It's all coming together! We have a rig that allows us to attach an ultrasound detector and recorder to a UAV (have a look here on how we did this), and we can use gliding flight to record without interference from the engine. So we gave it a go:
Flying the drone around on a loop we used a ultrasound source to produce noises that the on board recorders would pick up. The results are promising. When flying overhead at around 20 meters the detector was able to pick up the ultrasound signals emitted from ground-level. Since bats are often flying at tree height this gives us a good chance of picking them up in flight.
The glide slope we experienced was approximately 3.5:1, meaning that for every 1 meter altitude lost we flew 3.5 meters laterally. This isn't particularly good and the glide slope was reduced by the fact we were flying into the wind whilst gliding. Although this provides additional lift, the reduced ground speed has a large impact on the slope. In future flights we'll fly the glides downwind as much as possible.
The next step will be to plan and fly an autopilot mission using the gliding method to try and record an ultrasound source.
It's worth noting that we don't think this is the ideal solution and would much prefer to be able to record all the time so we're looking at other options for this.