8,400 Dark Events in 45 Days. The Nav Status at Gap-Start Cuts the Actionable Queue by 14%.

The Setup

AIS darkness is not homogenous. A vessel that stops broadcasting while moored at a terminal — power shut down, crew ashore — produces a short, explainable gap. A vessel that stops broadcasting while making 12 knots through open water is a different problem. The field that separates these two populations costs nothing to query: nav_status at the moment the AIS transmission stopped.

Over 45 days, 8,412 dark events were recorded across tracked vessels. Fourteen percent — 1,178 events — began when the vessel's last transmitted nav status was "moored" or "at anchor." Eighty-six percent — 7,234 events — began from a vessel that was underway.

The Chain

The duration split follows directly. Moored-origin dark events resolve in a median of 2.1 hours. Underway-origin dark events resolve in a median of 19.3 hours. The mean for underway events is 31.7 hours, indicating a long tail: some gaps run for days before a vessel reappears.

The risk tier distribution amplifies this. Among moored-origin dark events, 91% clear at tier-0 or tier-1 (no anomaly or monitor). Among underway events, 23% escalate to tier-2 or tier-3 — a rate that falls to 3% in the moored cohort. The tier-3 pool — events that combine duration, speed, and geographic context into the highest concern category — is 94% populated by underway-origin events.

This is not a surprising result. It is a straightforward one that most alert queues ignore. Dark events are typically sorted by duration, risk score, or geography. Nav status at gap-start is not a standard column in most screening workflows. The consequence: moored vessels burning off a two-hour power cycle sit in the same first-response stack as vessels that disappeared at sea.

The Implication

The triage math is simple. If a vessel went dark while moored, the prior probability of evasive behavior is low. If it went dark underway, that prior is substantially higher. Applying nav status as a pre-filter before duration or risk tier reduces the actionable queue by approximately 14% with minimal false-negative risk — the moored cohort produces almost no tier-3 events.

At current alert volumes, that filter removes roughly 1,165 low-signal events from analyst attention per 45-day cycle. The number scales linearly with fleet size. For operations running shift-based screening, that reduction translates directly to recovered capacity.

The second-order implication involves the subtype gap noted previously: only 3% of dark events carry a behavioral subtype classification. Nav status does not substitute for subtype — it is a coarser signal. But it is available now, on every row, without adjudication. It is a free pre-sort.

What to Watch

The nav_status field degrades at congested anchorages. Vessels reporting "moored" while technically anchored — a classification inconsistency common in certain flag states — inflate the moored-origin cohort modestly. Separating "moored" and "at anchor" as distinct states tightens the analysis further. Anchor events have a median duration of 3.8 hours versus 1.9 hours for berth-moored events, but both remain well below the underway baseline of 19.3 hours.

Three zones drive the highest underway-origin dark event concentrations: Singapore Strait transit corridor (27% of all underway-origin events), US Gulf approach corridors (18%), and the Hormuz exit zone (11%). These concentrations are consistent with prior coverage on chokepoint opacity; the nav-status lens does not change the geography, only the triage order within it.

Limitations

Nav status is self-reported and not validated. A vessel intending to evade detection can transmit "moored" while underway. This is not a theoretical risk — nav_status inconsistencies with speed-over-ground data appear in 4.1% of events. The nav_status pre-filter is a triage tool, not a detection method. Events that survive the filter still require duration and geographic scoring before a disposition decision.

Dark events data reflects the trailing 45-day window. Nav status is derived from the final AIS transmission before each gap. Duration is calculated gap-start to gap-end or vessel reacquisition, whichever is earlier.