GPS Week Rollover – There is No Need to Panic

What is the GPS Week Rollover and why is it of interest (or concern) to GPS users? This article is being republished with the permission of xyHt Magazine and author Gavin Schrock.  Updated March 7, 2019.

Don’t panic, but …

Graphic of GPS Week Rollover

GPS Week Rollover

GPS typically works so wonderfully for so many uses because, on the surface, it seems as simple as receiving data from multiple satellites and resolving positions in your receiver—without pain or anxiety—whether your receiver is a cheap chip on a cell phone or a high-precision, high-ticket survey rover.

The signals, codes, and messages of GPS (the U.S. Navstar constellation—this does not affect the other GNSS constellations) had been brilliantly designed for efficient transmission. One element, as defined in the legacy GPS navigation message (LNAV, ICD-200), uses 10 bits to count GPS Week Numbers with a limit of 1024 numbered weeks, or a little under 20 years, before rolling over to a new “week zero.”

The second scheduled GPS week number rollover (WNRO) will occur, according the rollover info on gps.gov, at 18 seconds prior to the 0000Z boundary (Coordinated Universal Time) between April 6 and 7, 2019—around midnight GMT, or Saturday afternoon of the the 6th in North America. If a GPS receiver is running firmware or software that cannot resolve that calendar ambiguity, it could have a flashback to 1999—the time of the first cycle of 1024 weeks from the 1980 zero week.

If you remember the 1999 WNRO, odd articles in consumer tech publications claimed that it could be “worse than Y2K!” (guffaw!). But there were real instances of receivers that thought they were back in 1980, causing issues that were troubling but nowhere near as problematic as the dire predictions.

Here is the good news in short form: almost without exception, gear sold since 1999 is (or has been updated to be) “compliant” with the 2019 WNRO. Note that if any RTN (real-time network) of any kind is comprised of varied brands/models of receivers, the respective network operator would need to check for compliance of each type. It is rare though that any RTN is running any legacy or non-compliant receivers. Other good news is that this does not affect any other constellations, and further that GPS modernization plans include adding a 13-bit message, that would not be limited to 1024 weeks like the legacy 10-bit message. Instead, the 13-bit message would not require a rollover for 157 years.

Users concerns: The primary concern of GPS end users (that we often hear) is of course: “Will my receiver stop working properly after the rollover?” Most users are confident in the highly skilled engineers who design and develop their high-precision GPS/GNSS rovers who have taken steps to ensure there are no ill effects of the WNRO on their gear. But it can be hard to reconcile such confidence with the sometimes-ambiguous messaging they get through some of the respective sales folks about WNRO, leap-seconds, etc.

Users (speaking among themselves and on forums) decry the blanket responses of “upgrade your firmware,” which can, and often does, get viewed as over-simplified and perhaps sales-serving. Of course, it is a best practice to keep one’s gear up to date, for not only constellation and signals changes but to take care of unrelated bugs and to take advantage of new features—but folks like to do updates as schedules and resources permit and do not want to have to jump into action over uncertainty about things like the WNRO.

We asked users what kind of response would be most reassuring, and most responded that they wish there were official statements issued and/or that they could hear directly from GNSS engineers.

We’ve reached out to the manufacturers of survey-grade GPS gear—and asked if we could hear back from respective engineering/developer teams—with questions about what receivers and respective firmware would be okay. We’ve received several responses by the time this post first went live, and we’ll post more on this page as we receive them before the rollover.

Note about OEM/embed receivers: while our inquiries about the WNRO were focused on receivers for surveying/construction, we have received tech notices about OEM/embed boards built for other uses.  Example.   If you have OEM boards or embed components we highly recommend contacting the respective manufacturers for WNRO information.

Responses (concerning surveying  and construction receivers) though, have been similar: If you have modern gear, say 2003 or newer, then you should have no issues at all. That does not mean legacy, pre-2003 gear will be rendered “boat anchors,” as some had weathered the 1999 WNRO just fine, and users have found ways to edit affected Rinex observation files manually or with scripts for post-processing. Most suggest that users consult tech bulletins and firmware release notes for their specific makes and models. Here are the responses so far, in the order received.

For the entire article click here.

Republished courtesy of xyHt Magazine

By Gavin Schrock, PLS   @schrockg

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