Frequently Asked Questions

A ship has the wrong destination, but it used to be correct!?
This is a feature of how AIS works. Consider a hypothetical tanker that has a regular run from Edinburgh to London. It sails from Edinburgh to London and is recorded by several ShipPlotters with a destination of London. She arrives in London and docks safely. Next day she is ready to leave, but navigating the Thames is tricky and the master is busy so he doesn't immediately update his AIS unit. There is a nearby ShipPlotter who gets a good signal and can easily track her, but he still records to old destination of “London”. Eventually the master has a little spare time and can update his AIS to show the new destination of “Edinburgh”. This update is soon picked up by the nearby ShipPlotter and Edinburgh appears on ShipAIS.com On the return journey, however, the master of the tanker chooses a route that takes him further offshore. The ShipPlotters along the coast can just about pick up the tanker but, because of the range, they only receive 1 in 3 of AIS broadcasts. This has relatively little effect on the “dynamic” data as this is broadcast every 10 seconds and so missing a few just means the position is updated every 30 seconds. The “static” information (which includes the destination) is broadcast every 6 minutes so missing them means that they may have to wait 18 minutes (on average) to receive it. Fortunately, much of the static information doesn't change often (eg length, name, IMO number) so ShipPlotter saves this information in its cache (shipinfo.txt).