It exists, is called ADS-B, is being implemented instead of completing the radar coverage above e g Russia and India and could have been in use already if "industry interests" (interests in other things than safety, of course) hadn't been delaying things. It's cheap, a lot cheaper than TCAS and operates using a TCP/IP-resembling protocol over the airwaves and DGPS.
Here is
a picture showing what it can look like when in use.
I've used it myself at my home PC, tracking test installations in aircraft and airport vehicles and it works great - to see a snowplow line during a winter storm at home in front of my computer with a cup of hot cocoa was a ballet-like experience.
I have also seen it on video when they tested it's ability to warn about and avoid runway incursions. When a vehicle entered the runway with an aircraft on final, the runway in the display flashed red and I think an aural alert was sounded. Would it have avoided the recent disaster in Linate and many other similar incidents? Undoubtedly.
As for tying up frequencies, it ties up only a few frequencies and they're outside of the normal airband. I don't know the specifics but the data transmission standard was laid down this spring at a conference in South America. If you want to look it up, it's called VDL mode 4.
Somehow, a lot of people still haven't heard about it. Amazing coincidence that it's getting very little publicity, don't you think?
Cheers,
/ft