[ntp:questions] garmin 18x and linux

unruh unruh at wormhole.physics.ubc.ca
Sat Sep 3 04:23:24 UTC 2011

On 2011-09-03, Chris Albertson <albertson.chris at gmail.com> wrote:
> On Fri, Sep 2, 2011 at 12:06 AM, unruh <unruh at wormhole.physics.ubc.ca>wrote:
>> No. ntp can use the GPS nmea sentences to time the clock to a few 10s of
>> ms. -- you may need to fudge the time offset to make up for the delay of
>> the nmea sentence.
> Yes, maybe NTP can but not with this Garmin 18 unit.   Some GPSes are better
> than others.  And like I said very good units are available for not much
> money.  These Garmin units have problems with their NMEA timing.   I tried
> to use an even earlier unit and had the same result.   Not worth knocking
> your head on a wall when good timing units are available for cheap.
> The problem is not just with the timing of the serial data coming out of the
> GPS.  There are FIFO buffers in the Serial port in the computer and in the
> OS.    PPs is handled by a very low latency interrupt driven interface that
> tie stamps the PPS.

I do not argue that pps is not better than nmea. It is. But that is
irrelevant. The question is whether or not nmea can be used to control
the system to a few ms.

If you are saying that this particular nmea unit has such huge
fluctuations, then it would be good to have some backing for that. 
The FIFO buffers are certainly able to better than 100ms. And the OS
does not take that much time. 

> Chris Albertson
> Redondo Beach, California

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