STS Transfer Detection

Detection mechanism

Ship-to-ship transfer (confirmed tier): two vessels within 500 meters of each other, both traveling under 0.5 knots, observed together across a span of 1 or more hours from the first sighting to the last, with no more than 4 hours between consecutive sightings. Verified when a paired draft change of at least 0.3 meters is also detected — one vessel's draft rising, the other's falling — within 1 hour before the encounter starts and 1 hour after it ends. A separate, stricter check aligned to Global Fishing Watch's published encounter definition (500 meters, under 2 knots, 2 or more hours, at least 10 kilometers from the nearest anchorage or port zone) is recorded as a pre-verification candidate, not a confirmed or verified event.