Description of SCAS calibrations and potential impact on science data

The Cassini magnetometer sensors are mounted on an approximately 11 meter boom to minimize interference from spacecraft-generated fields.  The magnetometer boom is a deployable coiled longeron boom resulting in some potential positional uncertainty of the magnetometer sensor orientation along the twist axis as well as lateral uncertainty due to the structural flexibility of the boom. The orientation of the sensors with respect to the spacecraft may change slightly after events such as spacecraft maneuvers or as the thermal environment changes with distance from the sun. The Science Calibration Subsystem (SCAS) is a means of determining the magnetometer sensor alignment. SCAS produces a bipolar square wave magnetic field that is detected by the magnetometer and can affect other fields and particle measurements as well. SCAS alignment checks were executed throughout the mission, starting during the Science cruise phase. The accompanying file lists the start and end times of SCAS activities throughout the mission.  

SCAS consists of two, perpendicular, coils rigidly mounted on the spacecraft body with a known alignment to the spacecraft axes. When commanded, SCAS produces well-defined magnetic fields which can be detected by the sensors and used to correct for any changes in sensor orientation.  Since the alignment of the SCAS coils with respect to the spacecraft is known (measured before launch) and the alignment of the spacecraft axes with respect to an inertial reference system is known, the relationship of the sensor outputs to the inertial reference system can be established. This relationship was also cross-checked by operation of the sensors in the well known Earth’s field during flyby.

The SCAS was designed to provide a bi-polar magnetic field parallel to the spacecraft X axis with a field strength 2.5 nanoTeslas (nT) (5.0 nT peak to peak) at the Vector Helium Magnetometer (VHM, outboard MAG sensor) and a bi-polar magnetic field parallel to the spacecraft Y axis with a field strength 2.5 nT (5.0 nT peak to peak) at the VHM.  These fields are superimposed on the ambient field and appear as steps in the data. 

Specific examples and more details on the use of SCAS for sensor alignment determination is included in the following reference.

References

Dougherty, M. K., S. Kellock, D. J. Southwood, A. Balogh, E. J. Smith, B. T. Tsurutani, B. Gerlach et al. "The Cassini magnetic field investigation." In The Cassini-Huygens Mission, pp. 331-383. Springer Netherlands, 2004.