The Ørsted Satellite

The Ørsted satellite, named after the Danish scientist Hans Christian Ørsted (1777-1851), is the first satellite mission since Magsat (1979-80) designed for high-precision mapping of the Earth’s magnetic field. It was launched with a Delta-II rocket from Vandenberg Air Force Base (California) on 23rd February 1999 into a near polar orbit. As the first satellite of the 'International Decade of Geopotential Research', the satellite and its instrumentation has been a model for other present and forthcoming missions like CHAMP and Swarm.

Ørsted external field science is coordinated by the Danish Meteorological Institute (DMI). The internal field science is coordinated by the National Space Institute (NSI). Satellite control is managed by the industrial company Terma A/S (Birkerød, Denmark).    

Satellite and orbit  

Ørsted was built as a joint effort of various Danish research institutions and companies with significant contributions from NASA, CNES, DARA, and ESA. The satellite weighs 62 kg, measures 34x45x72 cm and contains an 8m-long boom, deployed shortly after launch, carrying the magnetic field instruments. The satellite is gravity gradient stabilized; attitude maneuvers are performed using magnetic torquers. The Ørsted orbit has an inclination of 96.5°, a period of 100.0 minutes, a perigee at 650 km and an apogee at 860 km (decreased to respectively 99.7 min, 640 km, and 845 km after four years in space). The orbit plane is slowly drifting and the local time of the equator crossing decreases by 0.91min/day, starting from an initial local time of 02:26 on February 23, 1999 for the south-going track. The nominal lifetime of the mission was 14 months (2 months commissioning phase + 12 months science phase), but after more than 6 years in space the satellite is still healthy and provides high-precision magnetic data.

The purpose of the Ørsted satellite is high precision mapping of the Earth's magnetic field. 

Long-term stability of the Ørsted CSC vector magnetometer 

In-flight comparison of the scalar and vector magnetometer for March 15 - April 30  

An intercalibration between the two magnetometers (OVH and CSC) has been done separately for each day between March 15 and April 30. This gives 47 independent estimations of the calibration parameters and indicates the very high stability of the CSC vector magnetometer.

It is assumed that the CSC magnetometer is a linear instrument, which means that there are 9 parameters (offset and scale-value for each of the three sensor axes plus 3 non-orthogonalities transforming the non-orthogonal sensor axis to an orthogonal coordinate system). Dependence on the sensor and electronic temperature respectively has been determined pre-flight by Peter Brauer and Torben Risbo.

Due to an onboard software error, it is only possible to get housekeeping information, which about 50% of the time includes the temperature of the CSC sensor and CSC electronics (cf. Ørsted Newsletter #30, 1999, May 26). Therefore it was not possible to account fully for the temperature dependence of the calibration.

The results show a clear dependence especially on the sensor temperature, as expected. Therefore the calibration was redone, including the temperature dependence as estimated preflight. However, since temperature was only available for less than 50% of the time, the gaps (of several hours!) have been linearly interpolated. Although this is very coarse, this approach reduces the scatter to less than 0.3 nT for the offsets, less than 10 ppm for the scale values, and less than 1.5 arcsecs for the non-orthogonalities.

Plots of the calibration parameters, without and with temperature correction, show the stability of the vector magnetometer. The left column of these plots shows the offsets (top), scale values (middle) and non-orthogonalities (bottom) for each of the 47 days and for the three axes (Axis 1, Axis 2 , Axis 3). The upper right plot shows variation of sensor temperature (blue) and electronic temperature (green), and the middle right plot presents the achieved RMS misfit.

To test for a possible time difference between the two magnetometers, the calibrations were repeated but with a timelag between the OVH and the CSC as a free parameter. The right bottom plot shows the timelag for which the smallest rms misfit was achieved. It is believed that time differences greater than 50 ms can be identified in this way. However, from the present analyses there is no indication of a difference between the time stamp of the OVH and that of the CSC greater than 50 ms.

Publications

• N. Olsen,  L. Tøffner-Clausen, T. J. Sabaka, P. Brauer, J. M. G. Merayo, J. L. Jørgensen, J.-M. Lger, O. V. Nielsen, F. Primdahl & T. Risbo : Calibration of the Ørsted Vector Magnetometer, Earth, Planets and Space, 55, 11-18, 2003 (PDF)
• In-Flight Calibration Methods Used for the Ørsted Mission.
  In: Ground and In-Flight Space Magnetometer Calibration Techniques", ed.'s: A. Balogh and F. Primdahl, ESA SP-490, ESA Publishing Division, ESTEC, Katwijk, The Netherlands, 2001. ISBN 92-9092-800-X; ISSN 0379-6566. (PDF 1.44MB)