How well position fixes from GPS help in close waters?
There were six targets moving around the ownship in the radar picture on the left-handed side. All six targets were transmitting navigational information with their Automatic Identification System (AIS). Amongst the others, AIS of each target was sending positions fixed by their Global Positioning System (GPS).
The width of a lane in this section of the East Lamma Traffic Separation Scheme (TSS) was a bit more than 500 meters. Accordingly, the differences between a radar target and its AIS reported position in the image was ranging from 40 meters to 200 meters, some 0.2 to 1.1 cables.
The AIS position of each target and the overlaying chart were plotted on the radar display in reference to the position of ownship. Any error in the plot was compounded with those of the ownship and the target.
The bigger echoes on the left and the lower edges of the image were larger containerships anchored heading towards east. The smaller echo near the left was a smaller containership. Similar to ownship, these vessels have their superstructure and the navigation bridge located at the after part of the ship. Yet, their AIS seem to have their positions fixed towards the bow of the ship, well away from the navigation bridge.
There were two other fast moving objects running along opposite courses at the lower part of the image. These were high-speed crafts from the same company running a particular ferry service. Vessels were similar. Yet, their position fixes seem to have different errors even in close proximity.
The error of GPS fixes on different vessels could be very difference. In addition to those global errors, there were somethings local to each vessel affecting their GPS independently. In a shipboard environment, the accuracy of GPS may not be as good as expected. Even the use of differential corrections may not help in getting the accuracy down to tenth or even hundredth of meters. In summary, GPS in harbours should only be used with extreme cautions.