Without knowing any details about the environment you are working in, its hard to give too much specific device. However, the following are a few useful general tips for proper anchor and master placement:
The anchors, being at a level where they are obstructed by metal objects and people, will produce bad data. The UWB signal simply doesn’t penetrate metal objects or people. Thus the first path is so attenuated that all you get are multipath signals, and the first multipath signal is what gets timed. A future version of the demo code will attempt to detect when this occurs from signal properties and then deweight the anchor data in the results. Elevate the anchors to the ceiling level if you can. Higher is better as that avoids obstructions.
Master node needs to have good radio visibility to all nodes for configuration. It also needs to be in a surveyed and stable location or the results will be off. Plugging it into a laptop directly is usually not adequate, and typically orients the antenna flat. A USB extender cable to where the master is located in a more fixed location would help. Much like with anchors, having the master up high where there is less opportunity for obstructions to interfere with line-of-sight communications is better, but as the master has to be in range of all the anchors and all the tags, it is usually better to keep it a little lower than the anchors to ensure that there are no blind spots on the ground where the tags will be unable to communicate with the master.
Having only four anchors is marginal. If any one is compromised, it affects the results badly. Having more will be good.
Also, if your survey is off, then your accuracy suffers but it also causes jitter in displayed positions when a particular anchor varies. A bad survey means the solution is “fighting” between the data from the anchors to locate the best position and when an anchor leaves the fight, the position jostles a lot.
With anchors all on ceiling, make use of the bounding for the tags. If you bound the tags to be below the ceiling (say ceiling is 3 meters Z, bound tags to be 2.5 meters or less), then you will get reasonable Z axis data.
So, in short:
Fix master node higher, on cable, and with its antenna upward (not lying flat). Must be in surveyed location, must see all anchors as directly as possible, must see all possible tag positions.
Elevate anchors onto the ceiling.
Get more anchors.
Have an accurate survey.
Bound tags below ceiling.