Occurrence of Unknown Sensor Data within AIS Dynamic Messages

For more than a decade, the Automatic Identification System (AIS) has contributed to increasing the safety of navigation at sea. Despite the benefits of the system, AIS messages shared between vessels and the AIS dynamic data transferred to the Portable Pilot Units may contain unknown values of sensor data if the sensor data on board becomes either unavailable or undeliverable for any reason. In this paper, an experiment is conducted to analyse the performance of an AIS transponder during a virtual sea voyage. By altering the sensor data rate it is possible to cause the AIS transponder to output AIS messages with unknown sensor data. After performing the experiment, a generic approach is used in order to establish a correlation between the sensor data rate and the relative occurrence frequency of AIS unknown values. This leads to the formulation of a simple equation which describes the interdependence between the interval of sensor data provision, the age of the sensor data allowed by the AIS transponder and the percentage of unknown data within the AIS data output