A cluster sweeps a band of territory 2000km wide so, circling the planet every 90 minutes or so, it can revisit many areas several times a day. Moreover, unlike spy satellites fitted with optical cameras, rf satellites can see through clouds. Their receivers are not sensitive enough to detect standard mobile phones. But they can pick up satellite phones, walkie-talkies and all manner of radar. And, while vessels can and do illicitly disable their ais, switching off their communications gear and the radar they use for navigation and collision-avoidance is another matter entirely. “Even pirates don’t turn those things off”. Horizon also plans to compile a library of unique radar-pulse “fingerprints” of the world’s vessels, for the tiny differences in componentry that exist even between examples of the same make and model of equipment mean that signals can often be linked to a specific device. It will thus be able to determine not merely that a vessel of some sort is in a certain place, but which vessel it is, and where else it has been.

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