@ Pascal
Who said anything about "roving" or mobile GPS stations?
The stations are fixed on the ground - on AT&T mobile phone *towers*. Last time I checked, AT&T's cell towers were not "roving".

Mobile GPS stations to track planes in flight. That means two people moving randomly with one trying to track the other.
Doesn't sound like a good solution to me. In my forest orientation courses I was always told that if I got lost, I should stay put until help arrived. It would seem that it's easier to find someone if that someone is not moving.
I would have thought that such a reasoning would also be valid in reverse : it should be easier to track planes from a fixed location.
Like air traffic control towers, for example.
I have no issue with GPS tracking. Planes broadcast their position via radio anyway, so it makes sense to triangulate their position and progress from a GPS station. But I would prefer the GPS station to be immobile, in a known location.
A roving GPS locator is already subject to the built-in uncertainty of the US GPS system. Using that to track a plane that is also being GPS-tracked kind of compounds the error margin.
That doesn't sound too good for managing collision situations.

Most aircraft have been navigating with GPS for a decade. GPS ATC will not fix delays. The cause of delays is not in the air, it is on the ground.

We have eight lane freeways in the sky trying to exit onto a single lane exit ramp at rush hour - that ramp is our nation's airports.

With enough air traffic controllers, we can currently put more into the sky TODAY than can get down on time.

Until we improve or make better use of existing infrastructure - there is really no point to GPS ATC, or the so called "NexGen" ATC system. Especially at the bargain price of 1.8B.

So 1.8 billion is keeping costs low...oh wait, this is dollars.

Still, what would the cost be without AT&T's involvement?

so... will each base station be equipped with a UPS (uninterruptible power supply) - or are they placed so redundantly that you always pick up a signal at 5 stations anyway?
As we have seen, parts the US are relatively prone to power grid outages. If these cause the system to go down, it would be a scary thought to have all planes in that sector suddenly disappear from the air traffic control screens. In that case, would pilots just have to sort out their near-collision issues by themselves while under way? "Hey, are you going up or am I?" - "Your port side or mine?"