Since the beginning of April this year the Grace satellite duo (Gravity
Recovery and Climate Experiment) has been subjected to extensive testing in
the test facilities of IABG in Ottobrunn. In November the two climate
satellites are to be placed into orbit with a Rockot launcher from the
Russian cosmodrome Plesetsk. These satellites will enable researchers to
carry out gravimetric measurements of the Earth with an unprecedented
accuracy. For the first time this will make it possible to detect minor
changes in the gravity field caused by the circulating magma in the Earth’s
interior or by melting glaciers or changing ocean currents. Grace is based
on the German geophysical satellite Champ (Challenging Minisatellite
Payload) launched in July 2000. Champ and Grace were built by the European
space company Astrium in Friedrichshafen. Both projects are based on the new
satellite concept “Flexbus” developed by Astrium. It allows an extremely
cost-effective and fast production of satellites.
The system Earth.
The basic task of geological and climate research today is to understand the
Earth as an entire system. This consists of individual areas such as the
Earth’s interior, the oceans and ice masses, the atmosphere and the magnetic
field. These areas are, however, not independent of each other but subject
to extensive interactions. The magnetic envelope screening us against the
intensive particle wind from the cosmos, for example, originates in the
blazing hot layers of circulating magma in the Earth’s interior. The magma
in turn is responsible for volcanism on account of which large amounts of
gases affecting the climate, e.g. carbon dioxide, are emitted into the
atmosphere. They change the global temperature and have an influence on the
height of the sea level, for example.
Researchers are just about to understand this complex natural interaction,
the balance of which is the basis of our existence. Satellites play a
decisive role in the investigation of the system Earth, since only they can
observe the globe completely and over a long period. Particular progress has
been made by scientists in the measurement of trace gases in the atmosphere
such as ozone, CFC and nitric oxides. As far as the processes in the Earth’s
interior and the effects on the oceans and the atmosphere are concerned,
research is still in a primary stage. Champ and Grace are two epoch-making
missions started by geoscientists in cooperation with Astrium space
engineers.
..and its little irregularities
The Earth is not a perfect ball. The rotation of our planet induces a
centrifugal force which is strongest at the equator. For that reason, the
planet is stretched there and the Earth more or less resembles a rugby ball
or an ellipsoid: at the equator the diameter is 21 kilometres larger than
from pole to pole. Furthermore, deviations from a perfect ellipsoid, for
example on account of mountains and ocean trenches, also occur in smaller
scales.
Moreover, there are irregularities which are hidden within the Earth but
nevertheless they are of major importance for the system Earth in several
respects. Such anomalies, as researchers call them, have different origins:
they may occur in areas with very dense and heavy stones resulting in a
stronger gravity. In other areas the crust material is lighter which means
the gravity and thus the force of attraction is lower.
Each of these irregularities has an effect on the gravity of the Earth. If
the global gravity field is represented in a three-dimensional map, the
Earth looks like a potato.
For geophysicists such a map forms the basis for further research. It
allows, for example, to achieve a better understanding of the ocean currents
which are of great importance for our climate. In the same way, the
explosive question to what extent the sea level rises can only be answered
if the gravity condition of the Earth is known in detail. This is the major
task of Champ. Grace will also allow to measure even minor modifications of
the gravity field and their changes in the course of the years.
First Champ
A satellite enables us to measure the entire gravity field of the Earth. For
that purpose, it is just necessary to observe it closely on its orbit. If it
flies over an area with a high gravity, it is accelerated whereas over an
area with a weaker gravity it is slowed down. In this way it has already
been possible to carry out rough measurements of the Earth’s gravity field
during the past 20 years. At that point the possibilities of traditional
satellites had been exhausted.
Then Champ was developed by scientists from the geological research centre
in Potsdam (GFZ). What distinguishes this satellite is an onboard GPS
receiver. Its orbit can thus be tracked to millimetre accuracy. Champ will
be able to measure the Earth’s gravity field with an accuracy which is up to
a hundred times higher than in previous measurements. Furthermore, the
satellite is equipped with a sensitive magnetometer. Anomalies in the
Earth’s interior can thus for the first time be linked with changes in the
magnetic field. Scientists will investigate the questions why the Earth’s
magnetic field is shifting towards the west by 0.2 degrees and why it is
getting 0.1 per cent weaker every year. We know that during the past hundred
million years the magnetic field has changed polarity time and again and had
probably disappeared completely for a certain period. The causes of these
processes and the effects on life on the Earth are largely unknown.
Geophysicists in particular hope to be able to “look” into the Earth with
the aid of Champ. Irregularities in the liquid core of the Earth are
particularly interesting. Furthermore, scientists want to find out how
liquid magma circulates in the Earth mantle. This is a very slow process
which is, however, decisive for the development of the Earth’s magnetic
field.
..then Grace
When the engineers installed GPS in Champ, colleagues from the NASA Jet
Propulsion Laboratory were involved in the project. They were so impressed
that they decided to carry out a similar mission which they called Grace. It
consists of two identical satellites, in principle operating in the same way
as Champ but without performing measurements of the magnetic field. The twin
satellites will circle the Earth on a polar orbit at an altitude of 485
kilometres. They will follow each other at a distance of 200 kilometres.
An absolute novelty which is decisive for the mission is a microwave link
between the two satellites. It allows to determine the mutual distance with
an accuracy of several thousands of millimetres. This distance will change
continuously on account of irregularities in the gravity field. The
measurement accuracy will make it possible to measure small-scale anomalies
in the gravity field.
Compared to Champ, Grace will be able to increase the sensitivity once more
by ten to one hundred times. Grace is thus also suitable for the monitoring
of climatic changes. Due to the extremely high measurement accuracy,
researchers are planning to use Grace for the study of a variety of
phenomenon:
For a period of five years, Grace satellites with a size of 3.1 by 1.9
metres and a weight of 475 kilograms will orbit the Earth. Researchers
regard Champ and Grace as a new building block in the field of remote earth
observation and hope to be able to regularly study the Earth in this way in
the future. The prospects to achieve this goal are good, since the European
Space Agency ESA approves the construction of Goce (Global Ocean Circulation
Experiment). It would also be used to measure the Earth’s gravity field and
could be launched in 2005, i.e. immediately after the end of the Grace
mission.
Flexbus – Astrium’s new concept for small satellites
Champ and Grace are based on the Astrium-developed satellite concept
“Flexbus”. Flexbus starts from a mature base architecture for the satellite
bus combined with space-proven units and individual mission-specific
components. This allows an extremely cost-effective development and
production as well as a heavily reduced mission execution phase. Satellites
developed and built on the Flexbus concept are ready for launch only two to
three years after receipt of order. In addition to the development and
testing procedures for a mission, this concept also facilitates satellite
operation.
Compared to traditional construction methods, the costs for Champ could be
reduced by more than 50 % without any loss of quality, which is confirmed by
the scientists who are entirely satisfied with the mission process: ‘With
its Flexbus system, Astrium has gained a fantastic position on the world
market”, says Professor Christoph Reigber of GFZ Potsdam, the Principal
Investigator (PI) of Champ and Co PI of Grace.
This has already been demonstrated impressively by Astrium on the occasion
of the Grace contract awarded by NASA. Thanks to Flexbus and the missions
with a similar profile, large parts of the hardware, the documentation and
the software could be used reducing the development efforts considerably.
In addition to the development of the satellite platforms, Astrium is
responsible for the tests in Ottobrunn and for the integration of the two
satellites. The German aerospace centre (DLR), providing the ground stations
for mission control and the launcher rocket, is also involved in the
project.
After environmental tests in Ottobrunn, the satellites will be transported
to the Russian launch site Plesetsk situated 800 km north of Moscow. In
November 2001, the duo is to be launched into space with a Rockot launcher.
Eurockot Launch Services, a holding company of Astrium, is responsible for
the launch.
Friedrichshafen, June 2001/01009
For further information:
Astrium
Earth Observation & Science
Mathias Pikelj
Tel.: + 49-7545-8-9123
Fax: + 49-7545-8-5589
