Data Set Information
DATA_SET_NAME GALILEO ORBITER V MAG SUMM VENUS SUMMARY V1.0
DATA_SET_ID GO-V-MAG-4-SUMM-VENUS-SUMMARY-V1.0
NSSDC_DATA_SET_ID
DATA_SET_TERSE_DESCRIPTION
DATA_SET_DESCRIPTION Data Set Overview ================= This dataset contains data acquired by the Galileo Magnetometer during the Venus flyby. The data have been averaged down to twenty second samples from the 7.68 kB Low Rate Science (LRS) tape record mode. The flyby data were recorded and later played back during the early portion of the Earth 1 flyby. Limited space on the tape recorder required the magnetometer team to limit their high time resolution observations to a few short intervals in the pre-encounter solar wind, a main record period near closest approach, and then some post encounter solar wind data. Large gaps in the high rate data have been filled in with 16 RIM (~ 16 minute) averages from the optimal averager section of the instrument. 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 Venus Solar Orbital (VSO) coordinates. Primary Dataset Reference: Kivelson, M.G., C.F. Kennel, R.L. McPherron, C.T. Russell, D.J. Southwood, R.J. Walker, C.M. Hammond, K.K. Khurana, R.J. Strangeway, and P.J. Coleman, 'Magnetic field studies of the solar wind interaction with Venus from the Galileo flyby: First Results', Science, 253, 5029, 1518, 1991. [KIVELSONETAL1991] Primary Instrument Reference: Kivelson, M.G., K.K. Khurana, J.D. Means, C.T. Russell, and R.C. Snare, 'The Galileo magnetic field investigation', Space Science Reviews, 60, 1-4, 357, 1992. [KIVELSONETAL1992] Data ==== ------------------------------------------------------------------ Table 1. Data record structure, VSO Coordinates ------------------------------------------------------------------ Column Description ------------------------------------------------------------------ year Year = 1990 day Day of year (40 , 41) sec Second of day sc_clk S/C clock counter in the form rim:mod91:mod10:mod8 Bx_vso Magnetic field X component in VSO coordinates By_vso Magnetic field Y component in VSO coordinates Bz_vso Magnetic field Z component in VSO coordinates Bmag |B| Magnitude of B stBx_vso Standard deviation of the X component stBy_vso Standard deviation of the Y component stBz_vso Standard deviation of the Z component stBmag Standard deviation of the Magnitude of B npts Number of points in the average dqf Data quality flag Data Acquisition ---------------- The data for this dataset were all acquired in by the outboard magnetometer sensors in the flip left mode in the high field mode (ULHR). This mode has a digitization step size of 0.25 nanoTesla. However, these data are acquired at 30 vectors/second and then recursively filtered in the instrument. The high rate data that are recorded to tape have a sample rate of 4.5 vectors/second. If there is sufficient variation in the 6-7 input samples that make up a single output sample, then the effective digitization step size becomes much smaller. Data Sampling ------------- The high rate recorded data are not evenly sampled within a minor frame. However, these data have been averaged using an averaging routine to produce evenly sampled data. Each averaged value includes all available data from the prior and subsequent 10 seconds in the high rate data. Averages are not overlapping. The standard deviations and the number of samples in the average have been included with the final dataset. Coordinate Systems ================== The Galileo magnetometer Venus flyby data are being archived in Venus Solar Orbital (VSO) coordinates. The VSO X direction is taken along the Venus-Sun line, positive towards the Sun. The Z direction is parallel to the normal of the Venus orbital plane (Venusian ecliptic), positive northward, and Y completes the right-handed set (towards dusk). Data Processing =============== These data have been processed from the PDS dataset: 'GO-E/V/A-MAG-3-RDR-IRC-COORDS-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 6) Data were transformed into VSO coordinates 7) Data were averaged to 20 second sampling 8) Optimal averager data (in VSO coordinates) were merged in to fill gaps between record intervals. 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) - ] <> 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 [KEPKOETAL1996]. 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 VSO 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) 6) Once in an inertial coordinate system, SPICE software provides the subroutine which returns the transformation matrix to VSO (G_GSETRN) 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. 7) Standard non-overlapping 20 second averages and standard deviations were computed about the central time stamp. 8) Optimal averager data were taken continuously across during the cruise period near the Venus flyby. These data were corrected for offsets and phase delays associated with the averaging. The data were then transformed into VSO coordinated and merged with the 20 second averages, preserving only the data values during the record interval gaps.
DATA_SET_RELEASE_DATE 1994-01-01T00:00:00.000Z
START_TIME 1990-02-09T03:07:40.000Z
STOP_TIME 1990-02-10T08:34:40.000Z
MISSION_NAME GALILEO
MISSION_START_DATE 1977-10-01T12:00:00.000Z
MISSION_STOP_DATE 2003-09-21T12:00:00.000Z
TARGET_NAME VENUS
TARGET_TYPE PLANET
INSTRUMENT_HOST_ID GO
INSTRUMENT_NAME DUAL TECHNIQUE MAGNETOMETER
TRIAXIAL FLUXGATE MAGNETOMETER
MAGNETOMETER
FLUXGATE MAGNETOMETER
INSTRUMENT_ID MAG
INSTRUMENT_TYPE MAGNETOMETER
Magnetometer
NODE_NAME planetary plasma interactions
ARCHIVE_STATUS ARCHIVED
CONFIDENCE_LEVEL_NOTE Confidence Level Overview ========================= The sources of error which are the most significant are those associated with magnetic sources aboard the spacecraft, especially those with temporal or spacecraft orientation variations. The next greatest contributor of error is the data from the AACS which affects our knowledge of the spacecraft orientation and hence rotates the magnetic field vector. Lastly, telemetry or software errors which produce 'spikes' or bit errors in the data are error sources. In regions where the magnetic sources associated with the spacecraft are fairly constant, magnetic interference is probably reduced by data processing to better than 0.05 nT at the inboard sensors. In these same regions, sensor zero levels (offsets) are known equally well. The data processing software does a fairly good job of removing all currently identified sources of magnetic interference. However, there are some time intervals when the zero levels of the spin plane sensors show large variations (1-5 nT) on short time scales (minutes to hours). After a while (hours usually) the offsets return to their nominal levels. The source of these magnetic fields has not yet been identified. The method of removing offsets from the spin plane sensors does remove these effects, but the method of determining the spin axis aligned sensor offsets does not. In regions where large variations are detected in the spin plane sensors it is reasonable to assume that similar variations are taking place in the spin axis aligned sensor. The time period between Feb 10, 05:40 through 08:30 is the only interval in this dataset which shows rapid time varying offsets in the spin plane sensors. Our data processing software creates a data quality flag (dqf) which is an assessment of AACS and telemetry error source contamination of a given data point. This number is binary integer where each bit indicates the presence or absence of some error source. The number 0 represents the absence of all error sources which are tested. The higher order bit (larger number) error sources are considered to be more significant error sources. Data are examined for gradients in the field which might be associated with telemetry bit errors, for regions of bad AACS angles, and for completely missing data. If the error is considered completely unrecoverable, the data values are replaced with a missing data flag. In the case of a flag in the rotor spin angle, the vector components may be flagged but the magnitude is still valid. Here is a list of all of the error checks and the bits they set in the dqf field. DQF_GOOD_DATA 0 Good data DQF_BX_GRAD_WARNING 2^0 Component gradient warning DQF_BY_GRAD_WARNING 2^1 Component gradient warning DQF_BZ_GRAD_WARNING 2^2 Component gradient warning DQF_INTERP_ROTATTR 2^3 Missing rotor RA interpolated DQF_INTERP_ROTATTD 2^4 Missing rotor DEC interpolated DQF_INTERP_SPINDELT 2^5 Missing rotor Spin Delta interpolated DQF_INTERP_SCRELCON 2^6 Missing Relative Cone angle interpolated DQF_INTERP_SCRELCLK 2^7 Missing Relative Clock angle interpolated DQF_INTERP_ROTATTT 2^8 Missing rotor Twist interpolated DQF_INTERP_SPINANGL 2^9 Missing rotor Spin interpolated DQF_ROTATTR_FLAG 2^10 Missing rotor RA flagged DQF_ROTATTD_FLAG 2^11 Missing rotor DEC flagged DQF_SPINDELT_FLAG 2^12 Missing rotor Spin Delta flagged DQF_SCRELCON_FLAG 2^13 Missing Relative Cone angle flagged DQF_SCRELCLK_FLAG 2^14 Missing Relative Clock angle flagged DQF_ROTATTT_FLAG 2^15 Missing rotor Twist flagged DQF_AACS_TELEMETRY_HIT_FLAG 2^16 Telemetry hit in AACS record DQF_MAG_TELEMETRY_HIT_FLAG 2^17 Telemetry hit in mag record DQF_SPINANGL_FLAG 2^18 Missing rotor Spin flagged DQF_BX_GRAD_ERROR 2^25 Component gradient error DQF_BY_GRAD_ERROR 2^26 Component gradient error DQF_BZ_GRAD_ERROR 2^27 Component gradient error DQF_BX_FLAG 2^28 Component flagged DQF_BY_FLAG 2^29 Component flagged DQF_BZ_FLAG 2^30 Component flagged Magnetic field gradient warning or error levels are set during the data processing according to expected variances depending on the region of space. In the solar wind, gradient warnings are usually issued at gradients of 10 nT/sec and errors at 15 nT/sec. In the magnetosheath, these values may be 50 percent larger. In the inner magnetosphere, these dqf flags may be completely turned off. Similarly, AACS angles are interpolated across gaps during the processing if the gap length is relatively short (less than 10 minutes typically). If the gaps in spacecraft attitude are long, the AACS angles are flagged and not interpolated. Errors associated with AACS angles have various effects on the data. The rotor right ascension and declination are crucial to the understanding of the spacecraft orientation. Fortunately, these angles are slowly varying and can be interpolated to better than 1 degree of accuracy for long (many hour) time periods except near major spacecraft maneuvers. The relative clock and cone angles are used to remove scan platform interference. In their absence, no interference is removed (+/- 0.15 nT error possible in each component). The rotor motion spin delta is used to determine the instantaneous spin frequency for the phase delay computation. In its absence, the last known phase delay is applied to the current data point. The rotor spin angle and twist angle must be present in order to despin the data. These angles are generally not interpolated for more than ten minutes because the rotor spin period drifts over time periods on this order.
CITATION_DESCRIPTION Kivelson, M.G., Khurana, K.K., Russell, C.T., Walker, R.J., Joy, S.P.,Green, J., GALILEO ORBITER V MAG SUMM VENUS SUMMARY V1.0, GO-V-MAG-4-SUMM-VENUS-SUMMARY-V1.0, NASA Planetary Data System, 1994
ABSTRACT_TEXT Galileo Orbiter Magnetometer (MAG) calibrated averaged data from the Venus flyby in VSO coordinates. These data cover the interval 1990-02-09 03:07 to 1990-02-10 08:34.
PRODUCER_FULL_NAME STEVEN P. JOY
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