Correct me if I err, but for the life of me, I'm not quite sure how communications with or tracking of a "small GPS" unit can be maintained unless the unit is exposed and able to "lock on to" one (actually more than one) of the NAVSATS.

Then there's the additional problem....Unlike cellphones, pure GPS sets don't provide two way communication with the sats. They simply read and display the sat data on their internally-stored map or other display.