Fellow time-nuts,
Have any of you seen the reported problems?
Cheers,
Magnus
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The USCG NAVCEN has received multiple reports of GPS problems
beginning on or about 11-14 June 2009. During this timeframe, two of
the counters providing a truncated representation of GPS week in the GPS
navigation message reached their maximum value (255) and reset to zero.
This is a normal, documented behavior of the GPS signal, but past
observations have shown that some GPS receiver sets do not properly
process this event. NAVCEN recommends that anyone with a GPS receiver
which began experiencing problems during this timeframe consider
consultation with the receiver's manufacturer. Background information
follows:
GPS time is defined by a week counter as well as a time of week counter.
These counts, respectively, started at 0 roughly at midnight UTC, 6
January 1980. The GPS broadcast provides truncated representations of
the GPS week count in several different locations of the navigation
message, including:
- Subframe 1 (10 bits): WN: Resets to zero every 1024 weeks and is a
truncated representation of the week count corresponding to current GPS
time.
- Page 25 of Subframe 5 (8 bits): WNa: Resets to zero every 256 weeks
and is a truncated representation of the GPS week count corresponding to
the broadcast GPS almanac's time of applicability. NOTE: The GPS
almanac's time of applicability is, based on current MCS software,
projected roughly three days into the future relative to current GPS
time, and therefore can show an updated week count that is first
noticeable on late Thursday or Friday instead of Sunday. (IS-GPS-200D,
Section 20.3.3.5.1.5).
- Page 18 of Subframe 4 (8 bits): WNt: Resets to zero every 256 weeks
and is a truncated representation of the GPS week count associated with
the UTC time parameters. (IS-GPS-200D, Section 20.3.3.5.1.6)
It appears that certain types of receivers are continuing to have
problems with the rollover even past the rollover date. If your's
happens to be one of them, please contact your GPS equipment
manufacturer to see if they have a fix for you.
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