AIS is a sophisticated VHF radio based maritime tracking and
data communications system. Multiple independent entities such as vessels, buoys and coast stations automatically transmit and receive messages to and from each other.
AIS is an international maritime
data
AIS is a sophisticated VHF radio based maritime tracking and
data communications system. Multiple independent entities such as vessels, buoys and coast stations automatically transmit and receive messages to and from each other.
AIS is an international maritime
data communications technology
system defined by a series of
technical standards published
by the ITU and IEC.
AIS technology is optimised
for real time terrestrial
operation using a
sophisticated TDMA based
AIS based systems are used
around the world for reliable
vessel tracking and other
As specifically defined by the international ITU AIS technology standard (ITU-R M.1371), AIS uses defined marine band VHF frequencies as the base radio communications system. As such the natural laws of radio physics apply and dictate that the maximum tracking range of AIS tracking is the line of sight distance between the transmitting antenna and the receiving antenna. The line of sight distance between the two is in turn dictated by the height of each due to the curvature of the earth – the higher the antennas the further the distance and thus the greater the tracking range. Typically ship to shore tracking range is up to 30 nautical miles, and a good terrestrial coast station will see AIS targets out to a distance of up to 60nm – or even up to 200nm subject to radio propagation effects.
All radio transmissions travel in a straight line the direction of which is dictated by the transmitting antenna. Standard AIS antennas are orientated to ensure transmissions can be detected by coast stations and other vessels. Due to the curvature of the earth an unobstructed AIS transmission will continue to travel into space, thus making satellite reception of that transmission theoretically possible. The height of a satellite and its ability to orbit the earth means that this theoretical reception capability converts into a theoretical ability to receive AIS transmissions anywhere globally and thus enable tracking beyond terrestrial range.
The reliable range
of any AIS
governed by its line
of sight between
As described above, in theory all AIS transmissions reach space and can thus be received by a satellite with an AIS receiver. However due to multiple real world and required AIS technology operating protocol features, the actual successful rate of reception and successful decoding of AIS transmissions by AIS satellites is poor.
The first challenge is that there needs to be a satellite passing over the area at the time of transmission. Most constellations have relatively limited numbers today and thus orbit in a parabolic fashion to give global coverage during the course of a day, but perhaps passes over a given area every one to five hours. As the number of satellites with AIS grow and a typical 45 minute earth orbit time this will improve.
However the primary challenge for all AIS satellite systems is their ability to firstly detect an AIS transmission which has travelle