[time-nuts] How can I measure GPS Antenna quality?

jimlux jimlux at earthlink.net
Mon Nov 21 08:45:23 EST 2016

On 11/21/16 4:15 AM, Bob Camp wrote:
> Hi
> At spectrum analyzer bandwidths, the GPS signals out of the antenna are
> more than 20 db below the noise floor. You can’t see them with an analyzer.
> You need to run things into the equivalent of a receiver to turn it into anything
> you can see above the noise.
> What you will see on an analyzer is the noise out of the preamp. That will at
> least tell you that the output stage on the amp is working. It will tell you very
> little about the input stage to the amp and very little about the antenna on
> the other side of the preamp.
> One thing you want to know about a GPS antenna system is the
> stability of it’s phase center as things change (like sat angles). If the phase
> center moves, your solutions change between satellite readings. Another
> thing you want to know is how it rejects multipath. Neither one is easy
> to measure in a direct way.

The GPS folks do things like look at the correlation peak vs timing 
offset - you need the raw bits to do this, and you run a cross 
correlation against the spreading code.  A "good" antenna and location 
should show a nice triangular peak.  A "bad" antenna and/or location 
will show multiple peaks (corresponding to the multiple paths).

I think it would be fairly easy to collect the data - there's several 
"filter+threshold" widgets out there that could generate your data 
stream.  Then, doing the correlation just requires some fairly simple 
software: matlab could do it in a few lines, once you have some code (or 
a file) with the spreading sequence of interest.

Our receivers at JPL sample at 38.something MHz, so that the GPS 
frequency aliases to somewhat to the side of zero (so that even with max 
negative doppler, the signal is still positive frequency)

> A more formal method of testing would be to use a proper antenna test setup.
> Those normally are indoor systems that rotate things to test the antenna. Even
> there, the systems are only so good.

I haven't looked at whether a near field range could do it, but most 
indoor regular ranges don't have good enough multipath suppression for 
this kind of thing. The absorber on the walls is probably good to 30-40 
dB, but even then, there will be "something" that gives you a noticeable 

Another proposed solution is to run the antenna
> in the real world on a robotic arm and rotate it while in use. Again there are limitations
> to the process.

re: robotic arm

Or the way they do it for geodetic antennas - a grad student goes out 
and manually rotates the antenna to a new orientation.

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