| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains magnetic-field data acquired during the
cruise and tour phases of the Cassini mission to Saturn. The data
set begins with data collected on 16 August (day 228), 1999 and ends
on 17 November (day 321), 2005 when the instrument failed.
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
data set.
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 data set contains the magnetic-
field and calibration data recorded by the helium magnetometer when
operating in scalar mode. 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 SHM data
products which can be identified from the 'SHM*_C' in their file
names. The SHM submitted is already calibrated.
Data are received from Cassini in science or housekeeping telemetry
packets. Data from both these streams form part of this data set.
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 nano-Teslas.
The MAG data products in this dataset is:
Data Description
SHM_C Calibrated scalar data from helium magnetometer
The format of files containing this data product is described in
full in the archive volume SIS, found on this volume called the
'THE CASSINI MAGNETIC FIELD INVESTIGATION' by Dougherty et al.
Data
====
SHM data is given in the following columns:
1. time TAI (SCET in seconds since 12:00:00.000Z 1 Jan 2000)
2. B_SHM (in nT)
3. X_IAU_S (in Km)
4. Y_IAU_S (in Km)
5. Z_IAU_S (in Km)
Data Parameters
===============
Magnetic-field units
--------------------
The units of the magnetic-field data are given as
nanoTesla (nT). They represent the magnetic field values at
the given time.
MAGStatus
---------
There are no status words in the calibrated files
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 scalar values in MAG data files
are SCET in seconds since epoch 2000 in TAI (International Atomic Time)
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
Processing
==========
The processing software converts time from SCLK(1958) to TAI, subtracts
a small calibration correction from the scalar field values, and appends
XYZ position of Cassini with respect to either Earth or Saturn in
IAU_EARTH or IAU_SATURN coordinate systems respectively. Only valid
Scalar data points (LOCK bit = TRUE) are written to the output flatfile.
A delay of 0.7s is subtracted from each time tag, 0.5s because the input
time stamp is the end of the 1s interval over which the measurement is
made, and an additional 0.2s determined from a fit of ESB data to the
model field. The additional 0.2s delay is subject to revision. In order
to calculate the calibration correction to the SHM value of B, the
approximate angles of the field are needed, which can be obtained from
the FGM. So for each SHM value, an FGM vector avg is calculated over
[t-1sec,t], where t is the SHM time. The FGM vector is then transformed
into the SHM/VHM sensor coordinate system (nominal deployed orientations
are used). A total correction, including a Bloch-Siegert correction, is
then calculated.
Ancillary data
==============
Data such as analog, command validation, configuration image,
error counter and user-defined engineering data, are included
in this archive for completeness and are not required for
processing the magnetic-field data.
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