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?
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