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Comment by credit_guy | Hacker News Reader

So, if North Korea wants to develop an ICBM, the main challenges are 4: take off (which they appear to have mastered), miniaturization of the bomb, navigation and reentry vehicle. All the remaining 3 are quite difficult, and navigation in particular was traditionally done by a mix of inertial (i.e. based on gyroscopes) and celestial (i.e. based on the observation of stars). GPS makes all this unnecessary.

If NK uses the US GPS, there are most likely countermeasures in place, for example if the US detects an NK ICBM launch, then it stops or perturbs the GPS signal.

New providers of GPS complicate this problem.

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Is this just conjecture? It seems ridiculous that the US would rely on the fact they can control GPS satellites as form of missile defence... So shooting down their GPS satellites would also hamper there ability to launch ICBMs? Seems equally unbelievable.

It's a conjecture indeed, as many defense things are classified, but you can bet good money it is true.

> It seems ridiculous that the US would rely on the fact they can control GPS satellites as form of missile defense

The US most certainly would not entirely rely on that. It would only be one tool in the toolbox. Nuclear ICBM attacks are so dangerous, that the defense by necessity uses a multi-pronged approach. It starts from diplomatic pressure, and technology export controls (see for example the Wassenaar arrangement [1] that lists the controls for inertial and celestial navigation in chapter 7. The limits on the GPS receivers exist as well, unfortunately I can't point you to the exact source). The actual physical defense options against a moving missile are listed in [2], such as the Ground Based Interceptors, the Aegis system and the THAAD system. It's all but certain that in case of credible intelligence of an imminent hostile ICBM launch, the US has pre-launch options as well (the current NK ICBM used liquid fuel, and the launch facilities are known and monitored; you will notice that one or two days before any NK missile test the press already writes articles about the likely launch).

Considering all this, would the US neglect the possibility of altering the GPS signal in case it detects a hostile ICBM launch? For example, the reentry vehicle travels at many km/s. Early detonation by only half a second can reduce the impact of the bomb by a factor of 10 easily. If the missile has not GPS signal and only relies on less-than-state-of-the-art inertial and celestial navigation (which may be ok for horizontal coordinates, but not that great of altitude) that may make a huge difference. So my bet is not only that the US will promptly alter its GPS signal, but will have means to spoof the signal from the Russian and Chinese systems as well, and agreements in place with the European Galileo system too. However, as I said before, the problem is more complicated: the Russian and Chinese are not dumb, they most likely have their counter-counter-measures too.

[1] https://www.wassenaar.org/app/uploads/2018/12/WA-DOC-18-PUB-...

[2] https://en.wikipedia.org/wiki/United_States_national_missile...

The unencrypted GPS automatically disables themselves when they detect speeds of a missile to prevent exactly this.

That is a limit placed on the receiver, I can't imagine that being hard to get around with a custom receiver or maybe even just customized receiver firmware.

You cannot disable single receivers, they could only shut down or encrypt the whole system (at least for part of the world). Which could be standard procedure if they detect an incoming attack. With 3 other systems also available, that probably won't have much of an effect.

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