GPS Soundings

Based on radio signals from the GPS satellite constellation, normally used for positioning and navigation purposes, it is possible to determine vertical profiles of atmospheric temperature and humidity.

The GPS radio signals can be received by a low orbiting satellite. In vacuum radio signals normally propagate along straight paths, but when passing through the atmosphere they are bent slightly (up to around one degree). This bending angle can be indirectly measured through precise knowledge of position and velocity of the transmitting and receiving satellites and through measurement of the phase of the received radio signal. From the bending angle the refractive index can be determined, and thereby temperature and humidity at the point on the signal path which is closest to Earth. As the two satellites move relatively to each other, the atmosphere is thereby scanned, yielding a vertical profile of measurements through the atmosphere. Profiles measured in this way are called radio occultations.

The method gives a homogeneous coverage of the Earth's surface and is therefore very useful as data source for numerical weather prediction and climate research. Typically, a radio occultation satellite orbiting at 800 km circulates the Earth 14 times a day, and is able to produce around 500 GPS occultation profiles per day.

Moreover, the state of the ionosphere can be determined by this method, utilizing the fact that GPS signals are transmitted at two different frequencies and that the refractive index is frequency dependent for plasma (which the ionosphere consists of).

GPS radio occultation measurements are currently made by the european MetOp satellite, the american/taiwanese COSMIC-satellites, and other satellites.

Read more about the radio occultation method and its use for weather prediction and climate monitoring at GRAS SAF Project.

Contact: Georg B. Larsen, phone.(+45) 3915 7489, e-mail: gbl@dmi.dk