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 Availability" and is the purposeful degrading of the signal to make it less accuracy. The DoD said it will not do so, and has given assurances to the FAA (because planes are now highly dependent on it). The newest GPS III satellites no longer have the S/A hardware:

* https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_...

By 2000 people had figured out to get quite accurate results even with the S/A enabled, so the whole thing not as useful:

* https://www.aviationtoday.com/2000/07/01/the-real-reason-sel...

If there's a war on it's probably cheaper to just jam in the area of operations.

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.

> The US is able to block GPS signal to a specific area

The GPS system does not have any area denial capability. If the US wants to block it in a certain area they have to deploy terrestrial jammers just like anyone else.

Allies get decryption keys for the military high-accuracy signal.

> selectively denying some party GNSS access.

Military depends on an encrypted signal. If you have the encryption key, you can decode it even with a very high level of interference.

Disadvantage is the key is shared for all users, and if it leaks to the adversary, then they can jam the signal.

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.

>Everyone sharing four increasingly redundant systems starts to look like a waste of money

straight out of the "having two kidneys is a waste of energy" school of thought

You could buy yourself a dialysis machine "just in case" but you will have to give up something else to afford it. There is a trade off.

What level of redundancy is needed? Having three global systems seems reasonable, if it's an important service.

the problem with just one is that US will insist having a deadswitch on it, just like it did with Galileo.

so you must have your own if you don't want US to have a Damocles sword over you

Newer GPS satellites apparently don't support selective availability, so they would have no way of preventing access to only civilian devices. Of course, any of these systems can be jammed or shot down but that would affect anyone using it.

> In September 2007, the U.S. government announced its decision to procure the future generation of GPS satellites, known as GPS III, without the SA feature. Doing this will make the policy decision of 2000 permanent and eliminate a source of uncertainty in GPS performance that had been of concern to civil GPS users worldwide.

https://www.gps.gov/systems/gps/modernization/sa/

Why would a reformist country like China and Russia trust such a thing. We were also told that mass surveillance was a conspiracy/nutjob story, but the Snowden leaks has shown us a different world.

They wouldn't, and I can see why. I was just adding some information. As far as anyone in the public has been told, both GPS and Galileo will remain open for anyone to use, but depending on that requires trusting whoever runs it.

If I wanted my neighbours to trust my GPS system more, I'd tell them I don't have selective availability.

Sure. If you wanted GPS to be more widely used and depended on with no ulterior motives, you would also tell them you don't have selective availability. All of these systems require trusting the operators at the end of the day, and I'm not making the claim that one should depend on any single system.

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.

If everyone is thinking the same thing, then someone isn't thinking. (This time, I wasn't thinking, but I agree with the sentiment.)

In a perfect world yeah, but given the geopolitical and geoeconomical changes going on. The tradewar to seperate the Chimerica relationship, you probably want to own your own navigation systems.