[ntp:questions] Re: Fixed-location GPS mode?

David J Taylor david-taylor at blueyonder.co.not-this-bit.nor-this-part.uk
Tue Jun 6 06:46:05 UTC 2006


Tom Van Baak wrote:
> "David J Taylor" wrote:
>> I seem to recall reading somewhere that you could tell a GPS receiver
>> where it was, so that the PPS signal would depend on fewer
>> satellites - i.e. you didn't need four visible satellites to get a
>> complete 3D fix all the time.  Perhaps I simply mis-understood the
>> purpose of the "initial location" command?
>>
>> Did I dream this?  Does the GPS 18 LVC have this option and, if so,
>> what's the correct NMEA command to enable "fixed position"
>> operation?  Has anyone tried this - what improvements did you see?
>
> The GPS 18 LVC doesn't have a zero-D mode. However
> it is a very nice, compact, easy-to-interface, GPS timing
> receiver with a 1 us 1 PPS accuracy spec.
>
> Although this makes it a poor choice for work with atomic
> clocks it should be more than enough for NTP, yes?
>
> Recent GPS 18 LVC measurements I made showed it
> much better than spec; a couple hundred ns peak to
> peak and just 80 ns RMS for the 1 PPS output.
>
> /tvb
> http://www.LeapSecond.com

Thanks to Tom, Terje and Richard (hope that's everyone).

It seems the GPS 18 LVC does have a 2D mode (I hadn't known the correct 
term before) which can be enabled by the $PGRMC statement.  You need to 
tell it the height, of course.

I'm interested because my system only has a 180 degree view of the sky, 
and hence I would like to get the best performance with the fewest number 
of visible satellites.

I'm glad to hear the timing performance is good - it think the sensitivity 
could be a little better, though (it's not as good as my GPS 12XL with an 
external active antenna puck, used indoors).

Cheers,
David 





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