The shift of the NMR spectral line frequency in a proton free precession absolute scalar magnetometer using the omni-directional toroid container for a proton-rich liquid depends on the magnetic susceptibility of the liquid and on the direction of the external field relative to the axis of the toroid. The theoretical shift is estimated for water by computing the additional magnetic field from the magnetization of the liquid and comparing it to the theoretical field in a spherical container. Along the axis the estimated average shift is −0.08 nT and perpendicular to the axis the shift is +0.08 nT relative to that of a spherical sensor. The field inhomogeneity introduced by the toroid shape amounts to 0.32 nT over the volume of the sensor and is not expected to significantly affect the signal decay time, when considering the typical water line width of about 2.5 nT.

The compact spherical coil (CSC) vector-feedback magnetometer on the Danish Ørsted geomagnetic mapping satellite underwent extensive calibrations and verifications prior to integration and launch. The theory of the 'thin shell' calibration procedure is introduced. Spherical harmonic modelling was developed and tested over several years and used for Ørsted and other missions at test facilities in Europe, the United States and the Republic of South Africa. The verification of the test coil system using an Overhauser absolute scalar proton magnetometer is explained and the overall calibration results are given. The temperature calibrations are explained and reported on. The overall calibration model standard deviation is about 100 pT rms. Comparisons with the later in-flight calibrations show that, except for the unknown satellite offsets, an agreement within 4 nT was obtained. Finally an rf interference between the CSC and the Overhauser magnetometer is discussed, which may account for some of this discrepancy.

The requirements for precise global mapping of the Earth's vector magnetic field from a LEO satellite are discussed.

On Tuesday 23 February 1999, at 10:29 UTC, SUNSAT was launched into an 857×655 km, 96.47° polar orbit on a Boeing-Delta II rocket from Vandenberg Air Force Base in California, USA. Both SUNSAT and Ørsted were NASA-sponsored secondary payloads accompanying the USA Air Force Argos satellite. In the process it became South Africa's (and Africa's) first satellite in space. Although sponsored by several private industrial organisations, it is essentially a student project with more than 96 graduate students in the Department of Electronic and Electrical Engineering at the University of Stellenbosch providing the majority of SUNSAT's engineering development and operation since 1992. This paper reports on the magnetic field experiment on board the Sunsat satellite, consisting of two fluxgate magnetometers, called Orimag and Scimag, both built and calibrated by the Hermanus Magnetic Observatory. Orimag is mainly used for orientation control purposes on SUNSAT, while Scimag, mounted on a boom of 2.2 m is designed to perform geomagnetic field observations, employing standard navigation fluxgate technology.

The reference coordinate axes of a magnetic vector field sensor are related to the instrument digital output vector Ū by the calibration matrix and the offset vector Ō. In addition, this reference coordinate system must be related to (at least) two externally accessible optical or geometrical axes in order to be able to determine the precise orientation of the magnetic coordinate axes in an external reference system. Two methods for determining a reference axis in the sensor coordinates are discussed: (1) using a triaxial coil facility to measure the sensor orientation for two different positions, rotated about a fixed reference axis; (2) in the Earth's field the magnetometer sensor is rotated about a fixed axis into a number of (at least three) positions.

The Swedish micro-satellite Astrid-2 was successfully launched into a near polar orbit in December 1998. Despite the fact that the primary science mission was auroral research, the magnetic instrument was designed to accomplish high-resolution and high-precision vector field magnetic measurements, and therefore mapping of the Earth's magnetic field was possible. The spacecraft spins about a highly stable axis in space. This fact and the globally distributed data make the magnetic measurements well suited for the estimate of a magnetic field model at the spacecraft altitude (about 1000 km). This paper describes the initial analysis of the Astrid-2 magnetic data. As a result of the study of a single day (February 7, 1999), magnetically fairly quiet, it was possible to in-flight adjust the calibration of the magnetometer and find a magnetic field model fitting the scalar component of the measurements to better than 5 nT/sub rms/ for latitudes equatorward of 50/spl deg/. Several methods for field modeling are discussed in this paper under the assumption that the direction of the spin axis in inertial space is nearly constant, and this assumption is corroborated by the observations. The approximate inertial orientation of the magnetometer could then be determined simultaneously with the instrument intrinsic calibration and the estimate of main field model coefficients.

Space‐based, high‐precision magnetometry is essential for understanding a variety of phenomena ranging from secular variation of the Earth's main field, through the signatures of crustal magnetism and the effects of plasma currents flowing externally to the Earth. Ørsted, Denmark's first satellite, was launched on February 23, 1999 into a polar, low‐Earth orbit to provide the first near‐global set of high‐precision geomagnetic observations since the Magsat mission of 1979–1980 (see Magsat Special Issue of Geophysical Research Letters., vol. 9, no. 4, pp. 239–379, 1982). With the new mapping of the Earth's magnetic field, the International Geomagnetic Reference Field model (IGRF), a standard model used for navigation, prospecting, and other practical purposes, has been determined with improved precision for epoch 2000 [Olsen et al., 2000a; Mandea and Langlais, 2000]. The satellite has routinely provided high‐precision vector data since August 1999, and the mission is continuing well beyond its nominal 14‐month lifetime into 2001.

The pre-flight determination of the relative orientation between the CSC vector magnetometer and the Star IMager in the Magnetic Mapper Probe on-board the Argentinean SAC-C satellite is described. Key elements of the instrumentation are given and the inter-calibration is attained using a temporary reference magnetometer and the satellite instrument package. The relative orientation is obtained with accuracy in the arcsec range. As opposed to previous methods the orientation of the reference magnetometer need not be known a priori, but is determined to within a few degrees, which is enough for the accurate determination of the CSC/SIM Euler angles.

The construction of an orthogonal reference frame based on a set of three skew axes for a magnetic coil system and a magnetic sensor is discussed and presented. For a skew system, it is possible to define the coil axes and the magnetic axes as a dual set of axes that are linked to the system. Therefore, one orthogonal reference frame can be identified from the coil axes and another from the magnetic axes. Although this representation is not unique, it is the most intuitive visual representation as it is shown. The parametrizations based on the operation of the fluxgate transducer for the magnetic sensors compact spherical coil (CSC) (Ørsted satellite) and compact detector coil (CDC) (Astrid-2 satellite) are analyzed and identified. In principle, only the transformation matrix that orthogonalizes the sensor is needed, but it is customary to express this matrix as a function of some non-orthogonal angles. Therefore, most relevant conventions from the literature are presented for comparison. All the representations agree for the case of small angle approximation, which is valid for most sensors.

The Swedish micro-satellite Astrid-2 was successfully launched into a near polar orbit in December 1998. Despite the fact that its primary mission was the research of Auroral phenomena, the magnetic instrumentation has been designed to accomplish high resolution vector field magnetic measurements, and therefore mapping of the Earth's magnetic field is possible. The spinning of the spacecraft about a certain axis makes stabilisation in space possible. This fact and the well distributed data over the globe makes the magnetic data well suited for the estimation of the magnetic field model at the spacecraft altitude (circa 1000km). This paper describes the initial analysis of the Astrid-2 magnetic data. As a result of the study of a single day (February 7, 1999) magnetically fairly quiet, it was possible to calibrate the magnetometer and find a magnetic field model that fitted the scalar component of the measurements to less than 5nTRMS (for latitudes equatorward of 50o). Several methods for field modelling are presented in this paper with the assumption that the direction of the spin axis is nearly constant. The orientation of the magnetometer might be determined simultaneously with the instrument calibration and the main field model coefficients. Hence, apart from the scientific use of the magnetic data, the attitude of the spacecraft may be estimated with high precision.

Magnetic measurements taken by the Ørsted satellite during geomagnetic quiet conditions around January 1, 2000 have been used to derive a spherical harmonic model of the Earth's magnetic field for epoch 2000.0. The maximum degree and order of the model is 19 for internal, and 2 for external, source fields; however, coefficients above degree 14 may not be robust. Such a detailed model exists for only one previous epoch, 1980. Achieved rms misfit is <2 nT for the scalar intensity and <3 nT for one of the vector components perpendicular to the magnetic field. For scientific purposes related to the Ørsted mission, this model supercedes IGRF 2000.

The vector magnetometer sensor onboard the Astrid-2 satellite is made as a compact ringcore fluxgate sensor with single axis compensation. The ringcores used in the sensor are identical to the cores used in the fluxgate (CSC-) sensor in the high quality magnetometer onboard the field mapping satellite called 'Orsted'. To obtain good axial stability special attention is drawn to the mechanical construction of the tri-axial sensor configuration. Almost all parts of the sensor are machined from the glassy material MACOR(R) that has approximately the same thermal expansion coefficient as the core ribbon. The single axis compensated ringcore sensors are known to have some linearity problems with large uncompensated fields perpendicular to the measuring axis, This phenomenon is also seen for the Astrid-2 sensor, and from a coil-calibration of the flight-spare sensor we observe: non-linearities in the order of 2.1 to 4.3 nT(p). From the results of the calibration, an unfortunate magnetic coupling between two of the axes was discovered. This coupling is also associated with the ringcore geometry and large uncompensated transverse field, (C) 2000 Elsevier Science S.A. All rights reserved.