Universal Shipborne Automatic Identification System (AIS) Transponder

What is a Universal AIS?

Picture a shipboard radar display, with overlaid electronic chart data, that includes a mark for every ship of length 20m or greater (except, perhaps, commercial fishing vessels) within radio range, each as desired with a velocity vector (indicating speed and heading). Each ship " mark" could reflect the actual size of the ship, with position to GPS or differential GPS accuracy. By " clicking" on a ship mark, you could learn the ship name, classification, call sign, registration number, MMSI, and other vital information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation information, more accurate and more timely than information available from an automatic radar plotting aid, could also be available. Display information previously available only to modern vessel traffic service operations centers would now be available to every AIS-equipped ship. With this information, you could call any ship over VHF radiotelephone by name, rather than by " ship off my port bow" or some other imprecise means. Or you could dial it up directly using GMDSS equipment.

The Universal AIS is a shipboard broadcast transponder system, operating in the VHF maritime band, that is capable of sending such ship information as identification, position, heading, ship length, beam, type, and draught, hazardous cargo information, to ships and to shore. It is capable of handling well over 2, 000 reports per minute and updates as often as every two seconds. It uses Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure reliable ship-to-ship operation. The system is backwards compatible with digital selective calling systems, allowing shore-based GMDSS systems to inexpensively establish AIS operating channels and identify and track AIS-equipped vessels, and is intended to fully replace existing DSC-based transponder systems.

How Does it Work?

Each AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a standard marine electronic communications link (IEC 61162/NMEA 0183/2000) to shipboard display and sensor systems. Position and timing information is normally derived from an integral or external global navigation satellite system (e.g. GPS) receiver, including a medium frequency differential GNSS receiver for precise position in coastal and inland waters. Other information, if available, broadcast by the AIS is electronically obtained from shipboard equipment through standard marine data connections. Heading information and course and speed over ground would normally be provided by all AIS-equipped ships. Other information, such as rate of turn, angle of heel, pitch and roll, and destination and ETA could also be provided.

The AIS transponder normally works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or coastal or inland areas. Transmissions use 9.6 kb GMSK FM modulation over 25 or 12.5 kHz channels using HDLC packet protocols. 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.

Each station determines its own transmission schedule (slot), based upon data link traffic history and knowledge of future actions by other stations. A position report from one AIS station fits into one of 2250 time slots established every 60 seconds. 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 0 and 8 frames. When a station changes its slot assignment, it pre-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.

The required ship reporting capacity according to the IMO performance standard amounts to a minimum of 2000 time slots per minute, though the system provides 4500 time slots per minute. The SOTDMA broadcast mode allows the system to be overloaded by 400 to 500% and still provide nearly 100% throughput for ships closer than 8 to 10 NM to each other in a ship to ship mode. In the event of system overload, only further away targets will be subject to drop-out, in order to give preference to nearer targets that are a primary concern ship operators. In practice, the capacity of the system is unlimited allowing for a great number of ships to be accommodated at the same time.

The system coverage range is similar to other VHF applications; i.e. quasi-optically depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends and behind islands if the land masses are not too high. A typical value to be expected at sea is nominally 20 nautical miles. With the help of repeater stations, the coverage for both ship and VTS stations can be improved considerably.