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RE:GPS rollover day 8-22-99



At work, we have the capability to simulate the GPS week rollover. Some
receivers have failed. The ones that showed the incorrect date still gave a
correct navigation solution. Since the GPS satellites do not transmit calendar
date, it is not used in the navigation calculations. The software upgrades for
most receivers compare the software build date with the calculated date and add
an offset if the rollover is detected. For some of our older specialized GPS
receivers where upgrades are not available, we just modify the data stream with
a corrected date. We found that the  time given by the receiver was correct.
Only the date would be incorrect. We found that some receivers had to have their
almanac and other data cleared out when the rollover occured. The receivers
would quit tracking after the rollover. We suspect the receiver got confused
with the dates in the almanac when the GPS week went to 0.

Ivan

N4QWF@aol.com wrote:

> Wonder if anyone else has info other than this about the coming rollover
> problem?
>
> GPS Week Number Rollover Approaches
> The GPS Week Number count began at approximately midnight on the evening of
> 05 January 1980 / morning of 06 January 1980. Since that time, the count
> has been incremented by 1 each week, and broadcast as part of the GPS
> message. The GPS Week Number field is modulo 1024. This means that at the
> completeion of week 1023, the GPS week number will rollover to 0 on
> midnight GPS Time of the evening of 21 August 1999 / morning of 22 August
> 1999. Note that this corresponds to 23:59:47 UTC on 21 August 1999.
>
> Week beginning
> at 0000 GPS Time on GPS Week Number
> broadcast by satellites
> 08 Aug 1999 1022
> 15 Aug 1999 1023
> 22 Aug 1999 0
> 29 Aug 1999 1
>
> Once the rollover has occured, it is the responsibility of the user (i.e.,
> user equipment or software) to account for the previous 1024 weeks.
>
> Depending upon the manufacturer of your GPS receiver, you may or may not be
> effected by the GPS Week Number Rollover on 22 August 1999. Some receivers
> may display inaccurate date information, some may also calculate incorrect
> navigation solutions.
>
> Contact the manufacturer of your GPS receiver to determine if you will be
> effected by the GPS week number rollover. Contacts for some GPS receiver
> manufacturers can be found here or here.
> For more information, see the GPS Joint Program Office Rollover pages.
> ----------------------------------------------------------------------------
> ----
> <http://www.garmin.com/faqs/1.html>
>
> The Garmin page says our old units may require user intervention. We need
> to identify the version of our units' software. If it's 3.04, we may need
> new software. If it's 3.05 or higher, no problem. They have a download at
> this URL.
>
> Guess we'll just have to see.
>
> 73
> John
>
> "The cure of crime is not in the electric chair, but in the high chair."
> ...however, the electric chair works wonders with recidivism.
> ----
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