Mode S Operational Overview

Whilst traditional Secondary Surveillance Radar (SSR) stations interrogate all aircraft within their range, Mode S (Select) establishes selective and addressed interrogations with aircraft within its coverage. Such selective interrogation improves the quality and integrity of the detection, identification and altitude reporting. These improvements translate into benefits in terms of safety, capacity and efficiency – benefits which are key to supporting the future of the high-traffic density airspace of Europe.

In Europe, SSR Mode S is being implemented in two stages: Mode S Elementary Surveillance (ELS) and Mode S Enhanced Surveillance (EHS).

Note: Mode S interrogators must only operate with a correctly assigned code. See Mode Interrogator Code Allocation for further details.

Mode S Elementary Surveillance (ELS)

Within European Mode S airspace:

  • Aircraft intending to fly IFR as GAT must be Mode S ELS compliant.
  • Aircraft intending to fly VFR: requirements differ for VFR flights within the participating States. Consult appropriate AICs/AIPs for the states in which flights will be conducted.
  • State aircraft: Co-ordinated exemption arrangements have been established (consult the Civil-Military ATM Coodination Division).

Consult the ACID programme for further details.

Functionality

Aircraft compliant with Mode S ELS provide the following functionality (this is also referred to as "Basic Functionality"):

  •  Automatic reporting of Aircraft Identity. This is the callsign used in flight and is automatically presented to the controller.
  • Transponder Capability Report. A technical function to enable ground systems to identify the data link capability of the transponder.
  •  Altitude reporting in 25ft intervals (subject to aircraft capability).
  •  Flight status (airborne/on the ground). A technical function.
  • SI code capability. A technical function to identify transponders capable of operating within a Surveillance Identifier (SI) code ground environment (which permits a reduction in ground infrastructure complexity). Basic functionality with SI code capability is the minimum level permitted for operations in European airspace.

Benefits

Aircraft compliant with Mode S ELS provide the following operational benefits:

  •  Unambiguous Aircraft Identification. The availability of almost 17 million unique aircraft addresses, in conjunction with the automatic reporting of flight identity, permits the unambiguous identification of aircraft independently of any Mode 3/A code assignment.
  • Improved Integrity of Surveillance Data. Selective interrogation and the superior resolution ability of Mode S over existing SSR and MSSR installations will:

                                - Eliminate synchronous garble.

                                - Resolve the effects of over interrogation.

                                - Simplify aircraft identification in the case of radar reflections.

  • Improved Air Picture and Tracking. Radar controllers are presented with a better current air picture through system acquisition of flight identity and enhanced tracking techniques. The greater accuracy of Mode S radars (less random or systematic errors together with the production of more stable speed vectors) results in an improved horizontal and vertical tracking capability over current in-service SSR installations.
  • Alleviation of Mode 3/A Code Shortage. The situation concerning SSR code shortage in the EUR Region is reaching a critical stage. The unique aircraft address ability of Mode S will, in conjunction with other measures, help to ease this problem.
  • Improvements to Safety Nets (e.g. STCA). The ability of Mode S to eliminate synchronous garbling, to produce a more stable speed vector and to acquire aircraft altitude reporting in 25ft increments (if supported by compatible barometric avionics), provides valuable improvements to the quality of safety nets. These improvements should reduce the number of nuisance alerts and enhance the integrity of separation assurance.
  • Increased Target Capacity. In order to handle current and forecast traffic increases, Mode S radars are able to process many more aircraft tracks (approximately double the number) than conventional MSSR installations.

Mode S Enhanced Surveillance (EHS)

Within European designated EHS airspace:

  • All fixed wing aircraft, having a maximum take-off mass greater that 5,700 kg or a maximum cruising true airspeed in excess of 250 kts, intending to fly IFR as GAT, must be Mode S EHS compliant.
  • Aircraft intending to fly VFR: requirements differ for VFR flights within the participating States. Consult appropriate AICs/AIPs for the states in which flights will be conducted.
  • State aircraft: Co-ordinated exemption arrangements have been established (consult the Civil-Military ATM Coodination Division).

Functionality

Aircraft compliant with Mode S EHS provide basic functionality features (see above) plus the following eight downlinked aircraft parameters (DAPs):

DAPs

Benefits

In addition to the benefits for Mode S ELS, identified above, aircraft compliant with Mode S EHS also provide the following operational benefits:

  • Improved Situation Awareness. A clearer air picture, enhanced tracking and access to pertinent information direct from the aircraft will enable the controller to benefit from quicker and more accurate recognition of airborne events.
  •   Progressive Reduction of R/T Workload per Aircraft. There is scope for R/T usage between controller and individual aircraft under service to be reduced following the progressive introduction of Mode S Enhanced Surveillance. It applies in particular to the current requirement for SSR code verification procedures and also where system enhancements and/or the display of downlink aircraft parameters obviate the need for certain voice communication exchanges (e.g. “BAW721, what is your heading?”).
  • Safety Enhancements. Access by controllers to aircraft intent DAPs (Selected Altitude) will afford cross-checking of climb/descent instructions and help the early identification of potential level bust incidents.