Ask the expert

 In Articles

We address a particular question that has arisen in a recent investigation, this time it is a collision between two vessels that was reconstructed using AIS data on our Rapid Replay platform.

Can it be assumed that position accuracy reported by AIS for two vessels involved in a collision at a particular location will be similar?

The short answer is no.

There are several factors affecting the accuracy of GNSS (Global Navigation Satellite System) positions reported by AIS. Some of these will be common to the two vessels, some will not.

When positions are transmitted by an AIS unit they are always provided with a high or low position accuracy indicator which is calculated, when available information is received form the GNSS unit, by the AIS unit. When no information is received from the GNSS unit default indication is transmitted based on the position information being corrected or uncorrected.

Whilst local characteristics will affect the GPS accuracy of both vessels, the possible error for each is independent of the other vessel.

To submit a question for one of our experts please email

For more information on the use and accuracy of AIS / VDR data please contact Duncan Campbell


Duncan Campbell

Start typing and press Enter to search