The Corona-tracer uses Bluetooth to measure the distance between people wearing the Corona-tracers. But how does that actually work? We are going to explain that the Bluetooth signal in front of the body where the tracer is worn is stronger and therefore more sensitive than on the back. And that this is actually the pattern that the virus follows, because we infect each other through our breath or a cough. Bluetooth may not be the best technology for measuring distance, but it has the most advantages compared to other technologies, making it the best solution to keep track of who has been close to you.
As a car gets closer from afar at night, the headlights get brighter. What applies to light also applies to radio waves, because they are actually the same type of (electromagnetic) waves but with a different wavelength. The "brightness" (or as we keep calling it below "signal strength") is dependent on the distance with the inverse square law: when the car is twice as close, the headlights are four times brighter.
If one phone transmits a Bluetooth radio signal and the other phone measures the signal strength, the signal strength will become stronger the closer the two phones are. This would suggest one could quite easily measure distance this way.
Unfortunately, it is not that simple. The frequency used all over the world for Bluetooth (and for Wi-Fi), 2.4 GHz, is actually not very useful for radio traffic. For example, almost the same frequency (2.45 GHz) is also used in microwave ovens to heat up our food. It turns out that water absorbs this radio frequency very well. Water is a dipole, which means that one side of the water molecule is negatively charged, and the other side is positive. In an electromagnetic field - the radio signal - the direction of the electric field changes direction 2.45 billion times per second. The water molecule tries to follow that direction and therefore turns around as many times every second. All those fast-moving water molecules together cause a lot of heat in a microwave oven and that is exactly what we want.
The Bluetooth signal is billions of times weaker, so we cannot heat anything with it. Unfortunately, however, the radio signal is absorbed by water. And the tricky thing is that our body consists of more than 60% water.
Let us do an experiment. A test subject stands in the middle of a room and puts our Corona tracer on a lanyard around his neck. We can use a smartphone as a signal strength meter. We will first face the subject and measure a signal strength of -80dBm at 3.2 meters, which corresponds to 0.01 nW (nanowatt) (so the signal is 80000 billion times as weak as an 800 W microwave). We then walk around the subject and measure the distance at which we see a signal strength of -80dBm.
If we were to draw a line on the ground around this person, where that line indicates the distance at one particular signal strength value, you would expect a circle on the ground. But due to the absorption effect of water, the shape on the ground looks very different.
The red line indicates the distance with the same signal strength. The strength of the signal behind the person is much lower than at the front. The distance between the circles in the picture is always 1 meter. At the front, the signal reaches more than 3 meters, while at the back it does not get any further than 35 cm. So, the body has a huge effect on the shape of the emitted radio signal.
When the same person puts the Corona-tracer in a trouser pocket, the shape changes, as shown here.
In order for the Corona-tracer to work properly, it is therefore important that all users wear the Corona-tracer in the same position on the body, so that we can adjust the signal strength measurement in the Corona-tracer accordingly: that is why we recommend wearing it on a lanyard around the neck.
Therefore, we cannot define a circle of exactly 1.5 meters around the Corona-tracer within which other Corona-tracers are detected. If everyone wears the Corona-tracer on a lanyard around the neck, clearly visible to others, the shape is like the red line in the first picture. Since the distance on the side and back of the body is smaller than that on the front, we have set the distance on the front at 2.5 meters so that the distance on the side is not too small.
So, the answer to the question ‘how well can one measure distance with Bluetooth?’ is: actually, not that good at all. But it is a commonly used technology and it is featured in all smartphones, so it is mainly for practical reasons that we use Bluetooth. Since every tracer is the same, unlike smartphones (with apps based on e.g. PEPP-PT, DP-3T or the Apple/Google functions developed for Corona), we at least know the behavior of both the receiver and the transmitter, which provides optimal accuracy. The alternative GPS is in any case much less accurate, does not work reliably indoors, is more expensive and consumes more power. Wifi uses the same radio frequency as Bluetooth does.
The Corona virus spreads via breath and coughing, where the risk of infection in front of the body is the greatest. The shape of the Bluetooth signal matches that well.