# Z Coordinate Direction and Z Position Accuracy

I am not getting good accuracy along Z-axis (up to 1 m). I tried to set anchors at different height (1 feet apart) but that did not help…it leads me to the question:
Based on the manuals X is going from 0,0 at the bottom left corner (Initiator) to the right and Y goes from there to the top. Where Z is going? Into the page or out of the page?

Z should be up/out of the page.

If you set your anchors at 1 foot apart on the x/y axis would you expect good horizontal position? Getting good z values is tricky.

I started doubting what you said about direction of Z axes. The initiator anchor is located 2.44 meters above the floor and the tag is just at .57 m above the floor. So according to what you said the tax Z coordinate is -0.74 m…And yet I am getting the measurements (which are ok in X and Y but all Z measurements are positive, anywhere from about 0 to 1.37 m
BTW my anchors are 3-4 meters apart and differ in Z direction between them by 0.3-.5 m.

So with the differences like that - where do I even start to get more or less accurate Z measurements, something like 0.3-0.5 m ??

Hi Leon,

Andy’s pointed right but I think you missed the point. Try to understand GDOP: https://en.wikipedia.org/wiki/Dilution_of_precision_(navigation)
That will explain why you cannot get better results for Z if the anchors are 0.3-0.5 m separated in height. It’s a pure math and has little to do with UWB.

Cheers,
TDK

I have read that before but in a sense it defies the reality - most homes/offices has certain height of a ceiling (at least in US) and it is mostly does not exceed 2.7-3 m.
Now, how do I supposed to space the 4 Anchors along 3 meters?

1). 2 on the ceiling and 2 on the floor? (which basically flipping X-Y coordinate system 90 degrees)
2) Evenly spaced along 3 m - that is about 0.7m between them
3) Any other suggestions?

Unfortunately maths doesn’t care what height your ceiling is or whether the ideal geometry is possible in the real world or not.

Generally you’ll want to keep anchors off the floor since the chances are their view of the tags will be blocked a lot of the time if they are too low.

Generally you’ll want two at opposite corners of the room as high as you can get them and the other two as low as you can get them without restricting the lines of sight. That will give you the best mathematical solution that is still practical.

And just accept the fact that it’s not going to be as good in the Z axis.

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I get it. When you say “opposite” do you mean diagonally opposite?

I do discover more and more limitations and restrictions but that’s exactly the purpose of the entire “exercise”. For example: FCC restriction of placing static transmitters outdoors which practically eliminates use of the system outdoors. That makes accuracy in Z less relevant unless you are going to instrument a large warehouse where forklifts deliver goods to upper shelves like they do in Home Depot…

I followed your suggestion and indeed started getting 10-20 cm accuracy in Z coordinates. However, at the same time I am getting terrible accuracy along X-axes (50-60 cm which at the moment I cannot explain) and excellent accuracy along Y-axes…

Another correction.

So far I had 2 Anchors close to the sealing (92" from the floor) and close to the corners (2 feet from the corners) on one wall, and 2 others on the opposite wall 9" from the floor and not so close to the corners.
That where I was getting better Z accuracy but quite bad X-axes accuracy.

So I moved one Anchor from the low position to the mid position of 60" from the floor.
BTW, I always had line LOS.
In this scenario I get good accuracy along all axes (from 5 cm to 15 cm).

I also have a feeling that the orientation of the anchor has something to do with accuracy as well…can’t draw conclusions yet…

Hi securigy,

The FCC outdoor rule is not too clear. Have a look at the post below from mike ciholas.

Yves

Hi @securigy2,

can you tell me if you made any progress in optimizing your location results? As far as I understood, you have two anchors placed just below the ceiling, one anchor placed just above the ground and another anchor placed somewhere on mid level height, right?
Are the anchors close to the ceiling located at the same wall, while the remaining anchors are located at the opposite wall? Or did you distribute the anchors over every wall (assuming you have a rectangular room with 4 walls).