GPS L1 signal, obse...
 
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GPS L1 signal, observation of Doppler shift (not a surveying meas.)

am95405
(@am95405)
50+ posts Member

I recently wrote a blog and thought some of you here may enjoy the measurement. Nothing new ... in fact a very old measurement. I study surveying when time permits but the blog is part of my day job, which has nothing to do with surveying.

I'll summarize the main part of it here, which is the image below.

Figure3

The image shows the measurement of the GPS L1 signal over a four period. The x-axis is frequency, y-axis is time, colors are amplitude or power level. The arced or slanted lines you see are the L1 signal. If the satellites' position relative to earth was fixed, those lines would be vertical, straight up an down. For example, if I look at my local FM radio station's transmission, it's straight up and down. But the GPS satellites move relative to earth, so you get the Doppler shift, that is, the received frequency is changing continuously. Hence the arc shape.

And some interesting EMI or electromagnetic interference observed too.. who knows from what. Aliens maybe 😀 

I included a link to a paper about GPS history and the Doppler shift at the bottom of the article. I thought it was a fascinating read. 

 

 

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Topic starter Posted : April 16, 2020 7:59 am
jakkanen and Bill C liked
Bill93
(@bill93)
5,000+ posts Member

It seems more and stronger signals have increasing frequency, compared to those with decreasing frequency. Was your antenna shielded on one side by a building or trees?

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Posted : April 16, 2020 8:10 am
am95405
(@am95405)
50+ posts Member

It's a colleague's rooftop, if you click on the blog you can see the image of the antenna. I'm not sure I follow the comment, but the amplitude levels which are hard to read on the image are ~-150 to -160 dBm, so the whole range is only 10 dB. 

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Topic starter Posted : April 16, 2020 8:28 am

Bill93
(@bill93)
5,000+ posts Member
Posted by: @am95405

I'm not sure I follow the comment,. 

To restate: why don't I see as many lines leaning one way as are leaning the other way?

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Posted : April 16, 2020 8:34 am
am95405
(@am95405)
50+ posts Member

@bill93  (corrected my original message)  Either way, I still see the slants all towards the right, but with different slants. Do you see some slanted left?

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Topic starter Posted : April 16, 2020 9:21 am
Lurker
(@lurker)
200+ posts Member

@am95405  I may be interpreting the graph wrong, but I would expect to see a Doppler shift in both directions. the frequency becoming greater as a satellite approaches and decreasing as the satellite moves away. But the graph seems to show frequencies all moving the same direction over time.?

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Posted : April 16, 2020 10:26 am

Bill93
(@bill93)
5,000+ posts Member

 

I was being stupid at first.  Yes, they should all be leaning the direction shown.

When the satellite comes over the horizon it is getting closer to you so should have an up shift. At its highest point in your sky the distance isn't changing.  After it passes the highest position it is getting further so should have a down shift.

https://www.e-education.psu.edu/geog862/node/1786

But something still doesn't make sense.  Each line should have its greatest rate of change when it is nearest zero doppler (center of screen).  I don't see the correspondence in the picture, particularly missing in the ones on the left.  That leads me to wonder whether the frequency reference was accurate enough to put zero doppler in the center of the picture.

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Posted : April 16, 2020 10:35 am
Paul in PA
(@paul-in-pa)
5,000+ posts Member

There is a very large source of EMI in the sky every day. We call it the Sun. If you do some GPS planning along with celestial positioning you will find interesting happenings with GPS signals. An L1 signal following the same path as L1 frequency solar signals can merge in the same way as laser light and the signal you measure is outside normal expectations. If you follow the doppler shifts on full transit of a satellite you can see the change in sign as it passes your longitude overhead. Without using planning by looking at the CA or C1 signal values a satellite passing almost directly overhead has a measured value under 20,000,000 meters.

Paul in PA

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Posted : April 16, 2020 10:39 am
Bill93
(@bill93)
5,000+ posts Member
Posted by: @paul-in-pa

There is a very large source of EMI in the sky every day. We call it the Sun.

In a plot like pictured above, the sun would be a very fuzzy horizontal band across the whole frequency range and lasting a long time.  But I think it would be too weak to show on that plot.  The sun shines on everybody's GPS antenna for many hours a day without a problem.

Sun outages occur when someone is depending on a narrow antenna beam to receive a satellite and the sun appears too close to the satellite.

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Posted : April 16, 2020 10:52 am

am95405
(@am95405)
50+ posts Member

@bill93 I'll have to think about it. I'm not on an expert on calculating Doppler shift or satellite movements ... the frequency accuracy was ~20 Hz since the reference was locked to GPS.

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Topic starter Posted : April 16, 2020 11:46 am
am95405
(@am95405)
50+ posts Member

@lurker You are seeing that in the positive slope of the line (~arc). If I am not mistaken, this is a sideways view of the plot that is shown in the URL Bill referred to (a small snapshot in time, about 4 hrs).

Capture

 

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Topic starter Posted : April 16, 2020 1:09 pm
Bill C
(@bill-c)
200+ posts Member

@am95405 Thanks for sharing your blog post! The FieldFox looks like a great product, and this was a fun application for it. I grew up on the Hewlett-Packard 8566B spectrum analyzer, and back in 1984 or 1985 built an antenna and LNA and just managed to see GPS signals on the 8566B (I can't recall now if I was looking at L1 or L2). Years later, I happened to become an HP employee and was among the people who became Agilent, but I then moved on before Keysight arose from Agilent.

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Posted : April 16, 2020 3:11 pm
am95405 liked

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