[time-nuts] GPS for Spirent Smartbits

bg at lysator.liu.se bg at lysator.liu.se
Sun Feb 18 11:46:06 EST 2007


On Thu, February 15, 2007 22:33, Eduardo Jacob said:
> 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....

Who says GPS needs to be in position hold?  Position hold is one way to
get better performance enabling usage of only one sat at a time. You
should get sub 1us from almost any GPS receiver, also while moving.
Ofcause you need to see enough sats with good enough signal (wrt multipath
etc).

> We will
> think about connecting via 3G to an NTP server... and make some tests
> to see if results have any sense...

I would very much think this is a worse setup than a moving GPS receiver.

--

  Björn


> 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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