it's not so much about knowing where the exact location of the plane is.....there are web sites that pretty much track the planes up to 100 miles.

When the GPS personal unit first started becoming popular, pilots themselves started buying them and playing with them in the cockpit. Well, low and behold, planes started getting off course becuase they were actually interfering with the navigational systems in the cockpit. THAT is the reason they aren't to be used on board, in flight.