[time-nuts] GPS for Spirent Smartbits

Eduardo Jacob edu at kender.es
Thu Feb 15 16:33:28 EST 2007


Hello again

I have been thinking about this... It will no work,  as GPS needs to 
be in position hold which is not compatible to mobility.... We will 
think about connecting via 3G to an NTP server... and make some tests 
to see if results have any sense...

Eduardo


At 22:01 15/02/2007, you wrote:
>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.
>
>Eduardo/EA2BAJ
>
>
> >
> >>  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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