In this study, a guidance and an early warning method has been developed for navigation by using GPS data. Two dimensional navigation has been considered and the Istanbul Strait has been specifically studied as an experimental model. Despite modern navigation devices, there are still some problems for navigating of vessels in narrow waterways because of geographical structures and various disturbances. Even though all vessels have modern navigation devices, use of autopilot is not allowed in the Strait. Vessels have to be manually controlled according to rules. A human operator (helmsman) controls a vessel as a continuous system manually according to captain's command against random disturbances through strait of Istanbul and their behaviour affects the course and coordinates of the vessel. An algorithm has been developed to predict a vessel's position using artificial neural network utilizing GPS/radar data.