Investigate and define how to represent default values when submitted as a parameter value

[bergtwvd]

Currently, for all optional parameters the default value when not transmitting the parameter value is clearly defined. Typically when a parameter value is not transmitted, the default value that the receiver must assume is "not available". E.g. for the vessel draught: when transmitted it is a value in meters, and when not transmitted the receiver must assume "not available".

However, there might be cases when a value is always provided by the simulator, but it is unclear how to represent "not available" in this case.

Investigate and define how to represent default values when submitted as a parameter value.

[bergtwvd]

In order to not break the handling of optional parameters (and their default value) as defined in the FOM, no changes have been done, except for the ETAtime parameter in message type 5. Message type 5 is a commonly use message type, and ETAtime as well as Destination may be not available in the simulation. For Destination there is already a value that can be used, but not for ETAtime. In this case a special meaning is defined and described in the semantics of the ETAtime parameter for value 0, namely not available. It is up to e.g. gateways to handle optional parameters, apply default values for these where needed, and map each parameter value to ITU-R M.1371-5.