The Time Spread Problem with Satellite AIS.

Satellite AIS is one of the most dramatic changes in Maritime Domain Awareness since AIS first arrived over 10 years ago. For the first time, maritime authorities can look at a screen covering all of their EEZ or, indeed, the entire world and see where all the ships are… except that’s not really what they see.

There are several time-related factors that make Satellite AIS displays appear to be more than they really are. First, satellite data is delayed significantly from the time the signal is received by the satellite and when it appears on the user’s screen. After a satellite received the signal from a ship, it must store it on board the satellite until the satellite is within view of a compatible ground earth station. This can take from a few minutes to nearly 1 ½ hours. From there, the data collected (some time from an entire orbit of the planet) must be downlinked and then transmitted back to a central data center. This can take anywhere from a few minutes to 30 minutes or more. Finally, this data must (after waiting for available computer time) be processed into messages, timestamped and sent to end-users. All of this time is referred to as latency and, on average, takes about 2 hours for most systems.

A second important, but often ignored, factor concerns the Probability of Detection of a satellite. From orbit, an AIS satellite can “see” a circle of about 3,000 miles in diameter – an area of more than 7 millions square miles. Whenever there are more than about 1,000 ships anywhere in that field of view, the probability of detecting the vessel drops significantly. The very best and most expensive satellites can obtain about 85% detection of an area with about 5,000 ships in the field of view. Most satellites, regardless of the operator, detect closer to 15% on a single pass. This means that between 15% and 85% of ships will be missed on each pass. Additional satellite passes occur sometime later and pick up that percentage again. Statistically, after many passes, nearly all transmitting ships will be detected. Only after each detection does a dot appear on the user’s map display. The effect is that when a user looks at a screen and sees thousands of dots (or hundreds in a zoomed view), those dots represent the most recently reported and detected position of the ship. On average, the positions shown will be many hours old but with a huge spread of times than can range from a few minutes to several days. Each ship must be checked manually to see not only the position but also the time of that position and an estimate of its current position must be made.

When all these factors are considered, it is clear that satellite AIS users require training to understand what they are seeing on their screen. It is the operators in the field responsible for monitoring traffic, detecting anomalies and maintaining overall maritime domain awareness who must be able to interpret their screens properly. Without training, it is easy to look at a map filled with ship dots and assume that that is each one’s current position. Two ships that appear to be on a collision course or suspiciously close together are likely many miles away from each other if you check the time stamps.

Satellite AIS is a fabulous tool and represents a major advance in maritime security. The data can be misleading however, if you are not careful. Our TimeCaster Product was developed to address this particular problem is the only tool on the market than can provide a current location for every ship in the world.

Next time we will talk about Lost Ships on your AIS display.

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