It's not currently certified in the USA, but pay attention to this
thing called FLARM.
http://www.flarm.com/index_en.html
This device works from (FLARM equipped) aircraft to aircraft, draws
very little current (55ma), gives you rough relative direction, height
and distance of aircraft with conflicting course to you. It doesn't go
bonkers in a gaggle. And it's a backup GPS datalogger.
Sorry, it won't wash your wings and it isn't a satellite telephone,
but it's still a very good instrument.
I would be happy if FLARM was mandated on all aircraft, as opposed to
Mode C or even S and TCAS.
Perhaps, with sensible changes to laws regarding liability, this
excellent product will become available in the USA.
Jim