Aircraft transponders are a part of Secondary Surveillance Radar (SSR). These transponders reply to interrogation from a ground station with a code set by the pilot and instructed to do so by air traffic control. Due to increasing traffic in European skies it became necessary to replace the good old Mode A/C transponders with new digital Mode S transponders.

Mode S Transponders

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.

The first step in the introduction of SSR Mode S in Europe is known as Mode S Elementary Surveillance (ELS). Mode S Enhanced Surveillance (EHS), which builds upon the concept of ELS and consists of the extraction of Downlink Aircraft Parameters (DAPs), facilitates an increase in the safety and efficiency of the ATM operations.

Mode S Elementary Surveillance (ELS) Implementation Timescales

IFR Airborne Implementation
For aircraft flying IFR as General Air Traffic (GAT), the latest dates for the carriage and operation of Mode S ELS airborne equipment in designated airspace are as follows:

• New production aircraft to be compliant by 31 March 2007.
• Completion of aircraft retrofits by 31 March 2007.

VFR Airborne Implementation
All aircraft flying VFR in designated airspace are required to carry and operate Mode S ELS airborne equipment by 31 March 2005 with the following Transition Period:

• New production aircraft to be compliant by 31 March 2005, although there is now a general relaxation until 31 March 2008.
• Completion of retrofits, irrespective of date of first CoA issue, by 31 March 2008

This practically means that any aircraft with an old mode A/C transponder will not be allowed in European airspace after 31 March 2008.

Mode S Enhanced Surveillance (EHS) Implementation Time Scales

The requirements of Mode S EHS apply to IFR flights as GAT by fixed wing aircraft (having a maximum take-off mass greater than 5,700 kg or a maximum cruising true airspeed in excess of 250kt) in the designated airspace notified by:

• Germany and the United Kingdom with effect from 31 March 2005, and
• France with effect from 31 March 2007

A transition period of 2 years will be applied until 30 March 2007, during which a coordinated exemption policy will be applied through the EUROCONTROL Mode S Exemption Coordination Cell.

Flight plans and Mode S
Instructions for filling out a flight plan for an aircraft equipped with a mode S transponder can be read in this pdf file.

Authoritive text by: EUROCONTROL Transponder Mode S website