[LinuxPPS] Re: nmea driver

[Bernhard Schiffner]

Am Donnerstag, 31. Mai 2007 20:23 schrieb Udo van den Heuvel:

> 1 second is way big of an error. Something must indeed be wrong.
>
> > The pps output is a pulse of about 25 mS. on these boards, so by
> > changing the assert/clear on flag 2 I can change the offset to -1025.
> >
> > Any ideas?
>

1.) Find a NTP-Server you trust.
     (ntp2.ptb.de for me)
2.) Mark it as the only server in /etc/ntp.conf.
3.) Let the ntpd stabilze (will take hours)
4.) Keep in mind that the _accuracy_ is <= 1/2 ping-time to this server.
     (about 70 ms for analog modem connection)

5.) Connect your GPS-receiver to the PC
    (dont use it as server in /etc/ntp.conf)
6.) Make it start, organise a readout (minicom on /dev/ttyS*)

7.) Check the PPS-assert / clear time in the /sys/.... entry.
8.) One of them should be "best fit" to the full second (see 4.)
9.) _Only_ if you see this, you proceed with insert your device as server
     in /etc/ntp.conf.

It's my personal experience, that some GPS-receiver send wrong, not 
UTC-aligned PPS-signals if they have no position fix (no SV's in sight). 
But this is normaly marked as bit in the $GGA message.
So try to understand your $GGA messages ...

Bernhard