Pozyx

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Where to place the anchors

Rules of thumb

The following rules apply to distance based positioning and can be used for the Pozyx platform.

  1. Place the anchors high and in line-of-sight of the user.
  2. Spread the anchors around the user. Never on a straight line!
  3. Place the anchors vertically with the antenna at the top.
  4. For 3D positioning: place the anchors at different heights.

The rules explained

Rule number 1: Place the anchors high and in line-of-sight of the user.

The first rule is straightforward: placing the anchor high (on the ceiling or on the walls) increases the chance of receiving a good signal because there are less obstructions. Obstructions generally have a negative influence on the accuracy of the range measurements which has a direct effect on the positioning accuracy.

Rule number 2: Spread the anchors around the user. Never on a straight line!

A single range measurements will only give information in a single direction. This direction is exactly the direction from the user to the anchor. Because of this, it is best to spread the anchors such that they cover all directions. If the anchors are all on a straight line, the positioning error will be very large. This can be seen on the image below. You can see that a small change in radius (for example due to noise), will result in a very large change in the position of the intersection(s). In other words, the error on the range measurements is amplified! This is the same principle as in GPS, shere it is called the geometric dillution of precision (GDOP).

Rule nr 2.

Rule nr 3.

Rule number 3: Place the anchors vertically with the antenna at the top.

The Pozyx system uses wireless UWB signals for positioning and this requires an antenna. However, it is physically impossible to create an antenna that performs good in all directions. The monopole antenna (the white chip) on the Pozyx devices radiates omnidirectionally in the xz-plane, but doesn't radiate as well along the y-axis (see figure). To make sure that we have the best possible reception, it is recommended to place the anchor vertically. Note that the same is true for the Pozyx tags.

Rule number 4: For 3D positioning: place the anchors at different heights.

If the user and the anchors are all in the same horizontal plane, it is not possible to have a very good accuracy for the height (it is the same principle of GDOP in rule 2). For the pozyx system however, this rule is less important because the boards are equipped with an altimeter which can already measure the height.

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