VOYAGER MAGNETOMETER EDR PROCESSING SOFTWARE

Introduction

The purpose of the Voyager Magnetometer EDR Production System (VOYPROD) is to take raw experiment data from the Voyagers' magnetometers, decode it, and perform limited data analyses producing high quality summary or detail data that may be used for display or extended analysis. Voyager processing is currently performed on a DEC Alpha workstation running the OpenVMS operating system. VOYPROD processes Voyager magnetometer (MAG) instrument Experimental Data Records (EDRs) and Supplemental Experimental Data Records (SEDRs) to produce either a 48 second Summary Data Product or a Detail Data Product.

Background

VOYPROD decommutates EDRs to obtain integer magnetometer counts which are converted to units of magnetic field strength (gammas) by subtracting an offset value (zeros) and multiplying the resulting difference by the sensitivity value of the instrument as determined by the range setting in effect. VOYPROD uses SEDRs to provide navigation and pointing vector information necessary to transform decommutated magnetic field data into coordinate systems other than the onboard payload oriented system.

VOYPROD is currently capable of processing EDR's containing data from the following telemetry modes:

GS-x (encounter modes)
CR-1 (cruise mode)
CR-2 (cruise mode)
CR-3 (cruise mode)
CR-4 (cruise mode)
CR-5 (cruise mode)
CR-6 (cruise mode)
VIM-5 (current cruise mode)

The above modes represent all of the differring MAG telemetry modes used throughout the Voyagers' mission.

VOYPROD produces data based in either payload (PL), heliographic (HG) or planetary (S3=Jupiter, L1=Saturn, U1=Uranus, N1=Neptune) system coordinates. Data may be processed with an option to filter noise spikes using a population filter.

Processing

The following steps outline the basic sequence of Voyager EDR processing:

Read EDR
Unpack header block (rec. id, s/c id, tel. mode, FDS counts, data flags)
Convert selected time tags to integer time (yy/ddd/hh:mm:ss.fff)
Unpack sub-header block (MAG status words, plasma data)
Unpack science block (MAG counts)
Convert counts to gammas
Apply sensor and boom alignment matrices
Filter detail data based on standard deviation (optional)
Rotate (optional) detail gammas (Detail Data only) OR
Rotate (optional) 1.92 second averages while averaging detail gammas to create 1.92, 9.6 and 48 second averages (Summary only)
Write Summary or Detail record

Data Components

The Voyager MAG EDR is composed of three major blocks, the header, the sub-header or plasma block, and the science block containing MAG counts.

Header block

The header block provides basic information necessary to properly interpret the science and engineering data that follows it. Spacecraft identification, time tags, telemetry configuration, data status and quality are some of the parameters contained within the header block. Most fields within the header block are statically defined for the whole of the mission. An exception to this rule are the fields indicating data presence. These fields change size and format between modes.

Sub-header block

The EDR sub-header block contains MAG and plasma (PLS) command words, the MAG status words, and plasma data. MAG and PLS command words reflect the most recently commanded instrument states. The MAG status words provide information regarding operating state of the MAG instruments. These parameters include range, mode and calibration settings. Values extracted from MAG status words are used in production to map the correct sensitivity range to digital magnetometer values. Plasma data consist of eight bit words. They are unpacked from the sub-header block but are not employed in the MAG science processing.

Science block

The MAG EDR science block contains digital magnetometer counts. Counts are measured onboard using 12 bit words that may represent values ranging from 0-4096. Integer counts are converted to magnetic field units (gammas) by subtracting a zero offset, nominally 2048, from the measured MAG value and multiplying this difference by the sensitivity of the instrument. Counts are stored using a number of differring format methods dependent upon the telemetry mode employed. There are four basic format types used for recording science data; direct readout, averaged, delta modulated, and differenced.

Telemetry schemes

Direct Readout

Direct readout is the simplest method of representing MAG science data. The direct readout format contains a stream of 12 bit full word MAG counts that are representative of the data rate of the MAG instrument itself. The MAG instrument has a .06 second per sample data rate and thus direct readout data provides this level of resolution. The CR-1 mode is the only mode employing the direct telemetry format.

Averaged

Averaged data is written in a format similar to that of direct readout data. Detail data from the MAG instrument are averaged over a mode dependent interval then written as 12 bit full words. The CR-2 and CR-3 modes employ averaged formatting of MAG counts.

Delta modulated

The delta modulation scheme compacts the data stream by placing a number of 2 bit delta modulated words in between 12 bit reference full words. The 2 bit delta modulated words can be reconstructed to form 12 bit predicted values which are then fitted to the level of the reference words using a shift and pivot method. The CR-5 and CR-6 modes employ the delta modulation scheme of telemetry formatting.

Differenced

The differencing scheme compacts the data stream by placing a number of 6 bit difference counts in between 12 bit reference full words. The 6 bit difference words represent the arithmetic difference between a current MAG count and the prior count from the same instrument axis. Full words are reconstructed by adding accumulated difference words to the last full reference word encountered. The GS-x, CR-4 and VIM-5 modes employ difference formatting.

Data Products

Summary Data

Voyager summary data files provide a high resolution averaged magnetic field data in 48 second/9.6 second/1.92 second resolutions independent of the instrument telemetry mode and data sampling rate. A 400 byte header record is appended to the front and back of the summary data file describing the processing parameters used in creating the summary data product. These parameters include coordinate system (ie, payload, heliographic, planetary), single MAG or dual MAG, time tag type (spacecraft vs. event time), data filtering options, and other processing options. The 2272 byte length magnetic field records contain a 48 second interval of MAG data. The 48 second interval is generated by creating 1.92 second averages of detail data that are used to produce 9.6 second averages which are then averaged to create a single 48 second average for the summary file. All of these averages, 1.92 second, 9.6 second, and 48 second are contained in one summary file record. At the head of the summary record is a header block containing a data time tag, telemetry format, data identifier and further descriptions of the data. At the back of the summary record is a SEDR block that holds navigation information if the field data has been rotated out of payload coordinates.

Detail Data

Voyager detail data are written directly from the highest resolution data produced by the VOYPROD system. The resolution of this data is dependent upon the the telemetry mode being processed. Resolutions of data from the various telemetry modes are as follows:

     Mode    Primary  Secondary
              rate       rate
              (sec)      (sec)

     GS-x     0.06       0.12
     CR-1     0.06       0.06
     CR-2     0.12       0.24
     CR-3     0.24       0.96
     CR-4     0.30       2.40
     CR-5     0.24       0.96
     CR-6     1.92       3.84
     VIM-5    0.48      24.00