DEVELOPMENT, CONSTRUCTION AND ANALYSIS OF THE ORSTED FLUXGATE MAGNETOMETER

The experiments and theoretical considerations leading to the construc tion of a high-performance three-axis fluxgate magnetometer are descri bed. The magnetometer will be used (1996) in the earth's field mapping satellite named 'Orsted'. The fluxgate sensors are based on stress-an nealed metallic glass ribbons as core materials. It is shown that very simple physical models can be used to explain the fluxgate mode of op eration, thereby making it easy to calculate the overall sensor perfor mance from first principles. Special attention is drawn to the core ex citation current which is analysed on the basis of nonlinear electrica l circuitry. It is furthermore shown that the ring-core demagnetizing field obeys a simple cosine law which permits the calculation of the s ensor sensitivity with high accuracy. The sensitivity, that is the sig nal-to-noise ratio, is ultimately determined by the sensor noise which is about 15 pT RMS (0.06-10 Hz), corresponding to a noise power densi ty (1/f noise) of 6.2 pT Hz(-1/2) at 1 Hz. The actual magnetometer ope rating range and sensitivity is determined by the 1 bit resolution of the earth's field represented by the output from the 18 bit AD convert ed used in the instrument (+/-65 536 nT with 0.5 nT resolution). The m aximum attainable bandwidth is half the sensor excitation frequency (1 /2 x 15 kHz) but the Orsted magnetometer bandwidth is limited to 250 H z. The thermal stability of the sensor has been measured to be better than 1 nT in the temperature range -20 to +60 degrees C.