Anchor to tag minimum distance measurement

Hi ,

We were testing with four anchors and 4 tags setup. Our test area is 65 m and 148m.
In 65 m area on border position is less accurate(Has more deviation)
In 14
7 m area on border position accuracy is higher than 6*5m area.

What would be the issue?

Is there should be a minimum distance maintained between anchor-tag / anchor to anchor?


You can take below as a reference, although they do not directly talking about the minimum distance measurement.

the link is dead …can you repost the link or explain the answer here directly?

No, it is not dead. I can view it.

I copy it directly from there.


[color=#333333][size=small]This is a complex question without a clear answer. The modules from Decawave that are used in the LPS system specify the accuracy to be in the ±10 cm range when measuring the distance between two modules. There are a number of factors that change this though[/size][/color]
[color=#333333][size=small]1. Free line of sight. Any objects in the path of the radio waves will change the measured distance, depends on material and thickness.[/size][/color]
[color=#333333][size=small]2. Reflections. Metal surfaces can create strong secondary paths for the radio waves which may trick the module to trigger on the slightly longer path instead of the direct path. Reflections are actually helping the modules to receive the data of the packets (might be surprising) but is bad for the timing. We have found that the over all performance is better inside than in an open space outside.[/size][/color]
[color=#333333][size=small]3. Distance to anchors. There are non linearities associated to the received power of the signal, the error increases with power. There is compensation for this but the measurements are not as good when the modules are close. I would recommend a space where are anchors more than 3-4 meters apart to reduce the effect.[/size][/color]
[color=#333333][size=small]4. Interference with other radio sources. If there are other radio sources in the same frequency band they might interfere with the UWB signal that is very weak. The packets might be received but the timing can be off.[/size][/color]

[color=#333333][size=small]In the real world some packets are received as expected with good accuracy, some are received with bad timing and some are lost. We use sequence numbers in the packets to detect lost packets and try to make sure we use valid data fot the calculations. Packets with bad timing are harder to detect and the outlier detection could be improved.[/size][/color]

[color=#333333][size=small]The above is related to the time of flight measurements between two UWB modules. The next step is to estimate a position from this data and we use a kalman filter for this where we merge distance data from the LPS with other sensors such as the accelerometer. The kalman filter does a good job but noise and errors in any data will effect the end result. For instance unbalanced propellers increase the vibrations and the noise in the accelerometers (mainly Z) which might offset the estimated position.[/size][/color]

[color=#333333][size=small]It is also important that the anchor positions are correct. If the anchors are note placed at the coordinates that are configured in the system the end result will of course be wrong.[/size][/color]

[color=#333333][size=small]We have seen some “warping” of space that we do not fully understand. In some areas the estimated position can be offset a bit (up to 20-30 cm). It seems to be static and repeatable.[/size][/color]

[color=#333333][size=small]In your case I would start by looking at:[/size][/color]
[color=#333333][size=small]1. Is the bottom of your cage a metal plate? That might be problematic?[/size][/color]
[color=#333333][size=small]2. Are the bars in your cage metal?[/size][/color]
[color=#333333][size=small]3. It is hard to see in the image but it looks like the anchors are mounted on the bars in the cage? Assuming this is correct[/size][/color]
[color=#333333][size=small]3.1 Try to move the anchors further away, 3-4 meters from each other[/size][/color]
[color=#333333][size=small]3.2 The anchors seems to be mounted on the metal bars, try to mount them at least 10-15 cm from any wall or metal surface[/size][/color]
[color=#333333][size=small]4. Try to move the Crazyflie around without flying and see what position estimates you get (to avoid any vibrations)[/size][/color]