See also the Garmin Engineering and Joe Mehaffey's comments at end
> Which is it? "within one microsecond" or "ahead by several
seconds" or "ahead by 12 seconds"?<
>Also: Why does my "locked on" GPS sometimes read "right on" and sometimes differ from standard time by 11 or 12 seconds?<
GPS has become the world's principal supplier of accurate time. It is used extensively both as a source of time and as a means of transferring time from one location to another. There are three kinds of time available from GPS: GPS time, UTC as estimated and produced by the United States Naval Observatory, and the times from each free-running GPS satellite's atomic clock. The Master Control Station (MCS) at Falcon Air Force Base near Colorado Springs, Colorado gathers the GPS satellites' data from five monitor stations around the globe. A Kalman filter software program estimates the time error, frequency error, frequency drift and Keplerian orbit parameters for each of the satellites and its operating clock. This information is uploaded to each satellite so that it can be broadcasted in real time. This process provides GPS time consistency across the constellation to within a small number of nanoseconds and accurate position determination of the satellites to within a few meters.
Because of this process, GPS cannot tolerate the introduction of
leap seconds. Hence, in 1980, when the Department of Defense started keeping
time on the GPS satellites, its system time and frequency were set to agree
with UTC(USNO MC). At that time, TAI minus UTC was 19 seconds. Since
then, UTC has been delayed many leap seconds and GPS time has not. Hence,
GPS time is still very close to TAI minus 19 seconds. The
specification on GPS time is that it is to be kept within one microsecond of UTC(USNO MC) modulo one second. In other words, as a leap second is introduced into UTC(USNO MC) time, no such step occurs in GPS time. But GPS time is still steered to agree as well as possible with UTC(USNO MC), as if no leap seconds had occurred since 1980. In practice, the steering performance is much better than the one-microsecond specification; typically, it is well within 40 nanoseconds.
In order to provide an estimate of UTC time derivable from a GPS signal, a set of UTC corrections is also provided as part of the broadcast signal. This broadcast message includes the time difference in whole seconds between GPS time and UTC. During 1996 GPS time minus UTC time was 11 seconds. Also included in this message is the rate and time difference estimate between GPS time and UTC(USNO MC) modulo one second. This allows a receiver, in principle, to calculate an accurate estimate of UTC(USNO MC). The mission goal is 28 ns (1 sigma). Outside of the purposeful current degradation of the GPS signal (called Selective Availability, SA) by the DoD for security purposes, this calculation may have an accuracy of about 10 nanoseconds (ns) on an rms basis. Since USNO has been successful in predicting UTC to within about 10 ns, combining these two independent error sources yields a real-time potential uncertainty for UTC available from GPS at about the 14-ns level. In practice, SA prohibits achieving this accuracy level unless special clock systems and filtering techniques are employed. The SA degradation can be filtered away.
(Quoted from Hewlett Packard Application Note 1289: The Science of Timekeeping, by David W. Allan, Neil Ashby and Clifford C. Hodge. See the entire article at: http://www.allanstime.com/Publications/DWA/Science_Timekeeping/index.html
Marc Brett +44 181 560 3160 Western Geophysical
Marc.Brett@waii.com 455 London Road, Isleworth
FAX: +44 181 847 5711 Middlesex TW7 5AB UK
Start of Garmin quote>
Provided the unit has collected current leap second count from the navigation message, (current leap second difference from GPS time is only broadcast once in a 12.5 minute Nav. message), or current leap second has not changed since the last time the unit collected this variable, the time displayed on the front of the unit should be accurate to within 1 second of UTC.
>end of Garmin Quote
Joe Mehaffey comments:
This means that IF your GPS does not have (or does not save) the leap second offset from last time it was operated, your time may be off by perhaps 12 seconds until the complete NAV MESSAGE is received by the GPS. Jack and I have observed that "typically" Garmin GPS receivers display time which is delayed from about 1/2 to 1 second behind UTC. Lowrance GPS receivers are usually between 1 and 2 seconds delayed behind UTC. In both cases, this is a result of the display driver subroutine having low priority as the "GPS internal clock" is within a few nanoseconds of correct.
Similarly, the NMEA time output on the serial link is typically delayed a second or two depending on various factors.