1 Department of Automation, Technical University of Denmark2 National Space Institute, Technical University of Denmark
The description of the single axis magnetic gradiometer based on two Compensation Detector Coil (CDC) fluxgate ringcore sensors separated 20cm introduces the subject of magnetic gradiometry. Despite its good properties and high precision of less than 1nT, the calibration procedures are not straightforward. Firstly, the monoaxial character does not provide the vectorial information on the magnetic field. Secondly, one of the sensors measures the ambient magnetic field and this is used to compensate the field in both sensors. Several methods have been developed for its characterization and the calibration of the gradient measurement is achieved by the use of a magnetic dipole pattern of strength 2mAm2. In a coil facility, the gradient can be determined with an RMS value of 0.3nT/m.The ultra high sensitivity magnetic triaxial gradiometer has been constructed by employing another approach. Two independent Compact Spherical Coil (CSC) sensors are set up on an optical bench at a distance of 60cm. Each of the magnetometers is calibrated separately and has an absolute accuracy better than 0.2nT. The controlling electronics has been designed with space specifications and the same instrumentation is to be used in the forthcoming satellites CHAMP and SAC-C. Linearity, thermal, radiation, dynamic and calibration tests are carried out to qualify the magnetometer in order to ensure state-of-the-art performance with subnanotesla precision. The overall calibration of the gradiometer yields an omnidirectional absolute accuracy of 93pT/m.The scalar calibration of a vector magnetometer is explained thoroughly. The novel method is simple and it represents the most robust and unique way to estimate the characterizing 9 parameters of a vector magnetometer. Its power relies on the linearization of the parametrization by comparing the square of the intensities of the reference and of the uncalibrated magnetometers in the Earth's magnetic field. Using this method a CSC magnetometer can be absolutely calibrated with 0.2nT of accuracy.The absolute alignment of a vector magnetometer is also described. After the scalar calibration, the magnetometer intrinsic system is further related to an absolute coordinate system based on the stars by means of an advanced stellar compass. The pointing accuracy is better than 10arcsec.The magnetic gradiometry extends the subject of magnetic vectometry by the inclusion of differential magnetic quantities adding a new dimension for the identification of magnetic sources. Hereby, the development of mathematical tools for the interpretation of gradient measurements has been developed. The addition of gradient measurements in the inversion tools enhances the quality of the solution and offers the possibility of separating the geomagnetic field sources.By using tensor algebra the spherical harmonic expansion of the magnetic field in a curl free region and its associated gradient tensor are derived. This differential tensor quantity is then expressed by spherical coordinates. In the presence of magnetic sources or currents, the generalized expression of the magnetic field and gradient tensor has also been developed and it is described by the toroidal and poloidal functions.A space system based on tethered satellite technology is proposed for accomplishing a magnetic gradient mission. GRADSAT, two 20km separated magnetic instrumented satellites, combines state-of-the-art technology with advanced instrumentation to produce world class science.