PDS_VERSION_ID = PDS3
RECORD_TYPE = STREAM
DATA_SET_ID = {"GO-E-MAG-3-RDR-EARTH1-HIGHRES-V1.0",
"GO-E-MAG-3-RDR-EARTH2-HIGHRES-V1.0",
"GO-E-MAG-4-SUMM-EARTH1-SUMMARY-V1.0",
"GO-E-MAG-4-SUMM-EARTH2-SUMMARY-V1.0",
"GO-A-MAG-3-RDR-GASPRA-HIGH-RES-V1.0",
"GO-A-MAG-4-SUMM-GASPRA-SUMMARY-V1.0",
"GO-A-MAG-3-RDR-IDA-HIGH-RES-V1.0",
"GO-A-MAG-4-SUMM-IDA-SUMMARY-V1.0",
"GO-V-MAG-3-RDR-VENUS-HIGH-RES-V1.0",
"GO-V-MAG-4-SUMM-VENUS-SUMMARY-V1.0"}
PRODUCT_ID = "HR_PROC"
PRODUCT_TYPE = "DOCUMENT"
PRODUCT_VERSION_ID = "1"
PRODUCT_CREATION_TIME = 2003-02-03
START_TIME = 1990-02-09T03:07:40.000
STOP_TIME = 1993-08-28T23:59:27.023
INSTRUMENT_HOST_NAME = "GALILEO ORBITER"
INSTRUMENT_HOST_ID = "GO"
MISSION_PHASE_NAME = "JUPITER ORBIT OPERATIONS"
TARGET_NAME = {"JUPITER", "IO", "IO PLASMA TORUS", "EUROPA",
"GANYMEDE", "CALLISTO", "AMALTHEA"}
INSTRUMENT_NAME = "TRIAXIAL FLUXGATE MAGNETOMETER"
INSTRUMENT_ID = "MAG"
^ASCII_DOCUMENT = "HR_PROC.TXT"
OBJECT = ASCII_DOCUMENT
DOCUMENT_NAME = "GALILEO MAGNETOMETER CALIBRATION DESCRIPTION"
PUBLICATION_DATE = 2003-02-03
DOCUMENT_TOPIC_TYPE = "CALIBRATION DESCRIPTION"
INTERCHANGE_FORMAT = ASCII
DOCUMENT_FORMAT = TEXT
DESCRIPTION = "
This document describes the processing done to produce the above
data sets from the raw data."
END_OBJECT = ASCII_DOCUMENT
END
These data have been processed from the PDS dataset:
'GO-E/V/A-MAG-2-RDR-RAWDATA-HIRES-V1.0'
The 'raw data' product was created from the EDR dataset by
removing the data processing done by the instrument in space. The
raw data dataset contains the raw instrument samples which have
been recursively filtered and decimated as described above. In
order to generate the IRC processed data, the following procedure
was followed:
1) Sensor zero level corrections were subtracted from the raw
data,
2) Data were converted to nanoTesla,
3) A coupling matrix which orthogonalizes the data and corrects
for gains was applied to the data (calibration applied),
4) Magnetic sources associated with the spacecraft were
subtracted from the data,
5) Data were 'despun' into inertial rotor coordinates,
1) Zero level determination: The zero levels of the two spin
plane sensors were determined by taking averages over a large
number (about 50) of integral spin cycles. The zero level of
the spin axis aligned sensor was determined by a variety of
means. First, since the spin axis aligned sensor can be
flipped into the spin plane, the value of the zero level
determined in the spin plane can be used in the other
geometry. This works well if there are no spacecraft fields
and the zero level is stable. This is only valid in the solar
wind where the rotations are predominately rotational in
character. If there are spacecraft fields present
which remain constant over relatively long time periods (many
hours), then another method of zero level determination is used.
The spacecraft spin axis is along the Z direction, the data in
the X and Y directions have already had zero level corrections
applied.
Bm(z) = B(z) + O(z)
|Bm|^2 = B(x)^2 + B(y)^2 + Bm(z)^2
= B(x)^2 + B(y)^2 + B(z)^2 + O(z)^2 + 2B(z)O(z)
= |B|^2 + O(z)^2 + 2B(z)O(z)
= |B|^2 + O(z)^2 + 2O(z)[Bm(z) - O(z)]
= |B|^2 - O(z)^2 + 2O(z)Bm(z)
m = measured value - no subscript = true value
Now if |B| remains constant over a short interval and O(z)
remains constant over a much longer interval, we can take
averages and reduce this equation to:
|Bm|^2 - <|Bm|^2> = 2O(z)[Bm(z) - ]
<> indicates average value
Data can be processed using short averages of |B| until many
points are accumulated and then fit with a line in a least
squares sense. The slope of this line is twice the required
offset. The scatter in the data give an indication of the error
in the assumption the |B| and O(z) have remained constant.
Intervals with large rms errors are not retained. A file which
contains zero levels as a function of time has been provided as
an ancillary product with this dataset.
2) Conversion to nanoTesla simply requires dividing the instrument
data numbers by a constant scale factor. For the inboard high
range (low gain) mode the scale factor is 2. For the inboard low
range and outboard high range, the scale factor is 64. The
outboard low range data has a scale factor of 1024.
3) Calibration matrix applied: The determination of a calibration
matrix is too complex to describe here. The method employed has
been well described in 'A Fourier Transform Based Method for
Intraspacecraft Magnetometer Calibration', K.K. Khurana, E.L.
Kepko, and M.G. Kivelson, (in prep). [KHURANAINPREP]
4) After the data were initially processed (calibrated and despun),
it was clear that there were still coherent noise sources
remaining in the data. Dynamic spectra of the magnetometer data
revealed coherent energy at high order (2nd, 3rd, 4th) harmonics
of the spin period as well as some subharmonic frequencies. High
order harmonics of the spin period can be generated by spinning
about a fixed dipole source such as a source on the despun
platform. Subharmonic energy can be created by a dipole source
which spins with magnetometer but changes orientation at a
frequency which is near the spin frequency. The source of the
high order harmonics was modeled using 2-D (clock and cone angle)
Fourier transforms of high pass filtered data. This allows us to
resolve the source in terms of the relative spin phase and look
direction of the scan platform. Model fields associated with this
source (approximately 0.15 nT at the inboard sensors in the
lowest harmonic) have been subtracted from the data. A similar
approach was taken for the isolation and removal of sources of
subharmonic energy. Data were band pass filtered to isolate the
source signature and then resolved into components as a function
of the Energetic Particle Detector (EPD) motor position (look
direction). EPD interference (at about 0.05nT) has been removed
from the data on Dec 8, 1990. Both sets of interference
coefficients were calculated using data from the inboard sensors.
When the outboard sensors are in use, these values are
extrapolated using the inverse power law appropriate for the
source of each term.
It should be noted here that both of these interference
corrections are less then the quantization level for the inboard
sensors. Data resolution coming out of the recursive filter can
actually be better than that coming out of the A/D converter if
there is sufficient noise at the single bit level.
5) Despinning: Data are despun and checked in inertial rotor
coordinates before transforming to geophysical coordinates. Any
errors in the processing will be most readily apparent in
inertial rotor coordinates. The nominal transformation to IRC
from SRC is
(Bx) ( cos(theta) -sin(theta) 0 ) (Bxs)
(By) = ( sin(theta) cos(theta) 0 ) (Bys)
(Bz) ( 0 0 1 ) (Bzs)
Where s denotes spinning coordinates and theta is the rotor spin
angle.
Frequency dependent phase delays associated with the analog
anti-aliasing filter and the digital recursive filter have been
removed during the despinning of the data. The dominant frequency
in the spinning data is at the spacecraft spin frequency. The
phase angle delay associated from all known sources is computed
at the spin frequency and removed from the data during
despinning.
Analog Filter: Digital Filter (Nyquist Freq Fn = 15Hz):
1543 1/3
__________________ _____________________
s^2 + 55.5s + 1543 4/3 - exp(-PI*i*f/Fn)
s = 2*PI*i*f
Imaginary = 55.5s Imaginary = -sin(PI*f/Fn)
Real = 1543 + s^2 Real = 4/3 - cos(PI*f/FREQ_N)
f = frequency
delay = tan^-1(Im/Re)
In addition, there is an electrical delay associated with the A/D
conversion of about 1 millisecond. This delay is converted to an
angle using the instantaneous spin frequency. These 3 sources of
delay are then summed in to the quantity 'delay' and then the
despinning matrix becomes:
(Bx) ( cos(theta - phase) -sin(theta - phase) 0 ) (Bxs)
(By) = ( sin(theta - phase) cos(theta - phase) 0 ) (Bys)
(Bz) ( 0 0 1 ) (Bzs)
In order to create the processed GSE/GSM dataset the following step
was taken.
6) Data was transformed to geophysical coordinates: Data are
transformed from inertial rotor coordinates to the Earth Mean
Equatorial (equinox 1950) coordinate system. This system is
directly supported by the SPICE software provided by the
Navigation and Ancillary Information Facility (NAIF) at JPL as
inertial coordinate system 'FK4'. The angles required for this
transformation come directly from the Galileo Attitude and
Articulation Control System (AACS) data. The transformation
matrix for this rotation is:
-- --
|(cosTsinDcosR - sinTsinR) (-sinDsinTcosR - cosTsinR) cosDcosR |
| |
|(cosTsinDsinR + sinTcosR) (-sinDsinTsinR + cosTcosR) cosDsinR |
| |
|-cosDcosT sinTcosD sinD |
-- --
where
R = Rotor-Right Ascension
D = Rotor-Declination
T = Rotor-Twist - Rotor-Spin-angle (despun data)
Once in an inertial coordinate system, SPICE software provides
subroutines which return the transformation matrices to GSE
(G_GSETRN), GSM (G_GSMTRN), or RTN (G_RTNTRN) coordinate systems
for any ephemeris time. These matrices have been used to perform
the coordinate system transformations. The spacecraft/planet
(SPK) , leap second (TS), and planetary constants (PCK) kernels
required for these transformations have been archived in the PDS
by NAIF. These SPICE kernels are available on the CD_ROM which
contains the magnetometer data. The SPICE toolkit (software) can
be obtained from the NAIF node of the PDS for many different
platforms and operating systems. At the time of this archive,
the SPICE toolkit was available via an anonymous-ftp site at
naif.jpl.nasa.gov