“No, I based my comment on the mid-air collision risk being related to aircraft actually in the air. Do you have any info on hours flown by the Micro/LAA/Glider fleet?”
On a poor day, mid week, you are right, most aircraft will have transponders. On a good Saturday in June I would guess the balance would be at least 3 non to 1 with. If hours at my strip v local licensed airfield are typical then LAA / Micros would be flying more than C of A machines. The exception would be the 1% IFR guys who would manage a lot of hours but are very few in number.
If we want a GA/GA anti collision system right now we can have;
A Transponder with a LARS service (IF you can fit it £1600 - £5000 to fit)
Most of the UK is a LARS free zone at the weekends, which is when most GA flying takes place. A Transponder on its own has no GA/GA collision avoidance benefit, as PCAS equipment is not at all common. If we could get everyone a RIS at the W/E this would be an interesting option. Many aircraft cannot fit a Transponder for legal or technical reasons, a fact now accepted by the CAA.
A Transponder and a PCAS (IF you can fit it £2600 - £6000)
This will detect others with transponders. I would think that in the area around say Biggin, you would have a high % of transponding traffic, in the location I fly I consider 80% non transponder equipped at the weekend, which is when I do most of my 90 hours a year. Many aircraft cannot fit a Transponder for legal or technical reasons, a fact now accepted by the CAA
A FLARM system (£500)
(average value of a micro / LAA aircraft is around £10,000)
This would require GA to carry a portable battery powered unit, which can be used in all GA aircraft. No certification issues as it is totally portable.
Now lets look at what will actually happen in practice. Nothing at all! The CAA are not going to revisit the transponder issue as after many years it has realised it is not possible to get what it wanted. The “heavy” end of GA has invested in Mode S and will not accept a simple low cost solution. The “light” end of GA can only go FLARM, and will not do that unless there is a realistic chance of it reaching critical mass. Better brush up on you see and avoid, it is the only game in town for the time being
Rod1