It appears people take Mode-S for ADS-B, but it isn't. Mode-S is basically A/C plus an aircraft ID, 6 hex digits called ICOA ID, plus a flight ID. The later is commonly set to the callsign in GA (in most parts of Europe you get called by FIS if you enter bogus).
Do you need Mode-S in GA? Yes, you do. There are plenty of places around in european airspace where a transponder is mandatory. While this was unspecific in "the old days", under EASA regulations a transponder IS a Mode-S transponder, nothing else. So, if you travel with A/C only, by regulation you do not carry one. Currently, many or most places with transponder mandatory may grant you entering the airspace, but this is for courtesy and a special clearance (although often not called so) - they do not need to. What many people miss is the requirement for TMZ -> while you are free to enter with Mode-S equipped, you have to call for clearance if you carry Mode-A/C only! Even more, flying in transponder mandatory zones using only A/C transponder may be a violation and you may get fined. Take all the mandatory, equals Mode-S mandatory, airspaces - TMZ, CTR, FL>050 etceteras and you can count 3,2,1 and you are f###ed. I recently encountered several radio situations over Europe when entering a zone was not granted due to missing Mode-S and traffic situation. I do not know the regulations for IFR flight in the UK, but if the AIP states "IFR has to be equipped with transponder", as in other countries and in unified european airspaces, then subconsequently this also makes Mode-S mandatory for any IFR flight. So, if you have somebody progressing toward IR, she/he will immediately call for Mode-S (and may even cover a higher share of the equipment cost in a holders union, what is what we did some years ago - i.e. on top avionics checks to IFR only equipments was paid by the IR pilots, not all).
ADS-B protocol may settle on top of Mode-S with extendes squitter, 1090ES, technology and transmit additional information. There are more frequencies to ADS-B and other technologies, such as UAT in the US or 868 Flarm, but these are not all related to transponder technology. The most prominent additional data can be the GPS position of the aircraft, if and only if the onboard GPS unit is wired to the transponder.
Yes, Mode-S makes anybody listening to the signals aware of your identity, but this is only a quicker way, as your radar trace and a call to your departure field, or look in things as the German Reichshauptflugbuch, will reveal you callsign anyways.
Yes, you will get away without Mode-S for quite a while when flying VFR, but it will depend upon your destinations and FLs. There will be days you are unable to finish your flight as planned due to the missing Mode-S and you have to decide by yourself, when is the time right to put Mode-S in. My advice would be to wait until the first grumble in your stomach appears as a result of a cancelled flight, not earlier. If you failed to get a clearance for a CTR, or a TMZ, or being denied climb FL>050, or if you need service for the transponder broken or not working correctly, it is time for replacement. Until that day your flight profile will not cry for a new transponder. But, be aware if anybody flies to central Europe to check all transponder requirements explicitly on your path upon flight preparation.
Short, I doubt we "need" Mode-S in GA, but there's no way around.
Last edited by ChickenHouse; 12th May 2015 at 12:45.