[ntp:questions] Re: Question to NTP developers
ecoca at eed.usv.ro
Thu Aug 24 05:44:43 UTC 2006
Hal Murray wrote:
> What type of GPS receiver are you using? Have you turned on
> logging and looked in (both) clockstats?
> Some GPS receivers turn off the PPS signal when they know they
> don't have a good signal. Some leave it running.
Yes, I have turned on all possible loggings on ntpd. There is nothing
strange to see.
I have Garmin GPS18LVC - correctly configured (without PPS Diabled or
other things - like power savings features).
> Most GPS receiver give you some sort of status indication to tell you if
> the signal they are receiving is reasonable. For example, here are
> a few lines from a receiver that uses NMEA:
> 53963 17392.501 127.127.20.2 $GPRMC,044953,A,3726.0639,N,12212.1673,W,000.3,105.4,160806,015.3,E*6B
> 53963 17456.337 127.127.20.2 $GPRMC,045057,V,3726.0630,N,12212.1664,W,,,160806,015.3,E*7C
> 53963 17519.324 127.127.20.2 $GPRMC,045200,V,3726.0630,N,12212.1664,W,,,160806,015.3,E*7C
> 53963 17584.547 127.127.20.2 $GPRMC,045305,A,3726.0804,N,12212.1886,W,000.1,050.9,160806,015.3,E*69
> 53963 17647.534 127.127.20.2 $GPRMC,045408,A,3726.0806,N,12212.1892,W,000.0,064
> 53963 17712.586 127.127.20.2 $GPRMC,045513,A,3726.0805,N,12212.1929,W,000.1,096
> The V means bad, and the A means good. So the signal dropped out for
> somewhere between a bit over a minute and three minutes.
Not my case ! The receiver is mounted with almost 99% sky view. I've
verified this for 3 weeks of loggings after installing.
> One possible reason why your two boxes are not doing the same thing is
> that they aren't actually looking at the same data.
Hardware the two boxes are looking at the same data (as the signal are
hardwired connected at the same GPS).
The info above
> is only part of data that goes over the serial line. Normally NMEA
> receivers send a line of info every second (or every N seconds).
> ntpd only looks for info every 64 seconds. (handwave, that's probably
> not right but it's close enough) If you have two boxes looking
> at the same signal they may be out of phase.
My GPS is configured to send three strings (RMC, GLL and GSA) every
second - I've verified this.
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