How do ICBM early warning systems differentiate between a real threat and a launch of a normal rocket intended to go to space?
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Speed and trajectory. Orbital vehicles are going speeds and directions that absolutely scream, “I’m going into either a low-Earth orbit or a geosynchronous orbit.” Meanwhile, missiles heading for Detroit scream, based on the direction and speed they are going, “I’m headed for Detroit.” It’s super easy to predict where they are going, once they get going.
They can obviously have changes in speed and trajectory, but not big enough to go from one sort to the other. Agencies also communicate with each other. If they do anything even slightly sketchy that isn’t actually sketchy, they warn each other. Even North Korea give some warnings.
The early stages of a rocket launch is identical to an ICBM launch. It actually used to be the same rockets and a few are still dual use. But an ICBM shuts down its engines when it is still on a suborbital flight. This usually happens before ten minutes into the launch. An orbital launch however keep the engines on until it reaches an orbit. In some cases there is just seconds in difference. But this does indeed mean that you have to wait about ten minutes before you can be sure if it is an orbital launch or an ICBM launch. And by that time it might be too late to stop all warheads.
Another less reliable indicator is the heading of the rocket. Due to the speed you can not really change the direction of a rocket once it is fired so an ICBM have to be fired towards its target from the start. Orbital rockets can be launched into any number of orbits but they tend to be fired either straight east or straight north or south. So even before ten minutes you can get an idea of what the rocket is based on where it is heading.
It is also unlikely that an ICBM attack only include one launch. This would do limited damage and the rocket defenses would not need much advance notice to shoot the few warheads down. So a single rocket launch is likely not an ICBM but if they see more launches at the same time it is more likely to be an ICBM attack rather then an orbital launch.
But the most important indicator is that rocket launches are planned out well in advanced and is therefore published long before it happens. The people manning the early warning systems is therefore briefed about the launch and know what to expect. They still monitor the launch and look for any deviations but their assumption is that it is an orbital launch and not an ICBM.
In addition to other answers, people usually announce their space launches well in advance.
There was a case when Norwegians forgot to notify Russia about missile launch.
https://en.wikipedia.org/wiki/Norwegian_rocket_incident
And Russian president decided that 1 missile is probably not a missile strike, so called off the nuclear response.
There are teams of military personnel all across the world who track all the rocket launches using both ground-based radars and space-based radars 24/7. They have computer systems that calculate the trajectory of each and every rocket. They can pinpoint the location of the launch as well, so if a rocket was launched from some unknown location, it’s automatically a high threat regardless of the trajectory. If a rocket was launched at a commercial launch site, they still have eyes that are looking at the trajectory, to see if there needs to be an intervention or a warning to be relayed further.
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Multiple overlapping detection systems on both sides combined with a lot of historical and technical data. We know where most of their sites are, they know where most of our sites are. Any activity at those sites is closely monitored. That gives us indicators of what to look for. Anything below a certain threshold is sifted out through analysis.
Regular space launches are heavily scheduled events with a lot of activity and notice.
With ICBMs part of the strategy includes volume of fire.
While one ICBM could do terrible damage if it hits, a single ICBM would have almost zero chance of penetrating the defenses to reach its target. Enough ICBMs would have to be fired to overwhelm the targets defenses.
The systems act as a deterrent. In an actual attack the first thing would be to disable/bypass the early warning and defense systems.
The old joke is: If you can see it coming, you’re not the target.
through the study of its infrared signature , AI compares the launch with a library of known rocket/ missile signatures and can quite easily determine what the vehicle is fairly quickly.
Imperfectly…
That’s why the great powers take great care in announcing launches.
It just reduces the possibility that a launch is misinterpreted.
Even here, it’s a white-knuckle experience. Is it really a “friendly” launch?
Heres how the US does it, if they are not warned.
First is the infrared satellites that orbit the earth. They instantly detect the heat from a missile launch.
So a little blip appears in North Korea for example. Launch detected. But infrared isnt accurate enough to know anything other than some hot gases just got shot out of something.
Powerful high resolution radar stations called X Band are positioned around the world, especially aimed at countries that might attack the US. This radar will show you a high resolution of the missile as its launched.
They can very quickly work out its trajectory, based on the angle of launch and its speed. So they know what the missile is gonna do – and if its planning to come back down.
Then intelligence comes in – do we have any information that provides context to this launch? North Korean chatter about a missile test for 3 months? Okay maybe its that! But if there is a power struggle with hawks? Maybe an attack.
At this point the President is probably going to be directly looped in.
So then the question is – where did it launch from? How many? And what is it going to hit?
They will notice quite quickly that the missile is only going to land in the sea of Japan and so they don’t act.
Its 100% behaviour based to be honest. If the launch is detected from their land based launch pad where they do space stuff its probably a space rocket. If its coming from the nuclear silo, probably an attack.
If a missile is launched and is aimed at the United States or its allies, they will retaliate, and are unlikely to wait to see what kind of warhead it is.
The simple answer most people are overlooking here is talking. Allow me to demonstrate:
“Hey everybody, we’re putting a satellite in orbit tomorrow. Please don’t shoot it down, and don’t forget to smash that subscribe button on the web broadcast of the launch.”
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