The Grace mission logo. Photo: NASA

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.

An artist's concept of Grace satellites deploying from launch vehicle. Photo: Eurockot

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:

• Shifting of atmospheric pressure areas,
• Changes of ocean currents,
• Changes of the ice masses on Greenland and in Antarctica.

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.