Frequently Asked Questions

I know there's a particular ship out there, why do you not show it?
Don't trust what you read on the internet! I try my best to make the data as accurate as possible, but there are plenty of sources of error.
AIS is required for ships over 300GT that travel in international waters, so if they never leave the river then they do not need to carry AIS. This applies to the Mersey Ferries, some dredgers, some tugs and all ships under 300GT. For instance, Mersey Mammoth usually operates in the river without AIS, but turns it on if she needs to travel further afield.
Even if a ship is carrying AIS, it does not guarantee she will appear on this website. AIS data is a public radio broadcast, much like BBC Radio 4 is. If nothing is broadcast, the wrong information is broadcast or if interference or bugs corrupt the data, there is little I can do except to wait and hope that the correct data are broadcast again soon. I only receive the AIS data and cannot ask for corrected information to be rebroadcast.
I do have checks that are able to catch some errors or correct some missing information, so the data that appears on this site is not always the same as the broadcast AIS. However, the wrong information is sometimes entered into the on-board AIS units (eg wrong ETA or length in feet instead of meters) and the Maritime and Coastguard Agency do have a scheme to correct such errors (Marine Information Note MIN 231 (M+F)).