Mike,

The FLARM concept has been painfully obvious, from a technology point
of view, since the introduction of low-cost GPS. In fact, it could
even have been partially implemented with LORAN, but those receivers
were expensive and were never widely deployed.

Unfortunately, FLARM-type collision avoidance is only going to work if
it's deployed to virtually all aircraft, which would require the
authorities to insist on it. This won't happen: ADS-B is the chosen
approach.

It seems to me that TIS-B is most likely what will first begin to
provide us with the functionality we need - and actually get deployed.
TIS-B (Traffic Information Service - Broadcast) is a portion of
ADS-B, essentially a broadcast of the radar returns seen by ATC. At
the moment it has limited coverage in the US but you can receive it -
however you need to spend about $15,000 on avionics including a Mode S
transponder plus a display unit designed for bigger panels than we
have. Still, no doubt it will soon occur to some entrepreneur that a
TIS-B receiver without all the Mode S baggage, designed for display on
a PDA might well find a market (lots of Cezzna drivers think $15k is a
lot of money too).