MariTrace uses a combination of terrestrial (ground based) AIS receivers, and satellite receivers. The data from these two different types of receiver is merged. This is the data that appears on the MariTrace interface, or API.
Terrestrial receivers are great because they are comparatively cheap and can be placed strategically along shores or in ports. However, they do need to be fixed to a high point and they need both power and access to the internet so they can transfer the AIS data they receive to us. Terrestrial receivers can only get data from vessel for which they have a direct line-of-sight. This can cause problems with buildings or landscapes in the way and, eventually, the horizon.
The simple fact is that there is only so far that a terrestrial receiver can “see” and not every port in the world, or every part of the world’s shorelines and rivers, can practically be covered by terrestrial receivers. Accordingly, some areas are left without terrestrial AIS coverage.
A general rule-of-thumb is that even in the best-case scenario a single terrestrial AIS receiver’s range of how far it can “see” is about 40 nautical miles (74 km) so vessels out at sea, in rivers, or away from major populated areas are effectively lost to a terrestrial network.
To counter this, MariTrace also uses a network of coordinated satellites in low-earth-orbit to listen for any vessels that are away from the terrestrial network and send the data to us.
Different AIS providers will use different networks of terrestrial receivers, and different satellite receivers. Some will have no satellite receivers at all, and some will have no terrestrial receivers.
It can be thought of a little like the mobile telephone network. You may have a slightly poorer signal on your phone while your friend next to you has a great signal because they are on a different mobile phone network. However, if you move to a different location your signal may improve while your friend’s network is showing poorer coverage.
This does not mean that one network is better than the other overall. It is simply that at a given location, or even at a given time, one mobile telephone network might show an improvement over the other.
The same is true of AIS.
We know that there are cases when you will find a different AIS provider has a more recent position than us. However, the opposite is also true that we will often show a more recent vessel position than that provider.
Put simply; different AIS providers use different AIS networks, so different AIS networks will inevitably lead to different data.
Occasionally we will have a less recent position than another provider. Often, we will have a more recent position.