Data Set Overview
   =================
     The data set consists of calibrated observations.  The MAG measures
     the vector magnetic field.

     There are two principal coordinate systems used to represent the data
     in this archive.  The payload (pl) coordinate system and the SE
     coordinate system is a Spacecraft-Solar equatorial system.  The
     sun-state (ss) and planetocentric (pc) will also be used for Earth Fly
     By (EFB) and days 2016-177 through 181 Jupiter approach data. Cartesian
     representations are used for all four coordinate systems. The se, pc,
     and ss coordinate systems are specified relative to a 'target body'
     which may be any solar system object.  Primarily the 'target body' is
     Jupiter, but for EFB it is Earth.  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
   ===================
     Please see JNO_FGM_INST.CAT.

   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.

   Coordinate Systems
   ==================
     The MAG data are represented in the following coordinate systems:

     - spacecraft-solar equatorial
     - spacecraft 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.

Confidence Level Overview
   =========================
   Not applicable.

   Review
   ======
     The FGM data set was reviewed internally by the MAG team prior to
     release to the PDS. PDS also performed an external review of the MAG
     data.


   Limitations
   ===========
   The Juno magnetic field investigation was designed to measure fields
   to 16 Gauss per axis over 6 dynamic ranges of the instrument, the most
   sensitive of which is +/- 1600 nT with a quantization step size of
   0.05 nT (16 bit A/D). Moreover, the spacecraft magnetic requirement was
   not to exceed 2 nT static and 0.5 nT variable spacecraft-generated
   magnetic field. In very weak field environments, such as encountered in
   outer cruise, accuracy may be expected to be limited by sensor offset
   and spacecraft magnetic field variations. The combined (static)
   spacecraft-generated magnetic field and sensor offset may be continuously
   monitored in flight in the spacecraft x and y axis, since the spacecraft
   spins (nominally at 1 or 2 RPM) about an axis closely aligned with the
   spacecraft payload z axis. However, offsets in the z axis need be
   estimated using the Alfvenic properties in the solar wind (ref. Juno
   Magnetic field investigation paper in Space Science Reviews). Statistical
   in nature, estimates of z axis zeros are not continuously available and
   are less accurate than the x and y zeros. Also, variations in spacecraft
   field over a time span comparable to a spin period will also lead to
   larger errors.