title_body: Estimate position of a balloon attached to a quadcopter
upvoted_answer: The simple answer would be a potentiometer, as used for position feedback in a servo. You apply a voltage to the outer two contacts and measure the voltage at the moving, middle connection.
  The next step would be a rotary encoder. These should give you less friction and more accuracy, if you need it.
  A camera would require significant processing power, but might be viable on a larger drone. Measuring α would be tricky with a camera on the drone, but you could calculate it from the distance from the centre and the length of the tether.