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

David Woolley david at ex.djwhome.demon.invalid
Wed Oct 31 08:30:48 UTC 2012

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.

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