Sensing the earth´s atmosphere with occultation satellites

A pair of satellites in the same earth orbit but separated by about 60° in phase will have a radio path between them that intersects the atmosphere continuously. With a coherent transponder between satellites at a frequency of 5 GHz, changes of 3 cm in the phase path can be recorded, giving a sensitivity to changes in atmospheric density of 1:18 000. With a set of six satellites to sense density at five levels, sufficient resolution is acquired to define the vertical density profile for weather prediction equations. Three such systems in polar orbit will give the necessary world-wide resolution every 12 hours. Actual motion of the satellites due to orbit perturbations can be corrected for, giving air pressure accuracy better than 3 mbars with existing knowledge of the earth´s gravity and with long term averaging of data from the satellites´ own transponders. Below about 7 km, correction must be made for water vapor effects on the radio waves. Several techniques are available to extend the desired precision down to 5 km and may be able to give the necessary corrections down to 3 km.