diff options
Diffstat (limited to 'timebase.c')
| -rw-r--r-- | timebase.c | 23 |
1 files changed, 12 insertions, 11 deletions
@@ -7,22 +7,23 @@ clock be accurate to within one second. It would be nice to relax this to "accurate within one GPS rollover period" for receivers reporting GPS week+TOW, but isn't possible in general. -Date and time in GPS is represented as number of weeks from the start -of zero second of 6 January 1980 mod 1024, 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 -(theoretically) three-month boundaries according to rotational -bulletins issued by the International Earth Rotation and Reference -Systems Service (IERS). Historically all corrections have been issued -on six-month boundaries. +Date and time in GPS is represented as number of weeks mod 1024 from +the start of zero second of 6 January 1980, and number of seconds into +the week. GPS time is not leap-second corrected, and has a constant +offset from TAI. Satellites also broadcast a current leap-second +correction which is updated on (theoretically) three-month boundaries +according to rotational bulletins issued by the International +Earth Rotation and Reference Systems Service (IERS). +Historically all corrections have been issued on six-month boundaries. The leap-second correction is only included in the satellite subframe 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, UTC time reported from NMEA devices may be slightly -inaccurate between a cold boot or leap second and the following +cold-booted. Thus, the time reported from NMEA devices, although +supposed to be UTC, may be offset by an integer number of seconds +between a cold boot or leap second and the following subframe broadcast. It might be best not to trust time for 20 minutes after GPSD startup @@ -64,7 +65,7 @@ we can't know which rollover period we're in without an external time source. 4) Only one external time source, the host system clock, is reliably -available. +available, although it may not be accurate. 5) Another source *may* be available - the GPS leap second count, if we can get the device to report it. The latter is not a given; SiRFs before |