Hacker News Reader

Comment by tick_tock_tick | Hacker News Reader

How often has GPS, excluding intentional military operations, been unavailable?

Edit: Went looking around and it seem GPS has never suffered an outage. They have had issues with some satellites, even two days ago it seem, but never a true outage. http://navigationservices.agi.com/SatelliteOutageCalendar/GP...

View on HN

Yeah. Glonass (the Russian system) had an 11-hour outage back in 2014 because they uploaded an incorrect ephemeris to all their satellites and could only correct them when they flew over Russia. I think that was the worst GNSS outage of the modern era until this one.

Galileo has not declared Full Operational Capability (scheduled for 2020), it's still in the Initial Services stage. It's a bit like complaining that a beta has bugs.

In 2016 they did announce "the transition from a test system to one that is operational" [1] so I can understand why some people might find the distinction confusing.

[1] http://www.esa.int/Our_Activities/Navigation/Galileo_begins_...

The testing phase ended in 2016 - Galileo is a live GNSS network intended for actual use in navigation. The reason they haven't declared Full Operational Capacity is that they still haven't managed to deploy a full constellation of satellites. All of the official announcements and navigation messages from the satellites say that it's meant to be providing actual service, just with slightly degraded accuracy due to the missing satellites.

Not only that, the EU's controversial eCall initiative, which requires cars that go on the market after April 2018 to automatically call the emergency services with their locations and a live mic to the interior if they detect a crash, mandates the use of Galileo for positioning. Manufacturers can (anbd probably will) use other GNSS systems in parallel, but Galileo is the only one that's guaranteed to be supported by cars. This is for a system whose life-saving benefits are supposedly so important as to outweigh the privacy issues.

It was just a few weeks ago that there was a US midwest and eastern GPS timing precision error that prevented commercial jet's ADS-B from accepting it and causing them to give up throwing errors. ref: https://hackaday.com/2019/06/09/gps-and-ads-b-problems-cause...

That doesn't appear to be an outage. It appears to be equipment that requires GPS to have a particular level of precision, which may or may not have been chosen cleverly enough for times when GPS isn't working perfectly.

This ended up not being a problem with the GPS system at all -- it was actually a certain brand of receivers not dealing correctly with leap seconds[1] -- or, specifically, that it's been a while since the last leap second, which caused one of the GPS parameters (basically 'how long since/until a leap second' to roll over, which is not normally a problem, but thanks to a receiver bug, it was, on these specific receivers. This was covered in some detail on the LEAPSECS mailing list.[2]

[1] https://www.flyingmag.com/collins-receivers-struck-by-failur...

[2] https://pairlist6.pair.net/pipermail/leapsecs/2019-June/0071...

That's a pretty interesting set of data. Even one minute outages of satellites seem to be logged, even though that's probably pretty normal for receivers to have to deal with for other reasons.

Who are the consumers of outage information like this? Or is it simply informational?

There are networks of precise GPS receivers in known locations creating permanent reference networks [1] [2] [3]. High precision applications can gather 'observables' for a few hours then 'post-process' them, comparing them to the data from the permanent reference network, to achieve sub-centimetre accuracy.

Now, obviously these networks will have spotted the outage on their own if they're any good, but extra reports amount to saying "Yep, we saw that too"

GPS is also fairly old and run in the aerospace-and-military tradition where every outage requires an announcement made in NOTICES IN ALL CAPS because that's what pilots did in 1950 :)

[1] http://www.bigf.ac.uk/ [2] http://www.epncb.oma.be/ [3] http://www.igs.org/network

The "NOTICE ADVISORY TO GALILEO USERS" (NAGUs) are also all caps (here the announcement of service resumption [1]), like NOTAMs ("Notice to Airmen").

I understand that the Telex network used a 5-bit code with only caps [2], but today? Can't we use Unicode, or even some ASCII variant?

[1] https://www.gsc-europa.eu/notice-advisory-to-galileo-users-n...

[2] https://en.wikipedia.org/wiki/Baudot_code#ITA2

I'm surprised there are quite so many single satellite outages, and no whole-system outages.

What causes satellites to suffer single satellite unplanned outages? Uploading bad software? Bad data? Wrong commands for orbit adjustment?

One would think that after the 20th time these mistakes had been made, automated tests would be in place to prevent them happening again.

Yet it's impressive no whole-network outage has happened. Presumably after uploading data/commands to one satellite, it gets uploaded to the rest. Yet there has never been a time that everything looked okay till after it was deployed worldwide?

The majority of those outages are listed as single-minute, (or as planned downtime).

And there are 31 GPS satellites in orbit when global coverage is possible with only 24.

This, combined with a fully redundant control segment on the ground and a satellite 'autonav' feature that lets them run (with degraded accuracy) for up to 60 days without the control segment provides reasonably good protection against whole-system outages.

There's a bunch of maintenance stuff that happens regularly that's on that list... FCSTDV (stationkeepng burns), FCSTMX (scheduled maintenance, like switching to a different atomic clock or the like), that sort of thing.

There's a lot of recent UNUNOREF on the agi.com link in this thread's grandparent, which is basically "it broke and came back before we could send out a note". If you look closely at that list, though, you'll see that almost all of the recent events are a single satellite -- PRN18. I have no idea what's wrong with it (and AFAIK there's no way for the public to get the details), but it's been going unhealthy for ~2 minutes at a time for months now.

(I'm guessing they keep it live because either don't have another satellite positioned so they can put it into that slot easily, or because they don't consider whatever is wrong with it to be a huge problem.)

Reply on news.ycombinator.com