Having started as a crazy idea for just a small side project, the optical flow sensor quickly hacked together (we conduct our research on other topics) by a group of PhDs is now adopted by more and more systems. And because its an useful tool for research (not so much a research contribution), it made even a short paper. Optical flow is a pretty old and basic technique, but quite robust if the camera runs at very high rate - and the main success factor making the AR.Drone so robust and easy to fly.
Aside from the easy integration into different systems, the main benefit of a standalone design is the extremely low latency of the velocity output. In our latest Firmware version currently in testing the camera sensor runs at 450 Hz, which makes the output despite the relatively basic algorithm extremely robust. And it means we can bump the maximum speed to ~3 m/s per meter altitude, or in other words: Allow a maximum speed of 30 m/s at 10 m altitude and still allow a smooth precision landing at 3 m/s at 1 m altitude.
Because the ground distance noise feeds into the velocity noise, the next big step will be the integration of laser based altitude estimates - and with the Lidar-Lite having a really low price tag, it will be an ideal combination. The ultrasound ranging was in fact so far the biggest limitation and with this removed, we think an optical flow sensor should become the default in addition to GPS today - its a great complimentary technology to improve robustness and accuracy.
And by becoming a default, the costs of making the module will come down to a level where its a no-brainer to get one. Besides our own system a number of autopilot systems have been successfully interfaced (AutoQuad, MikroKopter and lately ArduCopter).
This video shows a scenario called urban canyon, where the multi path reflections of the environment and the blocked sky view by buildings make GPS reception extremely challenging and much less accurate than on an open field:
And of course there are many more cool videos on Youtube - I just couldn't embed all of them here.