On 2012-10-31, Richard B. Gilbert <rgilber...@comcast.net> wrote: > On 10/31/2012 5:04 PM, unruh wrote: >> On 2012-10-31, Richard B. Gilbert <rgilber...@comcast.net> wrote: >>> On 10/31/2012 4:30 AM, David Woolley wrote: >>>> Kennedy, Paul wrote: >>>>> I believe the answer to your question is 12.5 minutes. >>>>> >>>>> This is the time it takes to receive the full set of 25 almanac frames, >>>>> which contains the GPSTime/UTC offset (amongst other things). >>>>> >>>>> http://en.wikipedia.org/wiki/GPS_signals#Almanac >>>> >>>> I think he knows the time taken for the GPS receiver, which is a lot >>>> less than that. His concern is about how long ntpd takes once the GPS >>>> receiver is reporting the correct time. As noted, ntpd is not specified >>>> for this case, so makes no attempt to recover any faster than any other >>>> broken local clock case. >>>> >>>> The almanac you are referring to is a low resolution one to aid the >>>> receiver in finding satellites after a cold start. Once it has found a >>>> satellite, it should have a high resolution almanac for that satellite >>>> in about 30 seconds. Modern receivers tend to decode multiple >>>> satellites at once, which is how they get a fast start, so they may be >>>> fully acquired in 30 seconds. However, if there is no memory at all, it >>>> may take them some tome to find their first satellite, and locating >>>> subsequent ones may be slow until the full coarse almanac is received. >>> >>> >>> NTPD is a "slow starter"! Ideally, you will only start it once and >>> let it run for a few months. >>> >>> How slow is a "slow start"?. It can take NTPD up to ten hours to >>> synchronize within + or - 50 nanoseconds with whatever you are using as >> >> It will never get to within 50nsec. The interrupt processing is far more >> variable than that. You might get to withing a few micro seconds. >> >> >>> a time source. If you must boot your computer at 8:30 every morning, >>> NTPD is a poor choice! >>> >>> There is another "product" which will give you a "reasonable facsimile" >>> of the correct time in a very short time. I've never used it. I've >>> forgotten its name. Sorry about that. I'm sure that someone here >>> can recall the name I've forgotten! >> >> Chrony. >> It also gives better accuracy. >> > Accuracy long term or short term?
The standard deviation of the time as compared with a reference time is about a factor of three less than with ntpd. That is over a long term-- days and weeks. > > > _______________________________________________ questions mailing list questions@lists.ntp.org http://lists.ntp.org/listinfo/questions
