This article is part of the Pozyx Academy and is the fourth in a series of five articles that explain how the Pozyx Technology works and what to account for when installing the Pozyx System.
In this article, we will explain what line-of-sight (LOS) means in the context of ultra-wideband (UWB) positioning and how you should interpret it.
A great advantage of using ultra-wideband compared to optical or sound-based positioning like camera tracking or ultrasound, is that ultra-wideband is a radio technology that can easily penetrate objects. Pozyx relies on the transmission of an ID and timestamp over UWB for positioning, and not merely on signal strength like many other Bluetooth/Beacon-based solutions. This means that even if the signal is heavily obstructed, there is in most cases only a small loss of accuracy and range.
Clear line-of-sight is the ideal scenario for both the best range and accuracy. It means that the antennae of all anchors involved can see each other.
For wireless radio technology, the effect of obstacles on the radio performance depends on the material of the obstacle and its radio transparency. Signals will partially bounce off of obstacles causing reflections, and will partially penetrate the object. Depending on the type of material, the signal will pass through the object with little effect, or will be completely absorbed. When there are obstacles, we call this non-line-of-sight (NLOS).
Most materials can be split up into two categories: insulators and conductors. Because all radio waves are electromagnetic waves, materials in the conductor category will have a big impact on a signal. In the conductor class, the most common elements are metals, with the exception of saltwater.
Materials like wood, plastic, glass, cardboard, brick, etc. are all great insulators and are very transparent to radio waves. This means that if one of these materials obstructs the path between two Pozyx devices, the impact will generally be negligible. That does not mean the signal will go through a pile of logs or bricks without any impact, there are limits. But it does mean that a plastic casing or a none-lead-bearing wall will not affect your setup much.
Metals are the most common conductors. Conductors will reflect most of the radio waves and give the part that does go through a harder time passing. This gives you two negative properties:
• The signal will have less power and thus reduced range.
• The signal will spend extra time trying to get through the material, but since Pozyx positioning relies on calculating the time of flight, this will reduce accuracy.
This should not be interpreted as “any amount of metal will ruin the signal”. Thin sheets of metal will have an impact but can still be penetrated. Placing a tag on the back of a phone, near a battery, inside a car, or on a robot will still work fine, albeit with an accuracy reduction of a few centimeters. Concrete walls with plumbing, wiring, and metal supports structures, on the other hand, will cripple the signal. The signal will get through but your accuracy will be reduced and your range after the obstruction will be very short.
When metals are very close to the anchor, they can unpredictably alter the antenna's properties. Because of this, it is always recommended to keep the anchor at a minimum distance of 20 cm from metals.
Similar to metals, liquids absorb radio waves. The saltier the liquid, the more it absorbs the radio waves resulting in reduced range. Furthermore, radio waves travel slower in liquids (about 30%). Because Pozyx relies on calculating the time of flight of the signal, this will reduce the accuracy of several centimeters.
The size of the impact of the liquid obstruction on the signal is directly related to its volume. Holding your hand between two devices will have almost no impact. Placing a person or barrel of water between two devices will significantly reduce the range and result in range measurements that appear to be further away.
NLOS and reflections
As stated in the previous article about ultra-wideband, reflections have little impact on the accuracy of ranging because only the line-of-sight signal is used for calculations. However, in the case of severe NLOS, it may be that there is no LOS signal, and all that is received is the reflection. Because the reflection travels a much larger distance, this can result in ranging errors of several meters. Fortunately, there exist several ways to deal with these large NLOS errors using NLOS detection and mitigation techniques. At Pozyx we use several advanced techniques to eliminate the adverse effect of NLOS. This is available in all Pozyx systems and allows for operation in challenging environments such as warehouses or retail stores.
Read the next article on where to place the anchors to learn more about Pozyx’s technology.