[time-nuts] Distance between GPS Antennae

Magnus Danielson magnus at rubidium.dyndns.org
Sun Dec 18 20:13:13 EST 2011


Hej Björn,

On 12/19/2011 01:00 AM, bg at lysator.liu.se wrote:
> There are receivers doing all kinds of 'smart' stuff.
>
> 1) PVT could be the output from an internal (kalman) filter or a true
> single fix solution. A filtered solution will have less noise, and the
> ability to ignore a few bad solutions.
> 2) Pseudoranges could be smoothed by phase measurements, before PVT
> solution is processed. This gives lower noise, but does not increase
> accuracy.
> 3) The correlator tracking loops can be adapted if you know your receivers
> dynamic profile (say, stationary/walking/automotive/airborne). Higher
> bandwidth makes it possible to cope with high user dynamic. Narrow
> bandwith optimised for say stationary user will give less measurement
> noise.
> 4) Knowing the receiver dynamic profile, the internal kalman filter can be
> adapted to give less noise in a low dynamic use case.
> 5) On receivers with external frequency input, you can often tell the
> receiver how good your oscillator is, to allow the receiver to adapt its
> internal parameters accordingly.

RAIM also helps.

> Many receivers will enable you to tune its behavour in more or less
> explicit ways.
>
> One the topic in the subjet, I have not experienced trouble with short
> distance between antennas. But I prefer to use a signal splitter instead
> of multiple antennas. I have heard of installations where multiple GPS
> receivers did not work well together using closely spaced antennas.

For non-converting antennas (i.e. most of them) you have the antenna 
element, filter and amplifier. Amplifiers is in the range of 20-40 dB 
gain. Unless there is a bad design, the amplifier doesn't feedback onto 
the antenna element, in which case the antenna element would be a fairly 
well-matched radiator, but it would be a bad antenna for any receiver so 
it would not go un-noticed. However, the output side feeds onto the coax 
through connectors, leakage there could occur. While it is unlikely, you 
could get cross-coupling with the antennas amplifiers. Since both 
antennas have at least some filtering, this should show up as a raised 
background noise-floor as the filters allow (typically 2 MHz or 20 MHz 
centered around L1).if cross-coupling occurs. I really doubt that 
out-right oscillations occurs.

> Also some people have had trouble with receivers sharing the same antenna
> signal splitter.

And I assume that it is not the DC loading issue you are referring to?

Sufficient port isolation in splitters should keep issues down.

> I suspect it is a small problem today with modern receivers, but you might
> be unlucky having specific models that interfere with each other.

It can always be an issue, but maybe not the first to investigate.

Cheers,
Magnus




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