(Question in response to the comments.)
I have a fixed flat white rectangular whiteboard of dimensions 1 meter by 0.5 meter and pen. I want to track the position of the tip of the pen within the whiteboard, in real time.
I am allowed to have as many different sensors and processors on the sides of the whiteboard as I want, but there are three main restrictions:
The pen should operate freely; no wires or strings between the pen and the sensors are allowed.
The pen should be low power, ideally powerless.
The position of the tip of the pen should be determined with an accuracy on the order of 1mm.
What electronic setup would allow me to compute the position of the tip of the pen within the whiteboard with the above restrictions?