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 a 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 22.214.171.124.1.5).
- 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 126.96.36.199.1.6) 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 questions please contact:
CGSIC Executive Secretariat
USCG Navigation Center
or use our online "contact us" page.