I am not to sure that the inbuilt error of each wing-tip GPS will be the same, unless they are synchronised together.
At the moment I have my GPS telling me where my car is (it's parked on the drive outside the house.) and it has given readings of 55,44,40,43,50... feet away.
So what sort of calculations can you do if the Starboard-Wing GPS cannot accurately measure where Port-Wing GPS is.?