As you can see we had some issues with this method, the bottle made the drone very hard to fly, and the bottle soon started oscillating.
This doesn't seem to be a method that would work in practice as it would be very hard to pilot the drone, especially in the dark.
Our previous work showed that the drone kicks of a serious amount of ultrasound, enough to make recording from a ultrasound recorder on the drone useless. Our plan was to see if we could suspend a detector under the drone, far enough away to avoid interference. We used a water bottle, weighing the same as the detector in place of an expensive ultrasound detector, to avoid unnecessary accidents!