Correlating Vessel Detections with AIS
- Karen Farley
- Namrata Kolla
How does Skylight determine the identity of a vessel detection?
When viewing vessel detections in Skylight, you will note that some detections are labeled “AIS Correlated Vessel Detection” while others are “Dark Vessel Detection”.
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 AIS points near the detection, it is considered a Dark Vessel Detection and no vessel information is displayed.
In the below illustration, 1 and 3 are vessel detections and 2 is an AIS point. The Skylight correlation algorithm has determined the AIS point 2 was likely transmitted by the vessel in detection 1. Detection 1 is labeled AIS Correlated Detection and the vessel data available from AIS is displayed.
There is no AIS ping near detection 3 so the Skylight algorithm labels that detection “Dark Vessel Detection” and no additional vessel data is displayed.
(Skylight is contractually prohibited from sharing raw AIS points. Therefore, we do not display AIS point transmissions as shown above within Skylight.)
The Skylight correlation process searches over a circle centered on each vessel detection for overlaps. All intersecting segments from every vessel’s search radius will be used to compute all predicted vessel positions. AIS pings from vessels further than 1500 meters from the location of a vessel detection are disregarded as possible matches.
Why would a detection be labeled ‘Dark’ and then later be correlated?
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 24 hours.
Because Skylight is focused on displaying information as near real time as possible, it processes the 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.