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Comment by netsharc | Hacker News Reader

I would assume the US/NATO also jams GPS/use some sort of scrambler for the unencrypted signal to have some military advantage...

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You are quite correct, notification of exercise-related jamming is published in NOTAMs at least here in the UK. For example for Exercise Joint Warrior 171 this year:

Q) EGPX/QWELW/IV/BO/W/000/400/5836N00447W135

B) FROM: 17/04/02 07:00C) TO: 17/04/02 17:00

E) GPS JAMMING WI 135NM RADIUS 583600N 0044630W (FARAID HEAD). USERS MAY EXPERIENCE ERRONEOUS PSN INFO OR DEFAULT DEAD RECKONING MODE. PERIODS OF GPS JAMMING WILL NOT LAST LONGER THAN 40 MINUTES AT ANY ONE TIME.

Yeah, it's also possible that it was the Swedish armed forces/NATO doing the jamming as part of the exercise. In this case they would probably jam both GPS and GLONASS.

Seems politically quite risky though, what if some ambulance didn't reach their target or something like that? (Although I guess an easy workaround for that could have been tight coordination with the local rescue services - stop the jamming the moment there's an ambulance/fire emergency in the local area.)

Do ambulances typically operate at 2k-3k feet?

remote region ==> air ambulance?

I think the US military uses a different GPS signal that provides higher accuracy positioning.

The RF signal is the same, the additional accuracy comes from the use of cryptography.

The feature is called "Selective Availability", and has been disabled since May 2000.

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

So are you saying that the US military does not have a higher accuracy positioning system? I highly doubt that.

There are well-known ways to improve GPS accuracy, such as dGPS [1], inertial guidance, and beam-riding [2].

I think it unlikely that the US military has a secret second constellation of GPS-like satellites, operating on an undetectable secret frequency. Firstly, it would be difficult to launch and operate such a constellation in secret, and secondly, because augmenting the GPS data with independent, secondary sources operating with a different approach provides more redundancy and is proven technique for increasing accuracy.

The capability to limit accuracy in the satellite signal already existed – the fact that it was shut off is a strong signal that the perceived benefit is no longer important.

[1] https://en.wikipedia.org/wiki/Differential_GPS [2] https://en.wikipedia.org/wiki/Beam_riding

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