A Luxembourg-based team has come up with a really clever indoor navigation system
that combines ultrasonic beacons with IR to achieve absolute position ("synthetic GPS"), much like the NorthStar system we use but entirely DIY.
As they describe it: "The infrared flash of a single beacon arrives instantaneously at the robot that is equipped with a combined infrared/ultrasonic receiver. But the ultrasonic signal only arrives with a certain delay, due to the "slow" sound velocity, in comparison to the speed of light. The receiver is able to measure the time lag between both signals and deduce the distance between the robot and the beacon."
# participation of up to 7 beacons that may be placed at strategically interesting places
# the beacons send out strong IR and ultrasonic signals that don't require reflectors on the receiver side
# like RF-beacons, these beacons transmit their identification number, making localization easy
# always one beacon acts as the master beacon, sending out a synchronization message to all other slave beacons
The project is entirely open source and is in the proto-board stage (see below). It was designed to work with LEGO Mindstorms NXT and as such uses I2C output. But that, of course, is also readable by any microcontroller, so it could work as well with Arduino or Basic Stamp. Looks like a great opportunity for one of our PCB whiz's to make a small and light version for blimps!