Mode S-the future

After about 20 years of research, the Mode S system is becoming a reality. The airlines are installing the airborne equipment and plans are well in hand for the ground facilities. I expect Mode S to make a real contribution to the relief of airspace congestion. It is rather unconventional to start with the conclusions but these words actually concluded a presentation to the IEE by another author. It is not intended that this paper only looks back at the reasons for the slow adoption of Mode S but forwards to what Mode S can now offer. Secondary radar is a co-operative system where a ground interrogator transmits a coded interrogation to which a transponder in the aircraft transmits a reply. In current civil ATC systems the interrogation will request either an identity code or the aircraft altitude.