Postby jimh » Sun Nov 12, 2017 1:19 pm
On the general topic of reception of AIS signals by satellites in various earth orbits, it is unknown to me if the original design of the AIS system took any consideration of satellite reception, but it seems a reasonable assumption that, at some point, the incorporation of AIS MESSAGE 27 into the protocol must have been done with satellite reception in mind.
There are more problems with reception of AIS signals by a satellite than one might consider at first glance. The AIS signals were initially designed for a range of communication of perhaps 20 to 50-miles between ships. Satellites in Low Earth Orbit (LEO) typically have an altitude of more than 100-miles and up to 1,200-miles. A typical AIS LEO satellite might be in a orbit with a elevation of about 400-miles. As a result of this very high point of view, the satellite can see many more AIS transmitting ships than any terrestrial receiver. Since the coordination of transmission of AIS signals is in small groups of self-organized networks, the self-organized networks only cover ships that are within mutual receive-transmit communication. A satellite AIS receiver may receive signals from many ships, all participating in different self-organized networks, and these transmission are likely to overlap, using the same time slots for a transmission. The satellite receiver then is faced with trying to receive two or more signals at once, all in a single time slot.
SRT Marine, a leading manufacturer in AIS transceiver development and manufacturing, reported that a satellite AIS receiver may only successfully decode about 2-percent of the AIS transmissions it receives. The other 98-percent are unreadable due to interference from competing signals.
To overcome the poor receiver performance, several very complex methods have been proposed and perhaps one or more are already in use. One method to discriminate overlapping signals is to exploit the effects of Doppler Shift. Because the satellite is in relative motion to all the transmitters, the apparent frequency of the signal from a particular ship arriving at the satellite receiver will be shifted from the original frequency, based on its relative motion to the satellite's. The satellite receiver can re-tune its receiver slightly up or down in frequency to better receive a particular ship's transmissions.
Another method that can help a satellite separate AIS signals is use of a directional antenna. Electrically-steerable beam-forming antennas can be used to improve the signal strength from certain directions, allowing perhaps better reception of the AIS signals from ships in the antenna's main lobe while discriminating against interfering signals from other ships that arrive from a different heading,
Even more sophisticated methods have been proposed for improving the yield of successfully decoded AIS signals from overlapping transmissions. In one method, as soon as the first (usually strongest) signal is decoded successfully, the receiver then uses the decoded data to re-encode a new transmission, mimicking the original, and this regenerated signal is then injected into the original mix of radio-frequency signals being analyzed, but with a phase reversal. The stronger and now isolated signal is thus subtracted from the jumble of received signals, removing it from the mix. Now the next strongest signal can often be detected and decoded. If this is possible, then that second signal is then regenerated and subtracted from the incoming signals, perhaps yielding another layer of detectable and decodable signals.
Signals that are detected but when decoded produce an error may also be eventually successfully decoded by application of very complex mathematical analysis. With intensive computational processing, it may be possible to extract an error-free message.
And one manufacturer of AIS transceivers, SRT Marine, has devised their own trade-secret method of transmission that they say will significantly improved detection and decoding by satellite receivers, particularly by their own satellite receivers. Their method is not disclosed, but they claim a ten-fold improvement of successful message decoding, to 20-percent of all received messages up from 2-percent.