This was meant to be part of the #EUvsVirus hackaton, but due to rules I did not complete the submission. If so, I had to create a video for a library, which seemed a bit awkward. Perhaps I post a solution on Github in the future.
Inspiration
To facilitate social distancing it is necessary to get some measure of distance between persons. Often Bluetooth, Bluetooth LE or in particular Bluetooth LE Beacons can be utilized, but often with less accuracy than necessary or at least wanted. Is it somehow possible to augment use of Bluetooth, or perhaps increase the accuracy? That must of course be done with what already exists “in the box”, and the box is a cell phone. Or mobile, handy, or whatever it is called where you live.
The goal is to estimate when the Bluetooth is sufficiently strong that it is likely approaching the limit for social distancing, within some accuracy, and the likelihood for transfer of contagions increases. At this range Bluetooth is augmented with sound measurements to get a more accurate measurement. Those measurements can then be used for updating a model for how signal strength maps to distance, given the device, and then give a better range estimate.
What it does
A specially crafted sound can be transmitted from one cell phone and received by another. If the transmitting phone somehow tells the other party when it starts to transmit, then the receiving party can accurately clock the time for the sound to travel from one party to the other.
The specially crafted sound is a carrier frequency modulated with a direct sequence spread spectrum code, that is a binary sequence of chips. Such systems are usually employed for secure communication, but in this case we use it for its correlation effects. If we try to auto-correlate with wrong offset, then the accumulated sum would be -1, while with correct offset it would be N where N is the process gain or the length of the chip sequence. The difference can be quite large.
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