Global Positioning System (GPS) Radio Occultation (RO) senses the atmosphere using GPS radio signals that traverse the atmosphere as a moving receiver sets behind the horizon relative to the transmitting satellite. The radio raypath is refracted (bend) and its travel time is delayed due to variations of refractivity (Fig. 1). Precise measurements of the phase delay can reveal high vertical resolution structure of atmospheric bending and  refractivity, from which one can then derive atmospheric quantities such as air density, pressure, geopotential heights, temperature and moisture.