[time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

Attila Kinali attila at kinali.ch
Mon Nov 21 18:11:18 EST 2016

On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp <kb8tq at n1k.org> wrote:

> The reflection issue ahead of the antenna is a reflection of the signal from 
> a single satellite. The multipath 
> reflection makes that satellite appear to be further away than it really is. 
> In the case that the reflected 
> signal  is *stronger* than the desired signal, the multipath reflection 
> “captures” the receiver and the net
> solution is messed up. 

Even a weaker reflected signal can cause significant change of the
correlation peak and thus of the apparent distance of the satellite.
The multipath error envolope diagrams are usually for multipath to
direct path ratios of between 1:2 to 1:10 (mostly depending on what
the author wants to show or how much he wants to cheat).

> In the case of a mismatched cable, there is no “single satellite” issue. 
> Everything is impacted by the mismatch. 
> Even if the mismatch is pretty bad, the “primary” wave is the one that will 
> dominate at the receiver end. The 
> reflections will always be lower in amplitude. That effectively guarantees 
> that you don’t have a multipath 
> issue from the coax. 

As above, weaker signals can still cause quite a bit of change in the
correlation peak, but in this case it will not matter because the
reflection acts the same on all signals. I.e. the net result is a small
time offset (but no position offset). Unfortunately, there is one big
assumption in here that does not hold true: for all signals to be affected
the same way by the reflection, the receiver must be exactly linear.
But we know that many of the components in the signal path of the receiver
are distinctly non-linear functions. So there is a slight change of the
position (and thus time) due to reflections in the cable. But, as we are
usually dealing with minute differences in impedance, the reflected signals
are heavily attenuated. Assuming we have a nominally 50Ω

Malek's Law:
        Any simple idea will be worded in the most complicated way.

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