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.