Our algorithms did not manage to identify the destination of the vessel. The destination of a vessel transmitted via AIS is input by the vessel's crew. In many cases, the crew's input (abbreviations, mistakes etc.) is not in a format that the algorithm can interpret into something meaningful. Thus the NEXT_PORT_* fields remain empty.

The vessel's destination is not updated by the vessel's crew. Thus it is quite common that vessels leave towards a specific destination without having updated the AIS transmissions. This consequently leads to vessels reporting as destination the origin port or area. This is also considered invalid by our algorithms and the fields of NEXT_PORT_* remain empty.

The vessel's positions have not been updated for several hours. As this creates uncertainty regarding the vessel's destination, the algorithms do not update the NEXT_PORT_* fields which are consequently NULL.