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Availability: Archived at NSSDC, accessible from elsewhere


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

Data Set Overview

This data set contains magnetic field vectors acquired by the Galileo magnetometer before and during inbound shock and magnetopause crossings, the Io flyby on Dec 7 (341), 1995, and the part of the J0 orbital cruise. The data were acquired in the optimal averager mode of the instrument. These are raw data provided in their original EDR form. Processing done internal to the instrument remains in the data set.


The optimal averager mode of the magnetometer samples data by applying a recursive filter and decimate procedure to data that have been internally processed to correct for geometry, gains and zero levels. The data are then despun and filtered (see [KIVELSONETAL1992]). The magnetometer recursive filter has the form: B(t1-out) = [1/x] * B(t1-in) + [(x-1)/x] * B(t0) where B(t1-out) is the current sample after filtering, B(t1-in) is the current unfiltered sample, B(t0) is the previously filtered sample, and x is the filter parameter which is limited to powers of 2. The filtered data are then decimated on even increments of the spacecraft major frame counter (RIM). One RIM is approximately 60.667 seconds. Samples are taken at the end of an averaging interval. Comparison of optimal averager and higher time resolution data indicates that the time tag should be shifted back approximately 1/3 of a sample interval, towards the center of the input interval. Zero levels: These data have been averaged over several spin cycles. Any error in the onboard estimates of the zero levels would appear as a spin modulation in the spin plane sensor data prior to averaging. The averaging process removes any residual spin modulation correcting for any errors in the zero level estimates. The spin axis aligned sensor typically has a second zero level correction applied in the ground processing.


The data are stored in a rather complex and compact binary form called an EDR (Experimenter Data Record). The details of where all of the bits are located are contained in Galileo project documents (3-280, 3-640).

Ancillary Data

In order to rotate the MAG data into geophysical coordinate systems, several angles from the Attitude and Articulation Control Subsystem (AACS) are needed. The raw AACS EDRs are included as ancillary data to magnetometer data. In addition, in order to conserve bits to ground, frequently only the vector data were returned to Earth in the memory readouts. The instrument status words that contain the timing of the on commands are not always present in the data files. Also, there are times when the instrument was commanded to restart data acquisition before the time words were read out to reduce gaps in the data. In these instances, the time tag in the instrument status words is incorrect. For these reasons, the 'as run' spacecraft events files (SEF) which integrate all spacecraft commands are included as ancillary data. Command 35AV turns on the optimal averager at the next RIM boundary. Command 35DML loads parameters (like the optimal averager filter and decimate constants) directly into MAG memory. The following table lists the decimation rate (RIMs), filter constant (HEX), decimate constant (HEX), and corner frequency (Hz) for each of the optimal averager configurations. Rate filter decimate corner freq -------------------------------------1 0400 0000 1/134 2 0200 0001 1/268 4 0100 0003 1/536 8 0080 0007 1/1072 16 0040 000F 1/2144 32 0020 001F 1/4289 64 0010 003F 1/8679 128 0008 007F 1/17158 256 0004 00FF 1/34316 512 0002 01FF 1/68632 1024 0001 03FF 1/137264 A typical command pair looks like: 03203292:61:0 95-337/14:42:04.066 CMD,35DML,20LE4A, PRI,95-337/14:42:04.066,4766,00,80,00,07; << DIRECT MEMORY LOAD >>; 03203292:67:0 95-337/14:42:08.066 CMD,35AV,20LE4B, PRI,95-337/14:42:08.066; << OPTIMAL AVERAGER: ON >>; This pair configured the instrument to take 8 RIM averages with the 'on' command executing at RIM 03203293.

Coordinate System

The data are provided in spacecraft (IRC) coordinates. This coordinate system is roughly X parallel to the southward ecliptic normal, Z anti-earthward, and Y completing the righthanded set.

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

Alternate Names



  • Planetary Science: Fields and Particles

Additional Information



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



NameRoleOriginal AffiliationE-mail
Dr. Margaret Galland KivelsonData ProviderUniversity of California, Los
Dr. Margaret Galland KivelsonGeneral ContactUniversity of California, Los
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