O/T Using GPS to obtain UTC...

hippy

Technical Support
Staff member
#2
https://ics-cert.us-cert.gov/sites/default/files/documents/Memorandum_on_GPS_2019.pdf
A GPS device that provides UTC time does so by converting GPS Time to UTC using multiple parameters - including WN - conveyed in page 18 of GPS sub-frame 4. GPS devices with a poorly implemented GPS Time-to-UTC conversion algorithm may provide incorrect UTC following a WN rollover.
Updated: From a number of sources it appears "GPS Time counts in weeks and seconds of a week from this epoch". So it seems it is more critical than I imagined originally. A misinterpretation of what a GPS puts out rather than what it received ...
I'm struggling to think how week number would enter into any calculation for converting GPS time to UTC.

I thought week number was provided by GPS in much the same way that day of week is provided by a DS1307 RTC chip and similar, not because it's necessary, but simply to save those who want to know what it is from calculating it. I might be wrong.

I can't see anyone who doesn't actually need or use the week number provided by GPS to be affected by its roll-over to zero.

Given the last Week 0 started on Sunday, 22nd August 1999, I imagine the raw value isn't particularly much use for those who would use week number and year.
 
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AllyCat

Senior Member
#3
Hi,
in a nutshell, GPS Year is maintained as 10bit value, that rolls-over on 6th April 2019.
Not really, the (raw) GPS system counts in weeks (as hippy noted) so the 10-bit counter overflows about every 20 years. As the GPS system dates back to about 1980, the overlfow has happened once already, at about the same time as the Y2K "millenium bug", and was about as disasterous as that (wasn't). ;)

There are other "complications" associated with the raw (transmitted) GPS data (for example "GPS time" has now "slipped" by tens of seconds relative to UT/GMT) but these issues are generally handled by the "GPS Engine" anyway.

Cheers, Alan.
 

PhilHornby

Senior Member
#4
As the GPS system dates back to about 1980, the overlfow has happened once already, at about the same time as the Y2K "millenium bug", and was about as disasterous as that (wasn't). ;)
But the last time it happened, was before Bill Clinton removed the 'error' signal and effectively made it available to the general public. Presumably the military got their act together, but do today's $10 Chinese GPS boards handle things properly? I can't think of any way of testing them, prior to April 6th, to find out ....

As for Y2K, that wasn't a disaster, because of the team of highly skilled IT specialists who deployed hundreds of patches to millions of PCs...
(guess how I earned a living in 1999? :))
 
#5
…., but do today's $10 Chinese GPS boards handle things properly? I can't think of any way of testing them, prior to April 6th, to find out ....
That was the first thought that went through my mind, too.
As for Y2K, that wasn't a disaster, because of the team of highly skilled IT specialists who deployed hundreds of patches to millions of PCs...
(guess how I earned a living in 1999? :))
Not just PCs. I remember 1999 well - I was working in internetworking at the time and remember the thousands of router firmware upgrades. I also had to return from leave for 9-Sept-1999 (the date 9-9-99 was used last century to fill a mandatory date field when you didn't know what the real date was!)
 
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