Data Set Overview :  NEAR MAG RDR volume sets contain a single data set, from one instrument and one mission phase (defined in the phase table in /AAREADME.TXT).  This data set contains vector magnetic field data acquired by the fluxgate magnetometer (MAG) on the NEAR spacecraft during the EARTH phase. The data set begins on 1998-01-23T00:00:00.000 and ends 1998-01-26T23:59:59.999.  The data are processed science data provided in physical units (nT) in two coordinate systems Near Sun Orbital (NSO) and Eros Body Fixed (EBF). Instrumental and spacecraft effects have been removed from the data during processing. The data are provided in ASCII tables located in the DATA directory. Plots of the data are also provided for browsing purposes in the BROWSE directory. Spacecraft position is also included in the data tables and plot files.   Data/Parameters :  The UTC time tags assigned to each sample are the spacecraft event time of the magnetometer output of the A/D converter. The sample time has not been shifted to account for the time lag in the anti-aliasing filter. The UTC time tags have the format of year-mo-dmThr:mn:ss.ss, where year(I4) is year, mo (I2)is month, dm (I2)is day of month, hr(I2) is UTC hour, mn(I2) is UTC minute, and ss.ss(F5.2) is UTC second. DMET is the difference between magnetometer and housekeeping data samples. The difference is kept as small as possible for accurate evaluation of the time-dependent magnetic fields generated by currents flowing in the spacecraft.  ASCII tables labeled NSOyyddd.TAB (yyddd indicates year and day of year) contain the following Level 2 MAG data.  Column Name Description Unit ------ ----- ----------- ---- 1. UTC time, UTC see above 2. MET_DAY time, mission day 3. MET_HOUR time, mission hour 4. MET_MINUTE time, mission minute 5. MET_SECOND time, mission second 6. DMET housekeeping data time rel. to mag. second 7. B_X magnetic field NSO X component nT 8. B_Y magnetic field NSO Y component nT 9. B_Z magnetic field NSO Z component nT 10. B_TOT magnetic field magnitude nT 11. NSO_X NEAR position NSO X component km 12. NSO_Y NEAR position NSO Y component km 13. NSO_Z NEAR position NSO Z component km   ASCII tables labeled EBFyyddd.TAB (yyddd indicates year and day)contain the following Level 2 MAG data.  Column Name Description Unit ------ ----- ----------- ---- 1. UTC time, UTC see above 2. MET_DAY time, mission day 3. MET_HOUR time, mission hour 4. MET_MINUTE time, mission minute 5. MET_SECOND time, mission second 6. DMET housekeeping data time rel. to mag. second 7. B_X magnetic field EBF X component nT 8. B_Y magnetic field EBF Y component nT 9. B_Z magnetic field EBF Z component nT 10. B_TOT magnetic field magnitude nT 11. EBF_X NEAR position EBF X component km 12. EBF_Y NEAR position EBF Y component km 13. EBF_Z NEAR position EBF Z component km   Files labeled nso10day_yyyy_dd0_dd1.jpg (yyyy indicate year, dd0 is the first day and dd1 is last day of a 10-day interval) provide summary plots of MAG data in NSO coordinates averaged down to 10-min resolution.  Name Description Unit ----- ----------- ---- BX magnetic field NSO X component nT BY magnetic field NSO Y component nT BZ magnetic field NSO Z component nT BTOT magnetic field magnitude nT THETA mag. field direction from NSO Z axis degree PHI mag. field azimuth about NSO Z azis degree   Files labeled nso_ebf_yyddd.jpg (yyddd indicates year and day) provide daily plots of Level 2 MAG data in both NSO and EBF coordinates.  Name Description Unit ----- ----------- ---- X magnetic field X component nT Y magnetic field Y component nT Z magnetic field Z component nT T magnetic field magnitude nT THETA mag. field direction from NSO Z axis degree PHI mag. field azimuth about NSO Z axis degree R NEAR distance from the Sun AU SElong NEAR longitude in solar ecliptic coord. degree SElat NEAR latitude from solar ecliptic coord. degree Sunward Exc arcsin(NSO_X/NSO_R) degree Clock Angle atan2(NSO_Y,NSO_Z) degree Eros lat NEAR latitude in EBF coordinates degree Eros long NEAR longitude in EBF coordinates degree Eros-NEAR Distance between NEAR and Eros km   Processing :  Raw data contained in the Level 1 MAG dataset were processed by applying a series of corrections known from pre-flight calibrations and removing spacecraft fields evaluated in-flight. A summary of the calibration process is provided in the MAG_CALIBRATION document and more detailed descriptions can be found in [LOHRETAL1997] and [ANDERSONETAL2001]. Formally, the correction process within the spacecraft (SC) coordinates can be expressed as  B_final_SC : [T1](B_Obs_SC - B_SC)  Where B_final_SC is the calibrated field, B_Obs_SC is the measured field (i.e., the raw Level 1 data converted to physical units incorporating parameters obtained in the pre-flight calibration), and B_SC is the total satellite field. The matrix [T1] represents the correction in sensor orientations and gain factors resulting from the in-flight calibration [ANDERSONETAL2001].  The pre-flight calibration evaluated the linearity, orthogonality and cross talk of the MAG sensor block for each axis and each dynamic range. This calibration provided absolute gain calibrations to 0.1% (0.5%) in the least (most) sensitive range and orientation to 1 arc minute [LOHRETAL1997].  The major challenge in producing science-quality Level 2 MAG data was evaluation of spacecraft fields [ANDERSONETAL2001]. There are several known sources of spacecraft fields and each source was evaluated using the extensive in-flight data. The following table summarizes the sources of spacecraft fields together with their approximate magnitude, characteristic time scale and type of signal, and mitigation approach.  Source Propulsion valve motion. Magnitude ~100nT. Time Scale Propulsion events. Type of Signal Discrete pairwise jumps - zero net change. Mitigation Approach Flag Propulsion events. Correcting data. during propulsion events not necessary.  Source Terminal board: all spacecraft loads. Magnitude 25 nT. Time Scale ~10 min (heaters), < 1 s (NLR). Type of Signal Variable baseline with discrete jumps. Mitigation Approach Field is directly proportional to load currents monitored in spacecraft engineering data. Loops measured during assembly.  Source Digital power system shunts. Magnitude 5 to 10 nT. Time Scale Hours to months. Type of Signal Discrete jumps. Mitigation Approach Ratio between digital shunt and total solar array current identifies shunt step. Field fixed in each level.  Source Analog power system shunts. Magnitude 5 nT. Time Scale minutes. Type of Signal Gradual variations with intermittent discrete jumps Mitigation Approach Field is proportional to analog current within each of segments in analog shunt current corresponding to the six analog shunt resistors on the spacecraft.  Source Solar Arrays. Magnitude 30 nT. Time Scale hours to months. Type of Signal Gradual shift with discrete jumps. Mitigation Approach Field is proportional to total solar array current. Independent of digital shunt level.  Source Momentum wheels. Magnitude 1 to 2 nT. Time Scale 0.2 to 10s Hz. usually > 0.5 Hz. Type of Signal Superposition of four sinusoids corresponding to rotation rate of four wheels. Mitigation Approach Benign - wheel speeds kept above 0.5 Hz & monitored in engineering data. Speeds < 0.5 Hz occur during maneuvers. MAG 0.5 Hz filter elimiates contamination. 1/s sampling allows additional filtering if needed.   Predictive correction algorithms (see Software below) were developed to evaluate these fields using the available spacecraft housekeeping data. The corrections applied in the production of Level 2 MAG data include:  1. Subtraction of constant offset fields. 2. Subtraction of magnetic field generated by the system terminal board. 3. Subtraction of magnetic field generated by the analog and digital power system shunts. 4. Subtraction of magnetic field generated by solar array. 5. Correction of stepwise fields due to on/off of sensor heaters.  The final Level 2 MAG data were generated by rotating the field vector from spacecraft (SC) to NSO and EBF coordinates using the appropriate rotation matrix [T2] derived from information available in the SPICE volume:  B_final : [T2]B_final_SC   Sampling Rate :  MAG data were sampled at a rate that varied by command between 0.01 and 20 samples/s. During Eros observations the nominal rate was 1 sample/s. Brief, 10 minute, periods of 20 sample/s data were taken as part of weekly calibration checkout sequences. During cruise operations the MAG sample rate was commanded to coarse values of 0.01 samples/s or 0.02 samples/s. During full MAG instrument checkout the instrument was commanded through the complete range of sample rates. Full checkouts were performed after launch activation, after turn-on following the deep space maneuver and Mathilde fly-by, and prior to Eros orbit insertion.  The full-time resolution data can be found in the Level 1 MAG volume. The sampling rate presented in the Level 2 science data files differs from the original rate since it was necessary to remove spacecraft fields using housekeeping data that were sampled differently. The spacecraft housekeeping data were 44 times more voluminous than the magnetometer data and were recorded at a maximum rate of one sample/s. In practice, the spacecraft telemetry and data allocations allowed sampling of housekeeping no more often than one sample every 10s and typically 50s to 600 s between samples. The data rate for valid science data therefore actually ranged from two to four orders of magnitude less than the capabilities of the magnetometer instrument [ANDERSONETAL2001].  It is important to note that the MAG science data sample rate varied, often irregularly. This is due to the sequence of the command and telemetry processor (CTP) - spacecraft housekeeping data. For the Level 2 science data, MAG samples that were nearest to CTP samples were selected in order to accurately remove time-dependent spacecraft fields. There are a variety of reasons for occasional irregular sampling of CTP data: operations commands, commands from other instruments, and spacecraft autonomy. All of the available CTP data were used to maximize the MAG science records.   Range Setting :  The magnetometer had 8 ranges covering full-scale ranges from plus/minus 4 nT (range 0) to 65536 nT (range 7). A 20-bit A/D conversion for these ranges resulted in resolutions of 1 pT (range 0, limited by intrinsic instrument noise) to 2.0 nT (range 7). Range control was done either manually or automatically. Automatic range control provided transition to less sensitive ranges when any one axis exceeded 87.5% of full scale for 0.25 s and shifted to a higher sensitivity when the field on all three axes fell below 17% of full-scale continuously for 1 min. This ensured that the instrument follow rapid increases in field strength without rapidly toggling between ranges. During cruise, the instrument was operated in range 3 (plus/minus 256 nT full scale, 0.008 nT resolution) [ANDERSONETAL2001]. The range setting is not provided in the Level 2 data files. All necessary range scale factors have been applied in Level 1 and Level 2 data files. The range information is contained in Level 1 MAG files submitted to PDS. The range indicator ACTUAL_RANGE can be found in the Level 1 MAG data files MAGyyddd.FIT, where yyddd indicates year and day of year.  The magnetometer operated in range 3 (256 nT full scale) for almost the entire mission. Exceptions occurred during Earth flyby when the instrument cycled through all ranges, during instrument calibration and checkout, and during periods when the spacecraft field was larger than the maximum for range 3.   Ancillary Data :  NEAR attitude and location and EROS location, which are necessary for MAG coordinate transformation, are included in separate SPICE volumes. In each of the SPICE volumes that are organized by mission phase, there are the /DATA/SPK directory for EROS and NEAR position information and the /DATA/CK directory for the NEAR attitude information.   Coordinate Systems :  Two coordinate systems are adopted for Level 2 MAG data presentation.  NEAR Sun Orbital (NSO) coordinates are Cartesian coordinates defined with respect to the NEAR-Sun line and the orbit plane of Eros. The X-axis points from NEAR to the Sun, the Z direction is given by the Eros velocity vector crossed into X, northward normal to the Eros orbital plane, and Y completes the right-handed set.  Eros Body Fixed (EBF) coordinate system has its Z-axis along the Eros spin axis, the X-axis is in the Eros equatorial plane, and lies in the direction of the prime meridian. The Y-axis completes the right-handed set.   Software :  Software for production of science-quality Level 2 data from the raw magnetometer data contained in the Level 1 data is provided in MAG_SW document in this volume. The software consists of two packages, SDC_PROC and SCI_PROC. The packages perform all data processing needed as the result of sensor calibration and evaluation of spacecraft fields as described above under the subheading of Processing.  The SDC_PROC codes were used for reading Hierarchical Data Format (HDF) data files produced by the Science Data Center (SDC), extracting the pertinent magnetometer, command telemetry processors (CTP), and housekeeping data, and producing plots and data files for the NEAR MAG experiment. The files generated by these codes are used as input to the second software package SCI_PROC.  The main functions of SCI_PROC are: open and read the input files, apply corrections to the data, and produce plots and ASCII data files.  The codes are found in the in the MAG_SW directory of the present volume.   Media/Format :  This data set is released as a CDROM set.

Review : This data set is currently undergoing external peer review within PDS in August, 2001.   Data Coverage and Quality :  Gaps in data coverage exist for many reasons including telemetry outage. Gaps are not filled with flagged data.  The magnetometer was calibrated using in-flight data acquired during the Earth swing-by of January 1997 and during simultaneous solar wind measurements by the WIND from January 22-24, 1997 [ANDERSONETAL2001]. The calibration during the Earth swing-by resulted in less than 50 nT residuals in the Earth's main field of > 20000 nT, corresponding to an accuracy better than 0.3 percent. As for the absolute accuracy, the final magnetometer data are accurate to ~1 nT. This corresponds to ~0.4 percent of the full scale of range 3, which is used during most of the NEAR mission.   Limitations :  Samples with unusually large values, producing spikes when plotted, occur in the Level 2 MAG data. These are caused either by internal MAG calibration signals or by firing of the spacecraft thruster (propulsion events). The calibration signals have a shape of a box-car with a magnitude of ~470 nT and a duration of ~300 seconds. During Eros operations these calibrations were performed weekly. The thruster-induced spikes have varying amplitudes and durations and can be easily distinguished from the calibration signals. They occur at times of spacecraft maneuvers or momentum management activities.  The following table lists the dates and approximate times of propulsion events on the NEAR spacecraft during the Eros orbital phase of the mission. Propulsion events are indicated by the mnemonic adopted by NEAR Mission Operations.  Event Year Day of year Time UTC Orbital change/description (hhmm)  OIM 2000 045 1530 Orbit insertion OCM1 2000 055 1700 to 365 X 204 km OCM2 2000 063 1800 to 204 x 200 km MCM1 2000 075 2000 Momentum management OCM3 2000 093 0205 to 210 x 100 km OCM4 2000 102 2110 to 100 x 100 km OCM5 2000 113 1730 to 101 x 50 km OCM6 2000 121 1615 to 51 x 49 km MCM3 2000 131 1400 Momentum management MCM4 2000 138 1300 Momentum management MCM5 2000 145 1300 Momentum management MCM6 2000 152 1600 Momentum management MCM7 2000 159 1600 Momentum management MCM8 2000 166 1600 Momentum management MCM9 2000 173 1600 Momentum management MCM10 2000 180 1600 Momentum management OCM7 2000 189 1800 to 50 x 35 km OCM8 2000 196 0300 to 37 x 35 km OCM9 2000 206 1700 to 50 x 37 km OCM10 2000 213 1930 to 51 x 49 km OCM11 2000 221 2325 retrograde inclination adjust OCM12 2000 239 2325 to 100 x 50 km OCM13 2000 249 2300 to 100 x 100 km MCM12 2000 264 1700 Momentum management OCM14 2000 287 0545 to 98 x 50 km OCM15 2000 294 2140 to 50 x 50 km OCM16 2000 299 2210 to 50 x 19 km OCM17 2000 300 1740 to 198 x 67 km OCM18 2000 308 0700 to 198 x 195 km MCM13 2000 320 2000 Momentum management MCM14 2000 334 1730 Momentum management OCM19 2000 342 1535 to 196 x 35 km OCM20 2000 348 2015 to 36 x 34 MCM15 2000 362 1925 Momentum management MCM16 2001 009 1840 Momentum management OCM21 2001 024 1605 to 35 x 21 km OCM22 2001 028 0120 to 35 x 19 km OCM23 2001 028 1810 to 36 x 34 OCM24 2001 033 0851 Preparation for end of mission OCM25 2001 037 1743 Preparation for end of mission EMM1 2001 043 1514 End of mission - descent EMM2 2001 043 1858 End of mission - descent EMM3 2001 043 1914 End of mission - descent EMM4 2001 043 1930 End of mission - descent EMM5 2001 043 1941 End of mission - descent