Leap Second Implementation Confuses Some Receivers

February 2, 2015  - By
Image: GPS World

The United States Civil GPS Service Interface Committee (CGSIC) has issued a notice about a problem some receivers are having implementing the correct time. The U.S. Coast Guard Navigation Center has received reports of synchronization issues since the implementation of a leap second on Jan. 21. Users experiencing this problem should contact the receiver manufacturer for a firmware or software update.

Below is the text of the CGSIC notice:

All CGSIC: 2015 GPS Future Leap Second Implementation

The GPS 50 bit-per-second navigation message transmitted by each GPS satellite (specifically Page 18, subframe 4) includes the parameters needed to relate GPS time to UTC (Coordinated Universal Time).  That relationship is maintained through leap second implementation transitions by IS-GPS-200 compliant user equipment.  For leap second transition, user equipment must utilize the notice regarding a scheduled future delta time due to leap seconds (ÄtLSF), together with the week number (WNLSF) and the day number (DN), at the end of which the leap second becomes effective.

On or about Jan. 21, 2015, those GPS navigation messages began to include future leap second data which indicates an increase in the leap second to become effective at the end of June 2015.  IS-GPS-200 revision H, dated 24 Sep 2013 paragraph Coordinated Universal Time (UTC), documents the appropriate algorithm details to ensure correct utilization of the parameters above (including all potential truncated week number transitions and variations in time of processing relative to satellite upload timing near the future leap second effectivity).

The data upload for the June 30 leap second, initiated with SVN48/PRN07 at 18:33:56z on Jan. 21, was correctly executed. However, there are several receivers brands/models that seem to be mishandling this information and applying the leap second now. This is creating a negative one-second offset in faulty receivers. The U.S. Coast Guard Navigation Center has reports of these receivers causing synchronization issues with radios, computer systems, and data logging equipment.

Users experiencing issues with GPS receivers that began on Jan. 21 should contact the receiver manufacturer to determine if the latest firmware or software patch can correct the issue.

V/R Rick Hamilton
CGSIC Executive Secretariat GPS Information
Analysis Team Lead USCG Navigation Center

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