CNN--->>
Officials speculate Atta may have been in New York on September 10 to make a final visit to the World Trade Center to program the towers' location into a global positioning system, the source said.
Yes, I have seen multiple confirmations that Atta and Jarrah purchased the GPS equipment, and also additional corroboration of Atta's trip to the observation deck of the WTC to get a fix.
Not mathematical proof, but pretty solid.
From a different angle, why would they not use the equipment? It is easily available, highly accurate, and it doesn't take a rocket scientist to use it.
I have heard that civilian/sportsman level units are deliberately designed to go inop when the velocity exceeds a preset threshold, to prevent their use in missile guidance systems but I have been unable to get a fix from inside the cabin, whether sitting still or in flight and can't confirm this.
In any event, I know for a fact that military GPS equipment is not disabled in this way, and suspect that units are available for civilian aeronautical navigation.