This instrument measures the precise dual-frequency phase of electromagnetic waves transmitted from setting or rising GPS satellite. Combining this measurement with the satellites' position and velocity information, a series (e.g. 50 Hz in case of CHAMP) of small atmospheric induced excess phases (up to ~ 1 km near the Earth's surface) during the occultation event can be derived with a very high accuracy. The temporal variation of this series (atmospheric induced Doppler shift) together with the signal's amplitude can be converted to a vertical profile of bending angles. Assuming spherical symmetric distribution of the atmospheric refractive index, vertical profiles of the refractivity can be derived and then converted to vertical profiles of atmospheric parameters as pressure, temperature and, using independent knowledge of temperature, also of water vapor partial pressure. Main advantages of the calibration-free sounding technique are global coverage, high vertical resolution and all-weather-capability combined with high accuracy. Detailed explanations of the GPS receiver and GPS radio occultation technique are found in the PDF document (https://igscb.jpl.nasa.gov/projects/meetings/nw2k/abstracts/ext/wickert.pdf).