jack-oh:
Sounds interesting. Trying to get my head round the idea but a few questions pop up (if you pardon the pun):
1. IFRs may be flying in VMC or IMC, but VFRs can often be flying in what, for them, is VMC, but would be IMC for something bigger/faster (e.g. <140kts below 3000 feet: clear of cloud & in sight of surface).
2. If you're only giving traffic info to your IFRs on the VFRs, what rules would you have for IFRs passing close to VFRs? You're not providing separation, so how close could you take them?
3. This is a clearly a mandatory SSR environment. You'd have to not only say "you can't fly IFR if your squawk's bust" you'd actually have to say NO-ONE flies if their transponder's u/s because otherwise you lose all the benefits and have to go back to 5nm avoidance of all unknowns since you'd have no idea what height they were at. I guess the proponents of mandatory Mode S will say that's why it has to be for everyone because the benefits reduce sharply if only the IFRs are carrying it.
4. What do you do about autonmous VFRs who don't know their transponder's u/s and aren't talking to anyone? What about if it's only their Mode C u/s? Happens a lot in my experience with current equipment. Will Mode S be any better?
NS