Using an algorithm, Skylight matches AIS points with vessel detections to identify and display vessel metadata information for any vessel that can be matched or ‘correlated’ with AIS. If there are no recent AIS points near the detection, it is considered a “Dark Vessel” detection and no vessel information is displayed. This is simply another way to say the detection is not a known vessel.

A vessel detection may be dark for a number of reasons, including normal AIS gaps, a vessel that only transmits VMS or otherwise not required to transmit AIS, and those vessels that have disabled AIS.

To correlate vessel detections with known vessels, Skylight searches for AIS messages within 1500 meters of the detection location and matches the detection to the AIS message that is closest in time and space.

Why would a detection be labeled ‘Dark’ and then later be correlated?

A detection may appear as a dark detection, but the AIS tracks or other available vessel tracking information reveals a likely candidate for that detection that Skylight has not identified. The same detection may then be correlated after more time has passed.

There are two primary reasons this happens:

• More AIS data is received after Skylight already checked for AIS tracks in the vicinity of the detection.
• Skylight uses track “segments” rather than individual AIS positions to look for AIS vessels in the vicinity of a detection. Because of the way “segments” are generated, sometimes it takes longer for a track segment to appear near the detection.

If Skylight learns that a detection previously marked as “dark” can be correlated due to one of the above reasons, it will try to update the status of that correlation from “dark” to “correlated”. Skylight checks for updated AIS data in the vicinity of dark vessels every 3 hours for the first 24 hours, and then once a day for the next 9 days.

Because Skylight is focused on displaying information as near real time as possible, it processes the satellite image to identify vessels as soon as possible, even if it means that some of those dark detections may be possible to correlate with AIS over time. For this reason, we always recommend checking dark detections in Skylight with data you have in other tools (e.g. AIS data from other tools, VMS, radar) when possible.