Didn’t know what it was back then, but I shot a quick video of one of Google’s autonomous Prius (Prii?) before it drove away from in front of Joya in Downtown Palo Alto.
Looks like the sensor pod on top has a fixed and rotating component, as well as what I’m guessing is a stepper-motor style rotation counter on the left rear wheel. I also see what appears to be a GPS antenna (no surprise there) on the rear dorsal section of the roof.
I’m going to talk to some friends at Stanford’s vehicle lab and ee if I can pull apart what sensors are actually bolted on here.