John Kulp wrote:

What makes you think that GPS could decrease the needed separation?


Because that is exactly what it is designed to do?

Ah, no. GPS was not designed for that nor can it provide that. Most in
trail separation today is based on wake turbulence. Even if you got rid
of wake turbulence you still can't get less than 2.5-3 miles for jets
because that's how long it takes to land, slow down and exit the runway.
If it's dry. And that spacing doesn't allow departures to get out
between the arrivals. So you go to five miles and if everything works
out perfect that's barely enough room to get the jet departures out.
The plain simple fact of the matter is the limiting factor is lack of
runways. No amount of technology can force more airplanes onto the
runways we have now.