[time-nuts] PC time app

Hal Murray hmurray at megapathdsl.net
Sat Nov 26 01:58:17 EST 2011


david-taylor at blueyonder.co.uk said:
> Yes, although from some GPS devices the jitter may be worse than from
> Internet servers (depending on your connection).

I've been looking for "good", low cost GPS gizmos, preferably with no 
soldering required.  If anybody finds one, please let me/us know.

The best I've found is the Garmin GPS-18x.  It is only "good" if you can use 
the PPS signal and it requires some soldering.

All the low cost GPS unit's I've tested have horrible jitter.  They are 
crappy without PPS support.

In particular, the USB units don't have anything like PPS.  I'd call them 
good-enough if they worked as well as I hope.  The USB jitter is not a 
problem, at least for some/many people.  It's small relative to network 
delays.  I'd consider a USB device good enough to be interesting if it worked 
as expected.

The problem isn't just jitter, it's wander.  By that I mean very low 
frequency that's hard to filter out.  Ballpark number is 25 ms of jitter and 
100 ms of wander.  These are from SiRF chips on USB:
  http://www.megapathdsl.net/~hmurray/bb/gps/Holux-2.png
  http://www.megapathdsl.net/~hmurray/bb/gps/BU-353-gpgga.png

The older Garmin GPS-18 (non x) was good enough.  (but, unfortunately, not as 
sensitive and no longer available)
  http://www.megapathdsl.net/~hmurray/ntp/GPS18USB-off.gif


On the network....  I have a 384K DSL line.  It's mostly idle.  ntpd has no 
trouble filtering out the occasional poor samples when it happens to collect 
data while I'm loading a big web page.

On the other hand, I occasionally download CDs or such.  That takes hours.  
That ties up the line for long enough so that the queuing delays can confuse 
ntpd.  I've seen delays of 3.5 seconds.

There is a bufferbloat project working on that area, but it's going to be a 
lot of work.
  http://www.bufferbloat.net/
It's screwing things like VoIP.  If/when it gets fixed (or even improved) 
timkeeping will get better for free.




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







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