A collection of sources of z-errors when working with low cost rtk receivers

I think I would do it more like a mind map and collect any aspects, probably even as far as flight speed of a uav. But at the other hand that might lead to a real mess and it is better to separate everything in threads.

Probably an admin could introduce a new subcategory for that? Very ambitious, but I think this could provide real value to those people like me who are not surveyors but coming from another profession (and I would guess that a lot of REACH users belong to that category).

I think seperating Z from XY is important as the test and usage is different.

Also have one unified test setup that is performed. The one in the PDF i linked too is good but maybe a overkill as it focus on the mm aspect. A simpler one might do it, just to bring the calibration error within a cm or two (for the Z value)?

Ok, lets focus on z values and see how that works out.

Would you describe the setup? I would be interested to replicate that.

Are We here talking relative or absolute accuracy?

If you mean relative to another position or absolute on earth I would say both.
If I get it right the case of the antenna phase center has impact in both cases!?
It might be good the note down where the error sources applies.

I think there is some confusion because of different view points on the topic.I’m looking to get to know ALL sources of error in order to make proper field measurements and photogrammetry. I would even include banal things like applying a wrong pole height. @TB_RTK is far more focused and structured.

In another post, @TB_RTK mentions that weather could be one source of the Phase Center being offset. So, if you make your measurements in a limited time-window, meaning the weather is also the same, then that error would be close to zero.

For the Z-error, in a relative accuracy use case, you can also check this: Evaluating relative repeatable precision of the Reach RS+ - #22 by timmyd
Among the point collected here, I have a RMS in Z-axis of 1.6 mm (on controlpoints, checkpoints being 3.6 mm). From that I’d say that the relative accuracy is extremely good.

Now, for the absolute accuracy, I think the phase center is the least of our problems. Things such as baseline, observation time, and pure mechanical issues are much larger sourcees of error.

Thank you for the additional information!

Now, for the absolute accuracy, I think the phase center is the least of our problems. Things such as baseline, observation time, and pure mechanical issues are much larger sourcees of error.

That was what I was thinking before, but an error up “to almost 4 cm” […] “varying over time” and varying between the units would be a significant source of error.

Googling how to determine the phase center doesn’t give me a sure-fire way to determine this. It would be fairly easy (but still have some other significant errorsources) to establish that the units can disagree (like placing the units at the same height with a known distance in between), and thus concluding that the phase centers are off, but that won’t give us an absolute error we can use for post-processing, because, who are correct to start with?

Yeah, i think as the first step would be to get relative measures. Mixing locations, antennas, different weather etc is going to add insecurity to the variables.
Using two RS (or more) units and compare result from the same timeframe when processed against a vrs or refrence station is one way to determined a relative variation between different units. If no variation is measured and reported, a single mean average delta could be applied for all RS units.

Mount 6 RS units on the same leveled height and all record the same data and process all with and against the same refrence point and see if any units deviates

Does that sound right?

Well, yeah, but how to get 6+ units as a user? :smiley:

How many do you have? :sweat_smile:

edit: my point is, to first see if there is any deviations between units and for that you need minimum two… right?
If somebody is reporting deviation between units, we could all stop reporting because then we need individual offset delta`s

2, and yep! Will set up a rig when the darn rain stops :stuck_out_tongue:

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You know, those RS are waterproofed right :rofl:

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Yeah, but the operator is selectively not IP-rated currently :stuck_out_tongue:


Ya’ll make me laugh!! So how would we find the phase center? I understand what TB was saying as far as checking 2 units (along with my 4 virtual units :stuck_out_tongue_winking_eye:) on same mount to check for deviation. I will do that as well. But after we find any deviation between two units, what is the process details for determining the Phase Center on the Reach RS?

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If anyone find deviation of some relevance (like more then 10mm?), we are back to square one and each individuals have to perform a calibration of their own in some fashion, something like this https://www2.unb.ca/gge/Resources/gpsworld.february05.pdf


Ah, sorry mate. It wasnt my plan to derail your topic :see_no_evil::grimacing:
I can move them over to a new one?

No problem, I value your contributions, I didn’t think you hijack the thread but you have a more structured view of the topic which is very helpful.

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I think in order to solve the problem with finding the phase center we need to list the sources of error.

I have 3 Reach units and soon 1 Reach + unit. I guess a test setup would look like that:

  • mount all units on a perfectly leveled square mounted above a known point and measure the position for 24 h. Turn the square 180 degrees and measure again for 24 h.
  • if we get a deviation in the measure z value interchange the receivers and repeat the measurement, hopefully we get the same results.
  • if we substract the pole length we should get a value which is the sum of several errors, the phase center offset and other things influencening the measurement accuracy. If we could quantify each of the errors or apply some fancy statistics over a lot of measurements we could get the phase center offset value.

Can someone make a list of these things? I start with a random list:

  • phase center offset
  • not perfectly vertical mount
  • mount length change due to temperature differences
  • multipath errors
  • signal quality deteroiation due to clouds, sun bursts,…
  • number of satellites
  • baseline
  • electric noise

Before we do all of this, I think it worth asking Emlid…
@dmitriy.ershov & @egor.fedorov, any input on this, before we spent a massive amount of time?