J-Pod

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Duane Frymire

Active Member
Received my j-pod and like it a lot. Couple suggestions:

First, I would like the top to be able to rotate and lock in differing positions. It came with bubbles not calibrated, so I calibrated the bubbles and then when I set up the face of the LS is directly over one of the legs when I screw it on. This means I can't see the screen directly while setting up, and have to straddle a leg to look at it comfortably. I can fix that easy enough but will have to re-calibrate bubbles after moving the bracket. However, I also like to spin it so it faces north correctly after getting it level (what I've been doing using the LS on a prism pole with bipod); can't do that in current configuration.

Second, it came with standard steel points. All the adds/info show the mace leg grip. Not even an option shown to get the standard steel points. I expected it to come with mace leg grips. Is there a shortage and they will be sent to me when available?

Thanks,
Duane
 
Michael Glutting

Michael Glutting

Member
Hi Duane,

Thanks for the feedback, we are evaluating it.

Your three J-Pod ground tips, pictured below, are scheduled to ship on or before April 28th.

upload_2017-3-30_12-5-24.png
 
Jim Frame

Jim Frame

Well-Known Member
I also like to spin it so it faces north correctly
Click to expand...

I've mostly ignored this since there's so much noise inherent in RTK anyway, but I was surprised to see that the Triumph-LS L1 antenna phase center varies by over 13 mm at very high elevation angles. (By comparison, my clunky 20-year-old Trimble flying saucers max out around 9 mm.) The high SVs will mostly affect the up position, but even 60° SVs get into the 10 mm range at some azimuths. Maybe I'll start paying more attention to rover orientation, though it'll be hard to resist rotating to find the most comfortable viewing angle (generally putting the sun at my back) or to put the bipod legs downwind.
 
Adam

Adam

Well-Known Member
5PLS
I can't see any difference in my data that I can say came from facing another direction. I don't worry much over a couple hundredths. I agree with Jim, and the terrain and other obstacles usually decides which direction I am facing. If I were doing very tight work I might face it the right way but I would also want to rotate the pole 180 and average shots which would put one shot facing the wrong way anyway. I also notice the LS works better facing the certain directions based on terrain and vegetation in the woods, especially the mountains.
 
Jim Frame

Jim Frame

Well-Known Member
Error due to antenna orientation is kind of a crap shoot, especially with the short observation times typical of RTK. You can theoretically be oriented exactly wrong and yet be spot on if the SVs are located in positions with compensating phase center error.

Which begs the question: do the J-Field RTK algorithms account for azimuth and elevation phase center error, or is this only pertinent to post-processing?
 
Matt Johnson

Matt Johnson

Well-Known Member
5PLS
Jim Frame said:
Which begs the question: do the J-Field RTK algorithms account for azimuth and elevation phase center error, or is this only pertinent to post-processing?
Click to expand...

Right now only DPOS uses the absolute calibrations that correct for azimuth phase center variations. I'm hoping in the near future that the absolute calibrations can be used in the RTK processing as well and that the compass readings can be used so the receivers don't need to be aligned to north.
 
D

Duane Frymire

Active Member
Jim Frame said:
I've mostly ignored this since there's so much noise inherent in RTK anyway, but I was surprised to see that the Triumph-LS L1 antenna phase center varies by over 13 mm at very high elevation angles. (By comparison, my clunky 20-year-old Trimble flying saucers max out around 9 mm.) The high SVs will mostly affect the up position, but even 60° SVs get into the 10 mm range at some azimuths. Maybe I'll start paying more attention to rover orientation, though it'll be hard to resist rotating to find the most comfortable viewing angle (generally putting the sun at my back) or to put the bipod legs downwind.
Click to expand...
I do ignore the direction for many shots, but important points I orient. I generally start it in comfortable position then spin, sometimes using delay start. Seemed to me rotating 180 would introduce phase center error, but more could be going on than I realized according to this thread. Still, think I'll stick with my current SOP's until I hear absolute calibrations being used in RTK.
 
Nate The Surveyor

Nate The Surveyor

Well-Known Member
Jim Frame said:
the Triumph-LS L1 antenna phase center varies by over 13 mm at very high elevation angles.
Click to expand...

Jim, could you clarify this for me? Does this mean that the actual, non-algebraically corrected point of measure, is tall, above the top of the LS, and when you tilt it, that it skews the measurement?

Thanks!

N
 
Jim Frame

Jim Frame

Well-Known Member
The phase center variation isn't related to receiver setup, it's a function of the electrical characteristics of the antenna. The NGS antenna calibration site has ANTEX files for most antennas, including the Triumph-LS. The ANTEX file is basically an ASCII table that lists the phase center variation (in mm) for L1 and L2 in both GPS and GLO constellations as SVs go from elevation angle 0° (horizon) to 90° (zenith) and from azimuth 0° (north) around to 360° (north again). It's sort of a 3D representation of the possible SV positions with respect to the receiver antenna and the way in which the phase center moves as the SV moves.

The Triumph-LS ANTEX file can be found here. (I tried to upload a screenshot, but kept getting a "there was a problem uploading your file" error.)
 
Matt Johnson

Matt Johnson

Well-Known Member
5PLS
Jim Frame said:
The ANTEX file is basically an ASCII table that lists the phase center variation (in mm) for L1 and L2 in both GPS and GLO constellations as SVs go from elevation angle 0° (horizon) to 90° (zenith)
Click to expand...

The elevation data in the tables is actually in order from 90° (zenith) to elevation angle 0° (horizon). It is at low elevations where the large PCVs occur. Graphical representations of the calibration data is at http://anton.geopp.de/gnpcvdb/pcvdb/GNPCVDB.html

For example here is some T2 calibration data:

upload_2014-10-21_0-12-55.png
 
Nate The Surveyor

Nate The Surveyor

Well-Known Member
I had it explained to me at one time, that the actual antenna, is a "ghost" above the physical antenna. And, that it had a shape, that varied with what antenna and other circumstances.
Also, that it's shape varied some... some shaped like a cucumber, others like an egg, and still others like a small gourd.
When you rotate the antenna, that the actual center of the ghost, was not concentric with the physical antenna. And, that the "Phase center offset" represents this mapped difference between the ghost, and the physical antenna. So, when you rotate the antenna, 180°, you find this difference, x2.
So, this is what makes me ask... what does tilt have to do with it?
I went to your link.... shall we say, that is not exactly something I am familiar with.... But, thanks for the link!
Trying to learn...

N
 
Jim Frame

Jim Frame

Well-Known Member
Nate, it's not tilt in the receiver so much as it's the elevation of the satellite.
Click to expand...

SV elevation *and* azimuth. That's where receiver antenna orientation comes into play. In the graphic Matt posted, a GPS L1 signal coming in at 0° (horizon) at an azimuth of 180° would get a PCV adjustment of almost -2 cm, but if your antenna was actually oriented at 90° azimuth instead of 0° azimuth, the correct PCV adjustment would be about +1 cm, for a net error of 3 cm. (That's an extreme and unlikely example, because low SVs are usually masked out.)

I'm curious about that graphic, though. When I look at the GPS L1 (G01) table in the T2 ANTEX file, I don't see any values that large. It looks like the biggest ones are in the 3 mm range.
 
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