Currently requires a mode S tranny maybe, but surely a simple lightweight lowcost (OK that maybe unlikely) GPS transmitter/receiver/conflict detector could be developed. Given the tiny signals that GPS's receive, they presumably would not be required to belt out high power output signals unlike txp's, and accordingly, could work on batteries for non-elec aircraft.
There were three datalink technologies proposed for ADS-B.
1) Use the extended squitter of Mode S (i.e. its capability of sending data along with the response to interrogation)
2)
VDL Mode 4, a fairly complex system working on a couple of VHF frequencies, but allowing relatively low power systems to work. It was developed in Sweden and has been championed by Eurocontrol.
3)
UAT, a system developed by MITRE specifically for ADS-B and aimed at the GA user. It uses UHF, like the transponder. It's
under trial in Alaska for ADS-B.
The FAA
decided (also see
here) to use a combination of UAT and Mode S (called 1090ES) for ADS-B.
In Europe, since we'll almost all be required to carry Mode S transponders anyway, there's less of a pressing need for development of a low-cost low-power alternative link technology -- though of course there is a pressing need for development of a low-cost low-power Mode S transponder!