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

unruh unruh at invalid.ca
Wed Oct 31 21:04:01 UTC 2012

On 2012-10-31, Richard B. Gilbert <rgilbert88 at 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!

It also gives better accuracy.

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