[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 It's been replaced by the GPS 18x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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