Hal Murray wrote: > >>It isn't NTP which is the limit, but the GPS receiver acquiring lock from >>scratch after an indeterminate period of being switched off. The GPS >>could take minutes to lock and declare that it has valid time. > > From the spec sheet for the Garmin GPS 18x: > > Reacquisition: Less than 2 seconds > Hot: Approx. 1 second (all data known) > Warm: Approx. 38 seconds (initial position, time, and > almanac known; ephemeris unknown) > Cold: Approx. 45 seconds > > I believe that's typical of modern GPS receivers.
If "Cold" means there are no satellite data stored in the receiver then this can not be true unless some assumptions are made which may or may not be correct. The GPS timing frames are using GPS time, not UTC. GPS receivers can compute UTC from GPS time if they have received the UTC parameter set from the satellites, which contains the current UTC offset in seconds, plus a possible leap second announcement, plus some coefficients for a polynomial required to compute small offsets in a nanosecond/microsecond range. This UTC data set is part of the satellite's navigational messages which is repeated once every 12 minutes, so it may take up to 12 minutes after powering up the GPS receiver in cold mode until those parameters are available and thus the correct UTC time can be computed. Martin -- Martin Burnicki Meinberg Funkuhren Bad Pyrmont Germany _______________________________________________ questions mailing list [email protected] https://lists.ntp.org/mailman/listinfo/questions
