<P> An AIS transceiver normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas . AIS transceivers use two different frequencies, VHF maritime channels 87B (161.975 MHz) and 88B (162.025 MHz), and use 9.6 kbit / s Gaussian minimum shift keying (GMSK) modulation over 25 kHz channels using the High - level Data Link Control (HDLC) packet protocol . 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 without communications loss from other ships . The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations . </P> <P> In order to ensure that the VHF transmissions of different transceivers do not occur at the same time, the signals are time multiplexed using a technology called Self - Organized Time Division Multiple Access (SOTDMA). The design of this technology is patented, and whether this patent has been waived for use by SOLAS vessels is a matter of debate between the manufacturers of AIS systems and the patent holder, Håkan Lans . Moreover, the United States Patent and Trademark Office (USPTO) canceled all claims in the original patent on March 30, 2010 . </P> <P> In order to make the most efficient use of the bandwidth available, vessels that are anchored or moving slowly transmit less frequently than those that are moving faster or are maneuvering . The update rate ranges from 3 minutes for anchored or moored vessels, to 2 seconds for fast moving or maneuvering vessels, the latter being similar to that of conventional marine radar . </P> <P> Each AIS station determines its own transmission schedule (slot), based upon data link traffic history and an awareness of probable future actions by other stations . A position report from one station fits into one of 2,250 time slots established every 60 seconds on each frequency . AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions . Slot selection by an AIS station is randomized within a defined interval and tagged with a random timeout of between 4 and 8 minutes . When a station changes its slot assignment, it announces both the new location and the timeout for that location . In this way new stations, including those stations which suddenly come within radio range close to other vessels, will always be received by those vessels . </P>

Marine traffic global ship tracking intelligence ais marine traffic