[ntp:questions] Re: Effect of Antenna Cable Delay

Hal Murray hmurray at suespammers.org
Thu Sep 25 16:16:46 UTC 2003

>If there's a length of cable between the receiver and its antenna, what
>effect does that have on the location and the time that is derived?

You get the position of the antenna and the time at the antenna.

If you want to know the position or time at your computer, you might
want to try to correct for the cable.  How long is your cable?  Can you
measure the difference?  (Would the correction matter?)

When I plot the position of my GPS unit, it wobbles over about 100 ft
peak from the average.  A similar setup at work has a better antenna
position.  It's wander is only about 50 ft peak.

The speed of light in most cable is about 1/2 that in vaccuum.
Low loss cable is faster.  So it's somewhere between 1 and 2 ns per foot.
100 ft at 2 ns/ft would be 200 ns.  That's about at the limit of what
I can see.  My system has a roughly 10 microsecond pk-pk offset that
follows the daily temperature.  There are probably other errors that
are more important.  I'd expect better stability in an air-conditioned

Some (most?) fancy time-keeping boxes have a parameter to correct for the
antenna cable length.

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