I still think we're approaching the problem with the wrong technology. What's wrong with the gps based FLARM system? see ...
http://http://lbs.gpsworld.com/gpslbs/article/articleDetail.jsp?id=308235 It appears very practical, cheap in comparison and light. Power consumption is low and it is self contained. I'm guessing that because the system works aircraft to aircraft and not aircraft to ground, that the CAA ect have nothing to gain. It's the sort of system that could actually work if we all supported it but just like TCAS, will fail if only a few aircraft use it. It does though seem more likely to be practical than transponder based systems, or am I missing something obvious?
SS