Galileo Magnetometer Pre-Jupiter Calibrated Bundle Galileo MAG Gaspra High Resolution Calibrated Data Description PDS3 DATA_SET_ID = GO-A-MAG-3-RDR-GASPRA-HIGH-RES-V1.0 PDS3 DATA_SET_NAME = GALILEO ORBITER A MAG RDR GASPRA HIGH RES V1.0" START_TIME = 1991-10-29T06:39:32.033 STOP_TIME = 1991-10-30T08:46:43.771 DATA_SET_RELEASE_DATE = 1997-12-31 PRODUCER_FULL_NAME = MARGARET G. KIVELSON References: Kivelson, M.G., Khurana, K.K., Russell, C.T., Walker, R.J., Joy, S.P.,Green, J., Aiken, W.C. GALILEO ORBITER A MAG RDR GASPRA HIGH RES V1.0, GO-A-MAG-3-RDR-GASPRA-HIGH-RES-V1.0, NASA Planetary Data System, 1997 Collection Data Overview ================= This collection contains data acquired by the Galileo Magnetometer from the Gaspra encounter. The data are at the full instrument resolution for 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 physical units (nanoTesla) in 4 coordinate systems. One set of data files contain data in Inertial Rotor Coordinates (IRC = despun spacecraft). These files include many of the data processing parameters from the AACS system as well as the sensor zero levels. The other set of files contain magnetic field data in Gaspra-centric Solar Ecliptic (GaSE), Earth Mean Equatorial equinox 1950 (EME-50), and finally in Heliographic (RTN) coordinates. Trajectory data are provided as a separate collection for each of the geophysical coordinate systems. This collection contains the highest time resolution available for each observation. In cases where the satellite was not the primary target for a particular orbit the data may be at the survey (RTS) data rate. Primary collection References: Kivelson, M.G., L.F. Bargatze, K.K. Khurana, D.J. Southwood, R.J. Walker, P.J. Coleman, 'Magnetic Field Signatures Near Galileo's Closest Approach to Gaspra', Science, Vol. 261, p331-334, 16 July, 1993. [KIVELSONETAL1993] Wang, Z., and Kivelson, M.G., 'Asteroid interaction with solar wind', J. Geophys. Res., 101, 24479, 1996. [WANG&KIVELSON1996] 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, Spacecraft Coordinates (IRC) ------------------------------------------------------------------ Column Description ------------------------------------------------------------------ time S/C event time (UT) given in seconds since 1966 rotattt Rotor twist angle (EME-50) rotattd Rotor attitude declination (EME-50) rotattr Rotor attitude right ascension (EME-50) spinangl Rotor spin angle - inertial S/C coordinates spindelt Rotor spin motion delta screlcon Rotor-Platform relative cone angle screlclk Rotor-Platform relative clock angle Bx_sc Magnetic field X component in S/C (IRC) coordinates By_sc Magnetic field Y component in S/C (IRC) coordinates Bz_sc Magnetic field Z component in S/C (IRC) coordinates Bmag |B| Magnitude of B ------------------------------------------------------------------ Table 2. Data record structure, Geophysical Coordinates (GaSE, EME-50, and RTN) ------------------------------------------------------------------ Column Description ------------------------------------------------------------------ time S/C event time (UT) given in PDS time format YYYY-MM-DDThh:mm:ss.sssZ Bx_GaSE X component of B in GaSE coordinates (towards Sun) By_GaSE Y component of B in GaSE coordinates (towards dusk) Bz_GaSE Z component of B in GaSE coordinates (|| to ecliptic normal) Bx_eme50 Magnetic field X component in EME-50 coordinates By_eme50 Magnetic field Y component in EME-50 coordinates Bz_eme50 Magnetic field Z component in EME-50 coordinates Br Magnetic field radial component in RTN coordinates Bt Magnetic field tangential component in RTN coordinates Bn Magnetic field normal component in RTN coordinates Bmag |B| Magnitude of B Data Acquisition ---------------- The data were acquired by the Galileo Magnetometer in the normal Low Rate Science (LRS) manner except that the data were recorded to tape and played back during the Earth 2 encounter. The data were acquired by the outboard magnetometer in the flip left position and the low range (high gain) mode. 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. -------------------------------------------------------------------- Table 3. 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 * range is the opposite of gain Data Sampling ------------- The Galileo magnetometer samples the magnetic field 30 times per second. In order to reduce the data rate to the MAG LRS rate, the instrument performs an onboard averaging process. 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 The high 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 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 4 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. Gaspracentric Solar Ecliptic (GaSE) is a Gaspra centered coordinate system defined by the primary vector along the instantaneous Gaspra->Sun (GSun) line and the Earth's ecliptic north pole (ENP) as the secondary vector. In this coordinate system: X is the GSun unit vector taken to be positive towards the Sun. Y is the formed by the unitized cross product ENP x GSun Z completes the right handed set (Z = X x Y) such that the X-Z plane contains the ecliptic north pole. The Earth Mean Equatorial equinox of 1950 (EME-50) coordinate system is an inertial reference system. The primary vector in this system points from the Earth towards Aries at the reference epoch. The secondary vector is along the Earth's rotational axis positive towards it's north pole. In this coordinate system: X is a unit vector in the direction of Aries at the equinox of 1950 Y is a unit vector in the direction of Z x X Z is a unit vector in the direction of the Earth's equatorial north pole at the equinox of 1950. The EME-50 coordinate system is directly supported by SPICE as the 'FK4' inertial reference frame. The heliographic (RTN) coordinate system centered at the Sun. The primary vector in this system points from the center of the Sun to the spacecraft (R). The secondary vector in this system is the Sun's north rotational axis (Omega). R is along R, positive away from the Sun. T is along the cross product Omega x R N completes the right handed set (R x T) The magnetic field perturbation associated with the Gaspra flyby [KIVELSONETAL1993] is most easily understood in a coordinate system that is organized by the interplanetary magnetic field (IMF). The IMF coordinate system used by [KIVELSONETAL1993] to analyze the Gaspra flyby data takes data from the GaSE coordinate system and rotates about the Asteroid-Sun line (X) such that the average upstream IMF direction (between 22:30 and 22:33 UT) lies in the X-Y plane. This requires a righthanded rotation of 32.44 degrees about the GaSE X-axis to generate By_imf and Bz_imf from By_GaSe and Bz_GaSE. An IMF coordinate system is only valid for a short interval near the time interval that defines the IMF direction. [KIVELSONETAL1993] use this coordinate system only in the analysis of data acquired between 22:15 and 23:05 UT on the day of encounter (10/29/91). Ancillary Data ============== A subset of the Galileo interplanetary cruise magnetometer collection (GO-SS-MAG-4-SUMM-CRUISE-RTN-V1.0) has been supplied as an ancillary data product with this archive. The cruise data are provided to place the encounter data in context with large scale structures in the solar wind and IMF. These data are provided in RTN coordinates which is a standard coordinate system for solar wind data analysis. The time interval provided (9/10/91 - 11/24/91) spans roughly 3 solar rotations centered on the asteroid flyby. These data show that the Gaspra encounter occurred in an 'away' sector a day or so before a large field compression associated with a corotating interaction region. The data are stored as an ASCII table in the file 'CRUISE.TAB'. ------------------------------------------------------------------ Table 4. Data record structure, RTN coordinates cruise data ------------------------------------------------------------------ Column Description ------------------------------------------------------------------ time S/C event time (UT) given in PDS time format sc_clk S/C clock counter given in the form rim:mod91:mod10:mod8 Br Magnetic field radial component Bt Magnetic field tangential component Bn Magnetic field normal component Bmag |B| Magnitude of B R Radial distance of the spacecraft from the Sun LAT Solar latitude of the spacecraft LON Solar east longitude of the spacecraft avg_con Onboard averaging interval for the magnetometer data delta Magnetic inclination angle: delta=arcsin(Bn/Bmag) lambda Magnetic azimuth angle: lambda = atan2(Bt/Br) * 1 RIM = 60.667 seconds (spacecraft major frame) Data Processing =============== These data have been processed from the PDS3 collection: 'GO-E/V/A-MAG-2-RDR-RAWDATA-HIRES-V1.0' The 'raw data' product was created from the EDR collection by removing the data processing done by the instrument in space. The raw data collection contains the raw instrument samples which have been recursively filtered and decimated as described above. The processed data in IRC coordinates were rotated into geophysical coordinates using the SPICE libraries and kernels provided by NAIF. 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. 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. The zero levels removed from the data are given in the IRC data file. 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. 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. The source of the high order harmonics was modeled using 1-D (spacecraft clock angle) Fourier transforms of high pass filtered data. This allows us to resolve the source in terms of the relative spin phase 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. 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 .037 milliseconds. This delay is converted to an angle using the instantaneous spin frequency. These 3 sources of delay are then summed in to the quantity 'phase' 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) 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 allow a user to construct coordinate system transformation matrices for any ephemeris time. These matrices were constructed from the SPICE kernels by directly or indirectly extracting the primary and secondary vectors defining the coordinate system and then following the procedure outlined in the coordinate systems section of this document. The spacecraft/planet (SPK), leap second (TS), and planetary constants (PCK) kernels required for these transformations have been archived in the PDS by NAIF. The SPICE toolkit (software) can be obtained from the NAIF node of the PDS for many different platforms and operating systems." Confidence Level Overview ========================= Data quality assessment is a rather vague concept which we will try to address in a somewhat qualitative manner. Each aspect of the data processing sequence can be analyzed to determine its maximum possible error contribution. In theory, these errors could be summed to provide estimates of the maximum error for each data point. We have not taken our error analysis to that level. We believe that our calibrations (sensor geometry and gains) are good enough that they produce a negligible source of data error. In addition, we believe that the coordinate system transformations which are derived from the SPICE kernels and Toolkit are negligible sources of error in the magnetic field vectors. The sources of error which we feel 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 0.01 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. 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. 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.