Thursday, March 7, 2019

Fun fact: GPS uses 10 bits to store the week. That means it runs out... oh heck – April 6, 2019

GPS Week-Number Roll-Over (WNRO) problem

From The Register by Shaun Nichols

Nav gadgets will be Gah, Properly Screwed if you don't or can't update firmware

Older satnavs and such devices won't be able to use America's Global Positioning System properly after April 6 unless they've been suitably updated or designed to handle a looming epoch rollover.

The worldwide precise time broadcasting performed by the GNSS constellations (GPS, Galileo, GLONASS, Beidou)  is the most strategic and vital asset for all our society infrastructures.
picture courtesy of Marco Lisi, ESA

GPS signals from satellites include a timestamp, needed in part to calculate one's location, that stores the week number using ten binary bits.
That means the week number can have 210 or 1,024 integer values, counting from zero to 1,023 in this case.
Every 1,024 weeks, or roughly every 20 years, the counter rolls over from 1,023 to zero.

The first Saturday in April will mark the end of the 1,024th week, after which the counter (WN = Week Number) will spill over from 1,023 to zero.
The last time the week number overflowed like this was in 1999, nearly two decades on from the first epoch in January 1980.

You can see where this is going.
If devices in use today are not designed or patched to handle this latest rollover, they will revert to an earlier year after that 1,024th week in April, causing attempts to calculate position to potentially fail.
System and navigation data could even be corrupted, we're warned.
"GPS devices with a poorly implemented GPS Time-to-UTC conversion algorithm may provide incorrect UTC following a week number rollover," US Homeland Security explained in its write-up (PDF) of the issue this week.
"Additionally, some GPS devices that calculate the week number value from a device-specific date rather than the start of the current GPS Time Epoch may provide incorrect UTC at some other device-specific date."

As the Reg reader who tipped us off to the shortcoming noted, this could be a significant headache for data centers that use GPS timing for synchronization.
"Decent vendors should have patches. But who has been thinking about this?" our tipster told us.
"This could be a low-key Y2K style bug all over again, but with companies doing less preparation."

Fortunately, devices on sale right now should be prepared for this rollover and handle it gracefully.
Uncle Sam's GPS nerve-center says (PDF) receivers that follow the ICD-200/IS-GPS-200 specification should be able to deal with the week number overflow.
This basically means newer receivers built after, say, 2010 should be fine, provided they follow the specs and notice the rollover.

The creation of the GPS which serves as a reference to the epoch is January 6, 1980
About 20 years later, on August 21, 1999, the first Week Number Rollover
About 20 years later, on April 6, 2019, the next Week Number Rollover

To put it another way, if your gadget goes haywire in April, it's probably because of this.
If it works as normal: brilliant, it's not affected.
Consider yourself forewarned. also notes that the new CNAV and MNAV message formats will use a 13-bit week number to solve the epoch migraine right up until the planet becomes uninhabitable via climate change or we all blow ourselves up.

 Beware of the sudden profusion of sales of used GPS on ebay, which you will know with full knowledge of the facts that they will no longer work in a few days if that is not already the case.
source : 

Information from Furuno
For impacted GPS, a simple cold start (after resetting the almanacs/ephemerides) may solve the problem as it can permanently crash the GPS.
Note from Furuno : The date specified above is the date in which week number rollover occurs in our products listed above, and the date is independent from the one announced by the US Department of Homeland Security, namely, the 6th of April 2019, which is the date when GPS week number rollover occurs in the GPS as a system per se.
The difference stems from different initial dates of counting week between GPS system and the GPS chipset used in our products.

For devices unprepared for the counter overflow, a firmware upgrade will be necessary to keep the things working properly. recommends those unsure about their readiness for the turnover, particularly enterprises, should consult the manufacturer of their equipment to make sure they have the proper updates in place.

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