If I can see a ship as it passes a fixed harbor transponder then I can, on the ais engine, pinpoint exactly on the ship where the transponder is fitted. The accuracy appears better than the 10 metres or so I would expect from the gps system. On thinking about this I concluded that, as my gps and the target’s gps are getting their positions from the same satelites, the relative accuracy of the gps readings is more important than their absolute accuracy. The relative accuracy being considerably better. Since the ais system transmits position with a resolution of 1/10,000 of a minute (about 6 inches) what limits the relative accuracy I can expect?