NASA Logo, National Aeronautics and Space Administration
NASA Space Science Data Coordinated Archive Header



Availability: Archived at NSSDC, accessible from elsewhere


This description was generated automatically using input from the Planetary Data System.

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 payload (pl) coordinate system, the Sun-state (ss) coordinate system, and the planetocentric (pc) coordinate system. Cartesian representations are used for all four coordinate systems. The 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. 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 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.


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: (Connerney et al., Space Science Reviews, 2017, doi: 10.1007/s11214-017-0334-z)). A nuclear magnetic resonance magnetometer (Virginia Scientific Instruments) provided the absolute field strength measurements in the 16 Gauss range when it was working. 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. Through 2017 DOY 174, magnetometer observations in weak field environments (sensor range 0, +/- 1600 nT nominal dynamic range) are sourced from the outboard sensor. Subsequent to 2017 DOY 174, magnetometer observations in weak field environments (sensor range 0, +/- 1600 nT nominal dynamic range) are sourced from the inboard sensor to alleviate minor sporatic interference appearing in the z axis of the outboard sensor. This substitution is noted in the STS header that identifies the content of each record. The STS header should be consulted for file content each time a file is read, in the event that file content changes (this was the design purpose of the STS header). We anticipate further file format changes (in upper dynamic ranges) as additional corrections are introduced.

These data are available on-line from the Planetary Data System (PDS) at:

Alternate Names

  • JNO-J-3-FGM-CAL-V1.0


  • Planetary Science: Fields and Particles

Additional Information



Questions and comments about this data collection can be directed to: Dr. David R. Williams



NameRoleOriginal AffiliationE-mail
Dr. John E. P. ConnerneyData ProviderNASA Goddard Space Flight
Dr. John E. P. ConnerneyGeneral ContactNASA Goddard Space Flight
[] NASA Logo -