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Comment by amluto | Hacker News Reader

Perhaps they should also ask: if GPS is down in an area, do the cell towers continue to operate?

Often they rely heavily on GPS for time and frequency synchronization (think 5G TDD, where all operators need to be aligned in time or they jam eatch other‘s uplink). The GPS antenna is also often quite visible from the outside of a base station.

Still, what‘s also possible is to use PTP as backup (e.g., over eCPRI) and use the fiber / microwave backhaul to some master clock with, say, a ribidium time source as a backup, so I think it‘s not entirely unreasonable to assume that cell phone signals will remain available in the presence of a GPS outage.

> Perhaps they should also ask: if GPS is down in an area, do the cell towers continue to operate?

If the towers are uniquely identified, and are geo-located accurately, it could be possible to triangulate your position even if they are not operational from a make-a-phonecal perspective. They just have to perhaps keeps shouting "I'm here!".

Perhaps they'd at least need a semi-accurate time source? (Like a local rubidium clock, or at least NTP/PTP over a fibre backhaul.) On the other hand smartphones can geolocate themselves decently from Wifi AP signals, and those tend to not have timestamps with-in the broadcasts.

You make me wonder, are these systems programmed to fall back to Galileo EU, Russian Or Chinese (mainland) GNSS at all? Should we fall back to systems controlled by other countries?

> You make me wonder, are these systems programmed to fall back to Galileo EU, Russian Or Chinese (mainland) GNSS at all?

In the US, per FCC rules, it is not officially allowed to listen to other GNSS frequencies as they are not licensed:

> The Federal Communications Commission (FCC) rules require licensing of non-federal receive-only equipment operating with foreign satellite systems, including receive-only earth stations operating with non-U.S. licensed radionavigation-satellite service (RNSS) satellites.

* https://www.gps.gov/spectrum/foreign/

The FCC allowed Galileo use in 2018:

* https://www.fcc.gov/document/fcc-approves-galileo-global-nav...

I think the reasoning is that if it is not "officially sanctioned" then if there's interference with those signals it's not the FCC's problem. By allowing it they'll take noise on those frequencies more seriously in rules and enforcement.

I don't think I understand what allowed means in this context because on my OnePlus Nord phone, I have an app called GPS Test which shows me that I have so many GPS, so many GLONASS, and so many Galileo in view.

Strange thing is there is no Chinese GNSS in view. (I was on a short trip to Bali recently and iirc I did see Chinese GNSS on the app while there so I think the app and phone hardware is capable of picking up Chinese GNSS?).

> I don't think I understand what allowed means in this context because on my OnePlus Nord phone, I have an app called GPS Test which shows me that I have so many GPS, so many GLONASS, and so many Galileo in view.

That app may simply be reporting the presence of signals, but the geolocation functions and APIs may not actually use those signals in their calculations.

And if they are actually processing the signals, then they are breaking FCC regulations:

* https://selectcommitteeontheccp.house.gov/media/press-releas...

Legally they could get their FCC certification pulled and not be allowed to be sold in the US if they are doing that.

FAA certified avionics won't fall back from GPS to foreign GNSS constellations for authorized flight operations. If GPS is degraded then pilots have to fall back to older navigation methods. Those foreign constellations mostly all operate on similar frequency bands so if GPS is being jammed in a certain area then the other systems will likely also be impacted.

In the context of time synchronization, my gut feeling is no, as you can't guarantee that the atomic clocks on other GNSS tick at the same time as GPS. Even if they have the same frequency.

I can't find any benchmarks though that measures this

What do you mean by GPS being "down" in an area? The constellation is in medium orbit so no satellite is area specific. WAAS is area specific so if one of those transmitters fails then that can prevent certain aviation uses of GPS. Jamming can also impact a limited area.

> What do you mean by GPS being "down" in an area? […] Jamming can also impact a limited area.

Jamming can impact much more than just a limited area. Just ask anyone near a border with Russian:

* https://gpsjam.org

Like Estonia:

* https://www.wingsmagazine.com/estonia-gps-interference-2/

Each Russian jammer is only effective within a limited area. Russia. Russia has deployed multiple jammers.

Each jammer can cost as little US$ 10:

* https://hackaday.com/2020/09/08/teardown-mini-gps-jammer/

And a single one can mess up an entire airport until its found:

* https://trans.info/en/gps-jammer-that-brought-disruption-to-...

* https://insidegnss.com/fcc-fines-operator-of-gps-jammer-that...

Very cheap/easy to flood an area with noise.

Russian jammers regularly impact other countries

Tons of russian gps jamming around Krolewiec, cellphones still work. Afaik every cell tower runs a rubidium clock source just for this occasion.

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