1756-TIME Module Affected by GPS Week Number Rollover

This Tech Files note comes from a Rockwell Automation Knowledge Base article that is open to everyone regardless of TechConnect support status. To view the original Knowledge Base article, please click the link below.
https://rockwellautomation.custhelp.com/app/answers/detail/a_id/1082057

1756-TIME Module Overview

The 1756-TIME module provides accurate time synchronization on different interfaces by using Global Positioning System (GPS) technology. The 1756-TIME module obtains time from various sources, and provides time synchronization on other devices by acting as a gateway between different time synchronization methods and standards.

Learn more about how to use the 1756-TIME module in the User Manual.

Problem

There is an upcoming event, on April 6, 2019: a GPS Week Number Rollover (WNRO) on Coordinated Universal Time derived from GPS devices. The following organizations have also released information about this event: Department of Homeland Security’s Memo, or US Naval Observatory notification. The ControlLogix Time Module, 1756-TIME, obtains a time value via a GPS signal, and may be impacted by this event.

Cause

GPS Time as defined in the legacy GPS navigation message (ICD-200), uses 10 bits to count GPS Week Numbers. This representation can only cover a finite period of 1024 weeks (19.7 year epoch).

GPS Time is presently in its second Time Epoch which will end on April 6, 2019 at midnight when the week number rolls over to Week #0. 

UTC timing displayed and/or time tags of receiver data containing PNT information could jump by 19.7 years.

Solution

Rockwell Automation has released firmware for the 1756-TIME/B module that addresses this issue.  Firmware revision is v3.005 and is available on the Product Compatibility and Download Center.  The updated firmware extends the last data to 27/Sept/2036.

1756-TIME/A (series A) cannot be flashed with 1756-TIME/B firmware. After the rollover, a fault will not be generated, and the series A TIME module will continue to operate with the incorrect time. There will NOT be a firmware fix for the 1756-TIME/A (series A). Migration to 1756-TIME/B series B (at the customer’s discretion and cost) would be the only solution.

The 1756-TIME module provides accurate time synchronization on different interfaces by using global positioning system (GPS) technology. The Trimble Condor series C2626 GPS module is the GPS technology used within the TIME module to transmit the proper date and time to a receiver by supplying the receiver with current week and current number of seconds in the week. By using this information, the receiver translates date and time into the more typical format: day, month, year, and time of the day. However, the field that contains the week number is a 10-bit binary number field. This limits the range of the week number to 0…1023, or 1024 total weeks.

GPS week zero started on 6 January 1980. The 1024-week counter ran out and rolled over on 21 August 1999. The week counter then reset to zero and has been recounting ever since. The next time the counter will reach 1023 and roll over to zero is on 6 April 2019.

The Trimble Condor series C2626 GPS module is affected by the rollover. As a result, we have updated the 1756-TIME module firmware to revision 3.005, which checks the manufacture date of the Trimble chip and patches the GPS week behavior if it is affected. This updated firmware extends the last date to 27 September 2036.

In the future, the Modernized GPS Navigation (CNAV and MNAV) message will have a 13-bit week binary number storage, which for all practical purposes solves this ambiguity permanently.

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