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Posted: 11 Mar 2019, 12:47
Posts: 685 Joined: 07 Aug 2011, 14:17
Will your GPS malfunction on 6th April 2019?
Question of obscure answer, since it depends on how and when the problem has been faced by the receiver manufacturers, assuming that all of them have addressed this issue.

But, what is the problem?
Well, it's the one that in English is called GPS Week Number Rollover . It turns out that the GPS receivers don't receive from the satellites the information about the current date in year, month and day format, but as the number of weeks that have elapsed since a pre-established origin date and, for doing this, they use a field (of 10 bits) that can only notify values ​​between 0 and 1,023, that is, useful for only 1,024 weeks (slightly less than 20 years): this period of time is named epoch .
The GPS system began operating on 6th January 1980 , so its first epoch covered until 21st August 1999 ; now we are approaching the end of the second epoch, which will occur on 6th April 2019 .

So, what can happen?
On 6th April 2019 the week counter will be again set to ZERO , so those GPS receivers that are not programmed for it could start to work erroneously ... although not necessarily that very same day, because the manufacturer could have established as date of origin not the date of the GPS epoch but the date in which he built his firmware .
To alleviate this problem, new messages of type CNAV (civil) and MNAV (military) use a field of 13 bits to report on the number of weeks (useful for 8,192 weeks, just over 157 years), so the most modern GPS receivers (those that recognize such a field) will not be affected by the problem.

Yesterday night I transmitted my concern about this issue to the technical team of Garmin (on their website they say that their GPS devices should not be affected but, that if they were, it would not be in the positioning aspect, only in those that make use of the time functions: current moment, recording moment of the track , sunrise and sunset time..., which is not a trivial matter either):
This is what Garmin says in their web page:

What is the GPS Week Number Rollover (WNRO)?
The GPS system is world renowned for its ability to provide accurate and reliable positioning and timing information worldwide. The GPS satellites transmit to users the date and time accurate to nanoseconds. However, back in 1980, when the GPS system first began to keep track of time, the date and time was represented by a counter that could only count forward to a maximum of 1024 weeks, or about 19.7 years. After 1024 weeks had elapsed, this counter “rolled over” to zero, and GPS time started counting forward again. This first rollover occurred in August of 1999. The second rollover will occur on April 6, 2019.

What is the Effect of a GPS Week Number Rollover Issue?
For GPS devices that are affected, after the rollover occurs, an incorrect date and time will be displayed. This incorrect time will also be used to timestamp track logs, compute sunrise and sunset, and other functions that rely upon the correct date and time. However, the positioning accuracy will not be affected. The device will continue to deliver the same positioning performance as before the rollover.

Is My Device Affected?
For many years, Garmin has anticipated and prepared for this event. Regardless, Garmin has been performing exhaustive testing of current and legacy devices to determine if they will be affected by the GPS week number rollover. Our testing shows the vast majority of Garmin GPS devices will handle the WNRO without issues.

The response of the Garmin Iberia Technical Assistance Service:

Contact Garmin support if you have problems with the WNRO.

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