[time-nuts] Time servers on a well known web site.

[Hal Murray]

> Wonder too just how accurate it is, using a USB based (non PPS) GPS?

The picture looked like a GlobalSat BU-353.  I guess you could build a NTP 
server out one, but I wouldn't expect time-nuts quality.


I've been looking for low cost GPS units that work well with NTP.  I haven't 
found much.

USB has a bad reputation for timing because it's polled, but that polling is 
done in hardware on the order of a ms.  You can sanity check things by 
feeding a GPS with a serial connection to both a normal serial port and a 
RS-232 to USB gizmo.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18LVC-usb-off.gif
I think this says that USB works reasonably well and my kernel/whatever isn't 
getting the low-latency stuff right.

The old non-x Garmin GPS-18-USB was pretty good.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18USB-off.gif
The 18-x is pretty bad.
  http://www.megapathdsl.net/~hmurray/ntp/GPS18LVCx-off.gif

Most of the low cost USB units seem to use the SiRF chip sets.  They are 
horrible for timing.
  http://www.megapathdsl.net/~hmurray/ntp/GPSSiRF-off.gif

If anybody figures out how to get reasonable timing out of one of these 
things, please clue me in.


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