The dual technique magnetometer system onboard the Cassini orbiter is described. This instrument consists of vector helium and fluxgate magnetometers with the capability to operate the helium device in a scalar mode. This special mode is used near the planet in order to determine with very high accuracy the interior field of the planet. The four-year orbiting Cassini mission will lead to a detailed understanding of the Saturn/Titan system. In addition to the prime scientific measurement of the planetary field, the instrument will also make measurements of the planetary magnetosphere, and the interactions of Saturn with the solar wind, of Titan with its environments, and of the icy satellites within the magnetosphere.
The MAG instrument comprises a fluxgate magnetometer (FGM) and a vector helium magnetometer capable of operating in both vector and scalar mode (V/SHM). The instrument is intended to measure small changes in fields spanning four orders of magnitude with extremely high sensitivity. This goal is achieved in part by mounting the sensors on an 11-metre spacecraft boom; the V/SHM at the end of the boom, the FGM halfway along. The magnetometer boom distances the sensors from the magnetic field associated with the spacecraft and its subsystems, and especially from spacecraft-generated temporal field variations. Spacing the sensors at different distances along the boom allows the spacecraft fields to be better characterised and removed from the observations. However, mounting the sensors on a boom could result in their orientation with respect to the spacecraft axes changing from time to time, for example after spacecraft manoeuvres. A means of sensor-alignment determination has been provided by the Cassini project - the Science CAlibration Subsystem, SCAS. This system consists of two, perpendicular, coils rigidly mounted on the spacecraft body with a known alignment to the spacecraft axes. These coils produce well-defined magnetic fields on command which can be detected by the sensors and used to correct for any changes in sensor orientation.
Both magnetometers are capable of measuring the magnetic-field vector at rates from 0 Hz up to 10 Hz (VHM) or at least 30 Hz (FGM).
The VHM optimises low-frequency vector measurements in we