The Concept of Automatic Identification System - Essay Example

Automatic Identification System (AIS) Introduction On land, a four way intersection is considered a possible threat to collision for crossing vehicles. Traffic lights and stop signs are used to guide and control motion of vehicles to avoid accidents. At the sea, there are no roads or stop lights or traffic lights. Buoys are used to partially highlight passageways for ships especially in front of harbors and heavy traffic areas. Rules of the road were stated for ships to follow to clearly decide on the right of passage and avoid confusion to ships as to avoid collision at sea. 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. Buoyage System 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 batteries to operate the light at night. A bell buoy is a skeleton tower with a bell on top with several clappers attached or electrically operated horns. Figure 1: A nun buoy1 Figure 2: A can buoy2 Identification of buoys follows the guidelines of the International Association of Lighthouse Authorities (IALA). Two global systems are defined: IALA A and IALA B. IALA A covers Europe, Africa, Australia and most of Asia, while IALA B covers North and South America, Japan and Philippines. In IALA A, starboard buoys are green while port buoys are red for ships entering ports. In IALA B starboard buoys are red while port buoys are green for ships entering ports. Figure 3: IALA (A)3 Figure 4: IALA (B)3 The problem with buoys is their possibility of drifting away from their moored position due to conditions such as inclement weather. Buoys require high levels of maintenance due to their exposure to rough sea conditions in addition to their requirements for sea based maintenance. Radar Becon (Racon) A racon is a radar transponder used as a navigation aid to mariners. Racons are used to identify navigational landmarks such as buoys, turning points, offshore oil platforms, and hazards to navigation. The racon's distinctive mark is displayed on the ship's radar display at the position of the buoy or lighthouse carrying this racon (Bowditch 1977). Racons and their identifying marks are indicated on marine charts. Racons measure the frequency of every incoming radar pulse, and transmit a Morse code response less than 700 nanoseconds later back to the radar on that frequency. The Morse code response is encoded such that the length of one dash is equal to that of three dots, and the length of one dot equals that of one space. Figure 5: A racon showing a morse code Q4 The racon response is delayed to cause the displayed morse code to appear behind the echo from the the structure on which the racon is mounted. The morse character begins with a dash for identification as shown in figure 5. To conserve battery power, racons installed on buoys are programmed to operate 50% of the time. These racons are normally active for 20 seconds, and then off for the next 20 seconds. Racons installed on shore are programmed to operate 75% of the time. Racons are usually not programmed with a duty cycle greater than 75% to ensure that the response never completely masks an important radar target (USCG 2005). Racon range is line-of-sight range, which is over 19 nautical miles with actual range depending on mounting height, humidity levels, and racon receiver sensitivity setting. The anti-clutter rain control on a radar need to be turned off as not to mask a racon return. The detection range of a racon may also be reduced if the radar receiver is off-tuned. Tweaking the radar tuning control should correct that problem. Ramarks are radar beacons which transmit independently, without having to be triggered by a ship radar. A ramark response on a radar display gives no indication of distance, but instead extends from the ship's position to the circumference of the display. Automatic Radar Plotting Aide (ARPA) Technological advances in microelectronics have lead to the introduction of ARPA to shipboard radars. ARPA capable radars use microprocessors to process video data received by the radar. ARPA radars track and predict any contact's motion once detected by the radar. ARPA radars accept inputs from ship's navigation equipment such as gyro compass, speed log and GPS time and position information. ARPA radars are capable of automatic acquisition of detected ship contacts. The system calculates the course, speed, closest point of approach (CPA) distance and time of any detected contact. Figure 6: A Typical Shipboard ARPA System5 IMO has set its standards concerning usage of ARPA in its amendments to the SOLAS international convention as follows: "in order to improve the standard of collision avoidance at sea: Reduce the workload of observers by enabling them to automatically obtain information so that they can perform as well with multiple targets as they can by manually plotting a single target". The main advantage of ARPA is its automatic processing and evaluations of detected contacts. ARPA quickly provide mariners with expected relative motion of all detected ships to enable mariners make critical decisions in multi-contact situations. ARPA has decreased the work-load of watch standers in the bridge and enabled marines make better decision to avoid collision at the sea. The picture compiled by ARPA radars is still incomplete. Not all ships are plotted and tracked due to reduced radar sensitivity and number of possible contacts to be tracked. ARPA radars required human intervention and it is not completely automated. Automatic Identification System: The creation and legislation of AIS went through the following major stones. In 1997, the World Radio Conference (WRC), the international forum that allocates frequency spectrum, designated two frequencies to be used by AIS known as AIS 1 and AIS 2, VHF Ch. 87B and Ch. 88B. In 1998, the International Maritime Organization (IMO), Marine Safety Committee, at its 69th session set the standards for what they call "Universal Automatic Identification System". This system was intended to provide ships and competent authorities with information to accurately track ships nearby. The system was to be completely automated with minimum ship's personnel intervention. In 2000, IMO amended Chapter the Safety of Life at Sea Convention (SOLAS) to mandate the use of AIS on some domestic and most international ships. SOLAS is considered a major overhaul of navigation requirements on ships. SOLAS mandates carriage of AIS on board all ships of 300 gross tons and greater engaged in international voyages, cargo ships of 500 gross tons and greater not engaged in international voyages, in addition to all tankers and passenger ships (Arroyo 2003). IM's SOLAS Convention stated that the purpose of the AIS: "AIS shall provide automatically to appropriately e quipped shore stations, other ships and aircraft information, including ship's identity, type, position, course, speed, navigational status and other safety-related information; receive automatically such information from similarly fitted ships; monitor and track ships; and exchange data with shore-based facilities" (IALA 2002). AIS is intended to reduce risks of collision by permitting real time exchange of navigation information between ships and between ships and land based Vessel Tracking Services (VTS) centers. It is a shipboard transponder system that operates in the VHF maritime band to broadcast ship's report. It is capable of handling over 4,500 reports per minute and updates as often as every two seconds. It usesthe network protocol of Self-Organizing Time Division Multiple Access (SOTDMA) to ensure reliable ship-to-ship operation. SOTDMA is cell phone technology operated on the VHF marine band. An AIS system consists of a Central Processing Unit (CPU), three receivers and one transmitter radios, a GPS with its associated antennas to provide timing and position information, cable, BITE (Built In Integrity Tester) and a Minimal Keyboard Display (MKD). AIS also accepts inputs from on-board ship sensors, such as Global Navigation Satellite System, Gyro compasses, Rate of Turn indicator and an optionally speed, pitch, heel and roll sensors. AIS transmit and receive in two different radio frequencies to avoid interference problems. The system automatically connects itself to other ships with priority to closer ships than further away ones. Every minute is divided into 4500 time slot. Each ship picks randomly a time slot that is not taken by any other ship in its radio range. When a station requires changing its time slot it advertises its future slot. Ships wanting to join would pick randomly a time slot that is not taken by any other ship in its radio range. Figure 7: Allocation of Time Slots by ships6 Every two seconds an underway ship AIS would broadcast time, its unique identity number, course, speed, rate of turn and position. Every eight minutes, AIS would broadcast its radio call sign, name of ship, type of ship, dimensions, estimated time of arrival to destination. The future of maritime awareness is highlighted in Figure 8. An overall picture of ships' motion is compiled by integration of knowledge collected from different agencies interconnected through a coordinated simple and real time communication network. Maritime security and safety would be much advanced by use of such intelligence and surveillance of all vessels, cargo, and people that operate in the maritime domain. Figure 8: Maritime Domain Awareness7 Conclusion Traditionally, ships have relied upon visual lookouts, radars and cameras, for surveillance and tracking of navigational aids and other ships. Mariners have to be continuously alert for newly detected ships and navigation aids as to determine best action to take to avoid collision with other ships. In a multi contact situation, the decision to be taken must consider the relative motion of all detected contacts. Names of detected ships in addition to their course, speed and relative motion should be determined as to coordinate a solution to avoid collision. Modern advances in computer and communication technology have provided new revolutionary solutions to problems at sea. AIS utilizes advanced communication technology to create a Time-Division Network between ships at sea and Vessel Traffic Service Centers on land. Ships relay their basic information onto the network for identification and clarification of intended course of action. Voice communication using ships' name and GMDSS is also now possible by the information provided by AIS. Safety at sea and collision-avoidance of ships at sea has been greatly secured by the introduction and implementation of the newly created AIS system. References Arroyo, J., DuPont D. (2003). Expansion of AIS Carriage Requirements. Office of Vessel Traffic Management. New Orleans, LA Bowditch, N. (1977). American Practical Navigation. Defense Mapping Agency Hydrographic Center. IALA (2002). Guidelines on the Universal Automatic Identification System (AIS), Volume I, Part II, obtained from www.iala-aism.org Maloney, E. (1981). Dutton's Navigation and Piloting. Naval Institute Press. Annapolis, Maryland. United States Coast Guard (USCG). (2005). What is Automatic Identification System (AIS). Retrieved March 2007 from http://www.navcen.uscg.gov/enav/ais/AIS_Advisory.htm United States Coast Guard (USCG). (2005). Radar Becons. Retrieved March 2007 from http://www.navcen.uscg.gov/enav/racon.htm Read More