Hello all, I'm a recent recruit to this list, it looks very interesting
and I hope I can make a contribution. Though now retired I'm an
electronics & radio engineer and until last March worked for u-blox
(though not on the GNSS side of the business).
Looking at their product integration guides they don't mention antenna
match and cable length. They do suggest inserting a SAW filter between
the antenna and module especially with an active antenna, and of course
the group delay of a SAW filter makes it equivalent to quite a long
length of cable - at least 30 cm a nanosecond. Multiple reflections
will be attenuated at each reflection - if the return loss is say 6 dB
then you lose that much at each reflection, plus the attenuation in the
cable itself. So the effect of the cable plus mismatches will be to add
an extra channel impulse response that will be quite short as the
various echoes decay. Given that the system is working with wanted
signals well below thermal anyway, a couple of reflections plus a dB or
two of cable loss would make the largest echo ~15 dB below the wanted
signal. I suspect that this will cause rather small error in the
device's estimate of pseudo-range but this will be the same for each
satellite. Overall I doubt that the effect will be serious either for
positioning or timing - if it were I think (knowing u-blox) they would
make recommendations in the guide. By the way they don't even mention
it in the context of their high-precision devices.
(And I've just found an application note where they do discuss matching
but only in the context of maximising SNR, i.e. power transfer.)
But if one is really concerned then the obvious thing to do is mount the
module/chip right on the back of the antenna so there is no cable.
On 23/11/2016 12:00, time-nuts-requ...@febo.com wrote:
Re: Do reflections up/down the antenna cable cause a problem
with GPS? (Bob Camp)
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