AIS supplies identity and target-supplied motion data from participating stations, whereas marine radar measures relative motion by the own ship. Integrated displays may associate these separate inputs when position and motion criteria are met. The two systems therefore build their target pictures from different sources. An AIS target position comes from the target's electronic position-fixing receiver and may not coincide exactly with the target detected by radar.
Because AIS positions originate from the target's electronic receiver, they may not coincide exactly with the radar-detected target. This divergence means a vessel or object can appear on radar without AIS if it does not carry or operate the system, while AIS can sometimes show a target located in a radar shadow area.
