In Europe, Garrecht sells an ADS-B receiver that can also provide Classic
Flarm GPS data to a Mode S transponder for 680 Euros.

http://www.butterfly-store.de/en/TRX...eceiver,i4.htm

PowerFlarm already receives ADS-B and Mode C/S; so all that's needed is
filtering out the non-GPS data from the PF or other GPS feeds such as
advanced varios: Air Glide, CNV, LX.

The Europeans are waaaaay ahead on this.

If you peruse the Trig TT-2x Transponder Installation manual, you will
find configuration parameters for stuff such as wingspan, fuselage length,
location on airframe etc.

It's all great for ensuring wingtips don't snag some other airframe on
adjacent taxiways and aprons at major airports. 3m accuracy or better gets
critical in this case.

Once airborne 50m accuracy from the little guys would be just fine for the
heavy stuff that doesn't want to come within half a mile of any of us.

It's a real surprise to see the FAA's head firmly stuck in the sand while
the Europeans are producing solutions that will work for GA.