diff options
Diffstat (limited to 'www/NMEA.txt')
| -rw-r--r-- | www/NMEA.txt | 53 |
1 files changed, 42 insertions, 11 deletions
diff --git a/www/NMEA.txt b/www/NMEA.txt index 316e9f60..a36576ae 100644 --- a/www/NMEA.txt +++ b/www/NMEA.txt @@ -1,6 +1,6 @@ = NMEA Revealed = Eric S. Raymond <esr@thyrsus.com> -v2.6, Jan 2011 +v2.7, Jan 2011 This is a list of NMEA 0183 sentences with field descriptions. It is primarily intended to help people understand GPS reports. @@ -121,18 +121,45 @@ no valid data for it. However, many receivers violate this. It's common, for example, to see latitude/longitude/altitude figures filled with zeros when the GPS has no valid data. -Date and time in GPS is computed as number of weeks from the zero -second of 6 January 1980, plus number of seconds into the week. GPS -time is not leap-second corrected, though satellites also broadcast a -current leap-second correction which is updated on six-month -boundaries according to rotational bulletins issued by the +== Dates and times == + +NMEA devices report date and time in UTC, aka GMT, aka Zulu time (as +opposed to local time). But the way this report is computed results +in some odd bugs and inaccuracies. + +Date and time in GPS is represented as number of weeks from the start +of zero second of 6 January 1980, plus number of seconds into the +week. GPS time is not leap-second corrected, though satellites also +broadcast a current leap-second correction which is updated on +six-month boundaries according to rotational bulletins issued by the International Earth Rotation and Reference Systems Service (IERS). -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 20 years). The next rollover would fall in 2019, but plans -are afoot to upgrade the satellite counters to 13 bits; this will -delay the next rollover until 2173. +The leap-second correction is only included in the satellite subframre +broadcast, roughly once ever 20 minutes. While the satellites do +notify GPSes of upcoming leap-seconds, this notification is not +necessarily processed correctly on consumer-grade devices, and will +not be available at all when a GPS receiver has just +cold-booted. Thus, reported UTC time may be slightly inaccurate +between a cold boot or leap 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 20 +years). The last rollover (and the first since GPS went live in 1980) +was in 1999; the next would fall in 2019, but plans are afoot to +upgrade the satellite counters to 13 bits; this will delay the next +rollover until 2173. + +For accurate time reporting, therefore, a GPS requires a supplemental +time references sufficient to identify the current rollover period, +e.g. accurate to within 512 weeks. Many NMEA GPSes have a wired-in +assumption about the UTC time of the last rollover and will thus report +incorrect times outside the rollover period they were designed in. + +For these reasons, NMEA GPS should not be considered high-quality +references for absolute time. Some do, however, emit pulse-per-second +RS232 signals which can be used to improve the precision of an +external clock. See [PPS] for discussion. == Error status indications @@ -2229,3 +2256,7 @@ Only emitted by uBlox Antaris chipset. - [[[WAAS]]] "WAAS Information" http://gpsinformation.net/exe/waas.html + +- [[[PPS]]] + "Pulse per second" + http://en.wikipedia.org/wiki/Pulse_per_second |