[time-nuts] GPS for Spirent Smartbits

edu at kender.es edu at kender.es
Thu Feb 15 16:01:43 EST 2007

Hi murray

At a good price a few ms would be very fine. And you have given me an
idea. We already have a NTP server at the labo with a M12 (not the timing
version) now the question is to make it mobile... I already need a
computer for controlling the Smartbit... so I only need a switch and to
cables to make a mini-mobile-intranet... I'll check this befor jumping in
the big bucks wagon. Best regards and thank you.


>>  If you have an Internet access you can get a time reference with NTP
>> and even do timed captures.
>> We  would like to get 2 GPS timing systems in order to test influence
>> of handovers in Wimax on moving vehicles.
> What sort of timing accuracy do you need?
> Assuming a few ms rather than a few microseconds is good enough...
> I didn't see much about timing in that blurb.  It said it provides NTP
> service, but I didn't see anything fancy about where it got the time to
> "serve".  The ntp code is normally both a server and client, so I assume
> it
> can get the time from the net.  You can test that by pointing it at your
> local NTP server or one out on the net.  That should get you off the
> ground,
> but the accuracy may not be very good.
> The favorite low cost GPS unit in the NTP community is the Garmin
> GPS-18-LVC.
>  (There are two other models of the GPS-18, but they don't have the PPS
> signal.)  You can get them for under $100.  It requires some
> "assembly"[1].
> The usual trick is to steal power from a USB port.  For more info, start
> here:
>   http://ntp.isc.org/bin/view/Support/ConfiguringGarminRefclocks
> or feed >garmin GPS-18 ntp< to google.
> GPS may not work in a lab full of computers.  (You can test that with a
> normal hiking GPS unit.)  The clean solution is to put the antenna up on
> the
> roof.  Mine works most of the time inside my house, but only if it's up
> high
> near the ceiling.  You may have to locate the server near a window and/or
> kludge up some longer cables or ...
> I'm assuming you can find two old PCs to dedicate for an NTP server on
> each
> end.  It would be simpler if you could get identical hardware and software
> at
> both ends.  It might work if you run it on a personal machine but that
> will
> probably add more jitter if the user is doing anything.  You might have to
> experiment.  NTP doesn't need much CPU or memory.
> What I would use as a sanity check would be to slowly send short packets
> from
> A to B and measure the delay.  There will be two types of delay.  One is
> speed of light (and silicon processing).  The other is queuing.  The
> reason I
> said slowly is to make sure the test is not generating enough traffic to
> contribute to the queuing delays.
> If you make a histogram of the delays, the left edge should be the
> no-queuing
> case.  If the histogram in one direction matches the histogram in the
> other
> direction your clocks are probably good.
> 1] "Some assembly required" may be an American joke.  It refers to
> Christmas
> where the father has to assemble the new toys for the kid.  Sometimes it's
> not easy.  They usually print a warning on the box but it doesn't tell you
> how much work it will be.
> --
> These are my opinions, not necessarily my employer's.  I hate spam.

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