Last time we presented some of the challenges of the time spread between S-AIS reports and how they lead to a higher training level than is often perceived. This time let’s look at how your S-AIS display might be missing or hiding the most interesting ships.
As we have seen, AIS Satellites can take a great deal of time to accumulate a position report from every ship in a given region of the ocean. Another factor often missed by users is the question of how long a ship remains on the screen. A dot appears on the map representing a received AIS message from that ship. The next time a message is received from that ship, the old dot is erased and a dot at the new position is displayed. What happens when that ship is not heard from for a long time? All AIS displays will keep the dot on the screen for a certain period of time. Different software allows for different maximum times and whether or not the time is configurable. Some software that was designed for coastal AIS usage where messages can be received every few seconds, will erase a dot if the ship has not be seen in as little as one hour. Such systems are useless for S-AIS.
Software such as the highly regard GateHouse AIS Display (GAD) and OnlineAIS Display which we use at Maerospace for TimeCaster, are highly configurable. A ship can first be declared “missing” after a certain number of hours without a message; at which time it may be color coded to show its status. Later still, the ship can be declared “lost” if an AIS message is still not received, at which time the dot is removed from the screen altogether. All software packages will eventually erase a dot on the theory that enough time has passed that the ship must be far away. The default in many of these systems is 24 hours.
So, not only are the dots that are displayed on your AIS display based on a vast range of times, but those ships undetected for (typically) 24 hours, aren’t displayed at all.