Why would an event be removed from Skylight?
Out of Order AIS messages
One common reason an event is removed from Skylight is additional AIS data is received out of order and Skylight ‘learns’ the event did not actually meet the requirements. This happens most frequently for Potential Dark Activities.
In the greatly simplified example below, we can see how this might happen. There are four AIS position messages in the below diagram:
Message 1 was received at 10:15:00 and indicates the vessel’s location at 10:00
Message 2 was received at 11:19:00 and indicates the vessel’s position at 11:14
At 17:14 Skylight would generate a Potential Dark Activity since 6 hours had elapsed with no AIS update.
Message 3 arrives at 20:35:00 and notes the vessel’s location at 20:30. The Potential Dark Activity event ends and the duration is calculated to be 9h 16m.
Message 4 arrives at 23:45:00 with new information about the vessel’s location at 16:20. With this new, updated information Skylight learns that the vessel was not dark for 9h 16m as originally calculated. Instead, the vessel was dark for only 5h 6m which does not meet the threshold for a Potential Dark Activity. In this case, Skylight will remove the event.