In open water, ships sail under different courses and speeds, each following its schedule to arrive to its destination. A ship detected by visual lookouts or by radar at a range of 6 miles or less could have any course and is considered a threat to collision. Ships must make decisive quick changes in course and/or speed to avoid collision if such threat exists. Ships take time to change its course and speed since its advance in the water is a complex outcome of her heading, speed, weight, wind and current direction and speed. Decision how to pass or overcome a detected ship must be taken early in time before the detected ship is less than one mile away.
Once a contact ship is detected by visual lookouts or radar, the officer of the watch must analyze the threat of collision with this contact. He must determine the detected ship's course, speed and relative motion. He must decide whether this ship is on a collision course with his ship or not based on how close this ship would pass his ship and expected changes in course and speed. If action must be taken to avoid collision, he should determine who will maneuver according to the rules of the road to avoid collision.
Just imagine what should be done if more than one ship is detected. Automatic Identification system (AIS) was introduced to provide solutions to such complex scenarios. AIS provides a shipboard radar display, with overlaid electronic chart data, that includes a mark for every ship within radio range. Information pertaining to all detected ship is displayed as required. The size of the mark reflects the size of the ship. By clicking on any ship's mark, you could learn about ship's name, call sign, and classification. You could determine best maneuver to avoid collision with the detected ship instantaneously, using the displayed information such as course, speed, closest point of approach (CPA), time to CPA (TCPA) and position as latitude and longitude.
You could call upon any ship within the very high frequency (VHF) range using its call sign or name using VHF communication or Global Maritime Distress and Safety System (GMDSS). "Display information previously available only to modern vessel traffic service operations centers could now be available to every AIS-equipped ship" (USCG 2005).
Prior to the introduction of AIS, other navigation aid systems were used. The following aids to navigation would be discussed and compared to AIS: buoys, racons, radars, and Automatic Radar Plotting Aid (ARPA). The AIS system will next be described and explained.
A buoy is a floating device which is kept in some specific place in the water to convey certain information to ships passing by. It usually marks a danger to navigation or an administrative area to allow boats ship to navigate safely.
Size, shape, color and numbering are distinctive of each buoy's function and identity. Buoys that carry lights are also used at night time. Buoys carrying sound signals are used in conditions of reduced visibility such as fog (Maloney 1981). Valuable information is obtained from buoys when they are identified.
A buoyage system contains a number of different types of buoys. Each type is designed to meet the requirements of different conditions. A can buoy has the shape of tin can when seen from a distance. A nun buoy has the shape of a cone with a rounded tip. A lighted buoy is a short skeleton tower with a light at the top connected to