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

Tom Van Baak tvb at LeapSecond.com
Sun Nov 20 20:57:35 EST 2016

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.


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

> Is that even a sensible question?  Is there a better way to phrase it?
> The problem I'm trying to avoid is that the weather and the satellite 
> geometry change over time so I can't just collect data for X hours, switch to 
> the other antenna or move the antenna to another location, collect more data, 
> then compare the two chunks of data.
> The best I can think of would be to setup a reference system so I can collect 
> data from  2 antennas and 2 receivers at the same time.  It would probably 
> require some preliminary work to calibrate the receivers.  I think I can do 
> that by swapping the antenna cables.
> If I gave you a pile of data, how would you compute a quality number?  Can I 
> just sum up the S/N slots for each visible/working satellite?
> -- 
> These are my opinions.  I hate spam.

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