Overview:This data set contains the attitude data for the rotor of the Galileospacecraft. The data provided are derived from the Attitude andArticulation Control System (AACS) data downlink as provided by theGalileo project to the magnetometer team. This data set coversportions of the Jupiter Approach (JA) and all orbit operation missionphases (J0-J35). The data are discontinuous. The sampling frequencyvaries with mission phase. The coverage period is from1995-11-28T01:34 to 2001-09-21 (exact time of last sampleis currently unknown).Data Sampling:The data set is derived from 3 downlink sources, depending on the missionphase. During the recorded intervals, attitude data are played back withthe other recorded fields and particles data. During these periods data areavailable every minor frame (~2/3 second). Attitude data in these periodsare all sampled at the same time, no interpolation is required.When real-time Science (RTS) data are being acquired, attitude data areacquired simultaneously once every 5 major frames (~5 minutes).When neither RTS or recorded data are being acquired, AACS data areonly returned in the engineering stream, not as a true data stream. Thedata during these intervals are subcommutated. Right Ascension (RA),Declination (DEC), and Rotor Twist values are not synchronously sampled.In order to make the data from these intervals useful, the RA and DEC datahave been linearly interpolated to the time of the twist samples. Samplesfrom these intervals are available about once every two hours. Only theMAG and DDS instruments acquire data during these mission phases. Thissampling frequency is adequate given the long averages acquired by theseinstruments during these mission phases.Data Processing:When data are simultaneously available from all three AACS channels, noprocessing is applied other than to convert data numbers into radians.When data are not simultaneously sampled, the RA and DEC values arelinearly interpolated between samples to the time of the TWIST samples.Beginning in Phase 2A, the rotor spin angle was no longer included in thedownlink. This value is calculated on the ground by using the values ofRA, DEC, and TWIST and the difference between Earth's equatorial andecliptic poles.The EME-50 to System III (1965) rotation matrix is computed by using theSPICE SPK kernel S980326B.XSP (final prime mission reconstruction). Thesevalues can be reproduced by using the following pseudo-code fragment: INTEGER I, LU CHARACTER*24 SCLK DOUBLE PRECISION ET, E502J2K (3,3), J2K2S3(3,3), E502S3(3,3)CC Load KernelsC CALL CLPOOL() CALL LDPOOL ('P_CONST.KER') CALL LDPOOL ('LEAP_SEC.KER') CALL LDPOOL ('P_CONST.KER') CALL SPKLEF ('GLL_JUP.SPK', LU)CC Begin loop through data, reading a sclk and computing a matrixC READ (10, 999) SCLK CALL SCS2E (-77, SCLK, ET) CALL IRFTRN ('FK4', 'J2000', E502J2K) CALL BODMAT (599, ET, J2K2S3)CC Reverse sign of row three to make system left handedC DO 10, I : 1, 310 J2K2S3(3,I) : -J2K2S3(3,I) CALL MXM(E502J2K, J2K2S3, E502S3)C End loop, E502S3 contains the EME50 -> System III rotation matrixData:All data are archived in a single ASCII table.Column Type Description -______________________________________________________________time char Spacecraft event time (UTC), PDS time format.sclk char Spacecraft clock counter.RA real Rotor right ascension, EME-50. DEC real Rotor declination, EME-50. TWIST real Rotor EME-50 spin phase angle. SPIN real Rotor ECL-50 spin phase angle. A11 real EME50->SYS3 matrix row 1, column 1 value.A12 real EME50->SYS3 matrix row 1, column 2 value.A13 real EME50->SYS3 matrix row 1, column 3 value.A21 real EME50->SYS3 matrix row 2, column 1 value.A22 real EME50->SYS3 matrix row 2, column 2 value.A23 real EME50->SYS3 matrix row 2, column 3 value.A31 real EME50->SYS3 matrix row 3, column 1 value.A32 real EME50->SYS3 matrix row 3, column 2 value.A33 real EME50->SYS3 matrix row 3, column 3 value.Ancillary Data:There are several SPICE kernels used in the production of this data set.All of the SPICE kernels used to produce this data set are contained onthe MWG archive volume DVD in the EXTRAS/SPICE/KERNELS directory.The kernels (PDS PRODUCT_ID) used to create this were: S980326B.TSP - Prime Mission Reconstruction (JA - E12) S000131A.TSP - GEM reconstruction (E12-E26) S020128A.TSP - GMM (I27-I33) reconstruction, A34/J35 predict. MK00062B.TSC - Galileo spacecraft clock kernel. NAIF0007.TLS - Leapseconds kernel. PCK00006.TPC - Planetary constants kernel.Coordinates:The basic rotor coordinate system has its Z-axis aligned with the spinaxis (positive in the opposite direction as the HGA) and its +Y-axispointing outward from the spacecraft body along the magnetometer boom.X completes the right handed set. This coordinate system spins with therotor at a period of approximately 19 seconds.Earth Mean Equator and dynamical equinox of Besselian year 1950 (EME-50)is an inertial coordinate system used by the Galileo project fornavigation purposes. This system also commonly known as FundamentalCatalog (4) or 'FK4' by the SPICE software. At the reference epoch, thissystem has its +Z axis parallel to the Earth's north pole and its +X axispoints to the first point of Aries.System III (1965) is the standard planetographic (left handed) coordinatesystem as defined by the IAU for Jupiter [DESSLER1983]. The CentralMeridian Longitude is taken at 00 UTC on January 1, 1965. The rotationperiod for Jupiter is defined to be 9h 55m 29.71s (870.536 deg/day).

The AACS data are basically raw data that have been converted to physicalunits. There are some data spikes and dropouts (zero filled) in the dataset. In areas where RA and DEC values needed to be linearly interpolated,obviously bad points were edited out before interpolating. No attempt hasbeen made to interpolate data in the vicinity of spacecraft maneuvers.The spacecraft has a wobble that is less than a milliradian in amplitude.Linear interpolation, where applied does not reproduce this wobble. Thebasic attitude data is expected to be reliable at nearly the quantizationlevel of about 0.0001 radians.During the Phase 2A mission the rotor spin angle is no longer includedin the downlink. The difference between the spin angle and the twistangle is strictly a function of the rotor RA and DEC. This difference iscomputed on the ground and subtracted from the twist angle to generatespin angle where it is missing (after May, 1996).The rotation matrix data have been generated by software from the Galileoproject approved SPICE kernels. The software has been tested extensivelyand verified. Data can be considered to be reliable to the precisionimplied by the number of significant digits provided.