[time-nuts] GPS USB dongle for time server

[Hal Murray]

> I want to do a crude (as in to the second or so) time server inhouse to  add
> into a group of high accuracy servers.  This is so that I can go off  grid
> and still get updates.


> since USB by its nature won't have an accurate exact dedicated line to  let
> the GPS toggle to do a time hack to the software, I can see why  RS232 is
> preferable with the hardware signal lines they have.

The jitter from USB is far from the limiting factor.


> Any pointers to reading or comment on this would be appreciated.  I am
> probably asking the query before I should, as far as research, and
> appreciate any replies. 

Most of the low cost USB GPS units are using the SiRF III chip set.  They all 
have roughly 100 ms of jitter.  One complication is that it isn't random 
jitter, it's mostly long term drift with big steps to get back in sync.
  http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif

As long as your goal is nearest second, I think almost anything will work 
well enough.

The Garmin USB 18 was much better.  Unfortunately, it wasn't as sensitive as 
competing units and it's been replaced by the 18x which has the typical 
horrible jitter problems.  I don't have a graph of the 18x handy, but here is 
data from a USB 18.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18USB-off.gif
(The Garmin USB 18/18x units don't speak NMEA, but that's just SMOP.)

I tried a couple of thumb-drive size/shape units.  The don't work very well.  
I don't have any good numbers to back that up.


If anybody finds a good (for timing) USB GPS unit, please let me know.



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