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China's version of GPS now has more satellites than US original

asia.nikkei.com

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Re: China's version of GPS now has more satellites than US original

#71

Post by tyldum »

throw0101a wrote: post #57
raihansaputra wrote: post #31

Earlier quoted context omitted.

The US is able to block GPS signal to a specific area (I think it was done on the Gulf War) or disable the unencrypted GPS signal (only the US military can decode them). I assume each system has the same capability too. The worst case scenario would be all parties decided to block the unencrypted portion and leaving all civilians to be location-blind, including civilian aircrafts.

> The US is able to block GPS signal to a specific area ... Anyone can block the signal given how weak it is, and there are news reports of people doing so (and being charged for it). You can supposedly buy jammers online for not a lot of money. > ... (I think it was done on the Gulf War) or disable the unencrypted GPS signal (only the US military can decode them). What you are probably referring to is "Selective Ava…

Jammers are cheap and since a lot of expensive gear has GPS tracking these days, thieves are using simple jammers you plug into the cigarette outlet in cars. This is increasingly becoming a larger problem since they jam a relatively large area.

Re: China's version of GPS now has more satellites than US original

#72

Post by credit_guy »

onethought wrote: post #42
credit_guy wrote: post #38

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…

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...

Re: China's version of GPS now has more satellites than US original

#75

Post by makomk »

gmueckl wrote: post #51
gandalfian wrote: post #26

Earlier quoted context omitted.

Everyone sharing one system is excellent. Everyone sharing four increasingly redundant systems starts to look like a waste of money. We need a carpooling equivalent gps system. Still better than more bombs I guess.

It stops looking that redundant when one system goes down because both of its ground based refence clocks go down at the same time. This recently happened to Galileo. Nobody noticed because NavStar and GNSS covered for it.

NavStar GPS can keep on running for quite a while even if the entire ground segment fails. The current generation of satellites even have an Autonav mode that lets them synchronize their clocks and compute corrections to their orbital parameters using cross-links between satellites without any ground assistance. Supposedly it can provide almost full accuracy for six months after a complete failure of the ground segment. Galileo just isn't very robust by comparison.
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