*From:The Register (British IT News Site)*
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*Fun fact: GPS uses 10 bits to store the week. That means it runs out... oh
heck – April 6, 2019*
*Nav gadgets will be Gah, Properly Screwed if you don't or can't update firmware*
By Shaun Nichols in San Francisco
<> 12?Feb?2019?at?21:37
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.
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 2^10 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 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 GPS.gov 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.
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.
GPS.gov 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.
For devices unprepared for the counter overflow, a firmware upgrade will be
necessary to keep the things working properly. GPS.gov 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. ?
