AIS and radar can disagree because AIS position and motion come from the target's equipment while radar supplies own-ship measurements.
The Automatic Identification System transmits data directly from the target’s sensors, whereas radar provides measurements relative to the vessel itself. Since these two systems derive their information from different origins, the resulting positions may not coincide even when both devices are functioning correctly.
Furthermore, the built-in integrity test can report equipment malfunction but does not validate the accuracy of connected sensor data. Inaccurate connected sensors can transmit plausible data that the integrity test does not validate; routine checks and corroboration remain necessary.
AIS does not prevent collisions by itself and does not replace visual lookout, radar plotting, use of all available means or compliance with the COLREG.
