AIS is a relatively new technology (circa 2000 onwards) for which long-term records are infrequently kept due to the amount of physical disc space needed to store the transmitted messages.
Whilst AIS information provides an accurate representation of the received data, it is what ‘is not’ received that provides the greatest limitation. AIS-A provides characterisation of commercial shipping (AIS-A) but misses the bulk of non-AIS vessels, including:
- Commercial Vessels below 300 GT
- Recreational Vessels
- Fishing Vessels
- Military/Government vessels whilst on deployment.
It is considered likely that a 40 nautical miles reception radius for AIS-A will be achieved by an AIS network subject to various factors such as the power of the transmitted signal, the height of the transmitting ship’s aerial and meteorological conditions. Poor reception could be as little as 20 nautical miles, or as much as 350 nautical miles for powerful transmissions during appropriate atmospheric conditions.
Further limitations of AIS data relate to the quality of the received records, where potential sources of error exist within the data. For example, AIS transponders may be switched on or off during a ship’s passage or be defective, thereby not capturing the full transit. In addition, errors with the positioning system can provide inaccurate locations. Voyage data is largely user entered, and therefore has inherent limitations due to operator error or misrepresentation of information. Transmitted information such as Maritime Mobile Service Identity (MMSI) (vessel identification) numbers, vessel type or dimensions can also be incorrectly entered, thereby providing a degree of uncertainty.
AIS-B is a non-mandatory form of AIS typically used by small commercial craft, fishing vessels and recreational vessels. To prevent overloading of the available bandwidth, transmission power is restricted to 2 Watts, giving a range of up to 10 nautical miles. Information regarding use patterns by these types of craft from AIS sources alone will therefore significantly underplay the true frequency and use patterns.
