Alternative ways of doing localisation.
Awareness and interest in the individual positioning problem has increased, largely because most of us are using the GPS receivers in our phones for coarse grain localization. Below, we have listed a few common positioning techniques and have explained why we are unsatisfied with their use for individual positioning …
The Global Positioning System uses 24 satellite signals to calculate the receiver’s position. Its relatively coarse accuracy (within several meters) works well for vehicle applications and gross positioning. However, it requires a lot of computation power and works only outdoors. Therefore, GPS is not fit for individual positioning, but it is a very nice complement.
Signal strength methods
There are plenty of attempts to provide indoor positioning based on the strength of a radio signal. The principle is very simple: if you receive a strong signal, you are close to the transmitting antenna. Based on this rough indication, and provided you receive plenty of different signals from various sources (such as cellular and Wifi networks), you can use triangulation to estimate a position.
Countless startups are exploring this localization technique, but we were not satisfied with the accuracy and more importantly, the robustness of the method. A simple obstacle, like someone’s body, greatly affects the strength of the signal and the perceived location can suddenly jump away… until their body moves again.
Signal position methods
The figure below shows the effects of errors when measuring signal strength versus signal position. Signal strength methods analyze the height of the signal (Y axis), while signal position focuses on the time position (X axis).
Because signal strength is affected by many factors, the measured strength can vary significantly from the expected result. As shown in grey on the left hand part of the figure, these factors can seriously reduces the accuracy of the distance measurement. On the other hand, there are few factors that affect time measurement (on the right hand side). This makes signal position methods more dependable than signal strength methods.
With regard to signal positioning, let’s consider the modulation of a pulse in more detail. In a binary world, a pulse would be a square signal as pictured immediately below. However, in a radio world, things are a bit more complex and a pulse looks more like the sinusoid as shown below the square wave. The orange curve outlines the envelope of the pulse. Radio theory tells us that the wider the modulation bandwidth, the narrower the pulse width can be.
When measuring distances, the goal is to accurately find the pulse along the time axis. Hence it is important to minimize the width of the pulse, in the same way as one would pick a sharp pencil to make an accurate drawing.
Ultra wide band allows the creation of pulses which are 1000 times narrower than what is possible with Bluetooth. Ultra wide band is the sharper pencil by far, and that is one of the reasons we have selected ultra wide band for precise localization.
The table below summarizes the pros and cons of each positioning technique.