[ntp:questions] Reasons of NTP not to use GPS source

a.everett.000 at gmail.com a.everett.000 at gmail.com
Tue Apr 8 11:10:19 UTC 2014


On Monday, 16 September 2013 08:00:09 UTC+1, Igor Pavlov  wrote:
> Hi!
> 
> 
> 
> I am using GPS-receiver based on Geos-1m chip (
> 
> http://www.geostar-navigation.com/en/navigation_05.html)
> 
> 
> 
> I connected it to serial port and configured NTP.
> 
> It becomes unused by NTP: when do ntpq -p reuest ti puts "x" near
> 
> "GPS_NMEA(1)" record.
> 
> 
> 
> What reasons can be for this?
> 
> 
> 
> Example of "ntpq -p" output
> 
> 
> 
>      remote           refid      st t when poll reach   delay   offset
> 
>  jitter
> 
> ==============================================================================
> 
> xGPS_NMEA(1)     .GPS.            0 l   14   16  377    0.000  -303.07
> 
> 4.292
> 
> *stratum1.net    .PPS.            1 u   62   64  377   62.800  -68.052
> 
>  43.693
> 
> +dl120g7.naviteh 194.190.168.1    2 u   58   64  377   30.151  -100.04
> 
>  52.432
> 
> +89.221.207.113  192.36.133.25    2 u    -   64  377   10.006  -105.88
> 
>  64.279
> 
> 
> 
> 
> 
> -- 
> 
> Igor Pavlov

We find that the problem with many (not all) NMEA GPS receivers is that often too much data is transmitted between each PPS output. This can have the effect that the time output (ZDA) sentence can occasionally shift either side of its corresponding pulse output. This has the effect of a 1 second offset occasionally being added to time stamps supplied to the NTP daemon.
Sometimes, increasing the baud rate from the standard 4800 bps to 9600 or even 19200 bps can help by allowing more characters (data) to be transmitted between each 1PPS output.

Also, as previously mentioned, simply feeding a 3.3V or 5V pps output from a GPS receiver into a RS232 port will not work as the voltage levels are different. You will need a simple TTL logic to RS232 converter such as a MAX232 device to convert the PPS output signal to the correct voltage level.

Andy Everett
TimeTools GPS-Referenced NTP Servers
http://www.timetoolsglobal.com/information/gps-ntp-network-sync-products/



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