4 ms·
There are a number of minor errors in the post: GPS time doesn't have leap seconds. GPS time contains leap seconds, but it will not recognize any further leap
by dfc 11y ago
There are a number of minor errors in the post:
GPS time doesn't have leap seconds. GPS time contains leap seconds, but it will not recognize any further leap seconds. GPS time contains nine leap seconds-- the nine that occurred before creation of GPS time in 1980. GPS time is 19 seconds ahead of TAI (9 leap seconds plus the 10 differential between utc and tai from 1950 or 1960) and 17 seconds behind UTC.
This number (GPS-UTC) will grow as more leap seconds are inserted in the future. This is not the entire picture and it is important that people do not hard code this assumption into their programs. Leap seconds can also be removed, that is GPS-UTC can also shrink. This has never happened but there is no point in hardcoding incorrect ideas about time scales in your program.
ntpd will act like all of its sources are insane, with huge offsets and jitter values approaching -17000 milliseconds. Jitter values will not approach 17k milliseconds. Jitter is the variance between repeated measurements of a configured peer.
- filmor 11y agoRegarding your first point: Is GPS time defined for dates before 1980? If not it does not have leap seconds in the sense that it skips or inserts seconds at specific times. It just has a constant offset to TAI, I guess to maximise the number of leap seconds that GPS can handle in its 1 byte leap second field.
- charriu 11y agohttp://www.usno.navy.mil/USNO/time/gps/usno-gps-time-transfer http://www.usno.navy.mil/USNO/time/gps/usno-gps-time-transfe... says: "The GPS epoch is 0000 UT (midnight) on January 6, 1980." So no, gps time is not defined before 1980.
- protomyth 11y agoSo, in the time period GPS is defined it contains no leap seconds.
- dfc 11y agoIf that is the case why is it not the same as TAI?
- deleted 11y ago[deleted]
- filmor 11y agoThe GPS signal carries information about the GPS vs. UTC drift in a 1 byte field. As said, my guess is that they just wanted to maximise the use of this field by setting it to 0 instead of 19 at the beginning.
- rachelbythebay 11y agoYou might want to test some of those assertions. Specifically the jitter one. Go on, mess up a NTP host and watch what happens.