Ultrasonic Altitude Measurement
Ok another very simple way of measuring the altitude, at sea level the speed of sound is 340ms at 30,000m it is 300ms. Therefor we attatch ultrasonic transducers on a plank 1m apart facing eachother and a simple arduino counter. Transducer A pings at a signal and when Transducer B recieves it it pings back to A and so on. At sea level you should get a signal of 340hz whilst at 30km it will drop down to 300hz.