Could even see it through clouds using radar.
They had to use a fission reactor onboard those satellites to power the radar array
It consisted of 2 satellite types. US-A (active) and US-P (passive). US-A used radar and US-P used passive antennas so they can "see" signals from ships.
https://en.wikipedia.org/wiki/US-A
Late Cold War AShM's (SS-N-19 Shipwrecks) could use those satellites directly to update the target location in-flight
https://www.space.com/space-exploration/launches-spacecraft/...
Were they actually stopped? The linked Reuters article only says:
"The ship now sits idle in international waters but inside Denmark's exclusive economic zone, closely watched by Danish military vessels."
which makes it sound like it stopped of its own accord. It's also uncertain what, if anything Denmark or Sweden could do if it decided to bail.
I think the broad, practical answer to that is 'almost certainly'. I'm pretty sure if warships hadn't shown up and asked to come over and have a chat, the ship wouldn't still be sitting there. There's also probably a very technical legal answer which may indicate they weren't "stopped" at all based on the precise wording actually communicated and relevant maritime law.
Interestingly, a long time ago I got to experience being on a freighter in the South China Sea and having some sizable Singapore navy warships pull up and ask to have a chat and take a look around. Warships pulling up at sea certainly has a way of commanding your undivided attention. It's like being pulled over by a cop, except the cop is heavily armed commandos in an Apache attack helicopter.
I suspect the only reason the navy hasn't boarded that ship yet is because the captain is stonewalling and the government would prefer to avoid breaking maritime law (especially with everyone paying attention). However, if the government determines there's sufficient value in searching the ship, they may just have their maritime lawyers come up with the best excuse or "exigent circumstances" pretense they can and board.
The ship I was on was flying a Liberian flag but Indonesian owned. Given that we were in international waters and these guys weren't even customs, police or port authority (who are the dudes who usually pull over freighters), the captain could have technically denied their request but he didn't even consider it. He later told me, basically, if you are not 'the droids they're looking for' then you let them the fuck on board. As a commercial hauler there's simply no upside to making trouble for the Navy, they have a lot of ways to ruin your day and you might someday need their help if you've got an emergency (or pirates chasing you, which is not impossible in those waters). The only freighter captains who insist on exercising their rights under maritime law when facing a naval warship typically have a political motivation or maybe are actually pirates, spies or smugglers (of weapons or people - warships don't give a fuck about that undeclared container of stereos). Naval warships (at least in that area) also don't usually care much about expired paperwork, lapsed passports or even outstanding warrants. They aren't law enforcement and a ship like that costs over a million dollars a day to keep at sea. They are there under national command authority for reasons that are way above a freighter captain's pay grade. The captain and crew are usually just contractors hired to drive the ship. Our captain had been at sea 15 years and that was the first time he'd ever had a naval warship pay any attention to him.
So that probably also helps with not wanting to just escape. Where it's now it can at least escape through international water all the way to Russia if it wants to.
To me it feels like it's stuck between a hard rock and a stone...
https://www.svt.se/nyheter/utrikes/kina-redo-att-samarbeta-o...
"except the cop is heavily armed commandos in an Apache attack helicopter."
Apaches don't deploy at sea. They are land based beasties. They don't carry commandos either. Pilot and CPG - two crew. Perhaps you meant a Blackhawk, if you are thinking about an American setup, which I think has a maritime variant or two. Brits will come at you with a Lynx or a Merlin.
You'll probably never see an Apache at sea. Their usual targets will sink without any assistance Anyway, I think you mean marines in a Blackhawk. Commando is also a bit British.
I'm aware that Apache helicopters don't carry troops. My analogy was a quick aside and should have been clearer that I wasn't suggesting Apache helicopters deploying troops at sea. I was drawing a parallel to a more familiar circumstance because some might assume that a warship confronting a freighter at sea is similar to a routine law enforcement encounter like a police officer pulling your car over in your home town - where a well-informed citizen might reasonably assert their rights if the officer exceeds their authority.
However, a naval warship isn't typically used for law enforcement like customs, harbor police or immigration. A warship confronting a freighter at sea isn't at all routine, it's extraordinary. It would be more akin to having your car pulled over in your home town by a squad of special forces backed up by attack helicopters and tanks. Any rational person would immediately realize this ain't a routine traffic stop by local cops, it's something very fucking serious. A reasonable person might conclude today may not be the day to quibble over legal technicalities and instead just cooperate, at least until they figure out what the hell is going on. That's certainly what the captain of the freighter I was on decided. And as I stared down all those guns and twitchy locked and loaded sailors on the open sea, I fully agreed with him - despite usually being quite concerned about due process when I'm going about my business in the U.S.
The point being that the captain of that Chinese freighter refusing to cooperate with those warships is not akin to a civil rights enthusiast merely asserting their rights during a routine traffic stop. It's either somebody with a cavalier disregard for their own near-term safety or a person under orders they cannot afford to refuse, possibly reinforced by phone calls from family members suddenly in custody back home. Assuming that captain was being paid to drag his ship's anchor in a certain location, I suspect he now realizes it wasn't nearly enough to become an overnight seagoing celebrity who never captains a ship again. Not to mention being detained, possibly charged and having his ship impounded. Most people who hire freighter captains prefer boring, predictable pros who ensure their owner's ships arrive quickly and safely, not detained by warships on the international news. If he's lucky he'll only lose his license and be blacklisted for life by insurers.
Just the same way that if I say that there was this dude helping me out with carrying a dead tree, and it was like it was a elephant had strolled by and decided to pick up a toot-pick. I am not saying that it was a literal elephant.
But perhaps it is somehow detectable.
Basically things that make it harder for bad actors to maintain plausible deniability. So far, a lot of this has been done with relatively low cost and low risk for the perpetrators. The fact this Chinese ship was chased down and stopped before getting too far away was actually pretty lucky. If we formalize new systems and policies to ensure the instant a cable is cut there are navy ships converging on suspect ships - that will shift the cost calculus of the bad guys. If every cable attack requires a special submarine deploying a demolition team underwater near the target, it gets a lot more expensive and if caught there's little deniability.
If these kinds of attacks continue then I expect we'll see some extra-judicial direct action consequences imposed. Like a ship that gets away later having a mysterious accident causing it to slowly sink in deep water (thank goodness another passing ship was nearby to rescue the crew!). While I don't like it, when bad actors figure out a way to repeatedly avoid consequences, sometimes a little rule bending is needed.