Galileo Orbiter at Jupiter Magnetometer High Resolution Calibrated Bundle Galileo MAG Jupiter Calibrated Magspheric Survey Data Collection PDS3 DATA_SET_ID = GO-J-MAG-3-RDR-MAGSPHERIC-SURVEY-V1.0 PDS3 DATA_SET_NAME = GO JUPITER MAG MAGNETOSPHERIC SURVEY V1.0 START_TIME = 1995-11-06T00:21 STOP_TIME = 2003-09-21T18:00 PDS3 DATA_SET_RELEASE_DATE = 1997-07-01 PRODUCER_FULL_NAME = MARGARET G. KIVELSON Reference: Kivelson, M.G., Khurana, K.K., Russell, C.T., Walker, R.J., Joy, S.P., Mafi, J.N., GO JUPITER MAG MAGNETOSPHERIC SURVEY V1.0, GO-J-MAG-3-RDR-MAGSPHERIC-SURVEY-V1.0, NASA Planetary Data System, 1997 These data were originally archived in the following PDS3 data set: GO-J-MAG-3-RDR-MAGSPHERIC-SURVEY-V1.0 (https://doi.org/10.17189/1519668). Collection Overview ================= These collections contain browse plots and magnetic field vectors acquired by the Galileo Orbiter magnetometer during the magnetospheric survey portion of the mission for all orbits and satellite flybys (Io, Europa, Ganymede, Callisto, and Amalthea). These data were acquired in the optimal averager (opt/avg) and real-time survey (RTS) modes beginning during the Jupiter approach and continuing throughout Jupiter orbital operations. The collections cover the time period from 1995-11-06T00:21:30 UT (Jupiter approach) until the end of mission (September 2003). Sampling rates are variable and depended upon the downlink capabilities. With few exceptions, the data are provided at the full downlink resolution. The data are provided in five coordinate systems (IRC, System III [1965], JSE, JSO, and JSM). Parameters ========== Data Sampling ------------- Data acquisition strategies varied throughout the mission. During the Jupiter approach period, most particles and fields (MWG) instruments were off while the magnetometer acquired low resolution opt/avg data. During the bulk of the prime mission, the MWG instruments were allowed to acquire continuous RTS data whenever the spacecraft was inside 50 Jovian radii (Rj). When the MWG was not returning RTS data, MAG used its opt/avg capability to acquire low time resolution averages of the field. In general, these are ~32 minute averages, although some higher rate data (1-8 minute) were used to fill small gaps in RTS data coverage. Early in the prime mission, MAG was required to 'pay' for the bits its used to return the opt/avg data by not acquiring as much high resolution data. MAG did not return data from the MWG transauroral crossing recorded interval in the C3 orbit so that it could have continuous coverage during that orbit. After the fourth orbit, the MAG opt/avg data was return in the spacecraft engineering data stream at no cost to the team. The MWG acquired RTS data beyond 50 Rj in selected bit rich orbits (G2, G7, G8, C9, C10) during the prime mission. RTS, opt/avg, and snapshot data were all stored in the MAG internal memory buffer. RTS data were continuously averaged at MAG memory address 4800 and transmitted to Earth in real-time. When an opt/avg ON command was given data were stored in the MAG memory buffer beginning at address 4800 and continuing to higher addresses until Opt/Avg OFF was commanded, another Opt/Avg ON was commanded and the MAG buffer began filling at 4800 again, or the MAG memory buffer filled. Opt/avg data was returned to Earth via Memory Read-Out (MRO) as telemetry permitted. Since RTS and opt/avg data used the MAG memory buffer in different ways, they could not be collected simultaneously. Snapshot data, which completely filled the MAG memory buffer, could likewise not be collected simultaneously with opt/avg, and caused periodic spikes in RTS data. Snapshot corruption of RTS data is discussed in more detail in the 'Data Quality and Coverage' section of this document. For a more detailed discussion how MAG RTS, opt/avg, and snapshot data are collected, please refer to [KIVELSONETAL1992]. When high-resolution data were available, but the RTS data were either lost or corrupted, simulated RTS (sRTS) data have been generated from the high-resolution data. sRTS data were averaged down to the RTS rate, then interpolated to be continuous with the existing RTS data. During the prime mission, the RTS data rate varied, depending the downlink capability. MAG has several different possible RTS data rates depending on the telemetry format: --------------------------------------------------------------------- Table 1. MAG RTS Rates --------------------------------------------------------------------- Format MAG bit rate Time between samples Corner Freq. (bps) (seconds) (mf) (Hz) --------------------------------------------------------------------- A-D 2 24 36 1/34 E 4 12 18 1/34 F 6 8 12 1/17 G 8 6 9 1/17 H 12 4 6 1/17 I 18 8/3 4 1/17 Similarly, the opt/avg data can be acquired at different rates. The relatively high data rates fill the MAG internal memory buffer more quickly and are only used to cover short data disruptions. --------------------------------------------------------------------- Table 2. Optimal Averager Data Rates --------------------------------------------------------------------- Time between samples Buffer fill time Corner Freq. (RIM) (hh:mm:ss.ss) (day/hh:mm) (Hz) --------------------------------------------------------------------- 1 00:01:00.67 03:22 1/67 2 00:02:01.33 06:44 1/134 4 00:04:02.67 13:29 1/268 8 00:08:05.33 1/02:58 1/536 16 00:16:10.67 2/05:56 1/1072 32 00:32:21.33 4/11:51 1/2144 64 01:04:42.67 8/23:42 1/4289 The instrument has the ability to acquire longer time averages but these modes were not used at Jupiter. After the prime mission ended (December 1997), RTS data acquisition was limited to only a few days near perijove except for a few orbits. RTS data are typically in the 2 bps (24 sec/sample) telemetry formats. Opt/avg data after end of the prime mission, are primarily provided in 32 RIM (~32 min/sample) averages. Both the opt/avg and the RTS data processor compute field averages by applying a recursive filter and decimate algorithm to the data. The instrument applies a calibration, decimates vectors down to minor frame (mf) samples (2/3 second) and despins the data using the spin angle value broadcast on the spacecraft bus. The corner frequencies of the recursive filter are provided in tables 1 and 2 for the various sample durations. The effect of this process is that the spacecraft time tags both RTS and opt/avg data at the time of decimation (end of average), rather than at the effective 'center' of the average. As a result there is a time shift or phase delay between RTS or opt/avg data and data (e.g. LPW) which have not undergone the same filtering process. The magnitude of the phase delay is dependent upon the averaging interval (or rate) and frequency content of the data. Analysis of the filter response to a wave at the spin period (dominant frequency) has determined that the phase delay may be eliminated with the correction: [Corrected time] = [Sample time] - (Rate * (1/3)) Where the 'Sample time' is the time assigned by the spacecraft, and the 'Rate' is the sampling rate in seconds. Both 'Sample Time' and 'Corrected Time' are provided in the data files. The IRC data file (which does not include a 'Corrected Time' column) contains a 'Spacecraft Clock' column which corresponds to the 'Sample Time' (and not the 'Corrected Time'). The RTS and opt/avg data overlay high time resolution data acquired at the same time best when the 'Corrected Time' is used. MAG uses fixed gains to acquire data [KIVELSONETAL1992]. Gain states must be manually changed by sending a gain change command to the instrument. There are 3 ranges of field strengths that the instrument can measure: --------------------------------------------------------------------- Table 3. Magnetometer Ranges --------------------------------------------------------------------- Field Range (nT) Magnetometer, range min max --------------------------------------------------------------------- 1/64 - 32 Outboard low field 1/4 - 512 Outboard high field, Inboard low field 8 - 16384 Inboard high field While the outboard magnetometer's position on the boom does make it less susceptible to spacecraft fields, its zero levels were less stable than those of the inboard magnetometer. As a result, the outboard magnetometer was generally not used except when the magnetic field strength was very low. The outboard magnetometer was typically only used outside of 60 Rj (in the low field range). From 9-60 Rj the inboard magnetometer, low field range was generally used. Inside of ~9 Rj the inboard magnetometer, high field range was used. Processing ========== Browse data are primarily processed onboard the spacecraft by using estimates of the instrument calibration and zero levels. The calibration estimates cannot be improved in post processing due to the onboard averaging. Any errors in the sensor zero levels in the spin plane sensors will appear as harmonics of the spin period, possibly modified by frequency folding effects associated with the averaging windows. These effects can be removed in the RTS data in post processing. The long average intervals of the opt/avg data effectively removes these problems without need for post processing. Improvements are made to the zero level correction of the spin aligned sensor data for both RTS and opt/avg data in post processing. Data ==== These data are stored in multiple data files in order to facilitate use and electronic distribution. In general, data from a single orbit is included in a data file. For some of the more data rich orbits, further subdivision was provided. Data from the five coordinate systems are stored in four separate files. The file format for a particular coordinate system is independent of the data acquisition interval. All data files from a given coordinate system are identical in structure. All data files are ASCII, fixed field, white space delimited tables. The following tables (4a-d) describe the record structure of the various type of data files. -------------------------------------------------------------------- Table 4a. IRC Coordinates (Inertial Rotor coordinates) -------------------------------------------------------------------- Column Type Description -------------------------------------------------------------------- samp time char spacecraft event time sample was acquired from MAG memory, PDS time format sclk char spacecraft clock (rim:mf:mod10:mod8) Bx_sc float magnetic-field spacecraft (IRC) x-component By_sc float magnetic-field spacecraft (IRC) y-component Bz_sc float magnetic-field spacecraft (IRC) z-component |B| float magnetic-field magnitude rotattr float rotor right ascension rotattd float rotor declination rotattt float rotor spin phase (twist) angle spinangl float rotor spin phase angle RATE float data sample rate -------------------------------------------------------------------- Table 4b. System III [1965] Coordinates -------------------------------------------------------------------- Column Type Description -------------------------------------------------------------------- corr time char spacecraft event time of sample corrected for MAG filter response, PDS time format samp time char spacecraft event time sample was acquired from MAG memory, PDS time format Br float magnetic-field R (radial) component Btheta float magnetic-field theta (southward) component Bphi float magnetic-field phi (eastward) component |B| float magnetic-field magnitude R float s/c position - Radial distance from Jupiter center-of-mass LAT float s/c position - latitude ELON float s/c position - East longitude WLON float s/c position - West longitude -------------------------------------------------------------------- Table 4c. Jupiter Solar Equatorial (JSE) Coordinates -------------------------------------------------------------------- Column Type Description -------------------------------------------------------------------- corr time char spacecraft event time of sample corrected for MAG filter response, PDS time format samp time char spacecraft event time sample was acquired from MAG memory, PDS time format Bx float magnetic-field x (sunward) component By float magnetic-field y (duskward) component Bz float magnetic-field z (Jovian spin axis aligned) component |B| float magnetic-field magnitude X float s/c position - x (sunward) component Y float s/c position - y (duskward) component Z float s/c position - z (northward) component LOCHOUR float s/c local hour -------------------------------------------------------------------- Table 4d. Jupiter Solar Orbital (JSO) and Jupiter Solar Magnetic (JSM) Coordinates -------------------------------------------------------------------- Column Type Description -------------------------------------------------------------------- corr time char spacecraft event time of sample corrected for MAG filter response, PDS time format samp time char spacecraft event time sample was acquired from MAG memory, PDS time format Bx float JSO/JSM magnetic-field x-component By_jso float JSO magnetic-field y-component Bz_jso float JSO magnetic-field z-component By_jsm float JSM magnetic-field y-component Bz_jsm float JSM magnetic-field z-component |B| float magnetic-field magnitude X float s/c position - JSO/JSM x-component Y_JSO float s/c position - JSO y-component Z_JSO float s/c position - JSO z-component Y_JSM float s/c position - JSM y-component Z_JSM float s/c position - JSM z-component MLOCHOUR float S/C magnetic local hour These data were processed using SPICE kernels produced by the Galileo NAV team during the mission. All of the SPICE kernels used to produce these collections are contained on the 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) S030916A.TSP - GMM (I27-J35) reconstruction PCK00007.TPC - Planetary constants kernel (2000-04-24) MK00062B.TSC - Galileo spacecraft clock kernel Coordinate Systems ================== The data are provided in five coordinate systems. Data are provided in the spacecraft coordinate system in order to aid in the interpretation of particle instrument data. The other coordinate systems are provided for use in Jovian magnetospheric studies. The Jupiter spin axis is defined to have a right ascension of 268.05 degrees and a declination of +64.49 degrees in the J2000 coordinate system used by SPICE. Inertial Rotor Coordinates (IRC) -------------------------------- The IRC coordinate system takes the basic rotor coordinate system (Y along the boom, Z opposite the high gain antenna) which is spinning, and despins it using the rotor spin angle. For this reason IRC coordinates are sometimes referred to as 'despun spacecraft coordinates.' In this system, Z still points along the spin axis opposite the HGA (or roughly anti-Earthward), X is approximately parallel to the downward ecliptic normal, and Y completes the right handed set (pointing roughly towards dawn). System III [1965] Coordinates (SYS3) ------------------------------------ SYS3 magnetic field vector components form the standard right handed spherical triad (R, Theta, Phi) for a Jupiter centered system. Namely, R is radial (along the line from the center of Jupiter to the center of the spacecraft), and positive away from Jupiter. Phi, the azimuthal component, is parallel to the Jovigraphic equator (Omega x R) and positive in the direction of corotation. Theta, the 'southward' component, completes the right handed set. For SYS3 trajectory both east and west longitudes are provided. West longitudes are related to east longitudes by to the algorithm: west longitude = 360. - east longitude West longitude is defined such that it appears to increase with time for a stationary observer [DESSLER1983]. Note, however, that R, latitude, and west longitude constitute a left handed set. The SYS3 1965 prime meridian is the sub-Earth longitude of 1965-01-01 00:00 UT. The spin rate (which was determined from the rotation rate of the magnetic field) is 9 hrs 55 min 29.719 sec. (See [DESSLER1983] for a discussion on Jovian longitude). R is the radial (Jupiter's center to spacecraft center) distance. Latitude is planetocentric. Jupiter Solar Equatorial Coordinates (JSE) ------------------------------------------ JSE is a Jupiter centered cartesian coordinates system defined to have it's Z-axis along the Jovian spin axis, positive in the direction of angular momentum (northward). The X-Z plane is contains the Sun so that the X-axis is the projection of the Sun direction into Jupiter's equatorial plane (positive towards the Sun). Y completes the right handed set and points duskward. This coordinate system is sometimes called Jupiter centered, Sun longitude fixed coordinates. Jupiter Solar Orbital Coordinates (JSO) --------------------------------------- JSO is another Jupiter centered cartesian coordinate system. JSO is the equivalent at Jupiter of GSE coordinates system at Earth. In JSO coordinates, the X-axis points from Jupiter to the Sun. Z is parallel to the upward normal to Jupiter's orbital plane. Y completes the right handed set. Jupiter Solar Magnetic Coordinates (JSM) ---------------------------------------- Another Jupiter centered cartesian coordinate system, JSM is the equivalent at Jupiter of GSM coordinates system at Earth. In this coordinate system, the X-axis points from Jupiter to the Sun. The secondary vector defining this coordinate system is the centered magnetic dipole axis (M) which is defined to be tilted 9.6 degrees from the Jovian spin axis towards 202 degrees SYS3 west longitude. The X-Z plane is contains M. Y completes the right handed set. The Y-Z plane rocks at the Jovian spin period about the Sun-Jupiter line. Local Hour ---------- Local hour angle is the angle (HA) between the observer's (Galileo) sub-Jupiter meridian and the anti-sunward meridian, measured in the Jovian equatorial plane in the direction of planetary rotation. Local hour is the conversion of the local hour angle into units of decimal hours using the conversion factor of one hour to fifteen degrees of longitude. The following diagram is a graphic representation of local hour. Sun ^ | noon Planetocentric 12:00 Equatorial Projection | | | | Jupiter * * */ * | * (dusk) 18:00 -----------*--|--*------------- 06:00 (dawn) * |\ * * * * | \ | \ | \ |-HA--\ 00:00 + Spacecraft midnight Magnetic Local Hour ------------------- Magnetic local hour angle is the angle (MHA) between the observer's dipole meridian and the anti-sunward meridian, measured in the magnetic equatorial plane in the direction of planetary rotation. Magnetic local hour is the conversion of the magnetic local hour angle into units of decimal hours by using the conversion factor that equates one hour to fifteen degrees of longitude. The following diagram is a graphic representation of magnetic local hour. Sun ^ | noon Dipole 12:00 Equatorial Projection | | | | Jupiter * * */ * | * (dusk) 18:00 -----------*--|--*------------- 06:00 (dawn) * |\ * * * * | \ | \ | \ |-MHA-\ 00:00 + Spacecraft midnight Ancillary Data ============== There are several files that are provided in addition to the data files themselves that may be of value to the user. These include a detailed data gap listing (including reason for gap), a table of important spacecraft and instrument events, a discussion of instrument anomalies and resolutions, and a set of quick-look or 'browse' plots of the data. References ========== [DESSLER1983] Appendix B Coordinate Systems, in Physics of the Jovian Magnetosphere, ed. Dessler, Cambridge Univ. Press, New York, 1983. [KIVELSONETAL1992] The Galileo Magnetic Field Investigation, Space Science Rev. 60, 357, 1992. [KIVELSONETAL1996A] A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer, Science, 273, 337, 1996 [KIVELSONETAL1996B] Io's interaction with the Plasma Torus, Science, 274, 396, 1996. [KIVELSONETAL1997A] Galileo at Jupiter: Changing states of the Magnetosphere and first looks at Io and Ganymede, Adv. Space Res., 20, No 2, 193, 1997. [HUDDLESTONETAL1998A] Location and Shape of the Jovian Magnetopause and Bowshock, J. Geophys. Res, 103, no. E9, 20075, 1998." CONFIDENCE_LEVEL_NOTE = " Review ====== These data have been reviewed by the instrument team and are of the highest quality that can be generated at this time. Science results based on these data have been published in several journals (Science, Nature, JGR, etc.). After submission to the PDS, these data successfully completed the peer review process. Data Coverage and Quality ========================= Gaps ---- The magnetometer browse collections contain all of the survey data that were returned from the instrument. However, there are numerous gaps in coverage. Gaps can be caused by telemetry outages, insufficient downlink or uplink, conjunctions, instrument anomalies, spacecraft anomalies, commanding errors, etc. A detailed gap listing including a description of the reason for the gap is provided in a separate file with these data. Here is a list of some of the larger or more common gaps. JA: Gaps in coverage due to limited telemetry J0: Large gaps during probe relay and Io playback G1: MAG flip/commanding anomaly (3.5 weeks); spacecraft safing 1 week) J5: Conjunction, limited telemetry (5 weeks) E6: Instrument anomaly, radiation hit (1.5 weeks) G7: MAG flip anomaly (1 week) E12: AACS anomaly (7 weeks) J13: solar conjunction (6 weeks) E14: spacecraft safing (5 days) E15: ground station reallocated to SOHO for spacecraft anomaly recovery effort (3 days) E16: spacecraft safing (4 days) E17: ground station reallocated to Voyager 2 for spacecraft anomaly recovery effort (4 days) E18: spacecraft safing (2 days); ground station reallocated to NEAR for spacecraft anomaly recovery effort (2 days) E19: spacecraft safing (1.5 weeks); solar conjunction (3 weeks) I24: loss of telemetry during MRO (4 days) I25: opt/avg corrupted by shapshots (2 days) I27: spacecraft safing (2 days); solar conjunction (2.5 weeks) G28: loss of telemetry during MRO (4 days) G29: loss of telemetry during MRO (3 days) C30: SSI anomaly recovery/loss of telemetry (2 days); solar conjunction (2.5 weeks) I33: spacecraft safing (1 week); loss of telemetry for MRO (2.5, and 3 days); solar conjunction (3 weeks); loss of telemetry during MRO (4 days); spacecraft safing (4.5 days) A34: spacecraft safing (5 days); telemetry outage (4.5 days); end of spacecraft operations before impact (36 weeks) Other common causes of data gaps that are not related to telemetry outages and anomaly gaps are listed below. Gaps may have occurred throughout Jupiter Orbital Operations - whenever the conditions which caused them existed. Gaps between consecutive opt/avg segments: Gaps of at least two data vectors occurred between consecutive opt/avg data segments. The first and last vector in every opt/avg segment are underfiltered and have therefore been discarded. For the most common data opt/avg rate, this results in a gap of ~1 hour. These gaps may be larger than two vectors if there is any delay between the MRO of the data in the buffer and the subsequent opt/avg ON command. Snapshot corruption of RTS data: When snapshots (see [KIVELSONETAL1992] for more information) were taken the addresses in which RTS data were averaged were overwritten with snapshot data. Since snapshots were written to MAG memory in reversed byte order, the result was a spike which persisted over the course of few records due to the application of the filter. Whenever possible RTS data corrupted by snapshots have been replaced with sRTS data which were unaffected by snapshots. When high-resolution data are not available, however, snapshots result in a series of short (3-6 vector), regularly spaced gaps in the RTS. The MAG data quality (size of error bars) varies with instrument mode (digitization step size), sample rate, etc. The errors can be broken down into three basic types: zero levels, gains, and geometry. Simulated RTS Data ------------------ While sRTS data have been matched to the existing RTS data rates they have not been generated using the same process. sRTS data have not gone through the same recursive filter that was applied to the regular RTS data. sRTS has been generated for the following intervals: ---------------------------------------------------- Table 5. Simulated RTS (sRTS) intervals ---------------------------------------------------- High-Res. Orbit Obs. ID Start Time Stop Time ---------------------------------------------------- G1 G01-GAN 1996-06-27 06:07 1996-06-27 06:52 G1 G01-PSX 1996-06-30 02:01 1996-06-30 02:46 G2 G02-GAN 1996-09-06 18:33 1996-09-06 19:28 G2 G02-PSX 1996-09-11 02:38 1996-09-11 03:18 C3 C03-CALL 1996-11-04 13:15 1996-11-04 14:00 E4 E04-EUR 1996-12-19 06:35 1996-12-19 07:22 G8 G08-QRS 1997-05-06 13:00 1997-05-06 15:09 C9 C09-CALL 1997-06-25 13:25 1997-06-25 14:11 C9 C09-DAWN 1997-08-23 14:07 1997-08-23 16:09 C9 C09-TAR 1997-06-28 13:50 1997-06-28 14:51 C9 C09-DSK1 1997-07-04 14:09 1997-07-04 16:07 C9 C09-DSK2 1997-07-14 10:03 1997-07-14 10:48 C9 C09-DSK3 1997-07-23 13:11 1997-07-23 13:57 C9 C09-APJ 1997-08-07 11:06 1997-08-07 12:47 C10 C10-CALL 1997-09-16 23:49 1997-09-17 00:49 C10 C10-EQX 1997-09-18 22:35 1997-09-18 23:21 E11 E11-EUR 1997-11-06 20:09 1997-11-06 22:51 E12 E12-EUR 1997-12-16 11:43 1997-12-16 12:28 E14 E14-EUR 1998-03-29 13:05 1998-03-29 14:00 E15 E15-EUR 1998-05-31 20:42 1998-05-31 21:43 E15 E15-EUR 1998-05-31 20:42 1998-05-31 21:43 E18 E18-PSX 1998-12-10 19:36 1998-12-11 00:23 E19 E19-EUR 1999-02-01 01:49 1999-02-01 02:38 C20 C20-PJOV 1999-05-03 16:00 1999-05-03 18:00 C21 C21-PJOV 1999-07-01 23:52 1999-07-02 01:47 C22 C22-PJOV 1999-08-12 08:18 1999-08-12 13:06 C23 C23-PJOV 1999-09-14 14:36 1999-09-14 21:28 I24 I24-IO 1999-10-11 03:42 1999-10-11 06:41 I25 I25-TOR 1999-11-25 21:06 1999-11-26 05:54 E26 E26-EUR 2000-01-03 17:29 2000-01-03 18:30 I27 I27-TOR 2000-02-22 10:22 2000-02-22 12:15 I27 I27-IO 2000-02-22 13:05 2000-02-22 14:25 I31 I31-IO 2001-08-06 04:31 2001-08-06 05:28 I32 I32-RAMP 2001-10-15 15:31 2001-10-15 17:26 I32 I32-TOR 2001-10-15 21:53 2001-10-15 23:23 A34 A34-PSX7 2002-11-05 01:05 2002-11-05 05:45 Zero Levels ----------- In general, the zero levels of the spin plane sensors have been corrected to approximately 1/4 of the digitization step size (i.e. if the step size is 8 nT, the zero level is good to 2 nT) or 0.01 nT, which ever is larger. Spin plane sensor zero levels are continuously adjusted by removing spin period averages in the spinning reference frame. The spin aligned sensor zero levels are monitored and adjusted at least once per orbit. The spin aligned sensor zero levels are assumed to drift slowly and linearly. The exception to this statement occurs at times identified as 'offset anomalies' (see Table 7) where all three sensors in a triad show large (2-3 nT) jumps in the zero levels. These anomalies have been corrected in the processed data. In general, the spin aligned sensor zero level is reliable to 0.1 nT. Errors in zero level corrections appear as harmonics of the spacecraft spin period as modified by the averaging window. The nominal spacecraft spin period is 19 seconds and the most common sample rates are 24 and 12 seconds. Outboard Sensor Zero Levels --------------------------- Zero levels in the outboard sensor have proven to be strongly temperature dependent. Offsets changed linearly with temperature during the Earth-Jupiter Cruise mission phase where temperatures went from 280-220 K. At Jupiter, where temperatures have been lower than the instrument qualification ranges, offsets at Jupiter have experienced occasional episodes of instability. Offset changes may occur instantaneously or rapidly (over a period of minutes). In some cases the offset has drifted back to previous values over a period of days or months. In other cases the offset has returned to it's original value sharply. In all cases an attempt has been made to correct for these offset in the processing so that the data appear continuous. Table 7 contains a listing of major offset anomalies. Gains ----- The sensor gains were determined in ground calibration and verified in-flight. In-flight verification included comparison of Earth flyby data with model fields and monitoring for sensor gain changes using the MAG internal calibration coils. In general, the gains have remained stable throughout the mission. There has been a slight change (linear) in the absolute gain levels associated with the sensor temperature. Gains are believed to be known to substantially better than one percent. Geometry -------- Geometry corrections for sensor mounting and non-orthogonality are applied through the sensor coupling matrix. Although the sensor triads are rigid and are not believed to have changed geometry through time, the triad itself is flipped periodically to rotate the spin aligned sensor into the spin plane. Small variations (<0.1 degree) in the triad mounting with respect to the spacecraft coordinate system must be corrected after each sensor flip. This is done by computing a new coupling matrix. The data are despun by using the 'rotor spin angle' that is broadcast on the spacecraft bus by the spacecraft attitude and articulation control system (AACS). Despinning must be corrected in ground processing to account for delays in the MAG instrument electronics and filters. This task is accomplished by respinning the data using the raw AACS data and then despinning using a corrected spin angle. The total correction can be as large as two degrees. After ground processing, the MAG data are geometrically correct to better than one degree, and probably better than 0.1 degree. Optimal Averager AACS --------------------- Two of the rotor attitude angles supplied in the IRC data files, Rotor Twist and Rotor Spin, are always flagged when the opt/avg was being used to capture the data. The opt/avg produces extremely low sampling rates (typically >32 min/sample).In addition in the engineering stream the various rotor attitude angles are sampled asynchronously both from each other and from the MAG data. As a result any rotor attitude information provided at the MAG sampling times must be interpolated. While the other two angles vary only slowly, the twist and spin angles vary over 0-2 pi at the spacecraft spin period (~20 sec). Since the MAG data produced by the opt/avg represent averages over many spin periods it is not meaningful to assign specific twist or spin angles to the data. It is also not possible to 're-spin' the data, which is the purpose of providing Rotor Twist and Spin in the first place. Slew Tests ---------- Due to the continuing gyro electronics problems that were first observed in E12, a series of 'slew tests' were implemented in order to access gyro functioning. During these tests, because of the potential for anomalous behavior by the gyros, the spacecraft attitude is not known with any certainty. This poses problems for both the B-field components and AACS. Opt/avg data, because of the relatively short duration of the tests and the long averages of the data, show no real effects from the corruption. The RTS data, however, can show the effects very clearly. As a result the B-field components have been flagged when a slew test was performed with the instrument in RTS mode, and the components were preserved when the instrument is in opt/avg mode. The components were flagged in the RTS data even when no corruption was readily apparent. The flagged data are available in the Galileo Jupiter MAG raw bundle. The following table gives the timing of the slew tests. The times indicated correspond to the start and end of the group of commands that constituted a slew test. The interval in which the B-field components actually become corrupted was shorter than this. This corrupted interval started approximately with the 'SLEW TEST' and ended with the 'ENABLE SEQID,RESD HVEC' (see MAG EVENT TABLE). Mode indicates whether the instrument was in RTS or OPT/AVG mode. A value of 'N/A' indicates that the slew test occurs in a data gap. ----------------------------------------------------- Table 6. Slew tests ----------------------------------------------------- Orbit Start Time Stop Time Mode ----------------------------------------------------- E14 1998-05-13 12:40 1998-05-13 14:23 OPT/AVG E17 1998-11-07 17:00 1998-11-07 20:38 OPT/AVG E19 1999-04-27 21:32 1999-04-27 21:40 OPT/AVG I24 1999-10-13 16:42 1999-10-13 18:23 OPT/AVG I24 1999-10-27 12:00 1999-10-27 14:15 RTS I25 1999-11-28 07:27 1999-11-28 09:08 N/A I25 1999-12-09 01:00 1999-12-09 03:15 OPT/AVG E26 2000-01-05 19:12 2000-01-05 20:53 OPT/AVG E26 2000-02-07 15:00 2000-02-07 17:15 OPT/AVG I27 2000-02-23 17:52 2000-02-23 19:33 N/A I27 2000-03-27 16:00 2000-03-27 18:15 OPT/AVG G28 2000-05-22 00:12 2000-05-22 01:53 RTS G28 2000-06-23 19:30 2000-06-23 21:45 OPT/AVG G28 2000-08-31 07:42 2000-08-31 09:23 OPT/AVG G28 2000-10-18 22:12 2000-10-18 23:53 OPT/AVG G28 2000-12-07 21:42 2000-12-07 23:23 RTS G29 2000-12-30 01:32 2000-12-30 03:13 RTS G29 2001-03-07 17:00 2001-03-07 19:15 OPT/AVG G29 2001-05-14 16:00 2001-05-14 18:15 OPT/AVG C30 2001-05-26 01:00 2001-05-26 02:41 N/A C30 2001-07-05 23:00 2001-07-06 01:15 OPT/AVG I31 2001-08-07 20:00 2001-08-07 21:41 N/A I31 2001-08-23 23:00 2001-08-24 01:15 OPT/AVG I32 2001-10-17 03:40 2001-10-17 05:21 RTS I32 2001-11-19 16:00 2001-11-19 18:15 OPT/AVG I33 2002-01-18 18:00 2002-01-18 19:41 RTS I33 2002-04-03 16:00 2002-04-03 18:15 OPT/AVG I33 2002-06-05 16:00 2002-06-05 19:15 OPT/AVG I33 2002-10-07 17:20 2002-10-07 20:35 OPT/AVG I33 2002-10-24 17:30 2002-10-24 20:45 RTS A34 2002-11-13 23:30 2002-11-14 02:11 N/A A34 2003-01-07 22:00 2003-01-08 01:15 OPT/AVG Anomalies --------- Sector Data Loss Due to Thruster Flushes: Thruster flushes (RPM PROPEL LINES VENT) cause the loss of sector (rotor twist) data onboard the spacecraft. This loss results in the corruption of RTS and opt/avg data which depend upon sector data for their onboard despinning. As a result RTS and opt/avg data taken during thruster flushes should not be trusted. Where they have been identified intervals of data affected by this problem have been removed from these collections. Offset Anomalies: Table 7 lists the offset anomalies that occurred over the course of the mission. These anomalies typically consisted in an instantaneous jump in the zero-level over all three sensors, though in some cases there was a slower drift. The anomaly times represent the times at which this jump was evident in the data. In some instances, these events occcurred when the affected magnetometer was not in use. In these cases the offsets had one set of values when turned off, and a different set when turned on some time later. The time given for these cases is the time at which the magnetometer was turned back and the anomaly is indicated as having happend 'before' that time. While zero-levels for the inboard magnetometer did change over time, as evidenced by the below table the outboard magnetometer proved much more unstable than the inboard (URLR was the mode most commonly used in the outboard magnetometer). Because of its instability, the outboard instrument it was not used between C10 and G29. Had it been used additional anomalies may well have been observed in this time period. ---------------------------------- Table 7. Offset Anomalies ---------------------------------- Orbit Mode Time ---------------------------------- E4 URLR before 96-12-29 21:10 E6 URLR before 97-03-03 19:10 G7 URLR before 97-04-12 07:00 G7 URLR 97-04-16 15:19 C9 IRLR 97-06-22 16:05 C9 URLR before 97-07-02 10:00 C10 URLR before 97-09-25 19:38 C10 URLR 97-10-05 14:53 G29 URLR 01-02-07 06:11 C30 URLR before 01-05-29 14:00 I31 URLR before 01-08-12 02:30 I31 URLR 01-09-12 08:00 I32 URLR before 01-10-21 15:30 Jupiter Orbital Insertion (JOI): Prior to the JOI burn the spacecraft spin rate was increased in order to make it more stable for the firing of the main engine. In addition there was no high-resolution AACS through this interval. As a result of the variable spin rate and lack of high-resolution AACS it was not possible despin the MAG data. As a result only the magnetic-field magnitude is provided. Callisto 3 Orbit: The data from the inboard sensor, low field, flip right mode acquired during the third orbit was irrecoverably corrupted. Data from this orbit should be used with caution, particularly in applications where the field orientation needs to be known to better than a few degrees. At the beginning of the orbit, the inboard sensor was commanded to flip right. We believe that when this flip occurred, the sensor triad failed to lock into place properly. As a result, the sensor triad appears to 'jiggle' resulting in an unstable geometry. The extent of this jiggle is unknown but is believed to be on the order of a degree or so. One of the impacts of the unstable sensor geometry is that the sensor offsets, which are a combination of spacecraft fields and sensor zero levels, vary on short time scales. In order to reduce the spin harmonics attributable to offsets in this orbit, the RTS data have been averaged using a 120 second window on 24 second centers. Callisto 9 and 10 Orbit Opt/Avg: In orbits C9 and C10 the magnetometer was commanded into opt/avg mode without deselecting the instrument from RTS. As a result the Command and Data Subsystem (CDS) overwrote the opt/avg rate filter constant stored in MAG memory with a RTS rate filter constant. This resulted in the data being under filtered relative to the data sample rate. This under filtering shows up as spin-tone in the spinning sensors in spacecraft coordinates. In order to eliminate the spin-tone the raw data has been averaged (two point averages on one point centers). Averaging has been performed on the following intervals: ---------------------------------------- Table 8. Averaged intervals for underfiltered opt/avg data ---------------------------------------- Orb Start Time Stop Time ---------------------------------------- C9 1997-07-10T18:15 1997-07-11T10:58 C9 1997-08-01T14:50 1997-08-02T17:07 C9 1997-09-11T17:04 1997-09-11T21:01 C10 1997-09-29T11:36 1997-09-30T11:52 Europa 17 MAG and AACS instability: Between 1998-09-26T15:46:24 and 1998-09-26T16:42:01 the AACS data are unstable. Apparently, some form of AACS calibration was occurring. During this period, the magnetometer spin plane components appear to slowly rotate and then jump back to a correct orientation. This problem occurred on the spacecraft and has not been corrected in the ground system processing. Europa 19 AACS Data Gap: Due to a spacecraft anomaly AACS data from 1999-01-31 16:02:06 - 1999-02-01 01:49:37 (which were being stored in the Multi User Buffer) were never returned. The MAG data from this interval, however, were written to tape and returned later. Recovering the lost AACS - specifically the rotor right-ascension (RA), and declination (DEC) - was complicated by two Science Instrument Turns (SITURN's) which occurred in the missing interval. In between the SITURN's, RA and DEC were interpolated by the following means: 1/31 16:02:06 to 18:01:28 - set to last known value 1/31 18:01:28 to 18:26 - linearly interpolated 1/31 18:26 to 19:31:27 - assumed constant between SITURN's and determined by examination 1/31 19:31:27 to 19:49 - linearly interpolated 1/31 19:49 to 2/1 01:49:37 - set to next known value (taken from high-res. data, the E19-EUR recording) AACS instability Callisto 21 - Callisto 23: The AACS data for the Callisto 21, Callisto 22, and Callisto 23 perijove recordings (C21-PJOV, C22-PJOV, and C23-PJOV) were seriously corrupted. While similar problems are not evident in the RTS and opt/avg data, care should be taken in the use of these lower rate data as well. For Orbit 23 it was necessary to interpolate the rotor twist and spin angles of the intervals: 1999-09-17 05:26:41 to 06:08:30 and 1999-09-27 16:06:58 to 16:28:53. Io 25 Data Gaps Due to Corruption by Snapshots: Following recovery from spacecraft safing, MAG was inadvertently left in snapshot mode. As a result there are numerous short gaps where the RTS data were corrupted by snapshots, and a large gap where the OPT/AVG data were overwritten. This problem affects the interval 1999-11-26 04:32 to 1999-11-29 00:54. Europa 26 Data Rate Anomaly: The packet headers for the data from the time interval 2000-01-01 23:30:00.000 to 2000-01-03 14:08:39.742 indicated that the data were sampled at 18 minor frames (mf) or 12 seconds sampling resolution. This data rate matched the MAG RTS format reported in the sequence (format E). In the original 12 second data there were a series of systematic gaps. The end of each gap occurred where the packet of MAG data included a time tag. This would be the case if an incorrect sampling rate were applied to the data in packets where no time tag is given, then the data were to jump forward to the correct time when a time tag was given. Accordingly the data in this interval were re-time tagged at 36 mf (24 second) samples. This done the data became continuous across what were previously gaps -- both in time (the data are evenly sampled), and data value (the data contain no steps). The reason for this discrepancy was unknown. Io 33 Offset Instability: The spin-plane offsets in the outboard sensor (URLR mode) were highly unstable for the RTS data. This includes the following intervals: 2002-01-22 20:30 - 2002-01-31 13:00 2002-10-22 00:00 - 2002-10-31 12:30 While an attempt was made to correct the offsets, some problems may still exist (see 'Zero Levels'). Additionally, the zero level instability may indicate that the sensor was about to fail. For both of these reasons, the data from these two intervals should be treated with some suspicion. Io 33 opt/avg data timing: The timing of the opt/avg data in the interval 2002-Sep-28 13:00 to 2002-Oct-02 23:15 are somewhat questionable and the data should be treated with some caution. According to the planned sequence, the data collected after the opt/avg on command given at 2002-09-28 12:05, were due to be read out with the MRO scheduled for 2002-10-02 18:00. However, due to the spacecraft safing that occurred 2002-10-02 03:38 the MRO was not executed. Since the spacecraft was still safed, the subsequent opt/avg on command (2002-10-02 19:05) was also not executed and the data were not overwritten. Thus when the MRO commanded at 2002-10-07 10:00 was executed the data returned appear to have been from the opt/avg at 2002-09-28 12:05 -- and not from the opt/avg at 2002-10-02 19:05 as originally planned. The data in this interval have been timed according to this assumed sequence of events. Amalthea 34 data coverage: Spacecraft operations ended on 2003-01-15. As a result there is a large gap in coverage from about that time until 2003-09-21 when operations resumed briefly to record the spacecraft's plunge into the Jovian atmosphere. Between these two times an occasional single (8 hour) pass of data was provided as a monitor of instrument health. However, these monitoring data are not included as part of this collection. The reasons for omitting these data are that their sporatic nature limits their usefulness, and a lack of AACS data prevents them from being fully processed. These monitoring data are included in the Galileo at Jupiter MAG raw bundle. Jupter 35 AACS: The AACS returned in J35 had Earth Receive Times (ERTs) on 2003-09-21, but Spacecraft Event Times (SCETs) on 2003-01-22. In processing the J35 data it was assumed that the ERTs were correct and that the SCETs were incorrect, meaning that the values of RA and DEC were approximately correct. Due to the difficulty of precisely determining the correct timing of the AACS data, the RA, DEC, and rotor spin delta were set to the most common values for the interval, and the rotor spin angle, and rotor twist angle were calculated from them. Product Versions ================ The following table describes the various versions of the data files. -------------------------------------------------------------------- Table 9. Data product versions -------------------------------------------------------------------- Product Version ID Description -------------------------------------------------------------------- 1 original release 2 opt/avg data timing was corrected: a previous error in the time tagging of the opt/avg data resulted in that data being shifted earlier in time by one averager step. 3 JSO coordinates were added (and included with JSM coordinates); a 'CORRECTED TIME' column, which has a filter response time applied to the 'SAMPLE TIME' was added to the SYS3, JSE, and JSO/JSM coordinate data files; SYS3 coordinate file was modified to include both east and west longitude columns; a variety of changes were made to record formats Limitations =========== There are no particular limitations or warnings regarding the use of these data, in general, that are not described above. Users should review the PDS label files associated with individual data files for file (orbit) specific warnings.