In 2006, the CAA proposed to mandate Mode S transponders on all aircraft and gliders in all UK airspace. There was a huge response from the aviation community and the proposal did not proceed.
The one element which did proceed was to make Mode S the “means of compliance” in airspace where Mode A/C was previously mandatory. That is the London TMA and all airspace, including class G, above FL100. That was not an issue for most light aircraft as they do not normally have access to class A airspace and rarely fly above FL100. Gliders are exempt from all the transponder regulations so they were not affected by this at all.
That is background. Now here is some slightly technical stuff you can skip if you want but it would be helpful if you read it!
The CAA has said it needs to introduce Mode S to deal with 2 technical issues. First, because Mode A/C transponders respond to every radar interrogator within range they flood the 1090MHz band with responses causing interference. This is mainly a problem for commercial IFR traffic in congested areas such as holding stacks where responses can become garbled. Light aircraft have little effect on this. Secondly, the demand for Mode A codes for commercial IFR flights throughout Europe exceeds the number of codes available. In theory, mode S allows the single code A1000 to be used by all aircraft with their individual identities being obtained from the Flight ID code (FID) it transmits. However, in practice the FID cannot be used safely (airline pilots can’t be trusted to type the right number into their computer) so commercial aircraft still need Mode A codes to identify them and there are not enough to go round. So in technical terms, this is a commercial IFR issue but Mode S does not actually solve it.
Commercial aircraft carry Mode S EHS which sends out all sorts of data to controllers, for example it tells them what level the aircraft is climbing or descending to so altitude busts can be prevented and it allows a pictorial representation of stacks of aircraft in holds making for greater safety and efficiency. So mode S is a good thing for commercial operations and all airliners will soon be fitted.
Light aircraft can carry mode S ELS which sends out a limited amount of data. It sends your level (the code C), your unique aircraft address which allows the ATC computer system to track you and it sends the FID you have inserted, normally the aircraft registration. ATC screens display your position and altitude or flight level but not your FID; the label you may have seen on ATC screens comes only from a flight plan so you probably won’t have one. However, a controller can check your transponder FID (and your unique aircraft address) by putting a cursor over your radar return.
Any altitude encoding transponder will enable ATC to identify you if they need too and aircraft with TCAS (including military aircraft) will be alerted to your presence and are less likely to hit you. Thus, whilst transponders are a good thing, Mode S adds little for the VFR light aircraft going about its normal business.
Links:
What the LAA Thinks of the Consultation
Background & Technical Stuff
Why Mode S Transponders and Not A or A/C
Transition and Exemption Arrangements
Deadline 31st May 2008
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