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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