[ntp:questions] GPS NG type 46 refclock tests

David Taylor david-taylor at blueyonder.co.uk.invalid
Sun Dec 7 11:26:14 UTC 2014

On 07/12/2014 09:44, juergen perlinger wrote:
>> david-taylor at blueyonder.co.uk.invalid> wrote:
>>>    ln -s /dev/ttyAMA0 /dev/gps0
>> Sure that can work.  I haven't looked at Refclock 46 in any detail though.
> That's indeed the way it is intended to work. The GPSD-JSON driver uses
> the same naming scheme as the NMEA driver -- It makes switching between
> NMEA and GPSD a bit more easy while experimenting.
> NOTE:  You must also start GPSD on the same link (like 'gpsd -n
> /dev/gps0') or the driver will not find its device.
> If there is need for it, I'm willing to do the necessary stuff to let
> the driver do the link evaluation. Then the native device name would be
> used by the driver. This would still require the link for NTPDs sake,
> but GPSD could use ttySxy or whatever.
> Feel free to file a bug (well, a feature requirement) against the driver
> if you feel hampered by the current setup!

Thanks for the TVP/TPV correction.  You used to hear a lot about TVP but 
it seems to have gone out of fashion today!

I could not get this to work to start with, gpsd complaining that it 
could not find a device, but reconfiguring it as you suggested to look 
for /dev/gps0 rather than /dev/ttyAMA0 fixed that.  So it's working, but 
there is a significant problem...

The timing variation on the serial data from the Adafruit/MTK3339 GPS is 
quite large, and NTP won't sync to it alone, at least when there are 
other LAN or Internet sources available.  As NTP won't accept it (marked 
as "-" in the tally code), the PPS which it should offer is never 
accepted, so I have had to go back to using the type 22 ATOM refclock as 
well as the type 46 and LAN-based servers as well.  I might just as well 
use type 28 (shared memory gpsd) in addition to the ATOM driver as use 
type 46.  Perhaps it should at least pass PPS when it is available, or 
perhaps NTP should use its PPS irrespective of whether it is a avlid 
time or not, but that's completely against what appears to be good practice.

So it appears that type 46 does nothing for me.  Perhaps I'm missing 
something, though.  I had been thinking that type 46 could replace 
/both/ type 28 and type 22 drivers.

I've learnt a little more about Linux, though.

Thanks for your help, and there's no need to do anything further on my 
behalf.  It might help others were your note about reconfiguring gpsd 
after creating the symlink were on the type 46 Web page.

Web: http://www.satsignal.eu

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