If you run two antenna simultaneously then...

1. they both can't be at the same location and
2. What if the two antenna interfere one with the other.

I think maybe you need to collect data over a long enough period of tine
that wether averages out.  the satellite tracks repeate pretty much exactly

What you might want to know about an antenna is more than just S/N for good
locations but how it does with adverse conditions like multi path and a
nearby jammer and maybe gain vs. elevation and also dumb practical stuff
like if birds like to perch on it and if there is a way to route the cable
through the mast pipe or not

On Sun, Nov 20, 2016 at 2:13 PM, Hal Murray hmurray@megapathdsl.net wrote:

--

Chris Albertson
Redondo Beach, California

Hi Hal,

That's a very sensible question. I've often wondered the same, but I'm embarrassed to say I have never done a thorough job with it. You know the constellation repeats approximately every 24 hours so you want your X hours to be a multiple of days.

Looking at SNR seems obvious and may even be sufficient. Alternatively you could track the deviation of the per-SV timing solutions and draw conclusions from that. I suspect multi-path effects would show up in these residuals more than they would show up with just NSV (number of satellites received) or SNR (signal to noise ratio)

But in some respects, the bottom line is not NSV or SNR or multi-path or anything like that. What counts is only how well the 1PPS matches a local high-quality time standard (e.g., Cesium or better).

Another issue is that it's possible that the quality of a set of N antenna would sort differently for you than for me: different latitude, different sky-view, different weather. Some time nuts (not me) get lucky with a perfectly clear 360 degree horizon view.

I agree that N antennas and N receivers makes the experiment easier, because you might spend as much time validating that a set of receivers are all the same as later comparing various antennas.

With that in mind, consider the slice & dice (chopper) idea -- use a VHF relay switch / mux and round-robin N antenna across N receivers every, say, 10 minutes. That gives you 144 points per receiver / antenna pair per day and avoids the geometry and pre-calibration issues, as well as environment and local reference effects. Rubidium would be sufficient. It's possible the first minute of each segment may be weird (as the receiver switches from lost to lock mode), but you can handle that in your data reduction.

/tvb

----- Original Message -----
From: "Hal Murray" hmurray@megapathdsl.net
To: time-nuts@febo.com
Cc: "Hal Murray" hmurray@megapathdsl.net
Sent: Sunday, November 20, 2016 2:13 PM
Subject: [time-nuts] How can I measure GPS Antenna quality?

You need a definition of "Quality" to work with.

One definition might be "does it meet published specs? under what
conditions?"
Another definition might be associated with reliability and ruggedness.
Longevity in outdoor conditions.
Another might be with the antenna supporting your use case.
Another might be with suppression of reflections and spurious signals from
below the horizon.

So, the definition of "Quality" might change drastically with the use case
and from your expectations as to its cost.

If you have a definition of your quality criteria, then this crowd can
probably tell you what to measure and how to analyze the measurement data.

--- Graham

==

On Sun, Nov 20, 2016 at 4:13 PM, Hal Murray hmurray@megapathdsl.net wrote:

Sensible question but not easy to answer directly without a  spectrum analyser directly connected. Not having one of those I went the n receivers/ m antenna method since all I wanted was to get the best subset from what I had. My antenna are of the €5-€25 puck variety and I tested about eight 5V, 5/3V active Noname and Trimble antenna with  half a dozen receiver types. I am unlucky in having just a north looking sky view and in built up area which gets me significant multi path at certain times. So I first of all selected a period during the day where all receivers were reporting best SNR and max NSV and most stable 1PPS . I then measured the 8 antenna over the same time interval (IIRC it was 08h-11h) against the 1PPS of a PRS10 rubidium ref.  The GPS 1PPS was verified to see if it held the manufacturers stability spec which they mostly did. It was easy to see this against the rubidium signal. So I picked the best 4 and use them to feed my receiver pen via Mini-Circuits distributers ( There is a small signal loss here but in spec. 3db IIRC ). They have been in place for about 3-4 years at least without issue.

"The power of accurate observation is commonly called cynicism by those who have not got it. »
George Bernard Shaw

Yo Hal!

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray hmurray@megapathdsl.net wrote:

Yes, it is a good question.

I have been buying a lot of cheap GPS antennas for testing on RasPis.

I plug the antenna into a GPS, then just wait until the GPS gets a good
sky view, and then check the sat SNRs.  Replace with a different antenna
and repeat.

The difference in SNR between similar appearing antennas can be very
dramatic.  Some are clearly strong or weaker than others.

RGDS
GARY

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
gem@rellim.com  Tel:+1 541 382 8588

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.

If I put up a handful of antennas on the back porch, I can indeed hook them
up to various receivers and cables. I can take a bunch of data. There will
always be very real questions about location A being the same as location B.
You can swap all the antennas between all the locations and take weeks of
data in each location combination. You have fewer questions, but there are still
some.

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. 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.

Lots of approaches, lots of questions ….

Bob

In message 2974F356-3729-448E-A428-DC4D340FF068@n1k.org, Bob Camp writes:

I actually had a chance to do that once:

http://phk.freebsd.dk/raga/

I was very careful to space the antennas exactly 300mm apart, but that
didn't inspire them to align in any way.

In all fairness, those were "timing" receivers so nobody cared where
their phase-center was during production...

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

On 11/21/16 4:15 AM, Bob Camp wrote:

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)

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
reflection.

Another proposed solution is to run the antenna

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.

If you had 30 ft of rg59 outdoors seeing maybe 10 degC swings everyday,
would the propagation time be stable to ps? ns?

On Sun, Nov 20, 2016 at 7:04 PM Hal Murray hmurray@megapathdsl.net wrote: