; e.g. code verification procedures will not be required.

Improvements to ShortTerm Conflict Alert (STCA)

The ability of Mode S to eliminate synchronous garbling, to produce a more stable speed vector and to acquire aircraft altitude reporting in 25 ft increments (if supported by compatible barometric avionics) will improve safety. In addition, access to the downlinked aircraft’s vertical rate will produce early, accurate knowledge of aircraft manoeuvres.

Note: Whilst the ground system will benefit from altitude reporting in 25 ft intervals there is no intention to change the existing practice of displaying altitude information to the controller in 100 ft increments.

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Communication Protocols

Standard Length Communications (SingleTransaction)

Comm-A: Transfer of Information from Ground to Air.

Initiated from ground.

Comm-B: Transfer of Information from Air to Ground.

May be either ground or air initiated.

Extended Length Communications (up to Sixteen 80-bit Messages)

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Comm-C: Uplink

Comm-D: Downlink

Levels of Mode S Transponders

ICAO Aeronautical Telecommunications, Vol. IV, Annex 10 stipulates that Mode S transponders shall conform to one of four levels of capability:

Level 1	This is the basic transponder and permits surveillance based on Mode A/C as
	well as on Mode S. With a Mode S aircraft address it comprises the minimum
	features for compatible operation with Mode S interrogators. It has no data
	link capability and will not be used by international air traffic.
Level 2	This has the same capabilities as Level 1 and permits standard length data link
	communication from ground to air and air to ground. It includes automatic
	aircraft identification reporting. This is the minimum level permitted for
	international flights.
Level 3	This has the same capabilities as Level 2 but permits extended data link
	communications from the ground to the aircraft.
Level 4	This has the same capabilities as Level 3 but allows extended data link
	communications from the aircraft to the ground.

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Downlink Aircraft Parameters (DAPS)

Basic Functionality

•Automatic reporting of Flight Identity (call sign used in flight).

•Transponder Capability Report.

•Altitude reporting in 25 ft intervals (subject to aircraft availability).

•Flight Status (airborne/on the ground).

Enhanced Functionality

•Magnetic Heading.

•Speed (IAS/TAS/Mach No.).

•Roll Angle (system acquisition of start and stop of turn).

•Track Angle Rate (system acquisition of start and stop of turn).

•Vertical Rate (barometric rate of climb/descent or, preferably baro-inertial).

•True Track Angle/Ground Speed.

Future Expansion of Mode S Surveillance Services

When technical and institutional issues have been resolved the down linking of an aircraft’s intentions are recommended for inclusion:

•Selected Flight Level/Altitude.

•Selected Magnetic Heading.

•Selected course.

•Selected IAS/Mach No.

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