[time-nuts] GPS Time to Year?

Tom Van Baak tvb at LeapSecond.com
Thu Jan 22 16:06:43 EST 2009

> Hi:
> It's my understanding that the slowest time data from GPS is the 10 bit week 
> number.
> How does a GPS receiver come up with the current Year?
> -- 
> Have Fun,
> Brooke Clarke

Hi Brooke,

Good question. You are correct that this 10-bit week number
wraps once every 2^10=1024 weeks which is once about every
20 years. It's not the slowest GPS data, see below *.

Other GPS fields also wrap (more quickly) in a binary way.
Most wrist watches, for that matter, wrap 60 seconds, or 60
minutes, or 12 hours.

GPS week 0 started MJD 44244 = 1980-01-06
GPS week 0 (1024) started MJD 51412 = 1999-08-22
GPS week 0 (2048) starts MJD 58580 = 2019-04-07
GPS week 0 (3072) starts MJD 65748 = 2038-11-21

The ways GPS receivers come up with the current year are:

1) Since time moves forward only, the real year can't less than
what the year was yesterday. So when a GPS receiver on, say,
Sunday morning April 4, 2019 sees that the GPS week is now 0
while last night the week was 1023, the GPS receiver can be very
sure that the year is still 2019 and not 1980 or 1999 or 2019. As
long as a GPS receiver has NVRAM you're all set.

2) The real year can't be less than the year the GPS receiver was
manufactured. If the firmware sees GPS week 491, is has the
option to decide if that week means 0+491 (June 1989) or 1024+491
(January 2009) or 2048+491 (September 2028). With a +/- 10 year
margin the GPS receiver can pick the correct one.

On the other hand, those of us with boat anchor GPS receivers from
August 1999 know that firmware isn't always perfect.'

3) The real year can be obtained from external sources. Many GPS
receivers are now embedded into cell phone or internet-enabled
devices so obtaining a hint at the current date is easy. One may
even know the date&time well before the first GPS signal lock.

4) The real year can't have fewer leap seconds than the previous
year. So if you see that the GPS week number is 0 and the UTC
vs. TAI leap second count is around 20 seconds you know it's
GPS week 0 and year 1980. If the leap second count is closer to
32 seconds you know it's GPS week 0(1024) and so year 1999.
If the count is closer to, say, 44 seconds, then you can be safe
that it's GPS week 0(2048), or year 2019. Not perfect, but it will
work fine for our lifetimes and more.


*) The 8-bit leap second number is slower, wrapping once every
couple of hundred years. Note also that the new GPS data format
allows for wider bit fields for this and other parameters. See also:

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