nRF24L01 - Measuring distance using time elapsed For school usage, my team and I are working on a wristband that will alert the user if they are too close to another wristband (for social distancing). The problem is measuring distance using radio waves. Since I cannot use signal strength, I'm thinking about calculating distance from time taken to transfer some data and velocity (which will be around the speed of light). I discarded the idea of using ultrasound because it is too easily interfered with by other obstacles.
My plan is along the lines of this:

Sensor 1 and 2 are synchronised to a clock.
Both will run a
&quot;stopwatch&quot;, and the time values should be exactly the same at the
same time.
Sensor 1 sends out a message to Sensor 2. The message
contains the time at which the transmission started.
Sensor 2
receives the message, immediately records the time of reception, and
calculates the time taken for data to travel
d = v/t
Vice-versa

If the distance is smaller than the radius, a warning is triggered in the form of a vibration motor.
Limitations include clock speed; I intend on using a Seeeduino XIAO, an Arduino-compatible board. The clock speed is max. 48MHz, meaning that the smallest increments it can measure is 20.82 nanoseconds. I also calculated that it would take 13.33 nanoseconds to travel two metres (the former distance to keep in public). Hence, with the clock speed available to me, I can only measure 3m and beyond.
Assuming I was happy with this distance, would I be able to do this using one single Arduino sketch? I know that Arduino programs can only run one at a time. Would a constant background clock/stopwatch be too complex? Could you give me any other pointers?
Speed-of-light-delay measurements are not practical for the use-case you propose. As Majenko suggests in his comment, you'd need EXTREMELY accurate time measurement, as well as high speed radio-frequency signal processing. Any measurements you do through software will be far too inaccurate as a result of the software timing.
Apple's iBeacons (which are based on BLE) offer immediate, near, medium, and far distance readings that are based on signal strength. You should be able to achieve something similar with your own wristbands using BLE. The different distance thresholds would have some slop to them, but it should be possible to get decent results.