Geoscience Reference
In-Depth Information
of the CHAMP mission, but refer the reader to the previously mentioned
homepage.
As explained before, the measuring principle for CHAMP is satellite-to-
satellite tracking in high-low mode. The gravity field of the earth perturbes
the CHAMP satellite orbit. These perturbing accelerations correspond to
first derivatives of the gravitational potential
V
. This implies that the gravity
field of the earth may be derived from observed gravitational satellite orbit
perturbations applying numerical orbit integration (Montenbruck and Gill
2001) or using the energy balance principle (Ilk 1999, Jekeli 1999, Sneeuw
et al. 2002).
For further reading see Reigber et al. (2003), Seeber (2003: Sect. 10.2.2).
7.6.4
The GRACE mission
The information on the gravity recovery and climate experiment (GRACE)
mission has been extracted primarily from http://op.gfz-potsdam.de/grace.
The GRACE mission is a joint project between the U.S. National Aero-
nautics and Space Administration (NASA) and the Deutsches Zentrum fur
Luft- und Raumfahrt (DLR). The primary objectives of the mission are the
following:
•
determination of the global high-resolution gravity field of the earth,
•
temporal gravity variations.
In addition, another task is the determination of the total electron content
by GPS measurements to get knowledge on the refractivity in the ionosphere
and troposphere. The two satellites of this mission were launched simultane-
ously on March 17, 2002 from the Russian Plesetsk cosmodrome. The main
mission parameters of the two satellites are the following:
almost circular (eccentricity
e<
0
.
005) and near-polar (
i
=89
◦
)orbit,
•
•
initial altitude between 485 km and 500 km,
•
the two satellites are some 220 km apart (this requires orbit maneuvers
every one or two months to maintain the separation between the two
spacecraft),
•
design lifetime of the mission is five years (but extended operation is
envisaged),
•
the weight of each satellite is about 480 kg and the length about 3 m.
As with CHAMP, also the altitude of the GRACE satellites will decrease
in the course of their lifetime primarily because of atmospheric drag. The
