The central task of geosciences in the next decades will be studying and trying to understand the Earth as a system: a system composed of solid, fluid and gaseous parts which show large variations in space and time and between of which complex interactions take place on quite different time scales. Characterising such an extensive, complex and heterogeneous system requires very long and synoptic data series of the phenomena taking place within and between the various spheres of the system, beside of course the computer resources required for making use of such enormous data sets in the model development, improvement or validation. Global long term, synoptic data sets can systematically only be acquired by near Earth satellites, which however in general provide a limited resolution because of the reduced sensitivity with altitude. A combination of satellite observations with more regional data sets obtained on ground or in the first few kilometers in the atmosphere is therefore the suitable observation scenario to follow.

Despite the progress partially achieved rather fundamental questions concerning the exact nature of the dynamics of the solid Earth and the oceans and the interaction of the latter with the atmosphere are unsettled. In this context the global modeling of both the Earth's gravitational potential and the Earth's magnetic field are still basic areas of research in geodesy and geophysics, since both geopotential fields, together with the information derived from seismic wave propagation analysis, are the most valuable sources of information about the structure and composition of our planet and the evolutionary processes which continue to shape it.These global gravity and magnetic field models in conjunction with the temporal variations provide in addition basic reference information for e.g. navigation, surveying as well as for ocean circulation and sea level change studies, if a certain level of precision and resolution is achieved.

Presently used satellite systems, observation methods and analysis approaches limit the precision and resolution of both the gravity and magnetic field modeling to an extent, which at least would limit further progress in our understanding of the geophere and ocean dynamics. In a series of reports and recommendations from planning meetings and scientific workshops in North America and Europe these deficits, new observation techniques and predicted performances are described since the late 70th. A number of mission studies were carried out by ESA and NASA. Due to various reasons none of the proposed and studied dedicated gravity and magnetic field missions became reality until 1994, with the exeption of the MAGSAT magnetic field mission in 1979 - 1980.

Because of this situation and the fact that a special support programme for the East-German space industry was initiated by the German Space Agency DARA (now merged into DLR) in 1994 GFZ scientists (Pi Prof. Christoph Reigber) proposed in the same year a small satellite mission with science instrumentation and orbit characterisation that would allow to simultaneously measure and drastically improve the gravity and magnetic field modelling. The instruments proposed to fly on a spacecraft in a medium to low altitude orbit were:

• Gravity: a new generation GPS flight receiver for continuous tracking of the low orbiter by the satellites of the GPS constellation for accurately and continuously monitoring of the orbit perturbations and a high-precision three-axes accelerometer for measuring the surfaces forces accelerations.
• Magnetics: a high performance Fluxgate magnetometer set measuring the three components of the ambient magnetic field in the instrument frame combined with a star camera determining the attitude of the assembly with respect to a stellar frame and a Overhauser scalar magnetometer serving as magnetic reference.
• Atmosphere/Ionosphere: the instrumentation used for the recovery of the magnetic and gravity fields constitutes at the same time a powerful assembly of sensors for observing many parameters relevant for the characterisation of the state and dynamics of the neutral atmosphere and ionosphere. GPS radio-occultation measurements can be used for the derivation of temperatur and water vapor profiles. Electric field measurements are performed with a digital ion drift meter. Electron density is determined by GPS radio sounding and the density of the neutral atmosphere can be estimated from measurements of the high resolution accelerometer.

Against this background the three primary science objectives of the CHAMP mission are to provide:

• highly precise global long-wavelength features of the static Earth gravity field and the temporal variation of this field.
• with unprecedented accuracy global estimates of the main and crustal magnetic field of the Earth and the space/time variablity of these field components
• with good global distribution a large number of GPS signal refraction data caused by the atmosphere and ionosphere, which can be converted into temperature, water vapor and electron content.

With its multifunctional and complementary payload CHAMP thus aims at contributing to the following Earth system components:

• Geosphere: investigation of the structure and dynamics of the solid Earth from the core along the mantle to the crust, and investigation of interactions with the ocean and atmosphere
• Hydrosphere: more accurate monitoring of ocean circulation, global sea level changes and short-term changes in the global water balance as well as interactions with weather and climate
• Atmosphere: global sounding of the vertical layers of the neutral and ionized gas shell of the Earth and relationship with weather on Earth and space weather