Model

There are a number of different AIS messages that can be transmitted.

• Type 1, Type 2, Type 3 messages carry the current latitude, longitude, speed, heading, etc. of a vessel. These are generally transmitted more regularly as they provide the essential dynamic movement information.

• Type 5 messages carry the vessel name, IMO number, dimensions, type, destination, etc. These are transmitted less frequently as the information is less important and does not change very regularly.

Each message carries the MMSI number of the transmitting vessel. The MMSI number is unique to each vessel. However, over time the MMSI number assigned to a vessel can change due to the vessel changing country flags, for example. The old MMSI number can be reassigned to a completely different vessel. The model used in this version of HavenAIS is to match a Type 1, Type 2 or Type 3 message with the previously transmitted Type 5 message.

Every transmitted Type 5 message is recorded in the database. The same Type 5 message may be transmitted many times. Each Type 5 message is given a time of first transmission and time of last transmission. Throttling is used to reduce the size of the database by limiting the number of Type 1, Type 2 or Type 3 messages recorded in the database. There are two restrictions. Firstly the total number of messages per moving vessel per minute is limited. Secondly only a single message is recorded per minute for a stationary vessel.

Technical

• AIS signals received using a dual channel COMAR SLR 200 AIS receiver.

• Signals decoded using the Linux AIS decoder GRAIS.

• Database powered by MYSQL.

• Web pages powered by Apache and PHP.

Copyright

The base map tiles are provided courtesy of the OpenStreetMap project http://www.openstreetmap.org/ under the Creative Commons license http://creativecommons.org/licenses/by-sa/2.0/. The derived map is therefore subject to the same license.