Data Set Overview : This data set contains magnetic-field data acquired during the cruise and tour phases of the Cassini mission to Saturn. The data set begins with data collected on 16 August (day 228), 1999 and ends on 17 November (day 321), 2005 when the instrument failed. On 16 August, 1999 two days before the spacecraft commenced Earth swingby, the magnetometer boom was unfurled into its extended position; a configuration it maintained for the remainder of the mission. Magnetic-field data acquired prior to boom deployment are limited and are significantly affected by spacecraft noise. These data are of no scientific value and, consequently, do not form part of this data set.  Cassini carried two magnetometers: a fluxgate magnetometer (FGM) and a vector-helium magnetometer capable of operating in both vector and scalar mode (V/SHM). This data set contains the magnetic- field and calibration data recorded by the helium magnetometer when operating in scalar mode. The data are divided into time periods of one day and saved in files using the UCLA flatfile system. In this system, data are recorded in binary files that have associated text header files describing the format and content of the data.  The magnetic-field data are contained in the SHM data products which can be identified from the 'SHM*_C' in their file names. The SHM submitted is already calibrated.  Data are received from Cassini in science or housekeeping telemetry packets. Data from both these streams form part of this data set. The naming convention used for data files allows the telemetry source and date of acquisition to be readily determined from the file name. On any given date, the science and housekeeping data for a particular magnetometer cover the same time interval to within a few seconds. Science and housekeeping data files have identical formats and are processed in exactly the same way.  The magnetic-field data have been extracted from telemetry packets, assigned time tags, and formatted into simple binary tables of values in nano-Teslas.  The MAG data products in this dataset is: Data Description SHM_C Calibrated scalar data from helium magnetometer  The format of files containing this data product is described in full in the archive volume SIS, found on this volume called the 'THE CASSINI MAGNETIC FIELD INVESTIGATION' by Dougherty et al.  Data :  SHM data is given in the following columns: 1. time TAI (SCET in seconds since 12:00:00.000Z 1 Jan 2000) 2. B_SHM (in nT) 3. X_IAU_S (in Km) 4. Y_IAU_S (in Km) 5. Z_IAU_S (in Km)   Data Parameters : Magnetic-field units -------------------- The units of the magnetic-field data are given as nanoTesla (nT). They represent the magnetic field values at the given time.   MAGStatus --------- There are no status words in the calibrated files  MAG times --------- The Cassini spacecraft clock (SCLK) is a counter that advances by one tick nominally every 1/256 seconds. SCLK times have the format cccc:ttt, in which cccc specifies the number of full counts that have elapsed (one full count : 256 ticks), and ttt indicates by how many ticks the clock has advanced towards the next count, since the epoch 00:00:00Z 1 January 1958.  SCLK counts may also include a partition number, p/cccc:ttt. This number is initially 1 but is incremented during the mission if the SCLK counter is reset or somehow interrupted or altered. The following discussion assumes a partition number of 1. For other partition numbers, the determination of SCLK times requires knowledge of the time at which the current partition was initiated.  SCLK times are commonly recorded in MAG files as decimal counts. Time may also be represented in MAG files as Spacecraft Event Time (SCET) which, for Cassini, is Universal Time Coordinated (UTC). The relationship between SCLK and SCET/UTC is dependent on the count rate of the Cassini SCLK. Like most counter-based clocks, this rate is not constant but drifts with time. Consequently, conversion of SCLK times to SCET/UTC times requires knowledge of the drift rates. These rates are recorded in the SCLK/SCET coefficients file maintained by the Cassini Spacecraft Operations (SCO) team at JPL. As the Cassini mission progresses, the difference between SCLK and SCET will typically be of order tens of minutes.  Times in MAG data files  The times associated with magnetic-field scalar values in MAG data files are SCET in seconds since epoch 2000 in TAI (International Atomic Time)  Times in MAG header files  FIRST TIME SCLK time of first record in data file; derived from primary header of CHDO file  LAST TIME SCLK time of last record in data file; derived from primary header of CHDO file  SCLK (in ABSTRACT) SCLK count obtained from tertiary header of CHDO file; also converted into year, day of year, month, date, time format; may differ from FIRST TIME by some minutes  SCET (in ABSTRACT) year, day of year, month, date, time format; determined from corrected SCLK count; also converted into an equivalent SCET count of seconds since 1958  Times in MAG label files  START_TIME SCLK time of first record in data file; obtained from FIRST TIME in flatfile header  STOP_TIME SCLK time of last record in data file; obtained from LAST TIME in flatfile header  SPACECRAFT_CLOCK_START_COUNT SCLK time of first record in data file; determined from SPICE utility CHRONOS using START_TIME; format p/ssss.ttt  SPACECRAFT_CLOCK_STOP_COUNT SCLK time of last record in data file; determined from SPICE utility CHRONOS using STOP_TIME; format p/ssss.ttt  SCLK (in NOTE) SCLK count obtained from tertiary header of CHDO file; also converted into year, day of year, month, date, time format; obtained from flatfile-header ABSTRACT; may differ from START_TIME by some minutes  SCET (in NOTE) year, day of year, month, date, time format; determined from corrected SCLK time; also converted into an equivalent SCET count of seconds since 1958; obtained from flatfile-header ABSTRACT  Processing : The processing software converts time from SCLK(1958) to TAI, subtracts a small calibration correction from the scalar field values, and appends XYZ position of Cassini with respect to either Earth or Saturn in IAU_EARTH or IAU_SATURN coordinate systems respectively. Only valid Scalar data points (LOCK bit : TRUE) are written to the output flatfile. A delay of 0.7s is subtracted from each time tag, 0.5s because the input time stamp is the end of the 1s interval over which the measurement is made, and an additional 0.2s determined from a fit of ESB data to the model field. The additional 0.2s delay is subject to revision. In order to calculate the calibration correction to the SHM value of B, the approximate angles of the field are needed, which can be obtained from the FGM. So for each SHM value, an FGM vector avg is calculated over [t-1sec,t], where t is the SHM time. The FGM vector is then transformed into the SHM/VHM sensor coordinate system (nominal deployed orientations are used). A total correction, including a Bloch-Siegert correction, is then calculated.  Ancillary data : Data such as analog, command validation, configuration image, error counter and user-defined engineering data, are included in this archive for completeness and are not required for processing the magnetic-field data.

Confidence Level Overview : There is irregular timing in the data samples. This is caused by the limited resolution of the msec counter in the time field. For example, there are 128 FGM vectors in each science packet, the time of the packet corresponds to the time of the first vector in the packet. Times are calculated for the other vectors using the known onboard vector sample rate and average exponent. These calculated times don't have the reduced resolution of the msec counter. Thus at the first vector of each packet there can be a small time jump due to the msec counter resolution. This time difference is maximum 8 msecs (not 4msecs which is the resolution of the counter) because the DPU software only generates even msec values due to internal truncation.  Data Coverage and Quality ------------------------- There are routine events that cause data quality problems so chronological listings of them have been included in this archive: -calibration activities superimpose calibration data on top of science data and are documented in SCAS_TIMES.ASC (for more information on calibration activies refer to 'THE CASSINI MAGNETIC FIELD INVESTIGATION' by Dougherty et al in this archive) -data spikes may be seen at instrument range changes. Range changes are documented in RANGE_CHANGES.ASC -mode changes affect which sensors produce instrument data and spikes or rapid changes in data averaging may be seen at mode changes. The first few packets after the instrument is unmuted or after Science Packets recommence can be highly or incorrectly averaged. MODE_CHANGES.ASC lists the times of these events. Range changes are the only routine events that will normally effect SHM data.  Data gaps may be instrument related (e.g. a sensor turning on/off) or mission related (e.g. telemetry downlink problems). The former are documented in MODE_CHANGES.ASC. All mission related gaps are listed in GAP_FILEs - GAP_FILE_SCI_HK.ASC for science data in the housekeeping packets and in GAP_FILE_SCI_SD.ASC for science data in the science packets. Instrument related gaps of less than one day also show up in the GAP_FILES as well as in MODE_CHANGES.ASC  Most of the information in the GAP_FILEs are extracted from packets received reports and so follow a fixed-width tabular format. Multi-day gaps are not documented in the source listings and have been added manually with a different format including an explanation of the gap - if a reason is known. A summary of scientifically significant gaps is included below.  Gaps ----  SHM On 1999-230 Earth Swing-by 2000-209 Quiet Test no data collected 2000-267 Quiet Test no data collected 2003-236 SSR Library Test no data collected 2004-178 to 183 A VHM/SHM mode change was scheduled to occur at this time but it did not occur 2004-344 SHM test no data collected 2005-047T21:15:09 to 2005-048T04:29:00 SATURN INTFLD AND ENCELADUS FLYBY 2005-068T08:05:26 to 2005-068T15:09:18 ENCELADUS FLYBY, SATURN INTFLD AND TETHYS FLYBY 2005-088T23:59:45 to 2005-089T03:14:17 ENCELADUS FLYBY AND SATURN INTFLD A power failure and other technical problems resulted in 66% of science data from day 88 being lost. This includes a data gap from 19:30 to 24:00 during which the craft was flying by Enceladus and in Saturn's internal field. The SHM actually went on earlier than recorded but the first SHM data received is from 23:59:45. SHM on/VHM off 2005-104T19:00:25 to 2005-105T03:39:15 SATURN INTFLD AND MIMAS FLYBY SHM on/VHM off 2005-122T22:20:28 to 2005-123T05:54:19 TETHYS FLYBY AND SATURN INTFLD SHM on/VHM off 2005-141T02:35:26 to 2005-141T10:09:17 ENCELADUS FLYBY AND SATURN INTFLD SHM on/VHM off 2005-159T07:10:28 to 2005-159T14:44:19 SATURN INTFLD SHM on/VHM off 2005-177T12:13:29 to 2005-177T19:47:20 SATURN INTFLD SHM on/VHM off 2005-195T18:41:28 to 2005-196T02:14:19 ENCELADUS FLYBY AND SATURN INTFLD SHM on/VHM off 2005-214T02:21:27 to 2005-214T09:55:18 SATURN INTFLD AND MIMAS FLYBY SHM on/VHM off 2005-232T07:44:26 to 2005-232T15:19:18 SATURN INTFLD SHM on/VHM off 2005-248T07:26:24 to 2005-248T10:52:06 SATURN INTFLD SHM on/VHM off SHM off earlier than planned due to BIU sick 2005-266T17:30:24 to 2005-267T01:39:14 SATURN INTFLD AND TETHYS FLYBY SHM on/VHM off 2005-284T21:28:25 to 2005-285T05:36:16 SHM on/ VHM off 2005-322 V/SHM failed, data degraded data from or after this day is not suitable for science use and will not be submitted