The paper presents joint operation of AIS-AtoN and radar. There was brought forward a fact of divergence of the information visualized in radar in cases when AIS receivers are produced by different manufacturers. It happens that the AtoN AIS information is not displayed by radar screen. It is also essential that not every ship is provided with AIS equipment

According to the International Maritime Organisation (IMO) recommendation, when the target data from the automatic identification system (AIS) and radar tracking are both available and the association criteria are fulfilled such that the AIS and radar information are considered for one physical target, then as a default condition of radar equipment, the AIS target symbol and the alphanumerical AIS target data, including information on the closest point of approach (CPA) and time needed to reach the CPA (TCPA) should be automatically selected and displayed as this is more accurate than radar tracking data as the current values of true and relative vectors of a manoeuvring object can be presented without time delay which is characteristic for radar tracking. Research conducted at sea on two ships using real navigational equipment has shown that sometimes onboard AIS presents CPA of other vessels equipped with this device as unstable and inaccurate as the automatic radar plotting aid (ARPA). The source of these instabilities and inaccuracies is to be discovered and thus implemented into radar-navigational simulators used for radar training at operational and management levels. This article briefly describes the encountered inaccuracy and instability of the values of the CPA of the encountered vessel presented by AIS, on current shipboard systems, and the possibilities of their demonstration on simulation devices.