The most basic autopilot you can buy is the UNAV PicoPilot N, which is basically just a one-axis gyro and a GPS, with an old Pic processor inbetween. It's a "turn-rate leveler", which is to say that it keeps the wings level with the gyro and uses the GPS to correct for gyro drift (and to handle navigation, of course). It's twenty-year-old technology, but it works well enough on high-wing rudder & elevator trainers. I've got a couple of them (which cost an incredible $700 each at the time I bought them!), and my irritation with the dated technology, high price and closed system was one of the reasons for starting this site and our projects--I knew we could do better with open source.
I also have a long-standing love affair with Lego Mindstorms, which is the best robotics prototyping system out there. I've built a few Lego Mindstorms autopilots, but the truth is that for anything sophisticated it's better (smaller, cheaper and more flexible) to design custom hardware, as we've done with ArduPilot. But I've always been looking for a way to offer a Lego autopilot here that makes sense and could be built by many people.
Finally, it came to me. Rather than duplicate the functionality of our full-function autopilots in Lego, why not just make a Lego version of the basic PicoPilot? So here you have it: the hardware for a Lego Mindstorms turn-rate limiter autopiliot.
Basically, it's a one-axis gyro, a Bluetooth GPS (Mindstorms has Bluetooth built in and it's the best way to get serial data into it), and an ArduMUX to handle switching between RC and autonomous mode, interfaced with the NXT brick with a Hitechnic prototype board. All the functionality of PicoPilot, but open, pretty cheap, and Lego!
The components are as follows (along with the NXT controller). See picture above to see what's what.
--Hitechnic gyro sensor
--Hitechnic prototype board
--Mindsensors servo driver
--5Hz Bluetooth GPS (a different model is shown in the photo)
Total cost of these components is around $200 (if you already have a Lego Mindstorms NXT kit), which is half the price of PicoPilot! It's no ArduPilot, but if you're into Lego it's pretty awesome all the same.
Now that the hardware is in place, I'll turn to the software, modding my previous Lego autopilot code to handle turn-rate leveling. Given that this has a 5Hz GPS, I expect it will significantly outperform PicoPilot ;-)