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

jimlux jimlux at earthlink.net
Mon Nov 21 13:28:16 EST 2016


On 11/21/16 8:38 AM, Bob Camp wrote:
> Hi
>
>
>> On Nov 21, 2016, at 9:54 AM, Attila Kinali <attila at kinali.ch> wrote:
>>
>> On Sun, 20 Nov 2016 14:13:58 -0800
>> Hal Murray <hmurray at megapathdsl.net> wrote:
>>
>>> 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?
>
> If “sum the S/N” gives you a difference you should immediately ask why.
>
> Normal receiving antennas are a gain = directivity sort of beast. There is
> not a lot you can do about that. For a GPS antenna, you want to be able to
> receive over a hemisphere. You don’t really know in advance where the antenna
> will be used, so that’s how it’s done.
>
> Early on the designs had more gain straight up than at the horizon. That’s a
> bad thing. If anything, you want more gain at the horizon.

the satellites have a pattern that is "edge weighted" so that the 
received power on the ground is roughly constant into an isotropic 
antenna - that is, extra gain when the satellite is on the horizon is 
built into the transmit side of the link.

http://www.lockheedmartin.com/content/dam/lockheed/data/space/photo/gps/gpspubs/GPS%20Block%20IIR%20and%20IIR-M%20Antenna%20Panel%20Pattern,%20Marquis,%20Feb2014%20-%20publically%20releasable%20data.pdf

actually shows that the "peak" of the pattern is at around 45 degrees 
elevation (from the user)

And slide 32 shows that the variation (on the newer satellites) is on 
the order of 1-2 dB.




Signals are strong
> from straight overhead (short path, less atmosphere) and weak(er) at the
> horizon. They could easily give you a better “sum the S/N” number while
> actually performing worse in a location with sat’s mostly overhead. A “straight up”
> antenna might be wonderful at a location on the equator. It’s probably a disaster
> at a location on the arctic circle.

That might be. Also, there's a difference between "optimum combining for 
timing" vs "optimum timing for position/velocity".   I don't know that 
the geometry makes as much difference for timing, but it sure makes a 
difference for position.  That might actually be a rationale for higher 
transmit antenna gain at 45 degree user elevation: those are the 
satellites you want for a good fix: the one overhead isn't as useful.

A lot of GPS design decisions were based on use cases and requirements 
from 30 or more years ago: where were users likely to be, how 
sophisticated receivers were (e.g. a "pick the 4 strongest signals and 
hope they give good DOP geometry), what kind of antennas were easy (quad 
helix)





>
> The real answer for signal to noise will always be location dependent. If I’m in
> an urban canyon the only “sky” may be straight up. If I have a lot of terrestrial
> broadband noise close to the horizon, again straight up might be the answer.
> If my antenna is on top of a pole and I have a clean view 360 degrees around and
> down to < 5 degrees elevation, a straight up antenna is very much what I do
> *not* want.
>
> Even more complex: If I have a bunch of transmitters at a wide range of frequencies running at the
> same site as the GPS, I may want *really* good filtering ahead of the preamp. Those
> filters likely will have a temperature sensitivity.The filters create  loss ahead
> of the preamp so the noise figure (and thus S/N) take a hit. I get something I desperately need
> and trade it off against degraded performance in other areas.


This is very much the case.. we had a high performance survey quality 
antenna/preamp that in one location was unusable because of interference 
from close by signals (cellphones, I think).

The receiver had very little front end filtering, presumably for the 
stability reasons you mention above.



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