[time-nuts] Low cost GPS gadgets for timing

Hal Murray hmurray at megapathdsl.net
Wed May 6 10:24:30 UTC 2009


I'm looking for low cost GPS gadgets that are good for time keeping via ntpd.

Any suggestions?

I'm interested in not-so-good timing as well as the good stuff that attracts 
time-nuts.  As long as it's low cost.

USB doesn't support PPS and it has a bad reputation because it's polled.  But 
the polling is done in hardware with a 1 ms time scale.  It won't be great, 
but for lots of uses it's good enough.

The problem is that most of the low-cost GPS toys use the SiRF chip set.  It 
sucks for timing.  It looks like the NMEA sentences are sent from a timer 
with 100 ms ticks.
  http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif

The Garmin GPS 18 LVC used to be popular.  It's been replaced by the GPS 18x 
which is much more sensitive.  Unfortunately, the timing went way downhill.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18LVCx-off.gif

Anybody know how well the u-Blox chips work?  Are they used in any low cost 
units?  (USB ok.)


Just to make sure we are on the same wavelength.  I don't care about a 
constant offset.  I can easily correct for that.  It's the jitter that I 
don't like.  The time scale is wrong.  It wanders too slowly.  I can't filter 
it out.



-- 
These are my opinions, not necessarily my employer's.  I hate spam.






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