Radar may detect a physical target independently of AIS status because the two systems build their target pictures from different sources. AIS receives information broadcast by participating stations, while radar target information is measured by own ship's radar.
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. When AIS is switched off or provides false data, radar continues to measure the physical presence of the contact but does not automatically establish the vessel's identity.
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.
Consequently, radar confirms a marine radar track exists but does not automatically establish identity. The absence of valid AIS data leaves the mariner with a radar track that lacks automatic identification, requiring further assessment to determine the true nature of the contact.
