[ntp:questions] What is the NTP recovery time from 16s step in GPSserver?

Richard B. Gilbert rgilbert88 at comcast.net
Wed Oct 31 19:52:42 UTC 2012


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 
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!





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