We have already explained about the issue interference and the ability to transmit your callsign, the FID. We have taken advice from a technical expert about interference and found some interesting stuff. First, all the radars in the UK do or will operate in Dual Mode. Essentially this is one sweep of Mode S interrogation then one of Mode A/C, done for safety reasons. Mode S is good for interference as once the radar has established your position and track, it commands your transponder not to reply unless asked. However, because of the dual mode, Mode A/C interrogations are sent out and all transponders reply to that.
You may be getting the drift here: if all 15,000 affected aircraft and gliders were to fit Mode S they would all respond to Mode A/C and the electronic environment would collapse. So actually getting all GA to fit Mode S would be a very bad thing until such time as Mode A is switched off. Please bear this in mind for later. As Mode A/C will continue to work for some years and the information provided to the controller is very similar to Mode S, perhaps it would be sensible to allow Mode A/C to be fully used until such time as the system is switched off. There would be no increase in interference and we could transition to Mode S over time, unlike the proposal that we all fit Mode S now.
Finally in this section, an advertisement from the airline pilot community. We would like to discourage you from using Mode A without Mode C. Imagine you are flying an airliner near a UK airport. Your TCAS system detects all transponders nearby but screens out anything that is away from your level. Anything you see on the NAV display could be a threat. But if a transponder does not have Mode C, TCAS cannot screen it out so displays it as a possible threat. This is a real issue when you fly a non Mode C transponder under a TMA so if you are fitting a transponder, please consider a Mode C or S and if you have Mode A only, please consider fitting an altitude encoder.
Links:
What the LAA Thinks of the Consultation
Why Mode S Transponders and Not A or A/C
Transition and Exemption Arrangements
Deadline 31st May 2008