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Comment by javajosh | Hacker News Reader

>The first answer is ICBMs are, now a days, mostly an IR optical thing.

Source? This seems unlikely since ground-based visual tracking would be obscured by e.g. clouds. Is ICBM tracking all space-based and visual? (I can see why IR is useful to detect the initial boost, but it would also seem rather useless to track a small, passive object in freefall).

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DSP and more recently SBIRS are satellite-based systems, both use IR. If you track the missile to the end of it's burn you'll have a pretty good idea of where it's going after that -- ballistic trajectories are fairly predictable.

https://en.wikipedia.org/wiki/Defense_Support_Program

https://www.lockheedmartin.com/content/dam/lockheed-martin/s...

> small, passive object in freefall

That's probably anomalously hot as it plows back through the atmosphere.

I think radar stealth coatings are sufficiently advanced that they swung the balance back in favor of IR for tracking purposes.

Historically: it was hard / unknown how to make a flying object that resisted reflecting radar

Current: it's hard / unknown how to make a flying object that's resistant to radiating heat

>as it plows back through the atmosphere.

I would assume that if you're tracking it on re-entry that's a little too late.

Depends on what you want to do.

Launch - early warning, max time, low certainty of trajectory

Boost - trajectory and impact refinement

Re-entry - terminal maneuvering, impact prediction, terminal interceptor cueing

I'd imagine you'd have ground-based sensors for terminal interception refinement, but having a wide angle view to cover highly maneuverable glide vehicles would be useful too.

> Current: it's hard / unknown how to make a flying object that's resistant to radiating heat

There are techniques to reduce infrared signature, though:

https://en.wikipedia.org/wiki/Northrop_Grumman_B-2_Spirit#In...

The IR systems are space-based and look for rocket plumes in the ascent phase. Once a rocket gets above the troposphere there's no clouds to obscure it. It's an intensely bright IR source that easily stands out from the -60°C surrounding atmosphere.

Just a second of observing the plume's trajectory will tell the difference between and orbital or ballistic one. Ballistic missiles will also be launched over the poles to travel the shortest distance to their targets. So IR plumes over the North Pole either means Santa's upped his present delivery game or there's an incoming ballistic missile attack.

They're mostly talking about the initial launch detection which is indeed IR based because it's going to be nearly impossible to hide the signature of a rocket launch of any useful size. After that flight tracking is a lot of different sources like radar.

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