Neither system is universally more reliable because each builds its target picture from different sources and has distinct limitations. AIS receives information broadcast by participating stations, while radar target information is measured by own ship's radar; the two systems therefore build their target pictures from different sources.
A vessel or object may appear on radar without AIS because not all targets carry or operate AIS, while AIS can sometimes show a target in a radar shadow area. An AIS target position comes from the target's electronic position-fixing receiver and may not coincide exactly with the target detected by radar.
AIS can add identity and target-supplied motion information, while radar plotting supplies own-ship measurements of relative target motion; integrated displays may associate the two when position and motion criteria are met. MCA and IMO guidance require AIS to supplement systematic visual and radar observation and warn against giving AIS priority for collision-avoidance decisions.
