Satellite AIS vs terrestrial AIS: where the coverage breaks down
Every watch desk runs into the same moment. A vessel is tracking fine, then it isn't. Someone asks whether that's a coverage hole in the AIS network or something else, and the honest answer is usually "something else." Worth walking through what terrestrial and satellite AIS each cover, because the difference matters for how much you trust a gap on the plot.
Terrestrial AIS: short range, full detail
Terrestrial AIS runs on VHF, picked up by coastal receivers. Line of sight caps it at roughly 40 to 74 nautical miles depending on antenna height and terrain, less in a busy harbor approach with a lot of metal in the way. Inside that range it's dense: Class A transponders reporting position, course, speed, and static data every few seconds. It's the layer a port state control officer or a VTS operator relies on close to shore, and it's reliable because the receiver is always listening.
The catch is obvious once you say it out loud: once a vessel clears the receiver's range, terrestrial AIS stops hearing it. The shore station simply loses the VHF signal, and the record goes quiet for a reason that has nothing to do with whether the vessel is still out there working.
Satellite AIS: wide reach, built-in blind spots
Satellite AIS (S-AIS) is the fix for that range problem, in theory. A low-earth-orbit satellite listens for the same VHF broadcast from altitude, so it can pick up a transponder hundreds of miles from any coastline. That's genuinely useful for an EEZ with no shore infrastructure out past the continental shelf.
But S-AIS has its own limits, and they're the ones that trip up analysts who treat it as a continuous feed. First, revisit time: a satellite passes over any given patch of ocean only a handful of times a day, leaving windows of several hours where nothing is overhead to catch a broadcast. Second, message collision: in a high-traffic lane or a dense fishing ground with dozens of vessels transmitting at once, the satellite receiver can't always separate the signals, and messages get dropped rather than logged. Neither of those is a malfunction. It's the physics of listening to a crowded VHF channel from low orbit.
So S-AIS coverage offshore looks solid on a map legend and runs patchy in practice, with the patchiness worst exactly where the fleet density is highest.
The gap that matters
Here's the part that gets missed: both of these systems, terrestrial and satellite, depend entirely on the vessel broadcasting in the first place. A transponder that's switched off, a unit that's been disabled, or a vessel that never carried one produces the identical result as a revisit gap or a collision dropout: nothing on the AIS record. From the watch desk's side, a dark fleet vessel and a satellite blind spot look exactly the same.
That's the real problem with treating AIS coverage as something to patch by buying a better feed. You can stack terrestrial and satellite AIS, cross-reference both providers, and still never distinguish "the satellite missed it" from "the vessel wanted to be missed." An analyst ends up pulling AIS logs by hand, lining up timestamps against known revisit windows and known dead zones, trying to work out which unmatched gaps are infrastructure and which are a transponder turned off on purpose. It's slow, and it still leaves the actual dark fleet question unanswered: is there a vessel there right now or isn't there.
The one check that doesn't rely on the transponder at all is a direct look at the water. A vessel sitting in a fishing ground shows up in an optical satellite image whether or not it's broadcasting anything. That's the logic behind Illegal Fishing Detection: run daily high-resolution optical passes over a patrol area, flag every vessel that's visible on the water, and cross-check each one against the AIS record for that position and time. What's left over after the match, a detection with no corresponding broadcast, is the starting point for a dark fleet call instead of a guess about which kind of gap you're looking at.
If your desk is still sorting satellite dropouts from actual dark vessels by hand, it's worth seeing what a daily unmatched-detection list looks like instead.