On a Class A AIS device, you typically enter only one set of ship dimensions, not both internal and external. Here's how this works in detail:
1. Dimensions Required for AIS Configuration
Class A AIS transceivers require the entry of the vessel's dimensions as part of the static data transmitted periodically to other vessels and coastal stations. These dimensions are:
- A = Distance from GPS antenna to the bow
- B = Distance from GPS antenna to the stern
- C = Distance from GPS antenna to the port side
- D = Distance from GPS antenna to the starboard side
These measurements define the position of the GNSS antenna relative to the vessel's hull and are used by receiving systems (like ECDIS or ARPA) to graphically represent the ship's size and orientation.
2. No Separate "Internal" and "External" Dimensions
There is no concept of "internal" and "external" dimensions in AIS configuration. The system uses one set of dimensions only, regardless of whether:
- The GNSS antenna is internal (built into the AIS) or external (connected from another source)
- The AIS is integrated into a larger ship system or operates independently
What matters is where the active GNSS antenna is located, and the vessel dimensions must be entered accordingly.
3. Practical Tip
If switching between internal and external GNSS sources (e.g., between a built-in and a bridge-top antenna), you should update the dimensions (A, B, C, D) to reflect the position of the currently used antenna relative to the ship's hull.
You only need to enter the internal dimensions.