Yeh this was my initial concern with optical silhouette classification. The altitude and targets are quite small (and rapidly moving). I've thought about using optical flow analysis to track wingbeat frequency etc as I feel this might be a little simpler to acquire.
On wingbeat frequency I'm very curious to play around with mmWave doppler radar (there's some RF on chip stuff around), emitting a relatively isotropic signal might allow for a pretty wide field of view and I imagine you'd get a pretty reasonable wingbeat signal.
On wingbeat frequency I'm very curious to play around with mmWave doppler radar (there's some RF on chip stuff around), emitting a relatively isotropic signal might allow for a pretty wide field of view and I imagine you'd get a pretty reasonable wingbeat signal.