Determining distance between wireless sensors - sailing project
Just brainstorming here.

You could put barometric pressure sensors on the head and tack of the sails. If you measure the pressure difference you can calculate height. Might not be super accurate, but it would probably work to some extent. The short term change in pressure in the head sensor would be somewhat accurate while the sail is going up, even without reference to another sensor.

You could also put one sensor on the base of the mast to be your zero reference, and in that case, each sail would have just one sensor at the head. This would work better for conventional spinnakers which have considerable variation in tack height, depending on conditions.

Another way is to use something like an optical encoder on the mast. The detector would be on the head of the mainsail (this would only work for the main, probably not for headsails).

None of these things are going to be super reliable. In other words, I don't think this holds promise as an industrial type of system. It would be more like a teaching aid that could be kind of interesting (basically a gimmick). If you need rock-solid reliability, I think you will need to consider special halyards or something like that.