GPS Soundings

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

The radio signals can be received by a low flying satellite. Normally the radio signals propagate along straight paths but when propagating in the atmosphere they ar bent slightly (by fractions of a 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 of the point on the signal path which is closest to Earth. As the two satellites move relatively to each other, the atmosphere is scanned by near horizontal ray paths, yielding a vertical profile of measurements through the atmosphere.

The figure shows the concept of sounding the atmosphere by this so-called radio occultation method and below is shown a typical temperature profile.

The method gives a homogeneous coverage of the Earth's surface and is therefore very useful as data source for numerical weather prediction.

Moreover, the state of the ionosphere can be determined by this method but here the fact is used that GPS signals are transmitted in two frequencies and that the refractive index is frequency dependent for the ionosphere (plasma).

GPS radio occultation measurements are made a.o. by the Danish Ørsted, the German CHAMP, the Argentinian SAC-C, and the Australian FedSat satellites.

Read more about the GPS method and its use for weather prediction and climate monitoring. In addition, read more about  the GRAS SAF project.

Staff of The Division