[time-nuts] Distance between GPS Antennae

bg at lysator.liu.se bg at lysator.liu.se
Sun Dec 18 19:00:50 EST 2011


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.

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

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

--

    Björn

> Some GPS receivers automatically go into "position hold" mode if less than
> 1
> mph is detected for several seconds.  However, they usually have a command
> to disable that feature if you search the manual for the command.
>
> Connected by DROID on Verizon Wireless
>
> -----Original message-----
> From: Tom Van Baak <tvb at LeapSecond.com>
> To: Discussion of precise time and frequency measurement
> <time-nuts at febo.com>
> Sent: Sun, Dec 18, 2011 22:35:42 GMT+00:00
> Subject: Re: [time-nuts] Distance between GPS Antennae
>
> From: "Brent Gordon" <time-nuts at adobe-labs.com>
>>   The best answer is try it it and see what happens.  In my case two
>>   systems with patch antennas ten inches apart interfere significantly.
>>   Here are plots (using VisualGPS) of my position over a day or two.
>>   This is using a GPS from Sure Electronics.  Note that the standard
>>   deviation is 0 for over 100,000 samples.  As a side note, it must have
>>   some kind of heavy filtering going on to not show any position
>>   variation.  This kind of result is repeatable with this unit.
>
> Brent,
>
> That's really interesting. It looks like one receiver is in GPS
> hold mode (so you would expect the position to remain fixed)
> and the other is in a normal 2D or 3D mode. Can you look at
> the NMEA logs and tell which is which? Is this the same GPS
> receiver I played with (leapsecond.com/pages/MG1613S)?
>
> Thanks,
> /tvb
>
>
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