[time-nuts] time-nuts Digest, Vol 31, Issue 21
Dr Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Feb 7 06:22:02 EST 2007
Rob Kimberley wrote:
> Interesting paper. I agree that for nanosecond timing applications then
> antenna cable and matching are important. However, for frequency sync
> applications and non-critical timing, then the effects of the 75 vs. 50 ohm
> cable will not be noticeable.
> Rob K
>>> Those who maintain that its OK to use 75 ohm GPS antenna cable should
Except that just as varying multipath delays as satellite elevation
changes have an effect on the timing receiver output that is equivalent
to a frequency shift, the diurnal variation in the cable propagation
delay with temperature will also affect the timing error due to
reflections caused by cable mismatch. Thus the effect of the diurnal
variation of the cable propagation delay will thus have a complex effect
on the timing receiver PPS output timing. Variations in PPS output
timing are equivalent to frequency modulation of the PPS output.
If one is trying to achieve the frequency disciplining performance that
even single frequency GPS carrier phase tracking is capable, then such
effects are significant particularly if one is using the difference
between the phase and group velocities in the ionosphere to correct for
the propagation delay through the ionosphere.
This is one reason why it can be a good idea to stabilise the antenna
and cable temperatures.
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