| DATA_SET_DESCRIPTION |
Data Set Overview
=================
The data set consists of calibrated observations. The MAG measures
the vector magnetic field.
There are three principal coordinate systems used to represent the data
in this archive. The SE coordinate system is a Spacecraft- Solar
equatorial system and it will be used for cruise data only. The
sun-state (ss) and planetocentric (pc) will be used for Earth Fly By
(EFB) and Jupiter orbital data. Cartesian representations are used for
all three coordinate systems. These coordinate systems are specified
relative to a 'target body' which may be any solar system object (but
for this orbital operations will Jupiter). In what follows we will
reference Jupiter as the target body, but, for example, if observations
near a satellite (such as Io) are desired in Io-centric coordinates,
the satellite Io may be specified as the target body.
The SE coordinate system is defined using the sun-spacecraft vector
as the primary reference vector; sun's rotation axis as the secondary
reference vector (z). The x axis lies along the sun-spacecraft
vector, the z axis is in the plane defined by the Sun's rotation axis
and the spacecraft-sun vector. The y axis completes the system.
The ss coordinate system is defined using the instantaneous Jupiter-Sun
vector as the primary reference vector (x direction). The X-axis lies
along this vector and is taken to be positive toward the Sun. The
Jupiter orbital velocity vector is the second vector used to define
the coordinate system; the y axis lies in the plane determined by the
Jupiter-Sun vector and the velocity vector and is orthogonal to the x
axis (very nearly the negative of the velocity vector). The vector
cross product of x and y yields a vector z parallel to the northward
(upward) normal of the orbit plane of Jupiter. This system is sometimes
called a sun-state (ss) coordinate system since its principal vectors
are the Sun vector and the Jupiter state vector.
The planetocentric (pc) coordinate system is body-fixed and rotates
with the body as it spins on its axis. The body rotation axis is the
primary vector used to define this coordinate system. Z is taken to
lie along the rotation axis and be positive in the direction of
positive angular momentum. The X-axis is defined to lie in the
equatorial plane of the body, perpendicular to Z, and in the direction
of the prime meridian as defined by the IAU. The Y axis completes the
right-handed set.
Data in the vicinity of the moons of Jupiter (Io, Europa, Ganymede,
Callisto) may be provided in separate files in moon centered coordinate
systems, if it turns out that the mission plan affords an opportunity
to acquire data in the immediate vicinity of any of these bodies The
planetocentric and SS data follows the definitions above with the
reference body being the moon or target specified via option in the
command line All of the archived data files are simple and readable
ASCII files with attached documentation in a header that precedes the
columns of data. Files using a coordinate system centered on a target
body other than Jupiter are identified via the target body listed on
the command line which appears in the header along with an audit trail
of supplementary engineering (kernel) files.
The output from the processing program is in Standard Time Series (STS)
format. The Object Description Language (odl) header is included in the
STS file. There will also be a detached PDS label file describing the
contents of the STS file.
Each data file contains the observations collected on a given UTC day.
Instrument Overview
===================
The MAG Instrument Suite consists of two boom mounted observing
platforms (MAG Optical Bench, or MOB) each supporting a vector
Fluxgate Magnetometer (FGM) and two non-magnetic Advanced Stellar
Compass (ASC) Camera Head Units (CHUs). The ASC determines the
attitude of the MOB in inertial space and relative to the JUNO
spacecraft's Stellar Reference Units (SRU). The FGM was built at
the Goddard Space Flight Center (GSFC); the ASC was built at the
Technical University of Denmark (DTU).
The Juno FGM is fully redundant, with two identical power converters
providing power to one of two identical field programmable gate array
(FPGA)-based digital systems. Only one set (power converter and digital
system) is powered at a time; the other is a cold back-up. Either set
receives commands from, and transmits data to, either side of the
spacecraft command and data handling (C&DH) unit through redundant
interfaces. Two identical sets of analog electronics, both continuously
powered by either power converter, drive the outboard (OB) and inboard
(IB) sensors, via separate cables connecting the remote FGM sensors and
electronics box, and both are controlled by and communicate with either
of the digital systems. No single point failure can result in loss of
data from both OB and IB FGM sensors.
Each FGM sensor block uses two miniature ring-core fluxgate sensors
to measure the magnetic field in three components of the vector field.
Each of the two ring-core sensors measures the field in two orthogonal
directions in the plane of the ring core. With two such sensors,
oriented in planes intersecting at 90 degrees, all three components
of the vector field are measured (one component measured, redundantly,
by both). The sensor electronics uses negative feedback to null the
magnetic field in each core, providing linearity over the full dynamic
range of the instrument. The field in each ring core is both sensed and
nulled by a pair of nested coils within which the ring core resides.
Each coil nulls the field in one of the two perpendicular axes that
define the plane of the ring core sensing element. All elements are
maintained in precise alignment by a sensor block assembly constructed
of a machinable glass ceramic with low thermal expansion (MACOR) and
excellent mechanical stability. The FGM sensor block attaches to the
optical bench via a three point kinematic mount to maintain accurate
alignment over the range or environments experienced. The FGM sensor
block is designed to operate at about 0 degrees C, whereas the optical
bench and CHUs are designed to operate at about -58 degrees C to
minimize noise and radiation effects. The FGM sensor block is thermally
isolated from the optical bench via the three point kinematic mount and
individual thermal blanketing. The FGM sensor itself is impervious to
radiation effects.
The two FGM sensors are separated by 2 meters on the MAG boom, one
sensor (inboard, or 'IB' sensor) is located 2 m radially outward from
the end of the solar array and the other sensor (outboard, or 'OB'
sensor) is located at the outer end of the MAG boom. This arrangement
('dual magnetometer') provides the capability to monitor spacecraft-
generated magnetic fields in flight. The MAG boom is located on the
outermost end of one (+x panel) of three solar panels and is designed
to mimic the outermost solar array panel (of the other two solar array
structures) in mass and mechanical deployment. The OB and IB sensor
packages are identical. The CHUs measure the attitude of the sensor
assembly continuously in flight to 20 arcsec and are used to establish,
and continuously monitor, the attitude of the sensor assembly with
respect to the spacecraft SRUs through cruise, orbit insertion at
Jupiter, and initial science orbits. In addition to the extraordinarily
accurate attitude reference provided by the MAG investigation's multiple
ASC CHUs, the spacecraft provides (reconstructed) knowledge of the FGM
sensor assembly attitude to an accuracy of 200 arcsec throughout the
mission, using sensors on the body of the spacecraft and knowledge of
the attitude transfer between the ASC camera heads and spacecraft SRUs.
This provides a redundant attitude determination capability that could
be used if ASC attitude solutions are interrupted for any reason (e.g.,
blinding by a sunlit Jupiter obscuring the field of view for certain
geometries, radiation effects). If this redundant capability is required
at any time, the stability of the mechanical system (MAG boom, solar
array hinges, structure, and articulation strut) linking the body of
the spacecraft (SRU reference) to the FGM sensors (and CHUs) is an
important element in satisfying the spacecraft requirement.
The Juno MAG sensors are remotely mounted (at approximately 10 m and
12 m) along a dedicated MAG boom that extends along the spacecraft +x
axis, attached to the outer end of one of the spacecraft's three solar
array structures. This design provides the maximum practical separation
between MAG sensors and spacecraft to mitigate spacecraft-generated
magnetic fields which would otherwise contaminate the measurements.
A comprehensive magnetic control program is in place to ensure that
the spacecraft magnetic field at the MAG sensors does not exceed 2 nT
static or 0.5 nT variable. The separated, dual FGM sensors provide
capability to monitor spacecraft-generated magnetic fields in flight.
The JUNO sensor design covers the wide dynamic range with six
instrument ranges (see below) increasing by factors of four the
dynamic range in successive steps. The analog signals are digitized
with a 16 bit analog to digital (A/D) converter, which yields a
resolution of +/- 32768 steps for each dynamic range. In the table
below, resolution, equal to 1/2 the quantization step size for each
range, is listed in parentheses.
FGM Characteristics Dual Tri-Axial Ring Core Fluxgate
Dynamic range (resolution) 16.3840 G (+/-25.0 nT)
4.0960 G (+/-6.25 nT)
1.0240 G (+/-1.56 nT)
0.2560 G (+/-0.391 nT)
(1 G = 100,000 nT) 6400 nT (+/-0.10 nT)
1600 nT (+/-0.02 nT)
Measurement accuracy: 0.01% absolute vector accuracy
Intrinsic noise level <<1 nT (range dependent)
Zero level stability <1 nT (calibrated)
Intrinsic sample rate 64 vector samples/s
The data from each sensor can be in one of eight data formats. The
instrument intrinsic sample rate of 64 samples/second is supported
in data formats 0 and 1; averages over 2 to the n power samples
(n = 1,2,3,4,5,6) are supported in telemetry modes 2 through 7.
See the JNO_FGM_INST.CAT file for more information and
[CONNERNEYETAL2016] for full details.
Parameters
==========
The FGM powers up in operational mode and returns telemetry
immediately every clock tic (2 seconds). The FGM may be operated
in autoranging mode, or manual range commands may be sent to fix
the instrument in any of its dynamic ranges. Likewise any telemetry
mode may be selected, depending on telemetry resource allocation. In
addition, packets of engineering telemetry (in addition to science
telemetry packets) are telemetered at a variable rate, from one per
2 seconds to one per 512 seconds, per commanded state.
Calibration Overview
====================
The FGMs were calibrated in the Planetary Magnetospheres Laboratory
and the GSFC Mario H. Acuna (MHA) Magnetic Test Facility (MTF), a
remote facility located near the GSFC campus. These facilities are
sufficient to calibrate the FGMs to 100 parts per million (ppm)
absolute vector accuracy. An independent measurement of the magnetic
field strength in the 0.25, 1, and 4 Gauss ranges was provided by
Overhausen Proton Precession magnetometers placed near the FGM. Scale
factor calibration is extended to 16 Gauss using a specialized high
field coil and measurement techniques (see JUNO Magnetic Field
Investigation instrument paper). A nuclear magnetic resonance
magnetometer (Virginia Scientific Instruments) provided the absolute
field strength measurements in the 16 Gauss range.
Two independent methods are used to calibrate the magnetometers. The
vector fluxgates are calibrated in the 22' facility using a method
('MAGSAT method') developed by Mario Acuna and others. This technique
uses precise 90 degree rotations of the sensing element and a sequence
of applied fields to simultaneously determine the magnetometer
instrument model response parameters (the 'A matrix') as well as a
similar set of parameters (the 'B matrix') that describe the facility
coil orthogonality [instrument paper reference]. The second calibration
method (called the 'thin shell' and 'thick shell') uses a large set of
rotations in a known field (magnitude) to obtain the same instrument
parameters, subject to an arbitrary rotation [Merayo 2000 & 2001]. In
the 'thin shell' method, the sensor is articulated through all
orientations in a fixed, or known field magnitude. This can be done in
a facility like the GSFC 22 foot coil system, wherein any fixed field
up to about 1.2 Gauss may be utilized, or it may be done in the Earth's
field using the ambient field in a gradient-free region and a system
to compensate for variations in the ambient field (normally corrected
via a secondary reference magnetometer coupled with a Proton Precession
total field instrument). Application of this method in a coil facility
(with closed loop control for ambient field variations) allows for the
'thin shell' to be performed at many field magnitudes ('thick shell').
The MAGSAT calibration method provides the instrument calibration
parameters referenced to the optical cube mounted on the sensor
(or MOB) which defines the instrument coordinate system. These
parameters include the instrument scale factors, 3 by 3 instrument
response matrix (or 'A' matrix), and zero offsets for each instrument
dynamic range. The 'thin shell' method provides the same parameters,
but since the method conveys no attitude information, only the
symmetric part of the instrument response matrix is determined via
'thin shell'. Nevertheless, it provides a useful independent verification
of the MAGSAT calibration.
Inflight calibration activities are designed to monitor instrument
parameters, primarily zero offsets, and to monitor the relative
alignment of the magnetic field sensor platforms (the MOBs) and the
spacecraft attitude reference (Stellar Reference Units, or SRUs).
Spacecraft generated magnetic fields will be monitored using the dual
magnetometer technique and a series of magnetic compatibility tests
designed to identify the source of any magnetic signals (if any)
associated with spacecraft payloads. Since Juno is a spinning
spacecraft, spinning at 1 or 2 rpm nominally, any field fixed in the
frame of reference of the spacecraft (e.g., fixed spacecraft-generated
magnetic fields, sensor offsets, etc.) is easily identified. In practice
we apply an algorithm developed independently by several groups (Acuna,
Reviews of Scientific Instruments, 2002) to estimate bias offsets using
differences in the measured field. This method handily corrects for
biases in the spacecraft x and y axes, but since the spacecraft spins
about the z axis, biases in z must be estimated using different methods.
One technique utilizes the Alfvenic nature of fluctuations in the solar
wind, that is, the magnitude preserving nature of variations in the
field. Of course, not all fluctuations are Alfvenic (preserving
magnitude) so some care is taken in application of this method to select
appropriate events.
LEFT OFF
Coordinate Systems
==================
The MAG data are represented in the following coordinate systems:
- spacecraft-solar equatorial
- payload
- planetocentric
- sun-state
all described above.
Data
====
Data products contain the observations collected on a given
UTC day. Each coordinate system in a separate file.
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