Cortex Long Range Satellite Position Reports

Automatic Identification System (AIS) uses Very High Frequency (VHF) radio signals to broadcast vessel information including position, speed, course, and other relevant data to nearby vessels and shore stations. AIS can also transmit Long Range Position Reports (LRPR) to satellites, which then relay the information to ground stations.

The LRPR function is available on Class A AIS transponders, which are required for larger vessels, and is an optional feature on Class B AIS transponders. To send LRPR, the AIS transponder calculates its position using GPS or another Global Navigation Satellite System (GNSS) and transmits the data in a special message format designed for satellite communication.

The LRPR message (also known as AIS Message 27) includes the vessel's position, speed, course, and other relevant information, is transmitted at different frequencies (VHF 75 & 76), and are of shorter duration (17ms) than regular AIS messages to ensure that it can be received by satellites. Once the message is received by a satellite, it is relayed to a ground station where it can be processed and made available to authorized users, such as vessel operators, port authorities, and maritime authorities.

The LRPR function allows vessels to transmit their position information over long distances, even when they are out of range of Terrestrial AIS receivers. This can be especially useful for vessels operating in remote areas, where there may be limited or no AIS coverage, or for vessels that are crossing oceanic regions where there are no Terrestrial AIS receivers.

Consideration for Message 27 transmission suppression

Vessels transmit Message 27 once every 3 minutes. One transmission lasts only 17 ms and impact on and possible interference with adjacent channels is thus quite low. However, if there are many vessels transmitting Message 27 in the same area, interference may be observed. Because long-range AIS channels 75 and 76 are located on both sides of the distress and calling channel 16, interference should be avoided. In an area where vessels' AIS reports can be received via Terrestrial AIS base stations, the suppression of Message 27 transmissions should be considered.