Z-axis based on UWB signal

Estimation of Z-axis based on UWB signal is highly dependent on anchors deployment. Among line-of sight requirement between tag and anchors, placement of the anchors (GDOP) is the most critical factor achieving good accurate positions in Z-axis.

Let’s see an example with four anchors and one tag. If we consider ideal case where signal is travelling from Tag to Anchors with zero error – Z-axis is precisely calculated regardless anchor composition.

However, let’s consider the same scenario in more real word conditions, so we introduce signal propagation error in few Nano seconds:

Open Space (-1,1ns)
Moderate NLOS (-2,2ns).

Here are results:

Anchor Deployment	Anchor Coordinates [x,y,z]	Open Space Accuracy Error STD [x,y,z] m	Moderate NLOS Accuracy Error STD [x,y,z] m
3 anchors at the same height, 1 anchor is lowered.	[10;0;3], [20;0;1], [10;10;3], [20;10;3]	0.12; 0.09;1.39	0.29;0.17;2.71
Cubic constellation – Tetrahedron with 4 anchors	[10;0;0], [20;0;10], [10;10;10], [20;10;0],	0.08;0.06;0.19	0.16;0.15;0.29
Realistic Anchor placement with x:y ratio 2:1.	[10;0;2.7], [20;0;2.8], [10;10;2.5], [20;10;1],	0.09;0.08;1.61	0.38;0.33;3.96

*Tag coordinates for all scenarios is [15;5;1.123456].

Basically, only constellation providing sub-meter accuracy is deploying anchors into cubic shape, which is highly impractical outside the lab environment. Problematic part are anchors close to the floor – where they can be easily shielded by assets, moving people or vehicles and easily damaged.

Therefore, Sewio is not focused on Z-axis estimation via UWB signal and recommend using barometer approach where it is applicable. For other application where Z-axis with high accuracy is required we recommend extension of UWB [x,y] with other laser/lidar sensors for Z-axis.