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. The complete phase-out of older Mode A/C transponders is expected to take some time.
They also reply to interrogation by ACAS and TCAS systems onboard aircraft. Mode S types even exchange data with them so that aircraft equipped with these system know where you are and are able to avoid a collision.
Some manufacturers sell a special listening device (receiver) for aircraft transponders indicating signal strength and direction with altitude of the target. These are very helpful as they warn the pilot that traffic is detected within a certain set distance. You will still have to look and avoid the conflicting traffic as these systems will not issue commands.
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.
Both IFR and VFR implementations are differentiated here.
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:
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:
This practically means that any aircraft with an old mode A/C transponder will not be allowed in European airspace after 31 March 2008.
The requirements of Mode S EHS apply to IFR flights as GAT by fixed wing aircraft (having a maximum takeoff mass greater than 5,700 kg or a maximum cruising true airspeed in excess of 250kt) in the designated airspace notified by:
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.
Instructions for filling out a flight plan for an aircraft equipped with a mode S transponder can be read in this EuroControl Mode S ACAS programme.
Authoritative text by: EUROCONTROL Transponder Mode S website.
At the moment (2013) mode S has not been implemented completely in Europe and other parts of the World. Mode C is still used intensively in those areas.