The EUROCONTROL Mode S Programme

Conventional Secondary Surveillance Systems (SSR Mode A/C) employ technical principles which were originally developed over 50 years ago. However the ATC operational environment has changed significantly since such systems were conceived and the limitations of the surveillance technique have become increasingly apparent.

In the latter years of the 20th century, in the airspace above Core-Europe where the air traffic density is at its highest, the capability of the Air Traffic Management system was seen to be approaching the capacity that its surveillance infrastructure could support. To facilitate continued growth in air-traffic the development of the successor to SSR Mode A/C was initiated.

The EUROCONTROL Mode S Programme was established in the mid-1990s initially to support research and development activities. The Programme had defined objectives – to support the specification, development, validation and operational introduction of SSR Mode Select (Mode S) - a replacement to SSR Mode A/C. The programme achieved in full these objectives and was concluded in 2009.

What were the problems Mode S needed to address?

In addition to limitations regarding system performance (range and azimuth accuracy, height information in 100 ft increments, and restricted probability of detection) the main problems with operating SSR Mode A/C radars in airspace in which many aircraft are flying include:

  • Garbling. Replies from different aircraft at similar distances are often overlapping in time when they arrive at the radar. It is difficult for Classical SSR Mode A/C systems to make sense of these replies. This effect is known as garbling.
  • Mutual interference. All SSR systems operate on a common transmit (1030 MHz) and receive (1090 MHz) frequency. As a consequence they could receive unwanted signals from aircraft replying to other radars. This mutual interference can significantly degrade the efficiency of the radar.
  • Radio reflections. Signals reflecting off various obstacles such as buildings leading to a misinterpretation of aircraft position.
  • Lack of SSR Codes. The current SSR system was designed to support only 4096 individual identity codes (Mode A). With the predicted growth in traffic levels it was likely that ATC units could have run out of codes and, at peak periods, aircraft could have been held on the ground until a suitable code became available.

SSR Mode S is a backwardly compatible system – an evolutionary improvement which provides an ANSP with the tools needed to address the issues identified above, to improve system efficiency and provide surveillance capacity to support future traffic demands.

Many SSR Mode S systems are deployed and operational across Core-Europe and beyond. Mode S avionic equipage is now mandatory for flights conducted as IFR/GAT in many States and also for VFR flights in some designated airspace. The local mandates have been supplemented by Europe wide regulations published in 2011 by the European Commission.

This dossier provides a summary of Mode S technical and operational information. You may find the Mode S documents in the Surveillance library section.

Please note that the EUROCONTROL Mode S Programme is now closed. However for further information contact us.

In this dossier

There are two levels of SSR Mode S, Mode S Elementary...
Whilst traditional Secondary Surveillance Radar (SSR)...
SSR Mode S relies on a unique ICAO 24-bit aircraft address...
Note: Mode S mandates apply to airspace and all those...
The National Aviation Authorities (NAAs) are responsible...