Spacecraft Magnetic Field Model File for Mars Global Surveyor ABSTRACT: The Spacecraft Magnetic Field Model File contains the coefficients (spherical harmonic coefficients) with which one can estimate the magnetic field produced by the MGS spacecraft. SUMMARY: The Spacecraft Magnetic Field Model File is ascii text, in a format which conforms to JPL's naif kernel files (see below). The constants in this file are subject to review throughout the mission and may require updating in response to MGS Mag analyses of in-flight data, calibration data, and spacecraft maneuvers. All entries are to be updated by concatenation of "new" values to the original file, so that the "new" values follow the "old" in the file; naif kernel readers return the value of the last entry for a given parameter. FORMAT: The Spacecraft Magnetic Field Model file conforms to the JPL naif kernel files. All text not within data delimiters (\begindata and \begintext) is ignored by file readers, allowing for comments and doc- umentation. Entries following a data delimiter (\begindata) consist of "KEYWORD = VALUE" statements, where KEYWORD is a parameter symbol and VALUE is a number or JPL formatted time, if preceeded by an @ sign. The value associated with a particular keyword symbol is that value corresponding to the last entry for the keyword. The value field may contain a string of numeric values if enclosed in parentheses. Blanks or commas separate entries, and commas are translated as blanks. Additional information on file format and reading routines (Fortran) can be found in the naif libraries (/SFOC/naif/toolkit/etc/spicelib/env0) AUTHOR: J. E. P. Connerney, code 695 NASA/GSFC DATE: 24-FEB-2000 REVISIONS: 24-FEB-2000 07-MAR-2000 Added second dipole in spacecraft payload coordinates; added dipole offsets 27-MAR-2000 Use average HGA mod from HGA articulation sequences (case 1,3, and 4) 21-APR-2000 Add dynamic field of the solar arrays 05-JUN-2000 Use better HGA estimate and std. dynamic =============================================================================== The magnetometers on Mars Global Surveyor (MGS) are not boom mounted. They are mounted at the outer edge of the two solar panels, and both are about the same distance from the center of the spacecraft. In earlier missions, one of the two magnetometers was mounted at the end of a boom ("outboard mag") and the other mounted closer to the spacecraft body ("inboard mag"). The data acquisition scheme allows for more rapid sampling of one or the other magnetometer. The outboard mag was usually the primary mag, the inboard secondary. For MGS, we will preserve the terminology "outboard" and "inboard" mag for simplicity but we must define which is which. By definition, the MGS "OUTBOARD MAG" is on the S/C +Y solar panel the MGS "INBOARD MAG" is on the S/C -Y solar panel the S/C +Y solar panel is "SAP" the S/C -Y solar panel is "SAM" Each multiple length entry below models a separate component of the spacecraft. The model field associated with a spacecraft component is determined from analysis of in-flight data using Fortran program mgsmcal.f (see Connerney for details). Note that the frame of reference (coordinate frame) is that specified for the component, e.g., for the HGA (High Gain Antenna) it is the HGA coordinate system. The HGA and solar panels are articulated about the spacecraft (payload frame). The values in each KEYWORD = VALUES entry for static fields are dipole moments in units A-m**2, and offset in meters: MODEL = ( DMX DMY DMZ dx dy dz ) If we need to convert spherical harmonic coefficients (e.g., from mgsmcal program) in nanoteslas to SI units (A-m**2) cartesian dipole, we use the following: DMX = (0.01) g1,1 DMY = (0.01) h1,1 DMZ = (0.01) g1,0 so for example g1,0 = -156.027 g1,1 = -69.994 h1,1 = 86.559 then DMX = -.6999 DMY = .8656 DMZ = -1.5603 HGA_MOD = ( -.6999 .8656 -1.5603 0.0 0.0 0.0 ) The following model was a fit to case 1,3 and 4 HGA articulation, allowing for different offsets (as these tests required several orbits, due to non-illumination requirement). The sc body model, e.g., fixed offset dipole in payload coordinates, is set to zero below as it is not significant. HGA_MOD = ( -.133 .470 -2.107 0.407 -0.353 -0.103 ) Model in current use (below) is a more careful fit to each of the HGA articulation sequences, estimated error in each parameter (SD) estimated as follows: (0.18, 0.12, 0.13, .04, .05, .11) \begindata HGA_MOD = ( -.308 .490 -2.021 0.424 -0.431 -0.094 ) SC_MOD = ( 0.0 0.0 0.0 0.0 0.0 0.0 ) \begintext The following entries are models for the magnetic field, in sensor coordinates (attached to the solar panels), due to currents flowing in the power subsystem. The units are nT per Ampere of SAM_I or SAP_I (ib or ob sensor): These values were obtained from the illuminated flip tests, with HGA model fixed as per above. \begindata DSAM_MOD = ( -.156 -.085 .023 ) DSAP_MOD = ( -.211 .116 -.051 ) \begintext ===============================================================================