Determining the accuracy of ship position as a function of radar bearing

Maintaining maritime safety in areas in which vessel traffic is supervised is one of the main tasks of marine administration services. In these sea areas, coastal radar stations, which are part of the infrastructure of the system that supervises and controls vessel traffic, are used to monitor ships´ movements. When using radar to determine navigational parameters, one has no influence on the amount of energy that is reflected from a given object and returns to the radar antenna. The difference between the amount of energy that is reflected and absorbed by an object can be expressed by means of a factor reducing the quality of an observation that was made by using radar. This factor depends on a given object´s characteristic features such as the angle at which that object is inclined/positioned relative to an incoming radar wave (aspect), type of surface, the material the object is made of as well as its shape and size. Since all the coastal radar stations that are available in a given area make simultaneous measurements for one vessel, there is ambiguity in determining that vessel´s position fix, which is due to the fact that the vessel is “seen” by the radars at different angles. As for the process of combining data from multiple radar signals by systems that supervise vessel traffic, it is important that all of the radar stations determine a vessel´s position simultaneously with the least error. Given the analysed features of the objects that are being observed (ships), the reducing factor´s value depends on a given ship´s course and the bearing taken from a coastal radar station. In this paper, the authors attempt to show the influence of aspect on the accuracy of estimating a vessel´s position. Having analysed radar observations of vessels moving in an area that is covered by the VTS system, the authors present the relationships between the reducing factor´s value and a vessel´s course as well as its influence on the accuracy with which a ship´s position is determined by using the VTS system. Tests of the validity of the theoretical assumptions of this study were conducted in the waters of the Bay of Gdańsk. This bay is covered by a VTS system, which has been modernised and which now makes it possible to simultaneously use several radar stations that are evenly distributed around the body of water.