Magnetometer Constants File for Mars Observer Magnetometers
ABSTRACT: The Magnetometer Constants File contains all necessary
constants, such as scale factors and zero levels, required to convert
sensor telemetry in counts to engineering units, e.g., magnetic field
in nanoteslas.
SUMMARY: The Magnetometer Constants File provides all necessary
constants, such as scale factors and zero levels, required to convert
sensor telemetry in counts to engineering units. The File is ascii text,
in a format which conforms to JPL's naif kernel files (see below). The
file is initialized with constants resulting from MO Mag analyses of
ground calibration data obtained at the GSFC Magnetics test facility.
The constants in this file are subject to review throughout the mission
and may require updating in response to MO Mag analyses of in-flight
data, calibration data, and spacecraft roll 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. This file is
thus a concise history of the magnetometer constants.
The Magnetometer Constants File provides constants for two
magnetometers, designated Inboard (IB_) and Outboard (OB_) according
to the radial distance of each sensor triad on the deployed boom. Each
requires a zero table and a scale factor table to convert sensor
telemetry counts to magnetic field in nanoteslas. Both tables are
organized as 4 x N arrays, where N is the number of Mag ranges that
the sensor has available. Each row of the array has an entry for each
sensor axis (x,y,z) and range designation (0,1,2,3...) as follows:
x y z range
IB_ZEROS = ( 2038 2059 2051 0
2044 2050 2055 1
2047 2048 2049 2
. . . .
. . . .
2046 2048 2048 N )
IB_SCALE = ( 0.00182 0.00200 0.00200 0
0.00775 0.00795 0.00791 1
0.03097 0.03203 0.03222 2
. . . .
. . . .
31.70630 31.87791 31.90853 N )
Nominal Mag ranges are as follows: (+/-) nT range
4 0
16 1
64 2
256 3
1,024 4
4,096 5
16,384 6
65,536 7
But actual ranges may be expected to deviate from the nominal (design)
range by varing amounts, ranging from as much as 10% in the most
sensitive range (0) and decreasing to a fraction of a percent (0.3%)
in the highest range (7).
The Mag voltages are digitized with a single-ended analog-to-digital
converter with 12 bit resolution (2**12 steps = 4096) so the nominal
zero value is 2048. Actual zeros in the most sensitive ranges may differ
substantially (few hundred counts) from this value. In higher ranges,
the zero value more closely approximates the nominal 2048 midpoint.
A Mag sample 4-vector, consisting of x counts, y counts, z counts, and
range, can be converted to approximate magnetic field value in
nanoteslas by (1) subtraction of the zeros appropriate to the range
value (zero-corrected) and (2) multiplication by sense for the given
range (scaled). Small corrections to the field computed in this way
are introduced in the form of a matrix, historically referred to as an
"alignment matrix". This is somewhat of a misnomer, as this matrix is
not a rotation matrix. Rather, it allows small linear corrections to be
made to the "ideal" sensor output to compensate for sensor physical
misalignment between axes and cross-coupling between sensors, i.e., the
net effects of non-ideal behavior. We will call this matrix a
"rectification matrix" to make clear its purpose and distinguish it
from other necessary matricies which represent rotations, e.g.,
relative alignment of IB and OB sensors, and coordinate transformations.
This matrix is nearly equal to the identity matrix, and we carry a
fourth component (x,y,z,"0") as a convenience which preserves the range
information and reversibility.
IB_RECTN = ( 1.00112867 0.00222362 0.01009026 0
-0.00005261 0.99926096 0.00128650 0
-0.00229221 0.00026479 0.99959821 0
0 0 0 1 )
This matrix may be approximated by the identity matrix prior to
completion of calibration analyses.
An additional true rotation matrix rotates a vector in sensor
coordinates to spacecraft payload coordinates:
IB_S2PL = ( 1.00000000 0.00000000 0.00000000 0
0.00000000 1.00000000 0.00000000 0
0.00000000 0.00000000 1.00000000 0
0 0 0 1 )
Mag constants will be changed as necessary after analyses of in-flight
data, perhaps as often as a few times in a year.
FORMAT: The Magnetometer Constants File conforms to the format of 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: 25-AUG-1992
REVISIONS: 17-NOV-1992 Added magnetometer intrinsic sample interval
04-JAN-1992 Revised scale factors to reflect 12/4/91 calib.
Added sensor positions.
26-NOV-1996 Added calibration data for Mars Global Surveyor
instrument (MO S/N 001)
14-SEP-1997 Revise zero table at MOI per email from MHA
17-SEP-1997 Revise per John S./inboard temporarily
13-OCT-1997 Revise per Mario and John S.
==============================================================================
Initial (pre-flight) entries for the Mag constants appear below, as
determined from ground calibrations.
\begindata
REVISION = @26-NOV-1996-00:00:00.000
EPOCH_START = @25-NOV-1996-00:00:00.000
\begintext
The magnetometers have an intrinsic sample rate at which the input
is sampled. (The input may be averaged and decimated by the flight
processor according to the values uplinked for certain parameters.)
\begindata
SAMPLES_SEC = 32.
\begintext
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
(These are the zeros through MOI. We follow with new ones
after the text-off signal)
x y z range
IB_ZEROS = ( 2400.0 1850.0 1700.0 0
2135.0 1998.0 1961.0 1
2064.0 2035.0 2026.0 2
1989.0 2063.0 2025.0 3
2043.0 2066.0 2053.0 4
2045.0 2052.0 2048.0 5
2057.0 2061.0 2057.0 6
2049.0 2051.0 2049.0 7 )
OB_ZEROS = ( 2300.0 2150.0 2400.0 0
2110.0 2073.0 2135.0 1
2057.0 2064.0 2067.0 2
2044.0 2052.0 2040.0 3
2064.0 2056.0 2059.0 4
2051.0 2049.0 2049.0 5
2059.0 2057.0 2055.0 6
2050.0 2050.0 2049.0 7 )
\begindata
IB_ZEROS = ( 2072.0 1199.0 3152.0 0
2052.0 1832.0 2331.0 1
2043.0 2003.0 2117.0 2
2046.0 2036.0 2065.0 3
2048.0 2052.0 2054.0 4
2045.0 2051.0 2049.0 5
2057.0 2061.0 2057.0 6
2049.0 2051.0 2049.0 7 )
OB_ZEROS = ( 2350.0 3764.0 2798.0 0
2123.0 2484.0 2236.0 1
2073.0 2148.0 2116.0 2
2052.0 2072.0 2063.0 3
2061.0 2058.0 2058.0 4
2051.0 2051.0 2049.0 5
2059.0 2057.0 2055.0 6
2050.0 2050.0 2049.0 7 )
IB_SCALE = ( 0.0020121 0.0020117 0.0019969 0
0.0079143 0.0079032 0.007847 1
0.032190 0.032206 0.031983 2
0.12661 0.12652 0.12568 3
0.50043 0.49980 0.49697 4
1.96840 1.96350 1.95280 5
8.00600 8.00140 7.95960 6
31.49010 31.43470 31.27640 7 )
OB_SCALE = ( 0.002002 0.002001 0.002010 0
0.007865 0.007863 0.007904 1
0.032127 0.032034 0.032206 2
0.12615 0.12587 0.126610 3
0.49974 0.49832 0.50145 4
1.96240 1.95800 1.97070 5
8.01900 7.97710 8.03010 6
31.48910 31.34310 31.56840 7 )
\begintext
As of this date, the rectification matricies and sensor to payload
transformation matricies are derived by MHA from GSFC pre-flight
calibration data. The MAG serial number is recorded as SN002;
the low field (15k) rectification matricies are used here.
\begindata
IB_RECTN = ( 1.00000 0.0006822 -0.0025086 0
0.0057004 1.00000 0.0003112 0
-0.0068815 -0.0025975 1.00000 0
0 0 0 1 )
OB_RECTN = ( 1.00000 -0.0000591 -0.008906 0
-0.0041496 1.00000 -0.000979 0
0.0086519 0.0017138 1.00000 0
0 0 0 1 )
IB_S2PL = ( 1.00000000 0.00000000 0.00000000 0
0.00000000 1.00000000 0.00000000 0
0.00000000 0.00000000 1.00000000 0
0 0 0 1 )
OB_S2PL = ( 1.00000000 0.00000000 0.00000000 0
0.00000000 1.00000000 0.00000000 0
0.00000000 0.00000000 1.00000000 0
0 0 0 1 )
\begintext
The magnetometer positions in spacecraft coordinates, units meters,
currently a TBD item
x y z
\begindata
IB_POSN = ( 0.0 0.0 0.0 )
OB_POSN = ( 0.0 0.0 0.0 )
\begintext
==============================================================================