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LP MAGER SPINAVG MAGNETIC FIELD LUNAR COORDS 5SEC V1.0 (PDS)

NSSDCA ID: PSFP-00279

Availability: Archived at NSSDC, accessible from elsewhere

Description

This description was generated automatically using input from the Planetary Data System. Overview: ======== Magnetometer data records are time-ordered series of magnetic vector measurements. Each record consists of a time tag followed by six scalar values representing the magnetic field vector, measured in nanoteslas, in two different coordinate systems: selenocentric solar ecliptic (SSE) and body-fixed selenographic (SEL), followed by the rms deviation of the field magnitude, which is independent of the coordinate system. The spacecraft position is given in both of the above coordinate systems. These data are obtained continuously at 9 Hz and are averaged in 5-second intervals for this data archive. Parameters: ========== Magnetic field data are provided in units of nanotesla (nT). Processing: ========== Magnetic field data are sampled onboard at 18 Hz and averaged to 9 Hz before being placed into telemetry. In order to cover a very large dynamic range with 12-bit values, the full range of the instrument is divided into 8 sub-ranges. Range changing is performed dynamically onboard based on the ambient field strength. The first step in the processing is to extract the data from telemetry and form time-tagged magnetic field vectors. Then, occasional data spikes due to range changes or bad telemetry are flagged; these data points are not used in the offset determination. The instrumental offsets for Bx and By are calculated simultaneously for 1 minute data windows using a technique developed by M. Acuna. Time varying offsets are needed because the offsets drift slightly as the magnetometer temperature changes. (The MAG temperature is modulated as the spacecraft goes in and out of the Moon's shadow.) A constant offset is used for the Z component, because lack of spin modulation in that component precludes routine offset determination. Offsets are calculated separately for each data range and subtracted from the data. This correction reduces systematic errors in the X and Y components to less than ~0.1 nT. Since the Z component cannot be corrected in this way, systematic errors in that axis (due to temperature-induced offset drifts) can be as large as 0.5 nT. Instrumental gains, different for each of the 8 ranges, are then applied to convert to nanotesla. Next, the data are corrected for a slight misalignment between the magnetometer sensor axes and the spacecraft axes. The resulting sensor (SEN) coordinate system has its Z-axis parallel to the spacecraft spin axis, and its X-axis aligned with the magnetometer boom. The SEN coordinate system rotates as the spacecraft spins. The next step is to flag spurious data values. The first measurement following a range change is flagged, since the finite time needed to make the change often corrupts the first measurement in the new range. Other false spikes also appear in the data, most of which are attributable to occasional noise in the telemetry. A comparison technique is used to remove outliers. In practice, this effectively removes most spurious data values without eliminating any valid data. The next step is to ''despin'' the data from SEN to ''despun spacecraft'' (SCD) coordinates, which are defined such that the Z-axis is parallel to the spacecraft spin vector, and the direction of the sun is in the half-plane defined by X > 0, Y = 0. Despinning is performed using the reconstructed sunpulse data, which are corrected for spacecraft spin-up in the Moon's shadow. Next, the data are averaged in 5-second intervals; this reduces the data volume by a factor of 45. Finally, a rotation is performed from SCD coordinates to selenocentric solar ecliptic (SSE) and body-fixed selenocentric (SEL) coordinates using the spacecraft ephemeris data (the latitude and longitude of the spin axis obtained from files included in the PDS distribution of the LP Level-1 magnetic field data) and lunar ephemeris data obtained from the Jet Propulsion Laboratory's Horizons system (<http://ssd.jpl.nasa.gov/horizons.html>). SSE coordinates are defined such that the X-axis points from the center of the Moon to the center of the Sun, the Z-axis is parallel to Earth's ecliptic north, and the Y-axis completes the right-handed coordinate system. SEL coordinates are defined such that the Z-axis is parallel to the Moon's spin vector (north pole) and the X and Y axes intersect the lunar equator. The X-axis intersects the lunar equator at 0 degrees longitude, and is thus nearly aligned with the Moon-Earth line. (It is not exactly aligned because of the Moon's libration.) The Y-axis intersects the lunar equator at 90 degrees EAST longitude: SEL coordinates are right handed. Media/Format: ============ Data are archived on CDROMs in level 1 compliance with the ISO 9660 standard. Three CDROMs cover the entire mission. The data are provided as ASCII ''tables'' of 1-day duration in Selenocentric Solar Ecliptic (SSE) and Selenographic (SEL) coordinates. Date/time are given in 2 formats as described below. MAG Data: Naming convention: MAyymmdd.TAB Parameters: 1) time parameter 1: PDS date-time format of the mid-time of the 5-sec averaging window in spacecraft event time, i.e., Universal Time at the spacecraft. Example: 1998-11-08T05:50:42.5 2) time parameter 2: decimal day of the mid-time of the 5-sec averaging window in spacecraft event time, i.e., Universal Time at the spacecraft. 3) mag_field_SEL: Array[3] giving B-field components (nT) in the SEL coordinate system 4) mag_field_SSE: Array[3] giving B-field components (nT) in the SSE coordinate system 5) mag_field_RMS: RMS deviation (nT) of the field magnitude. Provides an indication of field variability for the 5-sec window. 6) Spacecraft SEL coordinates: coordinate array[3] (km) 7) Spacecraft SSE coordinates: coordinate array[3] (km) 8) ISUN - a parameter from the sunpulse file indicating whether the spacecraft is in the sun (0), in eclipse (1), or, if the sunpulse file was not available, the data were processed using the less accurate determination of the sunpulse time in the Level-0 data file and ISUN is set to 2. Note that this parameter contains erroneous values (0 <-> 1) from time to time. To reduce the occurrence of bad ISUM values the parameter was median-filtered with a 9-point window, which removes most of the errors. These parameters could be named PDS_time, decimal_day, Bx_sel, By_sel, Bz_sel, Bx_sse, By_sse, Bz_sse, B_rms, x_sel, y_sel, z_sel, x_sse, y_sse, z_sse, isun An appropriate format for reading the data is: format='( A21, f12.6, f9.3, 2(f8.3), f9.3, 2(f8.3), f9.3, f10.2, 2(f9.2), f10.2, 2(f9.2), I3 )' however, the records contain blanks between each parameters so that a format statement will not be required by most languages. Ancillary Data: ============== There are several ancillary data files provided with this archive. These include: Spacecraft Attitude data LP-L-ENG-6-ATTITUDE-V1.0 Spacecraft Ephemeris data LP-L-6-EPHEMERIS-V1.0 Spacecraft Position data LP-L-6-POSITION-V1.0 Spacecraft Command logs LP-L-ENG-6-COMMAND-V1.0 These data sets provide additional information about the state of the spacecraft and the instrument during data acquisition that may aid in the scientific analysis of this data set. Coordinate Systems: ================== The SSE coordinate system has its X-axis along the Moon-Sun line, positive towards the Sun. The Z-axis is parallel to the northward normal to the Earth's ecliptic plane, and Y completes the right-handed set. The SEL coordinate system used here is a Cartesian representation that places the Z-axis along the rotation axis of the moon, positive in the direction of angular momentum. The X-axis lies in the lunar equatorial plane at 0 degrees longitude, and Y completes the right-handed set. Software: ======== There are no software provided with this data archive.

Alternate Names

  • LP-L-MAG-4-SUMM-LUNARCRDS-5SEC-V1.0

Discipline

  • Planetary Science: Fields and Particles

Additional Information

Spacecraft

Experiments

Questions and comments about this data collection can be directed to: Dr. David R. Williams

 

Personnel

NameRoleOriginal AffiliationE-mail
Dr. Mario H. AcunaData Provider
Dr. Mario H. AcunaGeneral Contact
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