How to measure the angle of a rope on a moving airplane?

I've got a simple rc plane which I want to fly on a tether. The tether is attached to the bottom of the plane like this:

I now want to know what the orientation of the rope on the bottom of the plane is relative to the plane.

1: IMU

I've got an IMU which I'm building into the plane (together with a raspberry pi). I guess I could also attach an IMU on the rope and calculate the differences between the two, but since the plane will be moving a lot I guess this wouldn't be the easiest/best option.

I thought of attaching some kind of sensor on the rope about a meter below the plane and 4 sensors on the plane; one at the end of each wing, one in the front and on at the rear of the plane. If needed by the technology I could move the sensor on the rope closer or further away from the plane. It could be anywhere between 10 cm and 5 meters. I then need to measure the distance between the sensors on the plane and the sensor on the rope. Combining that information with the position of the sensors on the rope and the plane I can calculate the orientation of the plane on the rope. I wonder what kind of sensors I could use though. As far as I understand Lidar is not a good idea because measuring distance using light is difficult at such a small distance. That means maybe radar is a better idea, but I don't really know if this is feasible.

3: infrared

Another idea I had was attaching an infrared light on the rope and using an infrared camera on the bottom of the plane with blob detection to find the location and calculate the angles of the rope using that location. I'm not sure whether this would work by day though, and it might have some other disadvantages that I'm unaware of.

4: rotary encoders

I could also attach two rotary encoders on the plane and attached two very light wires on the rope. It would enable me to measure the distance/angle relatively precisely, but it would also make it quite fragile when landing the plane.

Precision

I don't need very high precision. If the sensor on the rope would be a meter away a precision of 10cm would be fine. Also, I could average the measurements of the four sensors and I guess that would make it more trustworthy.

All tips and ideas are welcome!

[EDIT]

@DaveTweed said this was an XY-problem (which I had never heard of, so I looked it up). So let me explain what I want to do. A company called AmpyxPower is flying RC-planes on a tether to make it create a pull on the tether. That way they can generate energy with a generator on the ground. I'm curious if I could build such a system. I know I'm really far from reaching that point, but the first part would be flying the plane on the tether in the wind. After this, I want to make it be able to fly in an eight-figure autonomously. I'm currently writing a first version of the code that should do this. It's in very early stage, but I'm just trying to collect the data that it needs. As said I now have an IMU, GPS (although I'm not sure how much that is going to help me) and I also want to know how the tether pulls on the plane. For that reason I want this information.

• You want to measure an angle but you specified your desired precision in terms of length. Can you clarify that? Also, how often do you want to get a measurement? Commented Dec 13, 2018 at 16:20
• connect the rope to a freely rotating bracket on the plane an measure with rotary encoder/potentiometer. Simplest I think. Commented Dec 13, 2018 at 16:20
• Connect the rope to an analog joystick at the bottom. That gives you two angles. Commented Dec 13, 2018 at 16:23
• @Oldfart - That is a brilliant idea! But I'm also afraid that the joystick wouldn't be able to handle the force. Any tips on using the joystick, but not putting the force on the joystick itself? Also, I'm afraid the joystick wouldn't be able to turn enough. For example: normally they can't be pushed all the way down to the table. Commented Dec 13, 2018 at 16:31
• This sure sounds like an XY problem. What is the purpose of the tether, and what is the airplane going to do with the angle information? Also, have you considered the effects of non-negligible weight and air drag on the tether itself? Commented Dec 13, 2018 at 16:53