DPOS and OPUS Comparisons

someone told me years ago that the LS should be facing south with the LS screen to the north,, back side facing south. Have I been doing it backwards?

Mark Wheeler said:

someone told me years ago that the LS should be facing south with the LS screen to the north,, back side facing south. Have I been doing it backwards?

Click to expand...

I believe you're correct. NGS drawing for reference...

Mark Wheeler said:

someone told me years ago that the LS should be facing south with the LS screen to the north,, back side facing south. Have I been doing it backwards?

Click to expand...

Screen to the North is correct. Your compass should be reading South

I guess that is an elementary thing that I should have learned, but have had the LS compass indicate N, when it should really be indicating south.... hmmmm.

I have went back and reduced the OPUS and DPOS files from several days ago. The horizontals on OPUS did not change any from the reduction days ago, but the verticals adjusted on some of them by thousands. I do not really have a timeline on how long you should wait to submit, to get the final reduced coordinates. Does anyone else have a timeline that should be held in order to get the final coords?

If this is significant, what part of a T3 base should be facing North?

Glen Yasharian said:

If this is significant, what part of a T3 base should be facing North?

Click to expand...

The side with the buttons and LEDs.

That's kinda funky.
So, we are supposed to be LOOKING south, on both the LS, and on the T3?
So, the hardware is all drilled the same.
And, the electronic antenna, (ghost) above the unit is NOT centered, on the physical center?
And,
This is so, because no 2 units are just alike?
So.... If I knew the correction, and direction of it, (the little correction placed inside each individual unit), we might, in theory, remove this correction, via an eccentric, variable mount, for each unit? And, thus direction would be completely compensated for? Or, is it also dependent on this proper direction for the measurements to each satellite? In which case, proper orientation is essential, for highest precision?
Curious minds want to know....
N

So, if we are looking NORTH on both t3, and LS, it does not self compensate? Ie, dgps?
So, the best answer is "look south young man"?
N

Nate,

Indeed, all Triumph receivers should be oriented with MMI (buttons) pointed North, AFAIK.

As for the calibration magic, the first thing is the PCO which is sort of the "average" point of orientation-independent reception of signals (the "ghost" is certainly a good word for it). In addition to that offset location, the phase center varies around that non-physical average point depending on signal orientation and frequency too...

Antenna calibration models account for both azimuth and elevation of EACH received signal with respect to an antenna-fixed reference frame. So for any given signal of a given frequency in a given spot in the sky, the receiver expects to apply a particular "correction" to that signal under the assumption that the antenna is oriented in the same direction as the model was developed for.

So if the antenna is modeled with a particular correction value for, say, a signal coming from due north at 30 degrees elevation, the only way it can apply the proper correction is if the antenna is oriented so the signal is hitting the antenna at that physical orientation, since the antenna/receiver doesn't measure the orientation of the signal itself. If the antenna is turned away from the expected reference orientation, the signal is still spatially coming from due north, 30 degrees elevation, but hitting the antenna in some other antenna-referenced direction. The receiver "hopes" the antenna is oriented correctly, looks up the azimuth/elevation of that satellite, and applies the expected correction from the model, but in this non-north-oriented antenna example the signal is now hitting the antenna at some other direction which would have a different correction that it should apply if only it "knew" it was oriented away from north.

That's where a magnetic determination of the antenna orientation could be immensely handy. Now the receiver knows how far off of North it's pointed, and can "simply" add/subtract that azimuth to it's lookup table to find the ACTUAL correction that it should apply for any given signal, no matter how the antenna is pointed... Realistically, one might imagine that, since PCV is dependent largely on elevation angle, tilt corrections should be applied to the calibration lookup in a similar manner as well, if out-of-level shots are being taken.

All of that said, in theory, for sufficiently similar base/rover antennas over sufficiently short distances, orienting both antennas in the same direction (South, or West, etc...) may indeed "self compensate" in a manner of speaking - in other words both receivers would apply the same "wrong" correction, yielding absolute positioning errors while giving adequately good relative measurements (all your measurements are for example now off by 5mm to the East, but it's the SAME 5mm East error for both receivers...). However, over the course of a day, as satellite locations change and different "wrong corrections" are applied to the signals, repeatability of measurements may get funky.

Hopefully someone can point out any errors in that whole monologue there. I'm no electronics/RF/NGS engineer

Anyway, for the time being, I quite happily spin my T-1 on the stick so that the lights and buttons are pointed north and mentally announce "Northern Lights" as part of my pre-shot checklist. "Look south, young man - that's the way it's always been done!" to squeeze out those extra mm of accuracy.

I’m imagining now, a d/d intersection, performed via total station. One is Leica -34 mm offset, one is Topcon, 30 mm offset. And, we rotated the unit, so as to swap the prism offset corrections.
makes sense. But, pointing north would be better. Unless you are in the Southern Hemisphere. Looking into the sun makes me tired.
also, it’s better to look north, when explaining things to others. Most people think spatially about maps, oriented to north.
believe it or not I know a surveyor, that drew his plants with a South arrow instead of a north arrow.

I think etweet did a great job of explaining the concept. And I looked at a couple of specfic examples using the antex entries for a T-LS, and I now believe that Matt's finding of 0.05' horizontal difference isn't unreasonable. It may be on the outer edge of what one is likely to find -- the possible combinations of SV signal directions and their PCV effects are too numerous for me to get a good idea of how they all might play into the resultant position -- but it seems within the realm of possibility.

@Alexey Razumovsky Do you see the possibility to tell web dpos to omit certain constellations from the processing. As we know at certain times of the day we have issues with Galileo using RTK, should we be concerned or have the abilities to omit Galileo from the Post Processing task.

I have continued to submit files to both DPOS and OPUS, and since OPUS is GPS only, could the variations that I am seeing between the 2 solutions be the other constellations. I would like to be able to have DPOS just process GPS so I could compare the post processing engines, to rule out the other constellations creating an issue.

I know creating a rinex for OPUS will turn off constellations, is that the path I need to take to get a file to send for DPOS, to just process GPS only?

Thanks

Create a rinex file with only GPS using JPS2RIN utility software.

Never mind. DPOS doesn't accept rinex. Sorry.

Looks like if I pick Geodis to process the file instead of Justin, I get all GPS and no other constellations, only issue I am still left with Geodis seems to use Geoid12B and not Geoid18, is that something that can be added, or changed. Is there a reason why Geoid18 is not used for Geodis. I will say that I was shocked to see how many CORS Geodis uses, which is significantly more than Justin uses for its computations.

Ok, since I figured out Geodis only uses GPS, I thought I would rerun the files and see if the consistency changed. The results are fairly significant. The Justin files are more prone to large Elevations swings, vs Geodis. The main takeaways are Geodis processes the files and the results are more consistent. The consistency is more inline with OPUS also. The elevations were the biggest differences. Since Geodis will only process Geoid12b and OPUS is using Geoid18, that may account for the difference in the elevations. The average point did not change all that drastic in the Horizontal, but fairly significantly in the vertical. Attached are PDF's showing the differences between the 2 different post processed files. Turning the base more directly toward north may improve the results, but best I can tell, the derived location of the base, is consistent with the published marks around it. I have tied into several fixed monuments and they check, relative to the base derived coordinate. The results of the newly derived Geodis / OPUS averaged coordinate, when staking out a fixed control monument, checked 0.01' Horizontally and 0.03' vertically. So it appears that if you can use Geodis, for multiple base files, that is the way to go.

Curious if there would be a reason NOT to use Geodis over Justin, when doing multiple days of reductions.

James Suttles said:

I guess that is an elementary thing that I should have learned, but have had the LS compass indicate N, when it should really be indicating south.... hmmmm.

I have went back and reduced the OPUS and DPOS files from several days ago. The horizontals on OPUS did not change any from the reduction days ago, but the verticals adjusted on some of them by thousands. I do not really have a timeline on how long you should wait to submit, to get the final reduced coordinates. Does anyone else have a timeline that should be held in order to get the final coords?

Click to expand...

James,
For what its worth OPUS does state that it can take weeks to get the final orbits.

I have reran all the OPUS files as well, to be sure they are not moving in great deal, and I have waited 24 hours since the collection times, with the exception of todays, processing. It will be 24 hours from 8:00 PM last night. I will reprocess the 8/10 file tomorrow in the AM.

Always use ellipsoid heights when comparing precision positions from GNSS. Orthometric just adds an additional layer of confusion to the comparison.

Agreed.