Emlid

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


#1

Hi,

I would like to start a random list of sources of z-errors (errors compromising the height measurement) when working with low cost RTK GPS receivers such as EMLID REACH. I would ask you to randomly contribute and I will later structure it systematically so that every one can benefit.


Antenna specification
#2

[Antenna hardware]: unknown position of the Antenna phase center (APC)

Antenna phase center is unknown for each antenna and needs to be calibrated to get the exact value. The build in antenna is an OEM model from Tallysman and phase offset could be uniq for each model, antenna phase offset highly depends on it surroundings. But its to be assumed phase center is close to ARP. A test could reveal that.
On my unites it varies between few mm to almost 4cm (reach with tin foil) and varies over time. Phase offset also changes over time.
image
https://www2.unb.ca/gge/Resources/gpsworld.february05.pdf
cited from TB_RTK


#3

Ok, cool . Should we focus on the Z values? and create a seperate one for the XY?


#4

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).


#5

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)?


#6

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

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


(Christian Grüner) #7

Are We here talking relative or absolute accuracy?


#8

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.


(Christian Grüner) #9

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+
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.


#10

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.


(Christian Grüner) #11

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?


#12

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.

E.g
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?


(Christian Grüner) #13

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


#14

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


(Christian Grüner) #15

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


#16

You know, those RS are waterproofed right :rofl:


(Christian Grüner) #17

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


(Timmyd) #18

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?


#19

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

:rofl:


#20

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