All the radio is done with the XBEE modules. I send a character to serial-out on one arduino and raise pin 13. When the other finds that character in it's serial-in, I start a timer on that arduino and stop it when it's pin 12 falls. With the speed of sound being around 300-400 m/s, that's a delay between the 2 of about 3.3 ms/m to 2.5 ms/m. I need to test the quality of this measurement (accuracy, reliability, resolution, linearity, etc) and it's maximum operating range, and test how these change under a few conditions like different temperature room, heat gun between the 2, etc.