I don't. The most accurate navigation system that doesn't require an off-platform signal is inertial (INS). INS units drift badly (relatively speaking), with

I don't. The most accurate navigation system that doesn't require an off-platform signal is inertial (INS). INS units drift badly (relatively speaking), with errors measured in tens or hundreds of meters after only an hour. I have personally seen data from a unit that was off by kilometers after a couple days without GPS correction.