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