Complementing AIS Transponders in River Navigation: Safety Aspects

摘要

AIS transponders emerged about 1993 as a Swedish improvement of weaknesses of IMO-approved radar-based DSC transponders of British origin. AIS evolved from the 4S transponder of the Swedish Navy for combined military air-land-water operations with a common transponding equipment for aircrafts, vessels and land vehicles. AIS transponders inherited the reduced channel capacity and the broadcasting of positions either of ships or aircrafts, as in the former DSC transponders. An own radio frequency, VHP Channel 72, was used originally, with approval of the ICAO, International Civil Aviation Organization. The inherent restriction of bandwidth in VHP channels (9.600 Bit/Second) was reduced by time division using world-wide GPS-synchronization (Time Division Multiple Access with Collision Detection and Self-Organizing Time Slot Assignment), similar to that of the original Aloha Radio Transmission Protocol adopted later by Ethernet. The time equivalence unit was shifted from seconds to minutes, substituting 30 time slots/second by 1.800/minute, however GPS operates with a standard time resolution of 1 second and not of 60 seconds. The advantages on coastal and sea operations of the 4S transponders were so convincing, as to promote the approval by the IMO as substitutes to DSC, with the new name Shipbome AIS Automatic Identification System in the marine frequency 156-174 MHz with 25 watt. This implies a considerable need for electric power, unsuitable for mobile equipment (pilot's luggage)