From 9215ff94d51f754f279866c2943144c469d90e69 Mon Sep 17 00:00:00 2001 From: Fred Wright Date: Fri, 15 Jul 2016 18:49:39 -0700 Subject: Corrects info regarding 13-bit week numbers. Also updates the "GPS Date Calendar" link, as requested by that website. Signed-off-by: Gary E. Miller --- www/hacking.html.in | 32 +++++++++++++++++--------------- 1 file changed, 17 insertions(+), 15 deletions(-) (limited to 'www') diff --git a/www/hacking.html.in b/www/hacking.html.in index 20a0d2ed..53deac3f 100644 --- a/www/hacking.html.in +++ b/www/hacking.html.in @@ -1192,11 +1192,11 @@ and bite on various future dates.

  1. The GPS radio format has a Y2K-style bug, the week counter - rollover, which happens either every 1024 weeks (roughly 20 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 time of - 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 2157.
    2. + 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.
  2. NMEA delivers only two-digit years.
    3. @@ -1216,20 +1216,22 @@ after a rollover. This can have side effects:

    have a recent ephemeris.
-

The public documentation is unclear, but it appears from a -reference in the Transmission Week Number section of IS-GPS-200 -PIRN-002 that whether you can get 10 or 13 bits is a function of the -satellite firmware revision, with 13 bits in the Block IIF and later -birds (the first of these was launched in May 2010). Of course your -receiver firmware also has to know that the extra three bits are -present; at time of writing in late 2010 this capability is very rare -and unavailable on consumer-grade receivers.

+

The new 13-bit week number is only provided by the new "CNAV" data, +which in turn is (or will be) available only in newly added GPS signals. +Based on the carrier frequencies used, only the newest of the new +signals (L1C) will be available to common civilian receivers, even with +compatible hardware and firmware. This signal is unavailable from +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.

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

-

When debugging time and date issues, you may find an -interactive GPS calendar +

When debugging time and date issues, you may find an interactive +GPS Date Calendar useful.

Hotplug interface problems

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