Your "went dark" alert is probably a coverage hole: sizing the missed-window rule for AIS

NO-DO — Sun, 15 Nov 2026 17:13:17 +0000

If you watch ships through a free AIS feed, sooner or later you'll want an alert for "this vessel stopped transmitting". The obvious rule is something like not received for 3 consecutive windows. We shipped roughly that, measured it, and it was wrong a lot of the time.

How much of the traffic does one window see?

Terrestrial AIS from community receivers is patchy even in busy water. We took 2-minute windows from a free stream and, for each window, looked at vessels we knew had been underway nearby during the previous hour. On 6 November 2026 a single window caught about 36% of them in the English Channel and 54% in the Strait of Gibraltar.

So "missing from the last window" is the normal state for a large share of ships.

What a fixed rule does with that

If a ship is received in any given window with probability p, and windows are roughly independent, the chance a perfectly ordinary ship is missed n windows in a row is:

P(false alarm) = (1 - p) ** n

With n = 3 and p = 0.36, that's 0.64³ ≈ 0.26. About one ordinary ship in four "goes dark" in the Channel.

Sizing n from the local receive rate

Pick the false-alarm rate you can live with (we use 1%) and solve for n:

import math

def min_missed_windows(p: float, false_alarm: float = 0.01, floor: int = 3) -> float:
    """Consecutive missed windows needed before flagging, given local receive rate p."""
    if p <= 0:
        return math.inf          # nothing is received here: never flag
    if p >= 1:
        return floor
    return max(floor, math.ceil(math.log(false_alarm) / math.log(1 - p)))

for p in (0.2, 0.36, 0.54, 0.8):
    n = min_missed_windows(p)
    print(f"p={p:.2f}  n={n}  fixed-3 false alarm={(1 - p) ** 3:.1%}")

p=0.20  n=21  fixed-3 false alarm=51.2%
p=0.36  n=11  fixed-3 false alarm=26.2%
p=0.54  n=6  fixed-3 false alarm=9.7%
p=0.80  n=3  fixed-3 false alarm=0.8%

The useful part is that p isn't a global constant. We estimate it per alert, near where the ship should be:

p = vessels received within 25 km in this window / vessels seen within 25 km in the last hour

Low p means you need to wait longer before saying anything; p = 0 means you have no basis for an alert at all.

The other three checks

The window count is only one of four conditions we require:

It was moving at its last fix: over 1 knot and not reporting "at anchor" or "moored" (nav status 1 or 5). Parked ships drop out for boring reasons.
We know where to look. Dead-reckon from the last fix (speed × elapsed time along course), capped at 6 hours. Past that the estimated position is fiction.
Others are being received there. At least 5 other vessels received within 25 km of the estimate in the current window. Otherwise it's a hole in coverage, not a quiet ship.
Enough missed windows, sized as above, never fewer than 3.

We apply it to class A transponders only; class B reception is too irregular for this to mean much.

What the alert can and can't say

Even with all four conditions, the honest wording is no AIS reception despite apparent coverage. A dead transponder, a power fault or a bad antenna look identical to a deliberate switch-off. The alert tells you where to spend analyst time next: a same-day Sentinel-2 pass if the sky is clear, Sentinel-1 radar if it isn't.

Where the formula is optimistic

Windows aren't fully independent: a receiver that goes offline takes out several in a row. That makes the fixed rule even worse than the formula says, and our n slightly optimistic.
The 25 km radius is a judgment call. Too small and you rarely have 5 neighbours; too large and you count ships a different receiver is hearing.

This is the logic behind the vessel watch in TRACEZ, the investigation tool we build at NO-DO. If you run something similar and picked different thresholds, we'd like to compare notes in the comments.