It is reported in the Facet data that it is possible to have a precision of 30 cm without corrections.
The screen normally reports that data, but in my measurements I have obtained discrepancies with respect to that value.
On the other hand, the specifications of the zed-f9p, downloaded from u-block, say that the precision without corrections would be 1.5 meters.
What is the reality?
The 30 cm reported on the screen, how real are they?
I await your comments
amlago:
It is reported in the Facet data that it is possible to have a precision of 30 cm without corrections.The screen normally reports that data, but in my measurements I have obtained discrepancies with respect to that value.
On the other hand, the specifications of the zed-f9p, downloaded from u-block, say that the precision without corrections would be 1.5 meters.
What is the reality?
The 30 cm reported on the screen, how real are they?
I await your comments
I would not trust a precision of 30 cm without any correction. In the other hand, what are the range of the discrepancies you obttained? maybe you can make a static point, and compare that way. I suspect is really around 1 meter. I just received a pair ox RTK express and waiting for survey poles and bipod to make some tests.
I have obtained position differences of around 1 m and more on occasion.
I have other professional equipment and without corrections, that is, in absolute mode, they inform you on the screen with precisions of 1 meter and more.
In other words, they tell you what to expect and what the theory predicts.
I don’t know then where the zed-f9p gets the 30 cm and less, of precision, when in reality we are in the environment of the meter and more.
Above u-block tells you that the precision in absolute mode is 1.5 m.
I would like the information on the screen of the Sparkfun rtk in absolute mode to be more accurate with reality. Of course it’s a zed-f9p issue.
I clarify that in relative mode, with rtk corrections, the precisions achieved are topographical and first level. In the environment of cm .
And the precision information displayed on the screen is totally credible and real. I have used a Facet in many jobs and I have more than proven it with many fixed points.
So my comments, I insist, are focused on when working in absolute mode, that is, without corrections.
It would be great if what I see on the screen was more in line with reality.
If I document the data collection at a fixed point, I will upload it.
I just remembered if I am in rover mode with 30 cm and go to base mode, using survey in, although in rover mode precisions of 30 cm or less will be reported on the screen, there is a precision jump to around 1 meter, which it’s much more believable.
In other words, the equipment knows that the precision is not at all around 1 meter, but sometimes it shows you something else. (the known 30 cm or less).
amlago:
I have obtained position differences of around 1 m and more on occasion.I have other professional equipment and without corrections, that is, in absolute mode, they inform you on the screen with precisions of 1 meter and more.
In other words, they tell you what to expect and what the theory predicts.
I don’t know then where the zed-f9p gets the 30 cm and less, of precision, when in reality we are in the environment of the meter and more.
Above u-block tells you that the precision in absolute mode is 1.5 m.
I would like the information on the screen of the Sparkfun rtk in absolute mode to be more accurate with reality. Of course it’s a zed-f9p issue.
I clarify that in relative mode, with rtk corrections, the precisions achieved are topographical and first level. In the environment of cm .
And the precision information displayed on the screen is totally credible and real. I have used a Facet in many jobs and I have more than proven it with many fixed points.
So my comments, I insist, are focused on when working in absolute mode, that is, without corrections.
It would be great if what I see on the screen was more in line with reality.
If I document the data collection at a fixed point, I will upload it.
I think there is a reason for this, and is there is a reason and knowing it, then u can use the data better. But i totally Agree with you that the info on the screen should be very close to the reality in terms of absolute accuracy that we are talking here, Relative position, (RTK, or rapid statics, or something else having some correction source) we all agree is whitin specs.
The HPA or Horizontal Positional Accuracy is what we display on the screen. It’s part of the HPPOSLLH message from the ZED-F9x. From the ZED-F9P interfaceface description (https://content.u-blox.com/sites/defaul … 008968.pdf) it is a “Horizontal accuracy estimate”.
I cannot say with authority how accurate the ‘accuracy estimate’ is but we regularly see better than 0.3m HPA with a TOP106 or equivalent antenna on top of SparkFun (good, clear view of the sky). This morning I’ve got 0.138m.
Comparing this against a PPP survey of the same antenna:
We see that the uncorrected ECEF coordinates coming from the ZED are approximately 0.7m when the HPA is reporting 0.2488m.
I recommend using the HPA as an estimate of accuracy and a quality indicator (a lower number is better, a high number indicates poor reception / lack of RTCM / etc).
Don’t forget that statements of accuracy are properly accompanied by a standard deviation or confidence interval. If it’s not, perhaps you best not take the number too literally. Use it as a quality indicator as @sparky suggested.
Is that purported 30cm a 1-sigma, 2-sigma, or 3-sigma value? Might need to dig into the U-Blox documentation to find out.
Manufacturers often provide the 1-sigma standard deviation value. Remember that means 68% of the observations are expected to fall with the provided value, eg. +/- 30cm, given that common statistical assumptions apply to the system. The other 42% would be expected to fall outside of the +/- 30cm.
Here’s a page I found with a quick web search that might help.
Also, the horizontal accuracy for GNSS receivers is generally noticeably better than the 3D accuracy.
Thanks for the feedback. Confidence intervals are not discussed at this time.
The u-BLOCK itself establishes 1.5 m for absolute positioning in its references on the zed-f9p and that does not even coincide with the 30 cm and less that we usually see and that we know from experience are not true. I always get in absolute mode, that is, without corrections, discrepancies of around 1 meter with corrected positions.
That is more like the 1.5m reported by u-block.
That is what I meant.
In short, I would like the screen to report more real values as reported by other professional level equipment that I own.
These professional teams only go down to decimeter or centimeter levels with corrections.
(I provided a screenshot of the u-block report above.)
I have gone further with the study of precisions using Facet and went on to study the RTK positions.
I placed the Facet at a well-known fixed point and got corrections from 3 ntrip stations from my country’s ntrip network, the IGM Uruguay network.
The first ntrip station is Leica GR10 and corrects only in GPS and Glonass, it was 2 km away and I got excellent results, the screen indicated 0.014 m and indeed the position obtained was around 2 cm with normal vision conditions, not excellent.
The second station was 50 km away, it only corrects GPS and Glonass and I obtained fixed positions with results on the screen of 0.02 m but the position differed somewhat more, reaching discrepancies of 0.10 cm.
The third ntrip station is 105 km away, it is a Leica GR30 station with the 4 constellations. I obtained relatively fast fixed results with the Facet, with 0.035 m on the screen, but they are misleading, since the position difference reached 0.25 m. Other equipment from another brand was fixed in a longer time and informed me of 0.28 m of horizontal error. Very similar to the actual error obtained.
Preliminary conclusions.
a) In normal conditions and short distances to the base, less than 10 km, the results are very good and the precision reported is similar to the real results of position difference.
b) In normal conditions or less than that, with distances to the base greater than 30 km even though the Facet says fixed position and the horizontal precision values indicated on the screen are small, such as 0.014 or 0.02 m, you must be careful and take the point repeatedly.
In this situation the actual position differences were much larger than the horizontal precision reported on the screen.
P.S. I love Facet and have spent a lot of time on it, reviewing and commenting on its firmware, both here and on Github.
I would like what is reported on the screen to be more credible, in order to work calmer with respect to the data collected in the field.
I know that the reported data comes from the Zed f9p and that already depends on the company that builds it and not Sparkfun.
We continue exchanging ideas. Regards
Thanks for posting your data! Your results look reasonable.
Manufacturers published RTK specifications are often something like “1cm + 2ppm”. That means 68% of the readings fall within +/- (1cm + 2ppm). And 32% fall outside that!
2 ppm would be 2mm error with a 1 million mm baseline.
Or 2mm error in 1km.
Or (105 x 2) mm in 105km
And the total 68% confidence error would be about
+/- ( 1cm + 210mm)
+/- 0.22 m
Whew hope I didn’t mess up the math.
Yes, the screen doesn’t say that. And we know 105km baselines are not the best way to do things. 
Returning to the issue of the precision reported by Facet on the screen, I am sending you results of a post-process with a file of one hour, under normal conditions and less.
When the post-processing achieved fixed positions, they were in the cm range, but several non-fixed positions were obtained with a margin of error of meters, while the screen reported ± 0.30 m.
I continue with the idea that the Facet reports precisions closer to reality, as other professional equipment does.
The strange thing is that the u-block itself reports precision without corrections of 1.5 m and then the famous 0.30 m is seen on the screen.
Ideas are accepted.
