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

Martin Burnicki martin.burnicki at meinberg.de
Wed Oct 31 10:52:40 UTC 2012

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.

What Paul wrote is correct. Almanac data for all satellites is only part 
of the navigational information sent repeatedly once every 12 minutes. 
Another piece of this data is the UTC parameter set which includes 
current UTC to GPS time offset, leap second information, etc.

In worst case. if a GPS receiver starts from scratch and starts to track 
the first satellite, transmission of the UTC offset parameters may have 
just passed, so the receiver will have to wait 12 minutes until those 
parameters are sent next time.

If you are just interested in your current geographic position then you 
don't have to wait 12 minutes since UTC parameters are not required for 
navigation. For navigation it is sufficient to be able to track 4 
satellites to determine position (x, y, z) and time offset t from the 
GPS system time.

So ideally the GPS receiver would tell if its UTC data is valid, or not. 
Just checking if UTC offset is > 10 as mentioned in a different post is 
a possible workaround if no such status information is available.

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

More information about the questions mailing list