Mode S

Mode S


Mode S is a Secondary Surveillance Radar process that allows selective interrogation of aircraft according to the unique 24-bit address assigned to each aircraft. Recent developments have enhanced the value of Mode S by introducing Mode S EHS (Enhanced Surveillance).


Mode S in its basic form has been standardised by the International Civil Aviation Organization (ICAO) for many years. It employs ground-based interrogators and airborne transponders and operates in the same radio frequencies (1030/1090 MHz) as conventional SSR systems with which it is backwards compatible.

Mode S has been deployed because the historical SSR systems have reached the limit of their operational capability. This takes the form of exceeded maximum number of targets, RF pollution, lost targets, identity errors and Mode A code shortage. Mode S is therefore an essential development for both SSR and the new ATM processes which will be essential in airspace subject to high levels of traffic density or absence of ATC radar coverage.

There are five sources of RF pollution:

  1. Acquisition Squitter
  2. Extended Squitter
  3. Airborne Collision Avoidance System (ACAS) transactions
  4. Self-generated Second Time Around Replies
  5. Replies from other Radar Interrogations

Mode-S employs airborne transponders to provide altitude and identification data, with Automatic Dependent Surveillance Broadcast (ADS-B) adding global navigation data typically obtained from a Global Positioning System (GPS) receiver. The position and identification data supplied by Mode S/ADS-B broadcasts are available to pilots and air traffic controllers.

Mode S/ADS-B data updates rapidly, is very accurate and provides pilots and air traffic controllers with common air situational awareness for enhanced safety, capacity and efficiency. Further, it can provide a cost-effective solution for surveillance coverage in non-radar airspace.

In Europe, SSR Mode S Elementary Surveillance (ELS) is rapidly being succeeded by Mode S Enhanced Surveillance (EHS).

Legal Requirements

Regulation (EU) No 1207/2011 requires that all flights operating as general air traffic in accordance with instrument flight rules within the EU are equipped with mode S transponders.

Mode S ELS (Elementary Surveillance)


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 aircraft callsign used in flight which is automatically presented to the controller
  • Altitude reporting in 25ft intervals (subject to aircraft capability)
  • Transponder capability report - a technical function to enable ground systems to identify the data link capability of the transponder
  • 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.


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. Mode S is the primary means of correlating radar tracks with system flight plans in automated ATC systems.
  • Improved integrity of surveillance data. Selective interrogation and the superior resolution ability of Mode S over existing SSR and MSSR installations eliminates synchronous garble, resolves the effects of over interrogation and simplifies aircraft identification in the case of radar reflections.
  • Improved air situation picture and tracking. Radar controllers are presented with a better current air situation 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 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 ease this problem.
  • Improvements to Safety Nets (e.g. Short Term 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 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 increases in traffic, Mode S radars are able to process many more aircraft tracks (approximately double the number) than conventional MSSR installations.

Mode S EHS (Enhanced Surveillance)


Aircraft compliant with Mode S EHS provide ELS functionality features (see above) plus some or all of the following downlinked aircraft parameters (DAPs):

  • Selected Altitude - the flight level which is manually entered in the FMS by the pilot. Selected Altitude provides an indication of the intended flight path and should reflect the ATC clearance with a few exceptions. It is used to improve controller situation awareness and conflict detection tools. The use of Selected Altitude values in Safety Net systems is expected to considerably reduce false alarms (an STCA study showed that by using Selected Altitude more than 90% of all false alarms could have been avoided) for aircraft engaged in vertical manoeuvres (level-off scenarios)and, thereby, to considerably increase the performance capability of the Safety Net systems. At the same time, the display of the Selected Altitude in the track label (either fully automated for cross checking with controller input or just presented as additional information) has proven to be an efficient tool to identify and mitigate the risk for potential level busts.
  • Roll Angle, True Track Angle and Track Angle Rate - these are technical parameters which may be used to enahnce the radar tracking capability and/or tactical trajectory prediction by the ground ATC systems. The Roll Angle can be used in conjunction with the True Airspeed by the surveillance sata processing systems to improve the recognition of horizontal manoeuvres and increase tracking accuracy. The True Track Angle, in combination with the Ground Speed, can be used to improve track initialisation (initialisation after just one plot and not after two or three plots as is currently the case), to increase tracking performance (particularly at the edges of the radar systems’ range) and to improve recognition of horizontal manoeuvre by monitoring changes in track angle. The Track Angle Rate (called also Rate of Turn) gives the turning speed of the aircraft. This parameter provides direct information to improve the recognition of horizontal manoeuvre and to increase tracking performance in surveillance data processing systems, better than a combination of roll angle and true airspeed. This leads to more accurate target positioning and a considerable error reduction for the predicted position.
  • Ground Speed - calculated aircraft speed relative to the ground. Information provided by ANSPs show that the value of this parameter is not providing a significantly better accuracy than the ground speed calculated by the surveillance data processing systems.
  • Magnetic Heading - the aircraft heading relative to magnetic north. Making this information available to controllers reduces R/T occupancy time as controllers no longer have to request the information from the pilot. The Magnetic Heading has the potential to improve horizontal manoeuvre recognition, either by the controller or by the surveillance data processing systems, via monitoring of heading changes.
  • Indicated airspeed (IAS) and Mach-number. Making this information available to controllers supports separtion provision tasks, reduces the R/T and hence the controller workload.
  • Vertical rate (barometric rate of climb / descent) - this parameter is not used operationally by ATC due to the significant variations cased by a number of factors, such as turbulence, small but rapid aircraft vertical movements, etc.
  • TCAS downlinked resolution advisories.
  • Barometric pressure setting (BPS)


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

  • Improved situation awareness. A clearer air situation picture, enhanced tracking and access to pertinent information direct from the aircraft enables the controller to benefit from quicker and more accurate recognition of airborne events.
  • Progressive reduction of R/T workload per flight. There is scope for R/T usage between controller and individual flight 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. “ABC123, report heading”.
  • Safety enhancement. Access by controllers to aircraft intent DAPs, such as selected altitude enables cross-checking of climb/descent instructions and helps the early identification of potential level bust incidents.

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