WHAT IS AIS TECHNOLOGY?
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
radio communications
protocol.
AIS based systems are used
around the world for reliable
vessel tracking and other
MDA applications.
THE PHYSICS OF AIS TRACKING RANGE
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
transceiver is
governed by its line
of sight between
the transmitting
and receiving
antenna.
SATELLITE AIS CHALLENGES
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 travelled from the transmitting vessel a distance of 600 miles to the satellite through the earth’s turbulent atmosphere – ambient radio noise from other radio systems and weather etc. This journey results in transmissions becoming corrupted and their strength significantly reduced. The second issue stems from the fact that all radio systems have a finite capacity which is defined by their frequency allocation and the size of each transmission. The AIS standards recognised this potential issue and as such uses a sophisticated TDMA radio access scheme which allows the available capacity at any given moment in time to be dynamically and automatically managed by the transceivers themselves and avoid two transceivers transmitting in the same space ‘slot’ at the same time. This TDMA protocol creates a fundamental issue for satellite AIS as it causes many transmissions to ‘collide’ before they reach the satellite. There are other subtle factors which also compound these issues.
The net result is that only a low percentage of AIS transmissions are firstly received and processed by AIS satellites. SAT-Trak technology works to overcome these issues and results in a significant increase in the number of transmissions which are successfully received and processed by an AIS satellite.
The SAT-Trak Advantage
SAT-Trak next generation technology optimises AIS transmissions from AIS transceivers for satellite reception, dramatically increasing the number of transmissions received by satellite and therefore more position reports per vessel.
The TDMA (Time Division Multiple Access) radio access scheme used by all certified AIS transceivers ensures high system reliability and integrity for ship to ship and ship to shore communications. The TDMA system shares the radio bandwidth by dynamically allocating ‘time slots’ to AIS transceivers within an area or ‘cell’. A typical AIS ‘cell’ is around 40 nautical miles in diameter. Satellite based AIS receivers have a huge 5000km diameter field of view, so from their perspective messages are received from many AIS ‘cells’ simultaneously resulting in message collisions with no decodable data. Additionally, the incidence of ambient radio noise can result in AIS messages being distorted and corrupted, further reducing the practical performance of AIS satellites.
SAT-TRAK technology dramatically increases the number of transmissions received by satellite
The reception of Class B units from space has been traditionally hindered because Class B transceivers transmit less frequently and with significantly lower power than Class A transceivers. As Class A populations increase within a satellite footprint, Class B transmissions become indistinguishable from background radio noise. By utilising multiple satellite data sources coupled with SRT technology, SAT-Trak allows terrestrial and satellite tracking of these small vessels without requiring expensive equipment and per‐bit message charges for the ship owner.
Class A
Class B
Identifier
AtoN
SAT-Trak technology is a major innovation in next generation satellite AIS which makes the detection of transmissions from AIS Class B and Identifier transceivers possible for the first time. This unique capability is fully supported by SRT AIS Class B and Identifier transceivers and utilises multiple satellite data sources to give superior S-AIS data coverage
SAT-Trak technology has enabled me to fit our fishing fleet with Identifier AIS transceivers and track them across our entire EEZ at an affordable cost.