[ntp:questions] Using parse driver rather then nmea driver?

Unruh unruh-spam at physics.ubc.ca
Mon Oct 20 18:40:20 UTC 2008


hal-usenet at ip-64-139-1-69.sjc.megapath.net (Hal Murray) writes:


>>I have a NMEA GPS device that outputs the correct strings.  However
>>due to where I have to position the device it doesn't always have a
>>lock on three satellites and then goes into Void mode.  However the
>>time values are still being collected (It always sees at least one
>>satellite).

>Which particular NMEA unit are you using?


>>Is it possible to write a parse (127,127,8,0) driver to read only the
>>time values and ignore the void message and thereby make it the
>>stratum 0 source?

>If your unit is doing what I think it's doing, you probably don't
>want to do that.  The GPS unit is telling you that it doesn't really
>know the time, but its best guess is xxxx.  Why would you want to
>use time that's known not to be good?  It would work OK if you
>didn't have to coast for very long.  But then your PC can coast
>just as well for short times, maybe even better.

IF you know your position, then a single satellite is sufficient to give
the time to high precision (ie, microseconds assuming you know your position to 300m acccuracy). However, I suspect that your unit will not differentiate between seeing no satellites whatsoever or seeing just one or 2. Many have an on board oscillator which they use to interpolate between valid sightings. 
Ie, it will depend entirely on your gps unit. 






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