The nominal GPS constellation consists of 24 satellites at an altitude of about 20,000 kilometres and distributed in six orbital planes around the Earth. An occultation occurs whenever a GPS satellite rises or sets on the Earth's limb as seen by the GRAS instrument. The Doppler shift of the received signals is directly affected by the refraction of the signal as a result of gradients of atmospheric temperature and humidity. Therefore, profiles of temperature, humidity and pressure can be derived from the refraction angles through the occultation phase. In the stratosphere and upper troposphere, where water vapour density is low, refraction is dominated by vertical temperature gradients. In the lower troposphere, the water vapour effects dominate refraction.

Credits: ESA - AOES Medialab