GPS week-rollover (was Re: Year 2k)

Arjen Kamphuis (
Sat, 28 Mar 1998 13:33:29 +0100 (CET)

At 17:50 27-03-98 -0800 Hal Finney wrote:
> GPS week rolls over from week 1023 to week 0000 at midnight 21-22
> August 1999 and approximately every 20 years thereafter
> Clearly documented in ICD-GPS-200, para 3.3.4(b)
> GPS receiver manufacturers responsible for their designs conforming
> to ICD-GPS-200
>So the problem occurs the night of August 21, 1999, when the week counter
>rolls over from 1023 to 0. This is part of the GPS spec but some
>receivers may not have been thoroughly tested for proper operation during
>this transition.
>The other slides indicate that the GPS satellites themselves have been
>verified to have Y2K and rollover compliance, but some of the ground
>support software is on older mainframes and has some Y2K issues.
>I gather that the ground software would not affect the operation of
>the satellites in terms of giving out good data, but would affect the
>military's own use of the satellites.

Thanks Hal, for digging that up so quickly. I got this reply from Garmin,
the manufacturer of my hand-portable GPS:

> Garmin took Y2K considerations into account from the beginning of its
> product development. As such Garmin products should continue to
> operate after the year 2000. Products that display two digits for
> year will display 00 for the year 2000, 01 for the year 2001, and so
> on. If you are interfacing a Garmin product to an NMEA device such as
> a plotter or moving map, you should check with the respective
> manufacturer to be certain that these products can properly receive
> the 2 digit date in the NMEA data.
> Another important event for GPS receivers will occur on August 22,
> 1999. This event, known as GPS week number rollover, is when the week
> number transmitted in the satellite navigation message will change
> from 1023 to 0. The GPS week number rollover will not cause
> navigation errors for Garmin products if operated during the rollover
> event.
> Some older Garmin products may need to perform an "auto locate" or
> "search the sky" operation in order to acquire satellites and perform
> navigation functions after the GPS week number rollover occurs. In
> order to facilitate our customers Garmin will issue instructions for
> each product a few weeks prior to the GPS week number rollover event.
> All Garmin products currently manufactured by Garmin are Y2K
> compatible and do not require any user intervention in order to
> operate after the GPS rollover event takes place.

I'll bring my GPS with me to the US in september, we'll see how it works
out in practice.


Arjen Kamphuis | "Here Be Dragons", read the ancient maps | in all the white spots that seemed large
enough to hold the fabled creatures.

let's go dragon hunting.

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