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

[David J Taylor]

From: Mark Sims

Yes.  It also works with GPSD so should be able to work with any device that 
GPSD supports.  It also works with most common GPS receiver native binary 
languages and provides full device control.


---

Thanks, Mark.  I look forward to playing with a copy.  I have a Garmin GPS 
60 CSx with Franson GPSgate turning its USB stream into a virtual COM5.


Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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

[David J Taylor]

The interpolation does not spread the data to where sats have not 
appeared...  what it effectively does is intelligently make the dots 
bigger...  but only along the azimuth "axis".


For each elevation angle, it starts at azimuth 0 degrees and searches 
forward until it sees a point with signal.  It then searches forward up to 
22.5 degrees looking for the next azimuth point at that elevation that has 
seen signals.  If it finds one, it colors in the arc between those two 
points (providing that the arc lies inside the clipping boundaries which 
prevents spreading the interpolation to areas that have not seen signals). 
As more sky data is collected over time  the space between adjacent azimuth 
points at a given elevation narrows and less "filling in" occurs.


Attached are two gifs,  one of the raw data from 24 hours and the other the 
interpolated data.  Note that areas near the horizon and to the north where 
no sats are seen are not colored.  The interpolation gives an excellent 
representation of your sky coverage.

=

Mark,

This looks to be a most useful and helpful display.  Will the program do the 
same from any serial NMEA source?


Thanks,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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

[Christopher Hoover]

> I'd like to try something like a Voronoi tessellation,  but that gets
rather nasty to implement..

There's a boost library...
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Re: [time-nuts] How can I measure GPS Antenna quality?

[Charles Steinmetz]

Mark wrote:


The tricky bit is interpolating signal levels between logged points (there is a 
display options for showing the raw signal level data).   Heather interpolates 
between adjacent azimuth points at each elevation angle.


I always wondered about the "filled in" (interpolated) plot.  To my 
observation, the orbital tracks of the satellites do not wander anywhere 
near +/- 22.5 degrees.  It is unclear to me that interpolating the data 
to areas of the sky where satellites never appear has any utility.


I suppose it makes trends somewhat easier to spot -- but that could also 
be done just by making the dots on the raw chart a bit larger, without 
suggesting a continuity of reception-space that doesn't exist.


Or am I missing something?

Charles


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

[jimlux]

On 11/22/16 10:13 AM, Mark Sims wrote:

Attached is a screen shot from Lady Heather showing various antenna signal 
displays.  The antenna was a cheap GPS/GLONASS patch antenna (mounted on a 3 
foot ground level tripod ) connected to a rather nice NVS-08 receiver tracking 
GPS, SBAS, and GLONASS satellites (typically around 22 sats).



very cool.

The usual "flat" plot for a 3 D pattern of a GPS antenna is to have the 
radius = 90-elevation angle (so horizon is outer border), angle is 
azimuth looking down on antenna, and color be power.  Somewhere I saw 
one that also integrated the multiple satellite tracks.


Another strategy is to plot it in u,v space.  Color is power, but u = 
cos(az)*cos(el) and v is sin(az)*cos(el)   (or sin(theta) where theta is 
the angle off boresight)



http://ww2.nearfield.com/amta/AMTA07-0092-GFM_SFG.pdf has a bewildering 
variety to choose from

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

[Jeff AC0C]

Mark,

Was that screen invoked from the LH application or via command line?  I 
don't see a mention of something similar on the LH help pop-up.


73/jeff/ac0c
www.ac0c.com
alpha-charlie-zero-charlie

-Original Message- 
From: Mark Sims

Sent: Tuesday, November 22, 2016 12:13 PM
To: time-nuts@febo.com
Subject: [time-nuts] How can I measure GPS Antenna quality?

Attached is a screen shot from Lady Heather showing various antenna signal 
displays.  The antenna was a cheap GPS/GLONASS patch antenna (mounted on a 3 
foot ground level tripod ) connected to a rather nice NVS-08 receiver 
tracking GPS, SBAS, and GLONASS satellites (typically around 22 sats).


The signal strength vs elevation mask plot shows a typical antenna 
characteristic where the signal strength drops off rapidly as the satellite 
elevation angle decreases.  The yellow marker tick is where the signal 
strength is 80% of the peak strength.   Swapping antennas can cause this 
angle to change by up to 15 degrees.  When Heather does an "autotune" 
function,  it uses this angle to decide where to set the antenna elevation 
mask angle.


The relative signal strength vs azimuth plot can be a bit deceiving... it 
shows great signal strength to the north,  but this is mainly due to the 
fact that the only times sats are visible there is when they are at high 
elevation angles.  Weighting the signal strengths by 1/ELevation angle does 
a better job of showing signal obstructions... There is a two-strory house 
25 feet to the west of the antenna.


The signal level vs az/el map gives the best information on the antenna sky 
view characteristics...  My best view is to the south east.   Lots of trees 
to the north,  house to the west,  lots of other ground level obstructions 
everywhere.









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

[bg]

Hi Atilla,
Read the document below
https://www.ngs.noaa.gov/ANTCAL/docs/NGSantcalprocedures.pdf
And the reference within
        antenna_README.pdf
Antennas are generally not calibrated individually by the user. You make sure 
the antenna you buy are in the ANTCAL or Geo++ lists. Any use of a snow cone 
should be with one the antenna was measured with.
Browse around for your favorite antenna vendor at
https://www.ngs.noaa.gov/ANTCAL/
MfG
     Björn
Sent from my smartphone.
 Original message From: Attila Kinali 
Now I wonder how the calibration data for mass produced geodetic antenna
are collected. I very much doubt they put them outside for a couple
of days to measure them exactly.

Attila Kinali
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Re: [time-nuts] How can I measure GPS Antenna quality?

[jimlux]

On 11/21/16 10:10 AM, Tom Van Baak wrote:

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation.


I agree. And besides, for those of us here in Oregon/Washington, the very 
ground is moving northwest at several inches per year (plate tectonics).



Is that movement in absolute terms? or relative to the NA plate?
Where I live in SoCal (on the Pacific plate side ), we also have an 
annual uplift on the order of 1cm.



http://www.unavco.org/software/geodetic-utilities/plate-motion-calculator/plate-motion-calculator.html

For 34N, 118W, 47.67mm/yr at 297.53 degrees(CW from north) 22.04N, 
-42.28E in local coordinates


-31.57 X, 30.68 Y, 18.31 Z using WGS 84



for 34, 119W (Ventura county, some 50 miles west) 48.04mm/yr total, 
-31.12 X, 31.51 Y, 18.62 Z


A little less X, a little more Y, because it's starting to "turn the corner"


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

[jimlux]

On 11/21/16 2:58 PM, Attila Kinali wrote:

On Mon, 21 Nov 2016 08:22:50 -0800
jimlux  wrote:


I'm not sure about whether an anechoic (which is really "hypoechoic")
chamber is going to get you the data you need.  Calibrating the chamber
to the needed level of accuracy might be harder than doing field
measurements.

[...]

  sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3
cm different path length (modulo wavelength) and 30 dB down will give
you a 1mm phase center error.  0.1 mm is -50dB.


Interesting. I haven't done the math, so I didn't think about that.
Yes, the reflections in the chamber would probably limit the resolution.

Now I wonder how the calibration data for mass produced geodetic antenna
are collected. I very much doubt they put them outside for a couple
of days to measure them exactly.


That's exactly what they probably do (put them outside) - assuming they 
have an individual cal at all.


A *good* antenna design is one where if the mechanical assembly is 
within manufacturing tolerances, it will have the same performance as 
all the others made to the same tolerance.



ftp://www.ngs.noaa.gov/pub/abilich/calibPapers/Goerres2006.pdf

Note that the residuals after cal were biggest at 0 and 90 elevation, 
and best in the mid elevations..










Attila Kinali



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

[Attila Kinali]

On Mon, 21 Nov 2016 08:22:50 -0800
jimlux  wrote:

> I'm not sure about whether an anechoic (which is really "hypoechoic") 
> chamber is going to get you the data you need.  Calibrating the chamber 
> to the needed level of accuracy might be harder than doing field 
> measurements.
[...]
>   sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3 
> cm different path length (modulo wavelength) and 30 dB down will give 
> you a 1mm phase center error.  0.1 mm is -50dB.

Interesting. I haven't done the math, so I didn't think about that.
Yes, the reflections in the chamber would probably limit the resolution.

Now I wonder how the calibration data for mass produced geodetic antenna
are collected. I very much doubt they put them outside for a couple
of days to measure them exactly.

Attila Kinali

-- 
Malek's Law:
Any simple idea will be worded in the most complicated way.
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Re: [time-nuts] How can I measure GPS Antenna quality?

[Tom Van Baak]

> For the equipment hobbyists usually have, the phase center is not that
> important. Most antennas have a variation <5mm. Even 10mm would lead to
> just a ~33ps variation.

I agree. And besides, for those of us here in Oregon/Washington, the very 
ground is moving northwest at several inches per year (plate tectonics).

http://leapsecond.com/pages/quake/

/tvb
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Re: [time-nuts] How can I measure GPS Antenna quality?

[jimlux]

On 11/21/16 8:38 AM, Bob Camp wrote:

Hi



On Nov 21, 2016, at 9:54 AM, Attila Kinali  wrote:

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray  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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Re: [time-nuts] How can I measure GPS Antenna quality?

[David]

When I was doing VHF and UHF direction finding antenna design, I would
drive out to the highest readily accessible hilltop for testing.  Once
I came up with a low sidelobe design, I started picking up things like
lamp posts, trees, and bushes in the parking lot, aircraft over LAX
and John Wayne airports 50+ miles away, etc. which limited testing
performance.

While a perfect test environment is handy for design, a GPS antenna is
going to be subject to all kinds of environmental limitations so I
would accept field testing which includes considerations like
multipath, temperature variation, and a generally hostile RF
environment.

On Mon, 21 Nov 2016 08:22:50 -0800, you wrote:

>I'm not sure about whether an anechoic (which is really "hypoechoic") 
>chamber is going to get you the data you need.  Calibrating the chamber 
>to the needed level of accuracy might be harder than doing field 
>measurements.
>
>It might just be because there's a ton of analysis software out there, 
>but the folks who really, really care about 0.1 mm shifts in phase 
>center seem to use field data in a well characterized site, and 
>accumulate it for a number of days.
>
>The GPS antenna folks at JPL, when they're testing a spacecraft antenna 
>for things like precision orbit determination (a basic choke ring sort 
>of thing) go out with the antenna and a test receiver on a cart in a 
>parking lot.
>
>Looking at it in terms of numbers:
>1mm is 1/150 wavelength, or about 2-3 degrees of phase.
>
>  sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3 
>cm different path length (modulo wavelength) and 30 dB down will give 
>you a 1mm phase center error.  0.1 mm is -50dB.
>
>Now, it's true that if you had a good spherical near field range, with 
>time gating, you can probably get rid of the reflections from the 
>chamber (and, in fact, you can do the measurements in a regular lab, or 
>your garage). But even there, it's tricky, because the probe calibration 
>has to be very good, and the structure supporting the scanning probe 
>also has to be accounted for.  You might be able to do it by doing 
>transmit/receive measurements on something like a spherical target of 
>appropriate size.
>
>I've done measurements on what was essentially an interferometer with a 
>2 meter baseline, in a conventional chamber on a conventional pedestal 
>(JPL Mesa 60 ft chamber  http://mesa.jpl.nasa.gov/60_Foot_Chamber/). 
>You could easily see -40dB specular reflections as the array rotated. 
>(and you could also see things like the ladder on the positioner behind 
>the antenna we accidentally left in there, even though it was behind the 
>horn antennas in the array)
>
>I think a good test using satellites and a very well characterized 
>comparison antenna in a open air test site is probably the easiest, and 
>most accurate, way to do it.
>Arranging your test on a post well above the terrain, and making sure 
>that the surface is flat is easy.
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Re: [time-nuts] How can I measure GPS Antenna quality?

[jimlux]

On 11/21/16 6:54 AM, Attila Kinali wrote:

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray  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?


There are multiple issues. As already mentioned, SNR is only a part of the
picture. What you are looking for is an uniform gain pattern over most
of the hemisphere, with a sharp decrease at low elevations. Then the
left vs right polarization ratio should be as high as possible over
the whole hemisphere (most antennas only have good polarization ratio
at the zenith and behave like a linear polarized antenna at low elevations).

Additionally to this comes the phase center stability. Ie that the phase
center is a fixed location, independent of azimuth and elevation. And this
is probably the hardest to measure.

Absolute (and probably the most precise) measures of these properties are
done in an anechoic chambers with a rotating antenna mount.



I'm not sure about whether an anechoic (which is really "hypoechoic") 
chamber is going to get you the data you need.  Calibrating the chamber 
to the needed level of accuracy might be harder than doing field 
measurements.


It might just be because there's a ton of analysis software out there, 
but the folks who really, really care about 0.1 mm shifts in phase 
center seem to use field data in a well characterized site, and 
accumulate it for a number of days.


The GPS antenna folks at JPL, when they're testing a spacecraft antenna 
for things like precision orbit determination (a basic choke ring sort 
of thing) go out with the antenna and a test receiver on a cart in a 
parking lot.



Looking at it in terms of numbers:
1mm is 1/150 wavelength, or about 2-3 degrees of phase.

 sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3 
cm different path length (modulo wavelength) and 30 dB down will give 
you a 1mm phase center error.  0.1 mm is -50dB.



Now, it's true that if you had a good spherical near field range, with 
time gating, you can probably get rid of the reflections from the 
chamber (and, in fact, you can do the measurements in a regular lab, or 
your garage). But even there, it's tricky, because the probe calibration 
has to be very good, and the structure supporting the scanning probe 
also has to be accounted for.  You might be able to do it by doing 
transmit/receive measurements on something like a spherical target of 
appropriate size.


I've done measurements on what was essentially an interferometer with a 
2 meter baseline, in a conventional chamber on a conventional pedestal 
(JPL Mesa 60 ft chamber  http://mesa.jpl.nasa.gov/60_Foot_Chamber/). 
You could easily see -40dB specular reflections as the array rotated. 
(and you could also see things like the ladder on the positioner behind 
the antenna we accidentally left in there, even though it was behind the 
horn antennas in the array)



I think a good test using satellites and a very well characterized 
comparison antenna in a open air test site is probably the easiest, and 
most accurate, way to do it.
Arranging your test on a post well above the terrain, and making sure 
that the surface is flat is easy.






The second way to do it, is to use a "known good" reference antenna and
use this as a comparison with a short (3-15m) baseline between reference
and antenna under test. For additional fancyness and to get better results
one can add the antenna onto robotic arm (like on the picture in [1]) and
get a more complete picture of the antenna. In this setup you want to have
an as fancy receiver as possible. At the minimum it needs to support carrier
phase data. The better receivers allow you to connect two antennas to the
same receiver and do a direct phase/amplitude comparison of the signals.

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation. Ie for the normal GPSDO that has a loop time constant
in the 100s to 1000s seconds and is using "normal" receivers, this is
completely drowned in the noise of the receiver's PPS output. Having good
sky view and as little multipath as possible is much more important.



Attila Kinali

[1] https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html



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

[jimlux]

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 
reflection.



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

[Attila Kinali]

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray  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?

There are multiple issues. As already mentioned, SNR is only a part of the
picture. What you are looking for is an uniform gain pattern over most
of the hemisphere, with a sharp decrease at low elevations. Then the
left vs right polarization ratio should be as high as possible over
the whole hemisphere (most antennas only have good polarization ratio
at the zenith and behave like a linear polarized antenna at low elevations).

Additionally to this comes the phase center stability. Ie that the phase
center is a fixed location, independent of azimuth and elevation. And this
is probably the hardest to measure.

Absolute (and probably the most precise) measures of these properties are
done in an anechoic chambers with a rotating antenna mount. 

The second way to do it, is to use a "known good" reference antenna and
use this as a comparison with a short (3-15m) baseline between reference
and antenna under test. For additional fancyness and to get better results
one can add the antenna onto robotic arm (like on the picture in [1]) and
get a more complete picture of the antenna. In this setup you want to have
an as fancy receiver as possible. At the minimum it needs to support carrier
phase data. The better receivers allow you to connect two antennas to the
same receiver and do a direct phase/amplitude comparison of the signals.

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation. Ie for the normal GPSDO that has a loop time constant
in the 100s to 1000s seconds and is using "normal" receivers, this is
completely drowned in the noise of the receiver's PPS output. Having good
sky view and as little multipath as possible is much more important.
 


Attila Kinali

[1] https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] How can I measure GPS Antenna quality?

[Poul-Henning Kamp]

In message <2974f356-3729-448e-a428-dc4d340ff...@n1k.org>, Bob Camp writes:

>If I put up a handful of antennas on the back porch, I can indeed hook them
>up to various receivers and cables.

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
p...@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.
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Re: [time-nuts] How can I measure GPS Antenna quality?

[jimlux]

On 11/20/16 7:41 PM, Mark Sims wrote:

When I was developing the precision survey code for Lady Heather,  I
used a lot of antennas.  My definition of antenna quality boiled down
to how well the results of a 48 hour survey compared to the cm level
survey point that I had for my antenna position (from a Ashtech Z12
receiver / matched choke ring antenna).



This is similar to the UNAVCO evaluaton approach
http://kb.unavco.org/kb/article/unavco-resources-gnss-antennas-458.html

"Antenna phase center variations can be characterized by mean phase 
center offsets and by phase and amplitude patterns for L1, L2 and L3 
(ionosphere free combination) tracking as a function of azimuth and 
angle. Mean offsets are defined as the average phase center locations 
relative to a physical reference point on the antennas (typically the 
base of the antenna preamplifier as used in RINEX files). The patterns 
are defined as the azimuth and elevation dependence to be added to the 
average phase center offsets. The sum of the mean phase offset and 
pattern gives the signal path delay for a given satellite elevation and 
azimuth. Precise knowledge of these phase patterns is essential for 
mixing antennas of different design where uncorrected effects can be as 
large as 10 cm in the vertical and 1 cm in the horizontal baseline 
components. The effect is more subtle for antennas of the same design. 
Here the problems arise over long baselines where the same satellite is 
observed at different relative directions and therefore experiences 
different delays at each site which can introduce solution scale errors. 
In addition, there is the issue of consistency of the phase patterns and 
offsets for each individual antenna of the same design."



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

[Bob Camp]

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



> On Nov 20, 2016, at 10:22 PM, Mike Cook  wrote:
> 
> 
>> Le 21 nov. 2016 à 02:57, Tom Van Baak  a écrit :
>> 
>> 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.
> 
> 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 
> distrib
 ut
> ers ( 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. 
> 
>> 
>> 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" 
>> To: 
>> 

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

[Gary E. Miller]

Yo Hal!

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray  wrote:

> Is that even a sensible question?

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
g...@rellim.com  Tel:+1 541 382 8588


pgp7LlWz_GQ1T.pgp
Description: OpenPGP digital signature
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Re: [time-nuts] How can I measure GPS Antenna quality?

[Mike Cook]

> Le 21 nov. 2016 à 02:57, Tom Van Baak  a écrit :
> 
> 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.

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 distribut
 ers ( 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. 

> 
> 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" 
> To: 
> Cc: "Hal Murray" 
> 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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Re: [time-nuts] How can I measure GPS Antenna quality?

[Chris Albertson]

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  wrote:

>
> 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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> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
> mailman/listinfo/time-nuts
> and follow the instructions there.
>



-- 

Chris Albertson
Redondo Beach, California
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Re: [time-nuts] How can I measure GPS Antenna quality?

[Tom Van Baak]

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" 
To: 
Cc: "Hal Murray" 
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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Re: [time-nuts] How can I measure GPS Antenna quality?

[Graham / KE9H]

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  wrote:

>
> 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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> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/
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