PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT PUBLICATION_DATE = 2000-11-17 NOTE = "INFO.TXT describes the contents of this directory." END_OBJECT = TEXT END The MAG_CAL directory contains PostScript files representing plots of data pertaining to the calibration of the Magnetometer. The information contained in these plots is explained in the document "A Note on the magnetic mapping of the MGS High Gain Antenna," which is included below. This document calls the files by their original names, as shown in the following table. The files have been renamed on this disk. Current Filename Original Filename ---------------- ----------------- H_*_D.PS hga_*_diff.ps H_*_M.PS hga_*_model.ps HGAAZELD.PS hga_azel_diff.ps HGAAZELM.PS hga_azel_model.ps The PostScript files were edited after their creation for inclusion on this disk. A legend has been added to the bottom of each plot, the time labeling has been changed in format, and some axis labels have been changed. =========================================================================== =========================================================================== A Note on the magnetic mapping of the MGS High Gain Antenna ============================================================== J. Connerney 08/02/00 GSFC The plots provided are intended to give you some insight into the High Gain Antenna (HGA) magnetic mapping and the model used to approximate the field of the HGA in our data reduction. One set of plots (hga_*_diff.ps) shows each of our HGA articulations, the dotted line is the DIFFERENCE between the vector fields measured by the two sensors in spacecraft payload coordinates. Subtracting one from the other removes (the much larger) variations in the ambient field and allows us to isolate movements of the HGA. The dashed line in the same plots shows the corresponding DIFFERENCE in the model field at each sensor location for comparison. NOTE that the model included a constant adjustment for each test that is not reflected in the plots. What you are interested in is not the constant offset of the data and the model, but how well the VARIATION in the model difference compares with the VARIATION in the data difference. Why do we allow for a constant offset from one test (HGA articulation) to the next? The tests must all be done when the spacecraft is in darkness (shadowed by the planet) in order to avoid confusion with the dynamic field associated with currents in the power subsystem. The time it takes to complete an articulation of the HGA is a large fraction of the eclipse period so we can only get one articulation per orbit. In the time between test, the static spacecraft field may have changed a bit, so we allow for this by allowing a constant (throughout any one HGA articulation) offset from eclipse to eclipse. The HGA model was derived from these data (differences in vector field at each sensor) using a generalized inverse matrix method (Connerney, 1981). We found that a simple offset dipole model (where offset is with reference to the HGA coordinate system) did as good a job in modeling the field of the HGA as did a quadrupole or octupole expansion (about the same origin) so this is the model we adopt for data reduction (see sc_mod.ker). Another set of plots is provided (hga_*_model.ps) to show the variation in the model HGA field at each sensor location for each of the HGA articulation tests conducted in early 2000. Again, it is the difference in these two measurements that appears in the other set of plots for comparison to the data.