[ntp:questions] What is the NTP recovery time from 16s step in GPSserver?
Richard B. Gilbert
rgilbert88 at comcast.net
Wed Oct 31 21:25:24 UTC 2012
On 10/31/2012 5:04 PM, unruh wrote:
> 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).
>>> 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.
Accuracy long term or short term?
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