Detection Rate
Detection rate is considered the most important characteristic of a satellite-AIS system. Without a very high-detection rate, the true value of S-AIS systems cannot be realized. Why is achieving a high rate such a challenge?...
Collisions
The AIS system was originally designed so that AIS messages would only be received by other AIS equipped ships or AIS equipped shore base stations within a distance of approximately 50 nautical miles. The AIS transmission protocol (a Time Division Multiplexing (TDM) protocol) was designed to prevent ships within nominal operating range (i.e. within a small “cell”) from transmitting simultaneously, thus avoiding garbled transmissions from overlapping messages within this approximate 50 nautical mile range. This is ideal for terrestrial AIS but poses additional problems for space borne receivers.
Figure 1: Field of view from an AIS satellite at 650 km altitude (purple) and 850 km altitude (red)
Due to the satellite’s large field of view from space, AIS transmissions from multiple cells may arrive at the satellite at the same time. These overlapping messages are difficult to separate into the individual messages. This is particularly true where ship traffic density is high.
When AIS message signals overlap or “collide” in this way, a “standard” AIS receiver of the type used terrestrially on ships and shore stations may not be able to extract individual messages. Some service providers have attempted to use receivers based on this terrestrial receiver technology in orbit with limited success.
The exactAIS™ system uses a special Advanced AIS receiver and sophisticated patented processing technology to “de-collide” the signals – thus extracting AIS messages from a much larger percentage of the ships in the field of view than the standard AIS receivers. The exactAIS™ system has been designed to provide maximum probability of ship detection by a single satellite pass in areas of high ship density.
Once the AIS signals recorded in orbit are downlinked to ground stations, they are routed the exactEarth Data Processing Centre in Canada. The AIS signals are then processed to extract AIS message data; the resulting messages are automatically filtered to select and sort messages for distribution to authorized customers according to the specific data attributes that have been purchased. The time to route the data to the processing centre and apply the algorithms typically takes less than half an hour.
