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# GPS Devices Might Stop Working This Weekend -- Week Number Rollover

Author Topic:   GPS Devices Might Stop Working This Weekend -- Week Number Rollover
dwise1
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 Message 1 of 22 (850186) 04-02-2019 8:21 PM

I should start by stating that as a software engineer, retired about a year ago, I worked very closely on a daily basis with several GPS receivers for over 20 years, so I know both personally and professionally that this is a real thing.

You may consider this a public service announcement, which is how Today I Found Out describes having been approached by Tom Tom to make this video, "How Does GPS Actually Work and Why Many GPS Devices are About to Stop Working":

That prompted me to open my GPS Week Numbers list on my phone (which I've rarely done since retiring over a year ago) to verify that, indeed, the GPS Week Number will roll over from 1023 back to zero this weekend, 07 April 2019. Like I said, I know that this is a real thing because I went through it the first time for 22 August 1999 -- kind of a dress rehearsal for the Y2K scare.

Basically, your GPS receiver determines your 3-D position by triangulating off of four satellites (optimally, but three could still work) and the precise time (contained in the satellite signals). Time is expressed in Week Number (WN), the number of weeks since the beginning of GPS time (06 Jan 1980), and Time Of Week (TOW), the number of seconds into the week. Given WN and TOW, the receiver calculates the exact date and time -- there's also a precise one-pulse-per-second signal, 1PPS, that the receiver syncs to to determine fractions of a second.

The problem stems from the WN counter, which is only 10 bits long. That means that it can only count 1024 weeks ranging from 0 to 1023 before the count rolls over (hence the term) back to zero. 1024 weeks amounts to about 19 years 8 months. The standard explanation for such short-sightedness is that nobody had expected the system to be around for so long, especially since it had started out as an experiment.

Part of how the GPS system works is that the orbital parameters for the 31 operational GPS satellites are downloaded into the receiver, so that it can calculate which satellites should be where in the sky at that given time. The receiver then uses that information to acquire satellite signals to use for its position solution. That means that if the receiver doesn't know the right date and time, then it will think that the satellites are where they are not which will completely mess up its position calculations. Bottom line: the receiver cannot work.

The first Week Number rollover, 22 August 1999, kind of took us by surprise. The receiver manufacturers came up with fixes that worked. I just assumed that they knew what to do from this point forward and so, coupled with the knowledge that it would happen again, they would know to always include those fixes in the new designs. Or maybe the manufacturers of products that use GPS receivers (eg, Tom Tom and Garvin) are just nervous and have their own fixes to detect and correct wrong dates coming out of the receiver.

At any rate, we're hearing buzz about the need to be sure that your firmware is up to date. Personally, I doubt that we will see many GPS-based devices fail, but if yours does then you will know what happened and then you can update your firmware. Or you can respond proactively and update before this weekend.

There are a couple more things to think about. This second Week Number rollover will occur at midnight UTC going into Sunday, 07 Apr 2019. Since I'm in the Pacific Standard Time time zone (UTC-8), it will happen on Saturday at 4PM (1600h). Everybody can apply their own time zone's offset for when the rollover will occur.

Second, testing whether our cellphone/mobile survived the rollover or not can be problematic. On the train in Germany, I would run the Maps app on my phone. I wasn't getting enough satellite signals to update my location, so it would just freeze until we passed through a train station at which time it would talk with the station's Wifi network, update to the station's location, and then freeze there. It turns out Wifi signals include location data whether you can connect or not. So if your phone is connected to Wifi, then it will get its location from that regardless of whether the GPS is working. It also gets time information from the cell towers (and maybe also from Wifi?) and might also get location information as well. Though it looks like switching to Airplane Mode will disconnect you from both the cell towers and Wifi, so that should allow you to test your GPS receiver.

I hope that this can be of some help. Even more, I hope that after rollover all your GPS-based devices continue to function without a hitch or hiccup.

 Replies to this message: Message 2 by Percy, posted 04-03-2019 10:27 AM dwise1 has responded

Percy
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 Message 2 of 22 (850199) 04-03-2019 10:27 AM Reply to: Message 1 by dwise104-02-2019 8:21 PM

Interesting. I'll be at home Sunday night, so I set an alert to check my cell phone position.

--Percy

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dwise1
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 Message 3 of 22 (850203) 04-03-2019 12:32 PM Reply to: Message 2 by Percy04-03-2019 10:27 AM

Like I tried to say, we've known for over two decades that this day would come and most manufacturers should have been competent enough to factor this day into their designs, you never know until it's put to the test. BTW, there are such things as GPS simulators which can simulate satellite signals for any date and any location in order to test GPS receivers, but they can be expensive so not every company owns one; it took several years before our company finally got one (so they've already tested all their products).

BTW, in New Hampshire (UTC-5) the rollover should happen at 7PM on Saturday, so you could check it then.

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Percy
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 Message 4 of 22 (850215) 04-03-2019 5:17 PM Reply to: Message 3 by dwise104-03-2019 12:32 PM

 dwise1 writes:BTW, in New Hampshire (UTC-5) the rollover should happen at 7PM on Saturday, so you could check it then.

--Percy

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dwise1
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 Message 5 of 22 (850303) 04-05-2019 10:35 PM Reply to: Message 2 by Percy04-03-2019 10:27 AM

Cell phone towers use precise frequency signals (eg, 10 MHz) and a precisely timed 1PPS (one pulse per second, on the second within a few nanoseconds) in order to support time-division multiplexing of your phone signals to send and receive them over a high-volume trunk line (made high-volume by that multiplexing). A cell tower gets those signals from a device, a rack drawer, like I had worked on for 20 years as a software engineer. That kind of box uses GPS as a time base with which to generate those signals. If that box loses GPS, then it loses that time base and, when the cell tower loses that box's outputs or they degrade too far, then the cell tower network will no longer be able to function.

That's the possible cell phone failure scenario I was thinking of before, but upon further reflection I remembered that it won't happen immediately -- keep in mind that I haven't thought about any of this for over a year since I retired. One of the features of a properly designed box is that it will do its best to maintain those output signals in the event of losing GPS -- I cannot speak for our competitors whose boxes are in most cell towers, but I do not doubt for an instant that they have designed their boxes properly as we have.

When a box loses GPS, it goes into "coast mode" during which it uses what it had learned to maintain the accuracy of its output signals. I forget the more exactly times, but it should be able to keep that up for several hours to even a day or two. At the same time, it should send a notification message to the computer that's monitoring it (which should exist) that something has happened and somebody had better respond to fix the problem.

So, does the owner of the cell tower know about this problem? He should. I do know that my old company has been receiving a lot of questions from its customers about this Week Number rollover.

So in terms of watching for cell phone service disruption because of this, we should not see it immediately and, assuming that cell phone network companies respond to problems when they do arise, we should never see it. But if this takes those companies by surprise, we could still see it through degraded or non-existent cell phone service.

BTW, even if the worst happens, the cell towers will still know their locations and (assuming that location data is transmitted from cell towers like it is from WiFi) then your phone would still receive proper location data. Unless something like an epically massive earthquake were to happen, but then Week Number rollover would be the least of our problems.

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dwise1
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 Message 6 of 22 (850340) 04-06-2019 3:33 PM

Forgot Daylight Savings
I forgot that we're on daylight savings right now in the US, so the times I gave are off by one hour.

Midnight UTC going into Sunday is Saturday at:

• 5PM Pacific Standard Time
• 6PM Mountain Standard Time
• 7PM Central Standard Time
• 8PM Eastern Standard Time

 Replies to this message: Message 7 by Tanypteryx, posted 04-07-2019 12:35 PM dwise1 has responded

Tanypteryx
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 Message 7 of 22 (850411) 04-07-2019 12:35 PM Reply to: Message 6 by dwise104-06-2019 3:33 PM

Re: Forgot Daylight Savings
I'm not really sure what was going to happen, but my 3 gps units are working as far as I can tell.

What if Eleanor Roosevelt had wings? -- Monty Python

One important characteristic of a theory is that is has survived repeated attempts to falsify it. Contrary to your understanding, all available evidence confirms it. --Subbie

If evolution is shown to be false, it will be at the hands of things that are true, not made up. --percy

The reason that we have the scientific method is because common sense isn't reliable. -- Taq

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dwise1
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 (1)
 Message 8 of 22 (850414) 04-07-2019 1:23 PM Reply to: Message 7 by Tanypteryx04-07-2019 12:35 PM

Re: Forgot Daylight Savings

Basically, a receiver without a fix would suddenly think that it was 22 Aug 1999 again, so it would start outputting wrong time/date values. Because the receiver uses that time data to predict where the satellites should be, it would very likely fail to acquire satellite data and therefore be unable to get a location fix. That would in turn impair the system using that receiver.

This article in Nature (https://www.nature.com/articles/d41586-019-01048-2?utm_so...) describes some of the data gathering stations that could be affected. In addition, cell phone towers use GPS-based sub-assemblies (our product), so failures there could have affected phone coverage.

This is a real thing. The only unknown is whether some receivers didn't get fixed.

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dwise1
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 (1)
 Message 9 of 22 (850453) 04-08-2019 2:34 PM

An Odd Coincidence
I noticed an odd coincidence, but I cannot think of how it could be related.

My phone (HTC M8 One, Windows 8.1) is paired with my car (2015 Honda Civic Hybrid Touring) through BlueTooth. They reconnect every single time I start the car. For that matter, in order to use the phone (it runs in safe driving mode) I have to stop the car and turn the car off.

I was helping teach a dance class when the rollover happened on Saturday, so the moment I stepped out of the studio I put my phone in Airplane Mode to disconnect it from cell towers (I don't connect to the studio's Wifi) and tested it (Maps worked as did a GPS app I have loaded). Then I turned Airplane Mode off and my phone reacquired cell towers. I did all of this will out of range from my car, which was also turned off at the time.

When I started the car, it didn't connect to my phone. Throughout the next day, same thing; the car even asked if I wanted to connect a phone. Finally this morning on the way to the gym, I pulled over and reestablished the connection. Now it works fine as before.

The timing is a weird coincidence suggesting that it's somehow connected to the Week Number rollover. But I cannot think of any way that GPS would affect BlueTooth.

 Replies to this message: Message 10 by Percy, posted 04-08-2019 5:16 PM dwise1 has responded

Percy
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 Message 10 of 22 (850475) 04-08-2019 5:16 PM Reply to: Message 9 by dwise104-08-2019 2:34 PM

Re: An Odd Coincidence
One test you could try as part of a process of elimination is to toggle airplane mode again and see if that causes any connection problems with your car.

My phone still thinks it's where I think I am.

--Percy

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dwise1
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 Message 11 of 22 (850486) 04-09-2019 12:59 AM Reply to: Message 10 by Percy04-08-2019 5:16 PM

Re: An Odd Coincidence
Tried that test. No effect: the phone connected with the car as it should.

Tonight I was well out of range of the car. When it happened the first time, I thought I was out of range, but maybe I was in range. I can try that tomorrow.

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Percy
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 (1)
 Message 12 of 22 (850612) 04-11-2019 8:32 AM

A Failure in New York City
From the New York Times:

quote:
On April 6, something known as the GPS rollover, a cousin to the dreaded Y2K bug, mostly came and went,...But in New York, something went wrong...

At 7:59 p.m. E.D.T. on Saturday, the New York City Wireless Network, or NYCWiN, went dark,...

The culprit was a long-anticipated calendar reset of the centralized Global Positioning System, which connects to devices and computer networks around the world.
...
Ms. Raphael [Stephanie Raphael, Department of Information Technology and Telecommunications spokesperson] admitted that technicians have been unable to get the network back up and running, adding, â€œWeâ€™re working overtime to update the network and bring all of it back online.â€
...
Approximately every 20 years, the GPS needs to reset the way it keeps track of weeks, and Saturday, April 6, was the date selected for the most recent reset. The reset also came with an improvement in timekeeping that means the next rollover will not be needed for about 157 years.

Maybe you already said, but what was the change that means the next rollover won't be for another 157 years?

--Percy

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PaulK
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 Message 13 of 22 (850614) 04-11-2019 8:48 AM Reply to: Message 12 by Percy04-11-2019 8:32 AM

Re: A Failure in New York City
The obvious change would be adding 3 more bits to the counter. So it lasts 8192 weeks before rollover instead of 1024.

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dwise1
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 Message 14 of 22 (850621) 04-11-2019 10:20 AM Reply to: Message 13 by PaulK04-11-2019 8:48 AM

Re: A Failure in New York City
According to an NBC report, that very change is being implemented.

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dwise1
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 Message 15 of 22 (850625) 04-11-2019 10:54 AM Reply to: Message 12 by Percy04-11-2019 8:32 AM

Re: A Failure in New York City
 Maybe you already said, but what was the change that means the next rollover won't be for another 157 years?

I thought I had already mentioned that, which I read in an NBC report. Guess it was to another group.

The original design, immediately set in concrete, was 10 bits for the week number count, limiting it to 1024 weeks, which is about 19.5 years. NBC reported that the system is being upgraded to a 13-bit counter which increases the limit 8-fold (23) to 8192 weeks, about 157 years. I'm not sure exactly how this upgrade is being implemented, since the entire system must continue to work seamlessly throughout the upgrade.

Regarding the NYT article you quote, I expected that most failures would be in systems that use legacy receivers. When my former company tested their GPS-based products, it was the few that used an old receiver (because the product was an old, early one) that failed the test. I suspect that NYCWiN used some GPS equipment from an early project.

Told all y'all that this was a real thing.

 This message is a reply to: Message 12 by Percy, posted 04-11-2019 8:32 AM Percy has acknowledged this reply

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