AIS, Radar and Nautical Charts: Different Sources of Information on Board

AIS, radar and nautical charts show different aspects of a ship’s surroundings. AIS transmits data broadcast by an equipped vessel; radar displays received echoes; a nautical chart shows charted coastlines, depths and waterways. The displays can complement one another, but they are not interchangeable representations of the same situation.

AIS: identity and transmitted data

The Automatic Identification System (AIS) transmits, among other things, a ship’s identification and movement data. On the receiving display, available reports can be associated with a target and tracked over time. This requires the ship to be equipped with AIS, the device to be switched on and data to be received. Not all vessels are required to carry AIS; even equipped ships may switch it off in certain circumstances.

Moreover, a report is not an independent measurement of every item of information displayed. Faulty or incorrectly configured position, speed and heading sensors can transmit inaccurate information. AIS data can therefore help identify and track a target, but cannot on their own confirm its actual position or that the way ahead is clear.

Radar: echoes from the surroundings

Radar transmits signals and processes their reflections from objects. The display provides echoes and their position relative to the ship; it depends, among other things, on the equipment, settings, operating conditions and the target’s reflectivity. An echo does not automatically carry a ship’s name, intentions or other identifying information. Conversely, an AIS target may appear without a corresponding radar echo that can be immediately identified.

The systems therefore answer different questions: AIS can provide identification and movement data reported by a received target. Radar can independently detect objects that are not transmitting an AIS report or whose report is not being received. In traffic separation schemes, AIS overlays can help identify targets in heavy traffic; however, the report does not replace assessment of radar echoes and the surrounding area.

Nautical chart: surveyed waters and reference

A nautical chart places a position in its geographical context. It contains charted depths, coastlines, waterways and other navigational features. Unlike AIS and radar, it is not a current observation of an individual moving target: an object that appears just now will not show up on the chart simply because it is in a charted area.

Chart information is based on surveys and must be kept up to date for a voyage and used in accordance with official notices. The chart helps put measured or reported ship positions in relation to the coast, depths and waterways; however, it does not automatically show every change in real time. As a result, the chart display, radar echo and AIS report may differ in detail, and no single image alone may explain why.

For navigation on board, these information sources should be considered together: AIS provides data transmitted by participating vessels, radar provides reflected signals from the surroundings, and the nautical chart provides the charted geographical framework. The International Maritime Organization warns that AIS may give an incomplete or inaccurate picture of traffic and must not be used as the sole source of information. Guidance from the UK Maritime and Coastguard Agency also stresses cross-checking electronic navigation information and the limitations of individual aids. None of the three displays can therefore replace a competent assessment of the overall situation.