The Ørsted Satellite Project

Langlais, B. (1), P. Ultré-Guérard (2), C. Vernin (3), M. Mandea Alexandrescu (4), and Y. Cohen (4), and G. Hulot (4)

1. Goddard Space Flight Center,Geodynamics Branch, Code 921, Greenbelt, MD 20771, USA


2. CNES, 2 Place Maurice Quentin, F-75039 Paris Cedex 01, France


3. CETP, 4 avenue de Neptuen, F-94104 Saint-Maur-des Fossés Cedex, France
4. Institute de Physique de Globe de Paris, 4 Place Jussieu, Tour 24-25, 2eme etage, F-75252 Paris Cedex 05, France

Abstract. Accurate measurement of the geomagnetic field is one of the very few ways to probe the properties of the Earth's core, mantle and crust. The interaction of the geomagnetic field with the solar wind also plays an important role in forming the external environment of the Earth and influencing the atmospheric processes. For all these studies, the need for high quality magnetic data evenly distributed around the Earth is obvious. In spite of this, only one high-precision satellite mission, MAGSAT, has been launched in the past, some 20 years ago. The successful launch of the Danish satellite Ørsted, on February 23, 1999, therefore appears as an important scientific event which will hopefully signal the beginning of a new era of continuous monitoring of the Earth's magnetic field from space.

Ensuring the success of this mission is critical. It requires several steps, the first of which is the commissioning of all scientific instruments on board the satellite. The present report deals with the commissioning of the Overhauser scalar magnetometer (OVH), for which IPGP is scientifically in charge. This commissioning relies not only on the data provided by the OVH instrument, but also on the position and time data provided by other instruments for which IPGP is not responsible.

Several problems have been encountered. Most of them have been solved, thanks to a close collaboration established with the Danish teams of the Ørsted Science Data Center and TERMA, the CNES and the LETI who built the instrument. Unfortunately, a few important problems remain. The most serious ones are not directly connected with the OVH instrument, which works in a satisfactory way as far as we can tell, but rather with the way the timing information is registered and stored on board the satellite and linked to the OVH measurement. The correspondence between the OVH dataset and the position of the satellite suffers from the fact that it relies on time parameters constructed in what appears to be a unreliable way.

This introduces artifacts within the OVH data when expressed as a function of the geographic location. These artifacts did not stop us from assessing the quality of the OVH data --- this data is very close to ground based forecasts --- but casts some doubts on the scientific value of the residuals (which of course contains the information justifying the launch of the satellite). It is to be feared that similar problems could be encountered with the Compact Spherical Coil vector magnetometer and the Advanced Stellar Compass, the data of which need to be precisely synchronized with that of both the OVH and the positioning system, before any scientific exploitation and interpretation can be made. It is our opinion that these problems should be solved before releasing any data within the international scientific community.