Author Topic: GPSDO Filtering on Elevation - Pros and Cons?  (Read 587 times)

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Offline jpb

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GPSDO Filtering on Elevation - Pros and Cons?
« on: May 26, 2019, 12:08:49 pm »
Lady Heather makes it easy to set an elevation filter for GPSDOs that allow it.

In my case I have the antenna on my garden fence so there are houses and trees around so the best signals are at elevations above 60 degrees though there are still a lot of good ones above 30 degrees.

If I set the filter to 60 degrees the advantage will be avoiding any signals due to reflections and restricting things to a more direct signal path (less atmosphere to pass through).

What are the downsides?

The obvious one is that you're not using data from a lot of good signals and I guess the receiver will need to switch satellites more frequently as they pass through the smaller window.

But I don't know the relative merits of the pros and cons and how clever the GPSDO is anyway - if it picks the best signals anyway the filter won't make much difference and might make things worse.
 

Offline texaspyro

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Re: GPSDO Filtering on Elevation - Pros and Cons?
« Reply #1 on: May 27, 2019, 04:17:19 am »
The most common cause of GPSDO jumps is when the receiver changes the group of satellites that it is tracking.   You want to minimize that.

If your elevation mask is too low,  you start getting into errors caused by multipath (signal reflection from the surrounding environment).  If it is too high, you won't see enough satellites.

Try this:
1) set the elevation mask to 0 (or a low value) using the FE command.
2) type CC to clear all the data
3) collect data for several hours
4) type ZE (or ZU) to show the signal level vs elevation plot

A yellow tick mark on that plot is where the signal level starts to drops off.  Set your elevation mask to that value or a bit lower (I usually use 2/3 to 1/2 that value).  The blue tick mark is the current elevation mask setting.

Another thing to try is to look at the satelite sky view map.  Find a satellite that is rising, but close to the horizon and looks like it is on a track to pass nearly overhead... say sat PRN 12.
Enter SQ 12... this will say to plot that satellite's az/el/sig level.
Enter VT...  this tells the plot routines to automatically rescale whenever the plot area gets filled.
Let it run until the sat disapperars below the horizon... maybe 6 hours.
Look at the blue signal level plot...  notice how the signal level gets noisy below a particular elevation.
Enter SQ 0 to turn off satellite signal/location plotting.
Enter GKA to turn off the signal level / position plots.

You can just pick any satellite and wait for a day for it to make a pass (well usually two passes, one will be at a rather low max elevation).


« Last Edit: May 27, 2019, 04:20:29 am by texaspyro »
 
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Offline jpb

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Re: GPSDO Filtering on Elevation - Pros and Cons?
« Reply #2 on: May 27, 2019, 11:35:54 am »
Thank you very much for your detailed answer. I will follow the instructions when I've finished my current ADEV measurements (I don't want to change conditions part the way through).
 

Offline FriedLogic

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Re: GPSDO Filtering on Elevation - Pros and Cons?
« Reply #3 on: May 30, 2019, 12:16:48 pm »

What are the downsides?

Another complication is that the closer you get to the poles the fewer high angle satellites you have anyway, so raising the elevation mask by much can really thin them out.

For testing GPS receivers it can also be useful to have 2+ of the same ones on a splitter on the same antenna for comparison. You can see how closely they perform normally, and then compare when you change something with one of them.
 
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Online edpalmer42

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Re: GPSDO Filtering on Elevation - Pros and Cons?
« Reply #4 on: May 30, 2019, 06:22:44 pm »
Another complication is that the closer you get to the poles the fewer high angle satellites you have anyway, so raising the elevation mask by much can really thin them out.

I always recommend that when planning your antenna installation, you should pick a location that has a clear view toward the equator to improve satellite visibility.  This becomes more important as you move further north or south.

Ed
 

Offline jpb

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Re: GPSDO Filtering on Elevation - Pros and Cons?
« Reply #5 on: May 30, 2019, 08:18:23 pm »
Another complication is that the closer you get to the poles the fewer high angle satellites you have anyway, so raising the elevation mask by much can really thin them out.

I always recommend that when planning your antenna installation, you should pick a location that has a clear view toward the equator to improve satellite visibility.  This becomes more important as you move further north or south.

Ed
Unfortunately in my case my study/lab is on the north side of my house but I moved the antenna to a garden fence with a slightly better view to the south and I seem to get a good number of satellites in view.
 

Offline Johnny B Good

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Re: GPSDO Filtering on Elevation - Pros and Cons?
« Reply #6 on: June 18, 2019, 07:36:02 pm »
Another complication is that the closer you get to the poles the fewer high angle satellites you have anyway, so raising the elevation mask by much can really thin them out.

I always recommend that when planning your antenna installation, you should pick a location that has a clear view toward the equator to improve satellite visibility.  This becomes more important as you move further north or south.

Ed
Unfortunately in my case my study/lab is on the north side of my house but I moved the antenna to a garden fence with a slightly better view to the south and I seem to get a good number of satellites in view.

 I've just recently (3 days ago) raised my mag mount puck antenna by a couple of metres to clear the "gable ended" brickwork of a dormer windowed attic bedroom on the eastward side of the ballasted biscuit tin I'd been using on the flat bay window roof of my 'office' (now my man cave in what is a 1st floor bedroom below the attic bedroom) to stick the puck antenna onto by way of an upgrade on the internally located antenna.

 Although this external location afforded a considerable improvement over the window ledge location, reception of SVs to the east was noticeably degraded which was obviously aggravating the positional errors as displayed by the deviation map plots. Since I wasn't too keen on drilling into the brickwork to mount a bracket for even a short lightweight mast whilst standing on a small bay window flat roof some 20 odd foot above our front garden, I decided to try a variation of the  original "Ballasted Biscuit Tin Support Pedestal" theme.

 Basically, I'd simply replaced the biscuit tin with an old metal drawer ballasted with four small car batteries with an 8 foot aluminium pole clamped with a couple of U bolts into the corner. The mag mount puck antenna then being stuck onto a screw jar lid held in place on the top of the pole using a countersunk screw into a wooden plug I'd hammered into the pole end. Effectively, a "permanent fixture" (courtesy of gravity) that rather neatly avoided drilling into the fabric of the house and any need to step out onto the roof from the attic bedroom window.

 It's about a foot short of clearing the ridge tiles but I'd wanted to avoid having the SMA connection between the antenna and extension cables (both 5 metre lengths) dangling just outside of my office window, exposed to our glorious English weather. As it happens, there's only about 8 inches of antenna cable left to spare in achieving my desire to keep the connectors on the dry side of the window opening.

 In spite of this shortfall in desired elevation, I'm now seeing a significant improvement in the sky map display which seems to have reduced the magnitude of the deviations being displayed by the deviation map. However, there's still a significant amount of deviant behaviour in the plot of successive fixes around a 'fixed location' which translate into phase deviations of as much as 20 to 30 ns peak to peak[1].

 By way of experiment, I tried using an elevation mask of 20 deg in place of the 10 deg mask setting I'd chosen over the 5 degree default. Oddly, this made the deviation map plots worse, rather than better. It would seem that a 10 degree mask with a reasonably clear all round sky view, is my optimum choice. In this case, it looks like SV quantity beats SV quality.

 Going from 5 degrees to 10 had dropped the typical SV count from 20 to 18 whilst a 20 degree mask had lost me another 4 or 5 SVs from the plot, leaving me with a count of 13 to 15 locked in SVs out of a total of 22 to 24 detectable SVs. Perhaps an increase in the elevation mask setting can improve accuracy with timing GPS receivers but that doesn't appear to be the case with a common or garden navigation only GPS receiver (NEO-M8N in this case).

 I've attached images showing the diagnostic displays in the u-centre application so you can judge for yourself. One thing to note about the SV signal strengths is that they're around 18dB or so down on the original genuine M8N module's signal reports (however, with an active indoor antenna, the signal strength reports are almost identical between the current fake unit and the original, now slightly damaged, unit). I don't think this apparent disparity makes any difference to their performance in regard of the variations of calculated positions in the deviation map display though.

[1] This last source of timing error is essentially a system limitation arising out of the need for the signals to negotiate an ionosphere of varying electron density, aggravated by the varying path lengths due to elevation angle of the individual SVs which can't be fully compensated for by the limited ionospheric correction data carried in the GNSS data packets.

 Never mind the the issues of non-integer division to derive a 10MHz or a 100KHz clock from a 48MHz XO or the sawtooth adjustments (complete with 'hanging bridges') used to keep the PPS transitions aligned to the nearest clock edge, it's this last issue of positional fix errors that demand such long (hours' to days' worth of) averaging times to smooth out (at least as far as the use of a common or garden variety of navigation only GPS receiver is concerned).

JBG
« Last Edit: June 20, 2019, 05:34:50 pm by Johnny B Good »
 
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