[time-nuts] GPS NTP update on Mac OS-X 10.7.4

[Hal Murray]

> I recently connected up a USB GPS to my Linux box and found it quite easy to
> get NTP to up from the GPS. I was hoping to get the same GPS to update the
> Mac running Lion 10.7.4, however, it has been a very frustrating uphill
> battle.

> Has anybody managed to get GPS NTP update working on OS-X? 

You will probably get much better answers to NTP questions over on the NTP 
lists.
  http://lists.ntp.org/listinfo
Down near the bottom is questions

GPS over USB generally doesn't work very well.  You will probably get better 
results over the net.

The problem isn't USB polling, it's firmware that adds about 100 ms of 
wander.  By wander I mean low frequency drift.  It's too slow to filter out.

Here is a graph from a SiRF III, the most common chipset in low cost USB GPS 
devices.  Most others are similar.
  http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif
If anybody finds a good one, please let me know.


> I did a sum link to /dev/gps0 and can cat the output and see the NMEA stream
> of alternate $GPRMC and $GPZDA messages, 

> I just cannot get it to accept the gps as a peer.

The GPRMC sentence has a flag to tell you if it is seeing enough satellites.  
Are you seeing A or V?  I think A is good and V is bad.



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