Ionospheric Sounding by Using Space Borne GPS

Introduction

The Earth's ionosphere extends from about 60 km up to the bottom of the plasmasphere at about 1000 km altitude. The ionospheric behaviour is affected by numerous factors such as solar radiation (wave length < 130 nm) and solar wind, thermospheric composition and neutral winds, electric fields, particle precipitation, electric currents or mass and heat flow. On the other hand, ionospheric processes affect significantly structure and dynamics of thermosphere and plasmasphere/magnetosphere systems.

LEO missions such as CHAMP carrying a dual frequency GPS receiver onboard, offer a good chance to derive vertical electron density profiles by using radio limb sounding technique. This technique has been demonstrated to be a powerful tool for remote sensing the Earth's neutral atmosphere and ionosphere by analyzing GPS radio occultation data obtained from the GPS/MET instrument, flown on the Microlab-1 LEO satellite.
 The German CHAMP satellite was successfully launched from the Russian launch site Plessetsk by a COSMOS rocket on July 15, 2000.
The main scientific objectives of this geo-science mission are related to precise gravity and magnetic field measurements and atmosphere/ionosphere sounding by using GPS radio occultation techniques.
Whereas the first neutral gas limb sounding measurements onboard CHAMP were obtained on February 11, 2001, the first ionospheric occultations were measured on 11 April 2001.
 

                    
                    The principles of  ionosphere sounding using ground and space based GPS measurements.

Since most of the LEO satellites use topside GPS antennas for navigation and precise orbit determination, it should be mentioned that this type of measurements opens a new area in sensing the topside ionosphere and plasmasphere. Applying data assimilation techniques, the tremendous amount of data should essentially improve our knowledge about the electron density structure of the outer ionosphere on global scale.

GPS measurements onboard CHAMP are planned to be processed automatically for retrieving ionospheric data products that will be offered to the world-wide user community via GFZ Potsdam (CHAMP@GFZ-Potsdam).