From 0402eb81691882b4d693f9009f1f7369b205fa1d Mon Sep 17 00:00:00 2001 From: Sanjeev Gupta Date: Tue, 28 May 2019 02:31:44 +0800 Subject: Update documentation on the 2019 GPS week rollover Also pulled in Gary's commit 41141c03a into NMEA.adoc Signed-off-by: Gary E. Miller --- www/NMEA.adoc | 18 +++++++++++++++--- www/gpsd-time-service-howto.adoc | 7 ++++--- www/hacking.html.in | 6 +++--- 3 files changed, 22 insertions(+), 9 deletions(-) (limited to 'www') diff --git a/www/NMEA.adoc b/www/NMEA.adoc index eb10cdd4..f3594fd2 100644 --- a/www/NMEA.adoc +++ b/www/NMEA.adoc @@ -210,12 +210,13 @@ second and the following subframe broadcast. GPS date and time are subject to a rollover problem in the 10-bit week number counter, which will re-zero every 1024 weeks (roughly every 19.6 -years). The last rollover (and the first since GPS went live in 1980) -was in Aug-1999; the next will fall in Apr-2019. The new "CNAV" data +years). The first rollover since GPS went live in 1980 was in Aug-1999, +followed by Apr-2019, the next will be in Nov-2038 (the 32-bit and POSIX +issues will probably be more important by then). The new "CNAV" data format extends the week number to 13 bits, with the first rollover occurring in Jan-2137, but this is only used with some newly added GPS signals, and is unlikely to be usable in most consumer-grade receivers -prior to the 2019 rollover. +currently. For accurate time reporting, therefore, a GPS requires a supplemental time references sufficient to identify the current rollover period, @@ -1241,6 +1242,17 @@ Field Number: Example: $GNGSA,A,3,80,71,73,79,69,,,,,,,,1.83,1.09,1.47*17 +Note: NMEA 4.1+ systems (in particular u-blox 9) emit an extra field +just before the checksum. + +----------------------------------------------- +1 = GPS L1C/A, L2CL, L2CM +2 = GLONASS L1 OF, L2 OF +3 = Galileo E1C, E1B, E5 bl, E5 bQ +4 = BeiDou B1I D1, B1I D2, B2I D1, B2I D12 +----------------------------------------------- + + === GSV - Satellites in view === This is one of the sentences commonly emitted by GPS units. diff --git a/www/gpsd-time-service-howto.adoc b/www/gpsd-time-service-howto.adoc index 3bee4720..c4ae7553 100644 --- a/www/gpsd-time-service-howto.adoc +++ b/www/gpsd-time-service-howto.adoc @@ -137,12 +137,13 @@ second and the following subframe broadcast. GPS date and time are subject to a rollover problem in the 10-bit week number counter, which will re-zero every 1024 weeks (roughly every 19.6 -years). The last rollover (and the first since GPS went live in 1980) -was in Aug-1999; the next will fall in Apr-2019. The new "CNAV" data +years). The first rollover since GPS went live in 1980 was in Aug-1999, +followed by Apr-2019, the next will be in Nov-2038 (the 32-bit and POSIX +issues will probably be more important by then). The new "CNAV" data format extends the week number to 13 bits, with the first rollover occurring in Jan-2137, but this is only used with some newly added GPS signals, and is unlikely to be usable in most consumer-grade receivers -prior to the 2019 rollover. +currently. For accurate time reporting, therefore, a GPS requires a supplemental time references sufficient to identify the current rollover period, diff --git a/www/hacking.html.in b/www/hacking.html.in index df9ddcb8..d7b870e6 100644 --- a/www/hacking.html.in +++ b/www/hacking.html.in @@ -1211,8 +1211,8 @@ and bite on various future dates.

rollover, which happens either every 1024 weeks (roughly 19.6 years) or every 8192 weeks (roughly 157 years), depending on whether your receiver can decode a 10-bit or 13-bit GPS week field. At the time of - this writing the last 0 week was in 1999, the next 10-bit wraparound - will be in 2019, and the next 13-bit wraparound will be in 2137. + this writing the last 0 week was in 2019, the next 10-bit wraparound + will be in 2038, and the next 13-bit wraparound will be in 2137.
  • NMEA delivers only two-digit years.
    • @@ -1241,7 +1241,7 @@ satellites earlier than Block III, which are currently (July 2016) not expected to begin to launch earlier than September 2016. Given that it takes years to launch a full constellation of satellites, it's highly unlikely that CNAV data with "operational" status will be available to -common civilian receivers in time for the April 2019 10-bit rollover.

    +common civilian receivers for some years yet.

    For these reasons, GPSD needs the host computer's system clock to be accurate to within one second.

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