Avoidance Route Calculation and Check for Obstacles

In this article it is shown how the avoidance ship route in a CAS (Collision Avoidance System) can be calculated in real time in case of a detected collision hazard. To determine the applicable COLREG rules, our earlier work is only briefly referenced here. The typical situation of the crossing of the ships is shown when one of the ships comes from the starboard side. The calculation of the passing point and the points of the avoidance route on the spheroid for this situation are explained. Since not all calculations are correct calculations on the spheroid after all, the approximations made are discussed and the resulting error is shown. A possible iterative procedure for improving the accuracy of approximations, if required, is mentioned. The calculations of avoidance route provide for each calculated point coordinates, time of arrival, as well as speed, heading and rate of turn. The article also shows how the developed system checks from electronic charts stored in database whether there are any obstacles within the calculated avoidance route. Two types of checks are explained. If the first, faster one shows an obstacle, this is checked with the second, which is computationally more complex. To increase the speed of such checks in the chart database, spatial indexing of the chart data is used. The basic principles of such indexing for geographic data are briefly explained here.