One day I will finnish this bigtrack project. The ultrasonic sensor is triggered from a 555 through a transistor to get the correct mark space ratio. The ultrasonic output is RC filtered and a then goes through a PCF8591 ADC to I2C. This means some loss of accuracy but I can read the distance asynchronously. I have also connected the battery supply via a 10v Zena to the ADC so that I get an accurate battery monitor. This is important as I do not want to discharge the 11.7v LiPo battery. (Will also add a LiPo battery alarm.) The GPS is a Garmin Geko 301 that connect to the serial port using very cheap max 232 level shifter powered of the Raspberry Pi's 3v3. The Geko has a compass (and barometer?). There are a couple of DC DC converters and voltage regulators. The PCF8591 is running of a supply that could also be used for the GPS. I have not yet connected the GPS supply so I am using the standard batteries. Hope to use some sort of web based interface for remote control. I have also used a PS3 webcam but the streaming is very delayed. I need to find a better streamer with less lag.