Cassini Magnetometer Raw Bundle
Cassini MAG Vector Helium Magnetic-Field Data at the Science Rate
Collection Description
PDS3_DATA_SET_ID = CO-E/SW/J/S-MAG-2-REDR-RAW-DATA-V2.0
PDS3_DATA_SET_RELEASE_DATE = 2019-05-16
START_TIME = 1998-12-30T11:45:23.275
STOP_TIME = 2006-01-01T00:00:04.957
PRODUCER_FULL_NAME = PETER SLOOTWEG
Collection Overview
=================
This collection contains raw magnetic-field data at the full science data
rate acquired during the cruise and tour phases of the Cassini mission to
Saturn. The collection begins with data collected on 30 December
(day 364), 1998. On 16 August, 1999 two days before the spacecraft
commenced Earth swingby, the magnetometer boom was unfurled into its
extended position; a configuration it maintained for the remainder
of the mission. Magnetic-field data acquired prior to boom deployment
are limited and are significantly affected by spacecraft noise. These
data are of no scientific value and, consequently, do not form part
of this collection.
Cassini carried two magnetometers: a fluxgate magnetometer (FGM)
and a vector-helium magnetometer capable of operating in both
vector and scalar mode (V/SHM). This collection contains the magnetic-
field data recorded by the vector-helium magnetometer (VHM). The data
are divided into time periods of one day and saved in files using the
UCLA flatfile system. In this system, data are recorded in binary files
that have associated text header files describing the format and content
of the data.
The magnetic-field data are contained in the VHM data products which can
be identified from the 'mrdcd' in their file names. The VHM data are raw
data which may be calibrated using the Cal02 software package provided in
the document collection of the cassini-mag-cal bundle and the calibration
files in the calibration collection of the cassini-mag-raw bundle.
The calibrated data may optionally be transformed into a range of
coordinate systems (see below) using the TransCal source code in
conjunction with SPICE kernels available from the Navigation and
Ancillary Information Facility (NAIF). Spacecraft attitude data
contained in the CHATT data files, in the data-att collection, which
can be identified from the 'ecdcd' in their file names may be used in
lieu of the SPICE C kernel.
Data are received from Cassini in science or housekeeping telemetry
packets. Data from the science stream form part of this collection. The
naming convention used for data files allows the telemetry source and
date of acquisition to be readily determined from the file name. On any
given date, the science and housekeeping data for a particular
magnetometer cover the same time interval to within a few seconds.
Science and housekeeping data files have identical formats and are
processed in exactly the same way.
The magnetic-field data have been extracted from telemetry
packets, assigned time tags, and formatted into simple binary tables
of values in engineering units (data numbers). The data are provided
in sensor coordinates. Software source code, in the document collection,
of the cassini-mag-cal bundle, may be used to convert these data
to nanoTesla and tranform them into alternative time and coordinate
systems.
The full set of MAG data products is
Data Description
VHM Vector data from helium magnetometer
The format of files containing these data products is described in full
in the volume SIS, found in the document-mag collection, in the
Cassini bundle, called the 'THE CASSINI MAGNETIC FIELD INVESTIGATION'
by Dougherty et al.
Data
====
Each row in the VHM/FGM binary tables is composed of the following
columns:
1. time (in SCLK counts since 00:00:00.000Z 1 Jan 1958)
2. X_VHM (in engineering units)
3. Y_VHM (in engineering units)
4. Z_VHM (in engineering units)
5. MAGStatus
6. VHMStatus
Data Parameters
===============
Magnetic-field units
--------------------
The units of the preprocessed magnetic-field data are given as
nanoTesla (nT). They represent the X, Y, and Z sensor values at
the given time. These values were converted into nT using the
following:
VHM: nT = (value - 8192) / 8192 * factor.
The value of the factor is determined from the range given with
each vector using the following table:
Range FGM Factor VHM Factor
0 40 32
1 400 256
2 10000 -
3 44000 -
MAGStatus
---------
The MAGStatus data are an array of bits that describe the status
of the MAG equipment, as set out in the following table.
FIELD SIZE NAME BYTE BIT
PacketType 1 Housekeeping/Science-data flag 0 7 MSB
SCAS 1 SCAS status 0 6
AverageType 1 Average Type (fixed/running) 0 5
SHMFlag 1 SHM Flag 0 4
VHMFlag 1 VHM Flag 0 3
FGMFlag 1 FGM Flag 0 2
ADCFlag 1 ADC Flag 0 1
MCI 9 Measurement Cycle Interrupt 0-1 0,7-0 LSB,MSB
Average 5 TimeCode Missing 2 2
sparebits 2 spare two bits 2 1-0
BIU Discretes 3-4 8
PROM 1 PROM program 3 7 MSB
ConfigEnable 1 Config-Enable 3 6
PSU_2 1 PSU 2 3 5
PSU_1 1 PSU 1 3 4
Processor_B 1 Processor B 3 3
Processor_A 1 Processor A 3 2
SleepMode 1 Sleep Mode 3 1
Reset 1 Reset 3 0 LSB
VHMStatus
---------
The VHMStatus data are an array of bits that describe the status
of the VHM, as set out in the following table.
FIELD SIZE NAME BYTE BIT
rg 1 Range (debugging only) 0 7 MSB
TimeStatus 4 time quality status 0 6-3
sparebits 2 spare bits 0 2-0 LSB
Digital 8 Digital Status word 1 7-0
CalibId 8 Calibration Id 2 7-0
CoordId 8 Field Angle data 3 7-0
MAG times
---------
The Cassini spacecraft clock (SCLK) is a counter that advances by
one tick nominally every 1/256 seconds. SCLK times have the format
cccc:ttt, in which cccc specifies the number of full counts that
have elapsed (one full count = 256 ticks), and ttt indicates by how
many ticks the clock has advanced towards the next count, since the
epoch 00:00:00Z 1 January 1958.
SCLK counts may also include a partition number, p/cccc:ttt.
This number is initially 1 but is incremented during the mission if
the SCLK counter is reset or somehow interrupted or altered. The
following discussion assumes a partition number of 1. For other
partition numbers, the determination of SCLK times requires
knowledge of the time at which the current partition was initiated.
SCLK times are commonly recorded in MAG files as decimal
counts. Time may also be represented in MAG files as Spacecraft
Event Time (SCET) which, for Cassini, is Universal Time Coordinated
(UTC). The relationship between SCLK and SCET/UTC is dependent on
the count rate of the Cassini SCLK. Like most counter-based clocks,
this rate is not constant but drifts with time. Consequently,
conversion of SCLK times to SCET/UTC times requires knowledge of the
drift rates. These rates are recorded in the SCLK/SCET coefficients
file maintained by the Cassini Spacecraft Operations (SCO) team at
JPL. As the Cassini mission progresses, the difference between SCLK
and SCET will typically be of order tens of minutes.
Times in MAG data files
The times associated with magnetic-field vectors in MAG data files
are SCLK counts since epoch 1958.
Times in MAG header files
FIRST TIME SCLK time of first record in data file;
derived from primary header of CHDO file
LAST TIME SCLK time of last record in data file; derived
from primary header of CHDO file
SCLK (in ABSTRACT) SCLK count obtained from tertiary
header of CHDO file; also converted into year, day of year, month,
date, time format; may differ from FIRST TIME by some minutes
SCET (in ABSTRACT) year, day of year, month, date, time
format; determined from corrected SCLK count; also converted into an
equivalent SCET count of seconds since 1958
Times in MAG label files
START_TIME
SCLK time of first record in data file; obtained from FIRST TIME in
flatfile header
STOP_TIME
SCLK time of last record in data file; obtained from LAST TIME in
flatfile header
SPACECRAFT_CLOCK_START_COUNT
SCLK time of first record in data file; determined from SPICE utility
CHRONOS using START_TIME; format p/ssss.ttt
SPACECRAFT_CLOCK_STOP_COUNT
SCLK time of last record in data file; determined from SPICE utility
CHRONOS using STOP_TIME; format p/ssss.ttt
SCLK (in NOTE)
SCLK count obtained from tertiary header of CHDO file; also converted
into year, day of year, month, date, time format; obtained from
flatfile-header ABSTRACT; may differ from START_TIME by some minutes
SCET (in NOTE)
year, day of year, month, date, time format; determined from
corrected SCLK time; also converted into an equivalent SCET count of
seconds since 1958; obtained from flatfile-header ABSTRACT
Preprocessing
=============
MAG data are raw data that have been preprocessed to extract them
from the telemetry packets received from the spacecraft. This
preprocessing converted raw voltages into magnetic-field
measurements with engineering units (nT-like) and assigned
individual times to each field measurement, the time of the first
vector in a packet corresponds to the start time of the packet and
the other vectors are calculated from that, for more information see
the 'Confidence Level Overview' in this file. Software source code,
that converts these data to calibrated nT and transforms them to
different time and coordinate systems is included in the document
collection of the cassini-mag-cal bundle. This software may or may not
compile using modern compilers.
Software
========
The software required to calibrate and transform MAG data into
alternative time and coordinate systems is provided as source code
for compilation on other platforms. This software may or may not
compile using modern compilers.
The packages are
Cal02 - calibrates vector data and transforms them from sensor
coordinates to spacecraft coordinates, and,
TransCal - converts SCLK counts into International Atomic Time,
transforms vector data from spacecraft coordinates
into alternative coordinate systems, and appends
spacecraft trajectory data to transformed data
records.
User guides giving full descriptions of the implementation
and operation of these software packages are contained in the
document collection of the cassini-mag-cal bundle. They are
called the 'Cassini MAG Data Processing Software: CAL02 User Guide'
and 'Cassini MAG Data Processing Software: TransCal User Guide' both
by Joyce Wolf of JPL.
Confidence Level Overview
=========================
The magnetic-field data that comprise this collection are raw data
derived directly from telemetry packets.
There is irregular timing in the data samples. This is
caused by the limited resolution of the msec counter in the time
field. For example, there are 128 FGM vectors in each science
packet, the time of the packet corresponds to the time of the first
vector in the packet. Times are calculated for the other vectors
using the known onboard vector sample rate and average exponent.
These calculated times don't have the reduced resolution of the msec
counter. Thus at the first vector of each packet there can be a
small time jump due to the msec counter resolution. This time
difference is maximum 8 msecs (not 4msecs which is the resolution
of the counter) because the DPU software only generates even msec
values due to internal truncation.
Data Coverage and Quality
-------------------------
There are routine events that cause data quality problems so
chronological listings of them have been included in the document-mag
collection, of the Cassini bundle:
-calibration activities superimpose calibration data on top of
science data and are documented in SCAS_TIMES.ASC (for more
information on calibration activies refer to 'THE CASSINI
MAGNETIC FIELD INVESTIGATION' by Dougherty et al in the document-mag
collection, located in the Cassini bundle.)
-data spikes may be seen at instrument range changes. Range
changes are documented in RANGE_CHANGES.ASC
-mode changes affect which sensors produce instrument data and
spikes or rapid changes in data averaging may be seen at mode
changes. The first few packets after the instrument is unmuted
or after Science Packets recommence can be highly or incorrectly
averaged. MODE_CHANGES.ASC lists the times of these events.
-date corruption of the on-board memory (SSR) shown through spurious
range changes lasting only one vector. The times of these corrupted
vectors are listed in SPURIOUS_RANGE_CHANGES.ASC
Data gaps may be instrument related (e.g. a sensor turning
on/off) or mission related (e.g. telemetry downlink problems). The
former are documented in MODE_CHANGES.ASC. All mission related gaps
are listed in GAP_FILEs - GAP_FILE_SCI_HK.ASC for science data in
the housekeeping packets and in GAP_FILE_SCI_SD.ASC for science data
in the science packets. Instrument related gaps of less than one day
also show up in the GAP_FILES as well as in MODE_CHANGES.ASC
Most of the information in the GAP_FILEs are extracted from
packets received reports and so follow a fixed-width tabular format.
Multi-day gaps are not documented in the source listings and have
been added manually with a different format including an explanation
of the gap - if a reason is known. A summary of scientifically
significant gaps is included in the data-coverage-gaps.txt file.
Gaps
----
Please note that the different phases of the mission are
described in the Cassini Mission Description, which is available in the
Cassini bundle, or from PDS.
References
==========
Asmar, S.W., and N.A. Renzetti, The Deep Space Network as an Instrument
for Radio Science Research, Jet Propulsion Laboratory Publication
80-93, Rev.1, 15 April 1993.
Cassini Mission Plan, Revision N (PD 699-100), JPL Document D-5564,
Jet Propulsion Laboratory, Pasadena, CA, 2002.
Dougherty, M.K., S. Kellock, D.J. Southwood, A. Balogh, E.J. Smith,
B.T. Tsurutani, B. Gerlach, K.H. Glassmeier, F. Gleim, C.T. Russell,
G. Erdos, F.M. Neubauer, and S.W.H. Cowley, The Cassini Magnetic
Field Investigation, Space Science Reviews, Vol. 114, Nos. 1-4,
pp. 331-383, September 2004
Dougherty, M.K., S. Kellock, A.P. Slootweg, and N. Achilleos,
CO-E/SW/J/S-MAG-2-REDR-RAW-DATA-V2.0, CASSINI MAGNETOMETER RAW DATA
V2.0, NASA Planetary Data System, 2019.
Kellock, S., P. Austin, A. Balogh, B. Gerlach, R. Marquedant, G. Musmann,
E. Smith, D. Southwood and S. Szalai, Cassini dual technique
magnetometer instrument (MAG), Proc. SPIE, Denver, Colorado, 2803,
141, 1996.
Smith, E.J., M.K. Dougherty, C.T. Russell, and D.J. Southwood, Scalar
helium magnetometer observations at Cassini Earth swing-by, J.
Geophys. Res., 106, 30129, 2001.
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