Mailinglist Archive: opensuse (1420 mails)

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Re: [opensuse] openSUSE + gpsd + PPS + NTP
Le 15/01/2014 17:00, Roger Oberholtzer a écrit :

The problem is not the accuracy of the GPS clock. Let's pretend it is perfect.
That is only internal. It must communicate this information externally
somehow. This is via serial, USB or ethernet. All three of those are non-
deterministic in that you do not know how long it takes for the data to get
from the GPS into your system. That jitter is a source of error. With the
serial port, the GPS can toggle a line that interrupts the client device,
allowing improved interpretation of the time when, eventually, it arrives.

I mean, what is the gps saying when there are no satellite visible? is it
transmitting anything? don't it transmit at least the time?

This is not standard. Some report whatever their clock says. Others transmit
nothing. We use the NMEA ZDA record for this. It seems to be transmitted even
when there are no satellites.

is this not enough? I hardly see hox you can get better result :-(

Until you see something like a GGA record, you
have to assume there has not been a sat fix and thus the time in the ZDA record
is not known to be current UTC time. The NMEA protocol is highly bad. Read
Eric Raymond's assessment at http://esr.ibiblio.org/?p=801

Eric being one of the best author I know, I see no other solution than the one I quoted in the pdf in my last post, that is radio time or physical timer, ntp should try to fix the transmission problems - what other solution??

jdd



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