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Get ready for the next GPS Week Number Roll Over

20 February, 2019

If you use GPS trackers in your business, April 6, 2019, is probably a date worth marking in advance. This day will mark a special rollover of the GPS week number, or WNRO, that helps receivers tell time more precisely. On April,6 or after it some GPS trackers may start to perform strangely.

The potential problem revolves around the way that GPS handles the week element of the data that forms part of the navigation signal. The week number is encoded into the data stream by a 10-bit field.  A binary 10-bit word can represent a maximum of 1,024 weeks, which is approximately 19.7 years.  Each 19.7 year period is known in GPS terms as an “epoch”. At the end of each epoch the receiver resets the week number to zero and starts counting again – a new epoch begins.

The first epoch started when GPS was launched in January 1980; hence the first epoch of GPS time came to an end on 21stAugust 1999. As we approach the end of the second epoch, which will fall on 6th April 2019, we may well see problems caused by the rollover.  Some GPS trackers that utilise the date and time function, may not be able to cope.

Should you be concerned?

Because this it is the second time the GPS week rollover will occur, many receiver manufacturers have prepared for it, and newer receivers will continue to operate without issue.

You should be concerned, though, if either of the following applies:

  • GPS device has been fielded for more than 10-15 years without firmware updates
  • GPS device is a core component of a critical timing system – ask yourself – what is the impact to my system if the GPS receiver stopped operating or put out wrong GPS or UTC information

In these cases, we want to verify that an issue will not occur. At a minimum, we recommend consulting your GPS device manufacturer to confirm that the issue has been fully tested and will not occur.

Can Gurtam fix the problem?

Unfortunately, we can't. From Wialon side, we’re unable to prepare for WNRO since we can’t predict which trackers will be affected and which data they will transmit to Wialon.

Will It Happen Again?

GPS is currently undergoing a modernization program to upgrade the signals with new modulation and data message structures. The newer message types (CNAV and MNAV) use 13-bit binary numbers to represent the GPS week number, so the issue should not occur in the future when more receivers are using the newer GPS signals.

Source: orolia.com

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