If Web VTS is getting a vessel's ais messages regularly then a vessel symbol is a blue triangle.
Sometimes you could see grey triangles too as below.
Web VTS displays a grey triangle instead of a blue one when it meets two conditions:
- the vessel has stopped a transmission of ais messages for more than 10 minutes (Web VTS has not received any ais messages from the vessel for more than 10 minutes);
- the vessel's speed was less than 0.5 knots at the moment when it stopped ais transmission.
Displaying grey triangles allows monitoring of position of vessels even they have stopped ais transmission.
When the vessel starts to transmit ais messages then its symbol becomes a blue triangle.
Web VTS has settings of how long to keep grey triangles on the screen. The default settings are 24 hours.