Trajectory data are provided in two Io centered coordinate systems, planetocentric (Iocentric) and IPhiO in a single data file. A browse plot of the flyby geometry in IPhio coordinates is provided in the BROWSE/TRAJECT/IO directory.
The spacecraft position is given in units of Io radii (1818 km) every twenty (20) seconds for the thirty (30) minutes near Io closest approach. Io-centric coordinates are primarily useful for understanding processes of Iogenic origin such as internal field generation [KIVELSONETAL1996A]. A substantial fraction of the published analysis of the magnetometer data acquired near Io use data and ephemeris information in the IPhiO coordinate system [KIVELSONETAL1996A],[KIVELSONETAL1996B].
The Iocentric coordinate system is a spherical, planetocentric coordinate system with standard J2000 definitions of range, latitude, and longitude. Longitude is measured in a right-handed sense from the sub-Jupiter meridian. Io is phase locked so this meridian is constant.
The IPhiO (Io Phi=X, Omega=Z) coordinate system is defined by using the two vectors Phi, the corotation velocity vector at Io, and Omega, the Jovian spin axis. Phi is positive in the direction of corotation and Omega is positive northward. Y completes the right-handed set (towards Jupiter). The direction of Phi is fixed at 95-341 17:45:58 UT or approximately the time of closest approach to Io. The basis vectors of the coordinate system are fixed at the epoch time. The spacecraft trajectory information takes the instantaneous range vector (R) from Io to the spacecraft and resolves it into XYZ components.
There are numerous useful variants of this coordinate system. One that has been used extensively is the Io, Phi=X, B (IPhiB) coordinate system. This coordinate system is a rotation about the IPhiO coordinate system X axis such that the Jovian background field lies in the X-Z plane of the new coordinate system. For this encounter, this requires a right-handed rotation of -3.013 degrees. Another variant that is commonly used is one that allows the coordinate system basis vectors to redefined continuously as Io moves. This system is primarily used when knowledge of the instantaneous corotation direction is needed. Users can generate all of these coordinate systems from the SPICE kernels that are included in the archive.
Data are stored in a single ASCII file (POS_DATA.TAB).
File structure is described by the detached PDS label files:
POS_DATA.LBL (Describes file parameters)
POS_DATA.FMT (Describes the columns in the data table)
Columns in all data files:
=====================================================
name type description
-----------------------------------------------------
time char Sample time in PDS time format number
R float Range from Io in Ri where 1 Ri = 1818 km.
LAT float Iocentric latitude of the S/C in degrees.
ELON float Iocentric east longitude of the S/C in degrees.
IPhiO_X float X component of the range to Io in IPhiO coordinates (Ri)
IPhiO_Y float Y component of the range to Io in IPhiO coordinates (Ri)
IPhiO_Z float Z component of the range to Io in IPhiO coordinates (Ri)
Various documentation files associated with these data can be found
in the CATALOG directory off of the root of the cdrom. Files of
interest are:
MISSION.CAT - Mission description
INSTHOST.CAT - Spacecraft description
POSIODS.CAT - Data set description