[LEAPSECS] GPS week-number data field size (was W1K GPS rollover for some time servers)

[Tom Van Baak]

Hi Peter,

GPS, as well as multiple other national GNSS systems, continues to evolve. There are new frequencies and new data formats. I suspect new receivers will be backwards compatible with old signals and formats for a long time. Sorry, I don't have carefully prepared set of links for you, but here's a couple:

http://www.gps.gov/systems/gps/modernization/civilsignals/
http://gpsworld.com/air-force-directs-early-civil-navigation-cnav-message-populated-l2c-and-l5-signals/
http://www.digitaltrends.com/mobile/gps-iii-explained-everything-you-need-to-know-about-the-next-generation-of-gps/
http://en.wikipedia.org/wiki/GPS_signals#Modernization_and_additional_GPS_signals

In that last link, note "The GPS week number is now represented as 13 bits, or 8192 weeks, and only repeats every 157.0 years, meaning the next return to zero won't occur until the year 2137. This is longer compared to the L1 NAV message's use of a 10-bit week number, which returns to zero every 19.6 years."

You even get DUT1 in:
http://www.navcen.uscg.gov/pdf/gps/IS-GPS-800.pdf
http://www.gps.gov/technical/icwg/
http://www.navipedia.net/index.php/GPS_Signal_Plan

There was once a time when your "sounds fraught with problems" would be my response too! But we survived going from 36-bit computers to 16 bits. And from 16 to 32, and 32 to 64. I once tapped ethernet coax cables with a drill and now we have fraught with problems RJ45 connectors. There there's USB 1 and 2 and 3. And IPv4 and IPv6. Cars evolved from 12 VDC cigarette lighter power sockets to fraught with problems 5 VDC USB sockets. My next car probably won't even have a cassette player!

So by now I'm now more confident that changes can safely occur in technology. The slower and more carefully planned the better. But adapting to new requirements, to new possibilities, to new performance is ok. This is one reason why I sympathize with those who want to abandon leap seconds. When leap seconds were proposed in the 60's, AC plugs were all 2-prong, radio chassis were hot, DOS didn't exist, 110 baud was standard, electronic voltages were 6.3 VAC (filament) and ~250 VDC (plate B+), there were no quartz wrist watches, or airbags. Evolution is ok. It might even be natural.

/tvb

----- Original Message ----- 
From: Peter Vince 
To: Tom Van Baak ; Leap Second Discussion List 
Sent: Tuesday, May 05, 2015 1:20 AM
Subject: GPS week-number data field size (was W1K GPS rollover for some time servers)


Hi Tom,



On 5 May 2015 at 00:40, Tom Van Baak <tvb at leapsecond.com> wrote:

As you know the GPS folks enlarged the 10-bit week number in the next signal spec so receiver manufacturers have less rope to hang themselves. 



Actually I wasn't aware of that, but am curious how it can be implemented in a working system.  All the old software will just be expecting a 10-bit number - will every GPS receiver need new software as of a certain date?  Sounds fraught with problems!


    Regards,


          Peter