**NSSDCA ID:** PSFP-00428

**Availability:** Archived at NSSDC, accessible from elsewhere

**Time span:** 1990-11-05 to 1990-12-31

This description was generated automatically using input from the Planetary Data System.

Dataset Overview ================ This dataset contains data acquired by the Galileo Magnetometer from the Earth1 encounter. The data have benn averaged down to twenty second resolution from the 7.68 kB Low Rate Science (LRS) real time telemetry mode. These data have been fully processed to remove instrument response function characteristics and interference from magnetic sources aboard the spacecraft. The data are provided in both Geocentric Solar Ecliptic (GSE) and Geocentric Solar Magnetic (GSM) coordinates. Primary Reference ================= Kivelson, M. G., C. F. Kennel, R. L. McPherron, C. T. Russell, D. J., Southwood, R. J. Walker, K. K. Khurana, P. J. Coleman, C. M. Hammond, V. Angelopoulos, A. J. Lazarus, and R. P. Lepping, 'The Galileo earth encounter: magnetometer and allied measurements', J. Geophys. Res., 98, A2, 11299, 1993 Data Columns ============ scet S/C event time (UT) given in the form yyyy ddd mon day hh:mm:ss.sss Bx Magnetic field X component in GSE or GSM coordinate <nT> By_gse Magnetic field Y component in GSE coordinates <nT> Bz_gse Magnetic field Z component in GSE coordinates <nT> By_gsm Magnetic field Y component in GSM coordinates <nT> Bz_gsm Magnetic field Z component in GSM coordinates <nT> Bmag |B| Magnitude of B <nT> stBx Standard Deviation of Bx stBy_gse Standard Deviation of By_gse component stBz_gse Standard Deviation of Bz_gse component stBy_gsm Standard Deviation of By_gsm component stBz_gsm Standard Deviation of Bz_gsm component stBmag Standard Deviation of Bmag npts Number of points use in average dqf Data quality flag Data Acquisition ================ The data for this dataset were acquired as part of the normal instrument calibration activities associated with the cruise to Jupiter. As such, the instrument was commonly configured in modes which required calibration even though they may not have been the optimal mode for science data acquisition. The Galileo magnetometer has 8 possible LRS acquisition configurations (modes). There are two sensor triads mounted 7 and 11 meters from the rotor spin axis (inboard and outboard) along the boom. Each of the sensor triads has two gain states (high and low). In addition, the sensor triads can be 'flipped' to move the spacecraft spin-axis aligned sensor into the spin plane and visa versa. Please see the instrument description for full details on the instrument, sensors, and geometries. The combinations of sensor, gain state, and flip direction form modes. _____________________________________________________________________ Mode Characteristics _____________________________________________________________________ Mode Name Acronym range quantization _____________________________________________________________________ Inboard, left, high range* ILHR +/- 16384 nT 8.0 nT Inboard, right, high range* IRHR +/- 16384 nT 8.0 nT Inboard, left, low range* ILLR +/- 512 nT 0.25 nT Inboard, right, low range* IRLR +/- 512 nT 0.25 nT Outboard, left, high range* ULHR +/- 512 nT 0.25 nT Outboard, right, high range* URHR +/- 512 nT 0.25 nT Outboard, left, low range* ULLR +/- 32 nT 0.008 nT Outboard, right, low range* URLR +/- 32 nT 0.008 nT __________________________________________ s/c clock date/time mode __________________________________________ 00562976:00:0 90-305/16:31 ULHR 00572976:00:0 90-316/17:00 ULLR 00578673:00:0 90-320/17:00 URLR 00586204:00:0 90-325/23:55 URHR 00592915:00:0 90-330/17:01 ILLR 00597439:00:0 90-333/21:15 IRLR 00610156:00:0 90-342/19:33 IRHR 00610509:00:0 90-343/01:30 IRLR 00615701:00:0 90-346/17:00 URLR 00618550:00:0 90-348/17:00 URHR 00624261:00:0 90-352/17:15 ULHR * range is the opposite of gain In addition to exercising the various instrument modes during the first earth encounter, numerous instrument calibration activities were performed. These include using both the internal and external calibration coils. Data corrupted by the use of the calibration coils or by the flipper motor have been removed from the processed data. These data have been archived with the Experimenter Data Records (EDR) and other Magnetometer team raw data archive products. Data Sampling ============= The Galileo magnetometer samples the magnetic field 30 times per second. These highest rate samples are recursively filtered and then resampled by the instrument at 4.5 vectors per second using a 7,7,6 decimation pattern. The Recursive Filter B(t) = 1/4 Bs(t) + 3/4 B(t-1) B = output field Bs = input field measured by the sensor t = sample time pattern is generated by doubling the spacecraft clock modulo 10 counter and then applying the decimation scheme. This gives 3 vectors every spacecraft minor frame (about 2/3 second) which are sampled unevenly. The first vector in a minor frame is sampled approximately 0.200 seconds after the last vector in the preceding minor frame. The other two samples are taken approximately 0.233 seconds apart. The time tag associated with a sample is the decimation time. Coordinate Systems ================== The Galileo magnetometer data are being archived in 3 coordinate systems. The first coordinate system is referred to as inertial rotor coordinates (IRC). This coordinate system has the Z axis along the rotor spin axis, positive away from the antenna and the X and Y axes lies in the rotor spin plane. In a crude sense, when the spacecraft is far from Earth, +X points south, normal to the ecliptic plane, positive Y lies in the ecliptic plane in the sense of Jupiter's orbital motion and positive Z is in the anti-earth direction. The spacecraft antenna (negative Z direction) is kept earthward pointing to about +/- 10 deg accuracy. Geocentric Solar Ecliptic (GSE) and Geocentric Solar Magnetic (GSM) are related earth centered coordinate systems. Both the GSE and GSM X directions are taken along the Earth - Sun line, positive towards the Sun. The GSE Z direction is parallel to the ecliptic normal, positive northward, and Y completes the right-handed set (towards dusk). For GSM, the X-Z plane contains the Earth's dipole moment vector, positive northward, and Y completes the right-handed set. GSE coordinates are commonly used for analyzing the solar wind near the Earth and GSM coordinates are used when analyzing data inside the Earth's bow shock. Data Processing =============== These data have been processed from the PDS dataset: 'GO-E/V/A-MAG-3-RDR-HIRES-V1.0' In order to generate the IRC processed dataset, 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, Lastly, in order to generate the processed data in GSE/GSM coordinates the data were transformed into geophysical coordinates and averaged to twenty second resolution. 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. 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) - <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 press). 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 comming 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 procedure was used. Data are 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

These data are available on-line from the Planetary Data System (PDS) at:

https://pds-ppi.igpp.ucla.edu/data/GO-E-MAG-4-SUMM-E1-GSE-GSM-COORDS-V1.0/

- GO-E-MAG-4-SUMM-E1-GSE/GSM-COORDS-V1.0

- Planetary Science: Fields and Particles

Questions and comments about this data collection can be directed to: Dr. Edwin V. Bell, II

Name | Role | Original Affiliation | |
---|---|---|---|

Dr. Margaret Galland Kivelson | Data Provider | University of California, Los Angeles | mkevelson@igpp.ucla.edu |

Dr. Margaret Galland Kivelson | General Contact | University of California, Los Angeles | mkevelson@igpp.ucla.edu |