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WHAT AIS DOES Because you need to know where a target is, rather than where it was, the usefulness of AIS data decays extremely rapidly with time. It must be updated and retransmitted every few seconds, at most. To accommodate this AIS uses a self-organizing time-division multiple access (STDMA) system that uses the precise timing data in the GPS signals to synchronize multiple data transmissions from many users on a single narrowband channel. It’s this STDMA technology that’s the key to AIS and the transponder at the heart of the shipboard AIS is sometimes referred to as the STDMA transponder. When received on each ship, the decoded data are displayed on the AIS receiver along with AIS reports from all other ships within range in graphic and text format. The information may be fed to the ship’s radar plotting systems and integrated navigation systems to “tag” radar targets with AIS signatures and can also be logged into the ship’s Voyage Data Recorder (VDR) for later examination. The system will also be handy for pilots as it will have the capabilities to transfer information to the pilot’s laptop computer, which, loaded with its own navigational software, will enable the monitoring of surrounding traffic without reliance on the ship’s system. Two AIS projects are currently underway in the U.S.: one on the Mississippi River at New Orleans (PAWSS) and one in the St. Mary’s River.
These stations will have the capability to provide a variety of services. They can monitor AIS transmissions as well as actively poll vessels requesting particular data, such as identification, destination, ETA and type of cargo. The AIS channels can also be used for coastal stations to broadcast tidal information as well as local weather forecasts and notices to mariners. AIS will also allow the monitoring of hazardous cargoes as well as commercial fishing, accident investigation and search and rescue operations. The hardware.
The three main components of Saab Transponder Tech’s R3 AIS vessel transponder are a GPS receiver, a VHF transceiver and a computer. The GPS provides accurate position and navigation data, the computer packages this with other data about the ship and from the ship’s sensors and broadcasts the data on a digital link via the VHF. Leica Geosystems (Leica Geo Systems, incidentally, has published a particularly useful guide, Marine GPS: The complete Guide to AIS)
The MX 421 is a type-approved high-precision GPS/DGPS “smart antenna” which was introduced by Leica in late 2000. It is the first marine GPS product to use a new Silicon Germanium (SiGe) receiver chip, developed jointly by Leica and IBM. In field trials, says Leica, the MX 421 has consistently achieved unprecedented positioning accuracies of under three meters RMS (root mean square) in an autonomous, non-differential mode. When used with the built-in dual-channel DGPS beacon receiver, the MX 421 yields accuracies of under one meter RMS. The MX 421 DGPS beacon receiver includes an H-field antenna for superior signal reception and interference rejection, and has been designed to meet all existing and projected IMO carriage requirements for GPS and DGPS. The MX 420 system’s Control and Display Unit (CDU) controls all GPS, DGPS and AIS functions. The display and operating procedures are similar to Leica’s MX 400 series GPS products. The CDU collects inputs from the ship’s gyrocompass, speed log and other sensors, organizes the data for transmission through the AIS transponder unit and collects and displays AIS data from all other stations. It also interfaces with the ship’s ECDIS, ARPA, VDR and other systems. The CDU keypad is used for entering static and voyage-related data for AIS reports, as well as system setup and all GPS/DGPS functions. The system provides extensive interfacing capacity, with eight user ports and additional dedicated ports for connection of the MX 421 smart antenna. There is even a separate dedicated port, which can be used for the ship’s pilot to plug a laptop computer into the AIS. |
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SHIP-TO-SHIP The main function of AIS is autonomous ship-to-ship reporting--each ship transmits its data to all other AIS-equipped ships within VHF range. All the above-mentioned data are fed from the ship’s sensors into the system where it is transmitted in “data bursts” on the dedicated VHF channel. (The frequencies are 161.975 MHz ( 87B) and 162.025 MHz (88B). Transmissions use 9.6 kb GMSK over 25 or 12.5 kHz channels according to International Electrotechnical Commission (IEC) document 61993-2).
COASTAL SURVEILLANCE: Shoreside automated installations will also be an important part of the system. The U.S. Coast Guard has been a proponent of AIS since its inception and has already taken steps towards implementation. 
INTEGRATED IN VTS:The Coast Guard is also planning to integrate AIS into its existing Vessel Traffic Systems (VTS). An initial project on the St. Mary’s River is part of a VTS/AIS modernization that the agency expects to have operational by this September. The Coast Guard will begin requiring AIS carriage in all its VTS areas as early as July 2002.
Many of them incorporate the AIS transponder developed by Sweden’s 
The new MX 420 system incorporates a high-accuracy MX 421 GPS/DGPS receiver and the IMO-compliant AIS transponder system, with a combined control and display unit. Optionally, the MX 420 AIS can be interfaced with an already installed GPS or DGPS receiver on board instead of the MX 421.