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Comment by jamessb | Hacker News Reader

GPS signals can be jammed [1] or spoofed [2] by terrestrial transmitters; and the satellites are controlled by the US military, who could in principle decide to deny access or degrade service.

Navigational errors that could result in grounding or collisions may endanger life (and be very expensive), so it is unsafe to rely solely on GPS, even if you have redundant receivers and power supplies.

[1]: https://en.wikipedia.org/wiki/Global_Positioning_System#Mili...

[2]: https://en.wikipedia.org/wiki/Spoofing_attack#Global_navigat...

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GPS could be degraded or shut off, but of course everyone noticed that weakness so we now have Russia's GLONASS, China's BDS and EU's Galileo. I don't know about the satnav units in ships, but at least modern smartphones receive and use all or most of them, no modern phone uses GPS alone.

Jamming and to a lesser degree spoofing are obviously still problems

> GPS could be degraded or shut off, but of course everyone noticed that weakness so we now have Russia's GLONASS, China's BDS and EU's Galileo.

All of which use the same very small, very narrow, band of frequencies—sometimes the exact same frequency of GPS:

* https://gssc.esa.int/navipedia/index.php/GNSS_signal

You jam L1 (1575.42 MHz) and you take out a whole bunch of services out.

It's one of the reasons why there's L2, L5/E5, and E6. But everything is in the Aeronautical Radio Navigation Service and Radio Navigation Satellite System bands.

These days I use GPS as a generic term to apply to any or all of the above, not specifically to the US implementation.

Wouldn't directional receivers be enough to mitigate the vast majority of jamming attempts?

Directional receivers aren't useful for GNSS. The receiver needs at least 4 satellites in view at all times in order to maintain an accurate 3-D position fix. The latest NavStar GPS satellites do broadcast an M-code signal which is somewhat more resistant to jamming, however that technology is only available to the military and has only been partially deployed.

https://www.gao.gov/products/gao-21-145

No, because GPS signals are extremely low power.

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