The intercalibration of the ASC and the associated instrumentation on the Oersted spacecraft

The main instrument for the mapping of theEarth's magnetic field onboard the Danishgeomagnetic satellite rsted is the "gondola"consisting og a compact spherical coilfluxgate vector magnetometer (CSC) and a star image (SIM). The SIM gives the attitude relativeto the rectascension-declination system bytaking a snapshot of the sky, the CSCmeasures the magnetic field vector in aninstrument coordinate system. The attitude relation (gondola quaternion) between the star camera system and the magnetometer system is a fundamental instrument constant to be determined by calibration with arc seconds accuracy. Intercalibration has been carried out at thegeomagnetic observatory at Brorfelde, Denmark.Snapshots of the sky was taken with simul-taneous readings of the CSC magnetometer andthe observatory variometer. The SIM attitudeis reduced to the local geomagnetic systemby time and astronomical data and the observatoryfield observation is transformed to the SIMcoordinate system. By comparison with the CSC readings the orthogonal transformation can be found. The procedure is equivalent to a magnetic calibration in a test coil system and the magnetometer constants can likevise be determined.Results from the calibration of the Ørsted instrument is presented.