| NAVCEN
GPS ADVISORY REGARDING GPS 256 WEEK ROLLOVER
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.
If you have comments or question please contact:
Rick Hamilton
CGSIC Executive Secretariat
USCG Navigation Center
703-313-5930
or use our online "contact
us" page.
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