title_body: Estimate position of a balloon attached to a quadcopter I'm trying to measure/estimate the position of a balloon, for that I need the two angles shown in the picture below. enter image description here
  
  The idea is to place a helium filled balloon on a quadcopter to provide lift. As a first step, I imagined a fixed platform with the balloon attached. Next, I would like to estimate the value of the angles α and φ, since the length of the strap is fixed, getting the position of the globe is simply trigonometry.
  The problem is that I am not sure how to measure such angles, what would be better and what type of sensor to use. I was thinking of some kind of optical sensor, but I'm not sure.
  Can anyone suggest me some ideas?
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.