[LinuxPPS] still the strangeness

[Udo van den Heuvel]

Bernhard Schiffner wrote:
> Do you have a "vaild and accurate position"?

Yes.

> What does the pps-signal do if this is not given (free running, not sent...?)

Reception is not optimal here yet (have to place the GPS on the roof).
I lose PPS sometimes for relatively short periods of time.
Amount of sats is then below 4 or 3 or so?

> How many satellites does your receiver see?
> What are the values of VDOP HDOP?

I would have to check. Currently the status has improved.
Later: 4 sats. hdop 3.5m.

> The statement doesn't say anything about the time difference between the 
> pps-signal and the start of the messages. This can be anything between 0 and 
> 999 ms.

That does not matter. Certainly as long as there is a steady stream of
PPS pulses and NMEA data right after each PPS. (as described in the manual)
The original time of a PPS can then be easily reconstructed, thus not
needing outside time sources to attach each pulse's time.

>> /dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4, Flags: low_latency
> May be this answer is independent from my remark.
> (What does low_latency say ? interrupt at the beginning of each received 
> charakter?)

No more fifo. Ans thus the old 8250 behaviour of 1 char per irq.

>> Does the NMEA driver this on NMEA data only?
> 
> !!! YES !!!

Of course, since it is a NMEA driver but it would be interesting to have
a good method (besides the difference in accuracy) to see if the PPS is
OK and received or not, from within ntpd.
(for now we use kernel debugging)

>> PS: progress...
>>
>> I did fiddle with everything. Upgraded firmware of the GPS18, checked
>> Next step?
> 
> Offset your local NMEA-receiver until it's in shape with the others.
> Forget about a lot of peers in ntp.conf in this state of affairs.

Only the few I posted. ntp.xs4all.nl should be closest but it does not
have an own GPS receiver.

> cat /proc/pps/0*/* must react!

It doesn't.
But I do have a feeling that I have PPS now.
Time offset was much closer to 200ms (one PPS length), jitter is lower.