The acronym “AIS” stands for Automatic Identification System. We at psicompany.com are principally involved in marine based AIS systems.
AIS was created to give vessel operators the ability to gather and distribute electronic navigational data information to other mariners, including those requiring navigational data from those who may be out of visual range and those who may have geographic obstructions in their navigational path.
Picture in your mind; a modern shipboard radar or an electronic chart display or chartplotter that includes a symbol capacity for every significant ship within VHF radio range, each as desired with a velocity vector (indicating heading and speed). Each ship symbol could reflect the actual size of the ship, with position to linked to GPS or differential GPS accuracy. By pointing and clicking on a ship symbol, you could acquire the ship name, speed and course, classification, call sign, MMSI, registration number, and much more. Historical ship’s plotting information, closest point of approach, time to closest point of approach and additional navigation information, more accurate and 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 can now be available to every AIS - equipped ship as we will discuss. With this navigational data, you could hail any vessel over marine VHF by specific name, rather than by ship off my starboard bow or some other imprecise means. Or you could access that vessel directly using GMDSS equipment. Also, you could receive from the vessel, or send to the vessel, short safety-related text messages.
The AIS is a shipboard broadcast system that acts like a smart transponder. It operates in the VHF maritime band, and has capacity of conducting 4,500 navigation reports per minute with navigational update capacity to every two seconds. It utilizes Self-Organizing Time Division Multiple Access (SOTDMA) modulation to meet a high broadcast rate potential, ensuring reliable ship-to-ship, or ship-to-shore operations.
Types of AIS:
There is a wide variety of pricing and feature differentiation when it comes to Marine AIS, as AIS falls into three distinct categories or types: “Class A” AIS “Class B” AIS and lastly receive only AIS
Also, AIS is available with either a built in display system or it can come designed to display it’s information on chartplotter, marine radar, or similar marine network display device.
Each “Class A” AIS system consists of two VHF TDMA receivers , a VHF transmitter, a VHF DSC receiver, and antenna system, and standard marine electronic communications connections to shipboard display and sensor systems. Ship’s Position and system timing information is sourced from a GPS receiver or a differential DGPS receiver for precision positioning in inland and coastal waters. Additional data broadcast by the AIS system, when available, is obtained from shipboard marine electronics through marine data link connections. Course and speed over ground and Heading information would be provided by all “Class A”IS-equipped ships. Other information, such as rate of turn, pitch and roll, angle of heel, and destination and Estimated Time of Arrival may also be included.
The AIS transponder capability works in continuous or autonomous mode, regardless of whether it is operating in the open seas or coastal and even inland areas. Transmissions use packet modulation over 25 or 12.5 kHz channels. To avoid radio interference problems, each AIS station transmits and receives over two radio channels. This allows radio channels to be shifted without communications loss from other vessels. The AIS design architecture provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations.
If the AIS network gets overloaded with AIS navigation data, the AIS system begins excluding data from its farthest reception range. So for instance, if you were operating in a major metropolitan shipping harbor, you may only receive AIS data pertinent to operating in that harbor. As the AIS data transmissions lessen, then the geographic scope of reception and transmission increases automatically.
From a practical radio propagation standpoint, the AIS system coverage range is similar to marine VHF radio applications, essentially depending on the height of the antenna. Its propagation is slightly better than that of marine radar. It is possible for AIS to gather data in visually obstructed areas and behind islands if the land masses don’t obstruct radio signals. A typical value to be expected at sea is nominally 20 nautical miles. With the help of radio repeater stations, the coverage for both ship and VTS stations can be extended over several hundred miles. 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 ultimately intended to replace existing DSC-based query systems.