Milspec Communication | GPS Y2K and Rollover Issues

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Global Positioning System Receivers
Week Number Rollover and Y2K Compliance

General Description | GPS Week Rollover | Y2K Compliance
Equipment Compliance Info | Related Links

General Description
There are 2 issues dealing with GPS compliance. The first is what is known as the GPS Week Number Rollover. The second is Y2K compliance.

GPS Week Rollover
GPS Week Number Rollover refers to the epoch week number transmitted in the navigation message by the NAVSTAR GPS satellites. The GPS calendar is based on a cycle of 1023 weeks. This means that at the end of week 1023, the calendar will rollover to week 0000 (zero), as opposed to counting up to 1024, and will continue to increment from 0000 (zero) then on until the next rollover event.

The rollover event will occur on 21-Aug-1999 at 23:59:47z.

The GPS week number rollover may cause navigation errors while the GPS receiver is operated before, during and after the rollover event. Check with the appropriate GPS manufacture to ensure continuous and reliable functionality of your unit during this event.

Some GPS receivers units may require the user to perform what is called an initialization, autolocate or a search the sky operation in order to reacquire satellite data after the GPS week number rollover occurs.

According to sources, the DOD, the governing unit responsible for GPS, has not yet determined how this event will occur. There are 2 possible scenarios. (a) All satellites in the GPS constellation will be updated simultaneously and (b), satellites will be 'grandfathered' in one at a time over a period of time. Figures floating around in the industry in regards to the update period is estimated to be near 4 weeks. (There will be more info on this as I get details.)

Possible symptoms and errors affecting GPS receivers for the rollover event include inaccurate position fixes and calculations, the generation of erroneous dates and difficulty in acquiring satellite signals and data.

GPS Y2K Compliance
GPS Y2K compliance refers to the change of date code as displayed and/or transmitted by the individual GPS receiver. Some GPS receivers do not make use of any notions of date and time which, in this case, the Y2K issues would not affect those particular units.

Other units do employ and make full use of both date and time code information. Such uses are date and time stamping of waypoints in the GPS receiver.

A Y2K issue of greater threat exists with GPS units connected to external equipment such as mapping units, computers, waypoint management software and radio equipment to name a few. Although date or time information may not be associated with any internal features of the GPS receiver, this timing information is continuously transmitted by the GPS receiver via the communication output port (RS-232 etc...). It is thus important to understand the behaviour of the various GPS units as conflict may arise when this GPS equipment is interfaced with external devices.

Some examples of timing output irregularities may be a total erroneous date output format, or a simple rollover to the year 00 (zero zero).

Furthermore, although the GPS unit may be Y2K compliant, the external hardware and software that the GPS receiver is interfaced to may not be Y2K compliant or may not accept irregularities in the year portion of the date code such as a rollover to the year 00 (zero zero) which could be interpreted as year 1900.

The Y2K rollover event will occur on 01-Jan-2000 at 00:00z.

Further complications?
Users of Precision Positioning (P) services and Encrypted (P) codes could experience additional problems. Although theoretical at this time, it could be possible that a GPS receiver experiencing a rollover from week 1023 to 0 may consider this as a ZEROISE condition. It is also feasible that the GPS may consider this as a ECM (spoofing) event. In any case, it would be prudent to have additional encryption keys on hand to reload and initialize GPS receivers during the rollover and Y2K events.

Equipment Compliance Information

• Rockwell Collins confirms that there will be no issues with Rockwell Collins supplied equipment.
• My tests show that the Trimble GPS Scout M+ does not appear to have any EOW rollover anomalies.
• Magellan NAV 1000M.

WARNING: The Magellan NAV 1000M did not pass the EOW rollover event. The 2 units that I have tested FAILED. This failure leads to the unit entering the re-initialize state after which the date field in the date/time group is erroneous. Manually downloading the almanac does not cure this problem. The almanac age is displayed as January 08, 1980. Furthermore, as you cycle through the setup functions, you will notice that each time you are at the date / time display, the date will vary and be erroneous. Some examples are (in mm/dd/yy format): 02/41/80, 02/43/80, 02/82/80 etc...

User Information Request
I would appreciate any feedback or information that users of GPS units and ancillary equipment might of collected or experienced in order to add your findings to this page in order to assist other users that might have questions or concerns related with GPS issues.

Click here to send your comments.

Additional Links:

NAVSTAR GPS JPO