The Automatic Identification System (AIS) is a short-range coastal tracking system currently used on ships. It was developed to provide identification and positioning information to both ＿＿（51）＿＿ and shore stations. Ships of 300 gross tonnage or more sailing on international voyages, freighter ships of 500 gross tonnage or more sailing in local waters, and all passenger ships ＿＿（52）＿＿ size are required by the International Maritime Organization to carry AIS equipment. AIS transponders automatically broadcast information at regular ＿＿（53）＿＿ . Navigational status data is transmitted every 2 to 180 seconds, depending on a ship's activity. In addition, voyage related data is broadcast every 6 minutes. These signals are received by AIS transponders installed on other ships or by land-based systems. The AIS signals have a horizontal range of about 40 nautical miles, ＿＿（54）＿＿ that AIS traffic information is only available around coastal zones or in a ship-to-ship zone. AIS communication takes place using two VHF frequencies, 161.975 MHz and 162.025 MHz. Although only one radio channel is necessary, each station transmits and receives over two radio channels to avoid interference problems and to allow channels to be shifted among ships ＿＿（55）＿＿ communication loss.