Re: [LEAPSECS] a system that fails spectacularly

From: Ed Davies <ls_at_edavies.nildram.co.uk>
Date: Wed, 07 Dec 2005 10:41:57 +0000

Francois Meyer wrote:
> ....
> I hardly understand how it is reasonably possible to use a
> GPS-derived UTC without taking into account the leap second
> information from the GPS navigation message.
>
> Unless the unit gets the UTC-GPS offset from the receiver
> just once at hardboot time and then forget about leap secs...
>
> Puzzling.

I doubt the unit deals with GPS time at all. Probably it
sets its own clock to the UTC value reported by the receiver,
leaving all handling of GPS time, UTC-GPS offsets, leapseconds,
etc, to the GPS receiver. Then, when the GPS receiver updates
its UTC estimate by one second early in the new year the unit's
clock is suddenly out by a second.

The fact that they write that UTC is adjusted in the first few
minutes of 2006 is a clue. Of course, the adjustment really
happens in the last minute of 2005. At a previous leap second
(1995/96) I logged the NMEA output of a Garmin 100 GPS receiver.
This (fairly old) receiver outputs fix information once every
two seconds. The change from odd numbered to even numbered
seconds happen a few fixes after midnight:

$GPRMC,235959,A,5137.56,N,00047.48,W,001.6,019.7,311295,,*07
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,235959,,,,,,T,,M,,N,*17
$GPRMC,000001,A,5137.56,N,00047.48,W,001.5,021.4,010196,,*0E
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,000001,,,,,,T,,M,,N,*17
$GPRMC,000003,A,5137.56,N,00047.48,W,001.6,024.1,010196,,*0F
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,000003,,,,,,T,,M,,N,*15
$GPRMC,000005,A,5137.56,N,00047.48,W,001.7,026.7,010196,,*0C
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,000006,,,,,,T,,M,,N,*10
$GPRMC,000007,A,5137.56,N,00047.48,W,001.6,025.8,010196,,*03
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,000008,,,,,,T,,M,,N,*1E
$GPRMC,000009,A,5137.56,N,00047.48,W,001.7,027.5,010196,,*03
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,000010,,,,,,T,,M,,N,*17
$GPRMC,000012,A,5137.56,N,00047.48,W,001.8,028.1,010196,,*0D
$GPRMB,A,,,,,,,,,,,,V*71
$GPR00,,,,,,,,,,,,,,*45
$GPGLL,5137.56,N,00047.48,W*75
$PGRMA,437,f,2*01
$GPXTE,A,A,,,N*3C
$GPBWC,000012,,,,,,T,,M,,N,*15

Ed Davies.
Received on Wed Dec 07 2005 - 02:43:35 PST