Space methods - Physical Geodesy - page 274

sight measurement of the range, range rate or acceleration difference between
the two low-orbiting satellites. The intersatellite link corresponds to accel-
eration differences between the two LEO satellites. Mathematically, this is
expressed by the difference of first derivatives of the gravitational potential
over a long baseline (i.e., the distance between the two LEO satellites).
In the case of satellite gradiometry, three-dimensional acceleration dif-
ferences referring to the very short baseline realized by the gradiometer are
measured. The gradient of gravity components corresponds to the accelera-
tion gradient. Mathematically, this is expressed by the second derivatives of
the gravitational potential.
Another feature inherent to satellite gravity missions should be kept in
mind: the amplification of errors by the factor ( r/R ) n +1 when transferring
the measurement comprising the signal and noise from satellite altitude to
the earth's surface. The factor ( r/R ) n +1 describes the field attenuation with
altitude. This error amplification effect is minimized by using an orbit as low
as possible and by not measuring the potential V itself or its gradient but
rather its second-order derivatives as in gravity gradiometry.

7.6.3 The CHAMP mission

The information on the challenging minisatellite payload (CHAMP) mission
has been extracted primarily from http://op.gfz-potsdam.de/champ.
The Geoforschungszentrum Potsdam initiated the CHAMP idea and has
the main responsibility. The primary CHAMP objectives are the following:

• mapping of the global gravity field, or, more specifically, to accurately

determine the long-wavelength features of the static earth gravity field
and its temporal variations (caused, e.g., by atmospheric mass redis-
tributions, ocean circulation, sea level changes resulting from polar ice
melting);

• mapping of the global magnetic field, or, more specifically, to accurately

determine the main and crustal magnetic field of the earth and its
space-time variations;

• profiling of the ionosphere and the troposphere, or, more specifically, to

derive from GPS signal refraction data information on the temperature,
water vapor, and electronic content of the atmosphere.

The CHAMP mission was launched on July 15, 2000 from the Russian Ple-
setsk cosmodrome. The main mission parameters of the respective satellite
are the following:

almost circular (eccentricity e< 0 . 004) and near-polar ( i =87 ◦ )orbit, •