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Comment by pietroglyph | Hacker News Reader

No one seems to have mentioned that the US has actually blocked GPS signals before for military reasons (during the Kargil war in 1999): https://timesofindia.indiatimes.com/home/science/How-Kargil-...

There's a good reason and precedent for China (and others; see GLONASS, IRNSS, Galileo) to be afraid of this.

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No, the US has never done any blocking in the GPS system. The article refers to a request by India for access to the military GPS signal and military equipment, which was not granted.

The US, as well as virtually all other technological powers, does have the capability to jam GPS signals using ground, sea, or air-based jamming platforms but those are not part of the GPS system.

Yup it did - I was flying aerial surveying at the time. Bad timing. Actually I didn't notice anything in the end so was all good but when I saw the NOTAM I couldn't believe my bad luck.

Edit - link: https://www.theregister.co.uk/2016/06/07/us_military_testing...

Not sure whether you're agreeing or disagreeing, but that refers to jamming done by electronic warfare units, not part of the GPS system, as I indicated above.

The United States actually does have the ability to selectively degrade signals. I don't know enough to comment on this capability in more detail (another comment seems to indicate that it is through jamming). A relevant excerpt (from 2000) is below:

> Additionally, we have demonstrated the capability to selectively deny GPS signals on a regional basis when our national security is threatened.

https://www.faa.gov/about/office_org/headquarters_offices/at...

As I indicated above, separate systems on electronic warfare platforms can jam GPS signals. This is not part of the GPS system, and is what the article refers to.

Building your own GNSS system does nothing to protect against electronic warfare platforms.

Selective availability was defeated a long time ago which is one of the motivations for Clinton formally turning it off in 2000. Also while I'm sure that the US military is fully capable of turning off GPS satellites however they want the efficacy of this is probably limited considering the coverage of GLONASS + EGNOS.

The point of that article is that a main justification for turning off selective availability was that the US could still deny access in certain geographic areas. Selective denial != selective availability.

> The US, as well as virtually all other technological powers, does have the capability to jam GPS signals using ground, sea, or air-based jamming platforms but those are not part of the GPS system.

As well as anyone that has access to basic electronic parts. It's very easy to swamp a 30 watt transmitter that's ten thousand miles away.

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