[time-nuts] GPS antenna length correction

Chris Albertson albertson.chris at gmail.com
Mon Apr 28 20:10:05 UTC 2014

On Mon, Apr 28, 2014 at 6:52 AM, Brian Lloyd <brian at lloyd.com> wrote:

> ..., additional antenna cable introduces a fixed
> delay value and hence a fixed constant that gets added to each path
> regardless of direction. It seems to me that this would produce a much
> "fuzzier" solution to position and/or variation in timing. Knowing cable
> length and propagation velocity, would allow the software to subtract that
> constant from all ranges and thus provide a more correct position and time
> solution. Is this not the case?
I think you have it exactly correct.  The antenna location determines the
geometry and the cable length adds only a fixed offset.  Many GPS receivers
have a command where you can tell them a "cable delay".    But also you
have to think about the other end of this.  How long is the cable that is
used for the output FROM the GPS receiver.  This cable introduces a delay
also.   You might even have a distribution amplifier of at least a TTL chip
acting as a driver or maybe a MAX232 chip doing level conversion.

There is a total system delay.  But really this only matters if you are
keeping absolute time of day.   Delay does not mater for frequency
measurement or for time interval measurement.


Chris Albertson
Redondo Beach, California

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