Hi Doc/ Tony / Ian,
If it was easy, we would have done it, believe me.
Yes, as Ian says, with an aircraft fitted with ADSB and Mode S, we get two sets of data, but Lee then filters out the Mode S using the 'hex' address, as the ADSB gives us exact positional data which are then presented to your nav system.
The problem arises with straight Mode S, which is used by a significant proportion of GA, as well as by a significant proportion of CAT instead of ADSB. Having no positional information (except altitude) we have to assess and present risk information based solely on the strength of the received signal. The problem is that signal is generated from a transponder the output of which can vary from about 70watts minimum at the antenna (e.g. Trig TT21 or similar) up to 500watts at the antenna for CAT. That's before we take into account significant variation between the fit of even the same transponder in individual aircraft, which can be considerable. The only reliable filter to differentiate between a close GA signal or a further out CAT one is the human eyeball I'm afraid.
Regards
Peter R
Mode S Development Tester