Data set Overview
  =================
 
   On Dec 15, 1984, the Vega spacecraft was launched to first flyby Venus
   at which time landers were released and then continue to a flyby of
   Halley. The magnetic field experiments MISCHA carried four fluxgate
   sensors, with three sensors mounted on a boom at the end of the solar
   panels and the fourth sensor mounted one meter closer.  During the
   cruise phase, the TRASSA-1 mode of the instrument (1 vector/2.5 min) was
   used.  The sensors were switched to the TRASSA-2 mode (1 vector/min)
   during the flyby which started roughly two days before the encounter.
   From 3 h before closest approach (CA) until 1 hour after CA, the HS-mode
   (1 vector/6s) and the DT-mode (1 vector/100ms) was used [DELVAETAL1991].
 
 
  Parameters
  ==========
 
   The MISCHA experiment delivered a time series of measurements of the
   magnetic field.   Four quantities were collected: three rectangular
   components (Bx, By, Bz) at one position and a fourth component, Bu, in
   the direction of Bx, but at a position closer to the spacecraft.  The
   field components measured by the sensor are composed of both the
   interplanetary contribution and the field from the spacecraft called the
   offset.
 
 
  Processing
  ==========
 
   Data are first converted from the Rohdata files to Flat Files with an
   ASCII .hed file (/DOCUMENT/PROCESS/DATACONV/flatspec); this is
   ''original'' data. A further processing is done to provide offsets and
   corrections for the spacecraft spin; two methods are used for this step
   which are outlined in the calibration paper [DELVAETAL1990] which is
   provided as a series of GIF images located in the 'DOCUMENT/CALIB'
   directory.   The main correction is to account for the magnetic field of
   the spacecraft and the offsets introduced by the spin of the spacecraft
   around one primary axis.
 
   Offsets were also determined through a correlation technique or through
   a set of linear equations. The actual method used is given through a
   number after the actual offset value (for the master data form) :
 
   example:
   22 dec 84 13 34 22             -> begin of time interval
   23 dec 84 08 17 00             -> end   ''  ''     ''
      5.5  0.0  0.5  9.5          -> offset values Ox,Oy,Oz,Ou
      0                           -> method (0-spin,1-Russell,2-Hedgecock)
      0.                          -> flag to be set (0.= NO problems)
 
 
  Data
  ====
 
  The data are provided in five forms: original, master, survey, FBT1SE,
  and FBT2SE.
 
  Original file
 
  The flat file of original data has components of the magnetic field, as
  measured by the MISCHA experiment, in the coordinate system of the
  spacecraft, termed PSeudo SOlar (PSSO).   In addition, a Bt and BUX (Bu-
  Bx) is provided as well as a clock time with epoch started on Jan 1,
  1965; a quality flag indicates missing data. This data is in the EXTRAS
  area.
 
 
  MASTER file
 
  The MASTER file of the magnetic field data, measured by the MISCHA -
  experiment on board of the Vega spacecraft, contains the following types
  of data: the magnetic field data, the position of the space- craft in
  heliocentric (solar) ecliptic coordinates and in heliographic coordinates
  from the prime meridian, information about the orientation of the
  spacecraft, magnetic field data after calibration and rotation to a
  coordinate system centered at the spacecraft with x - axis to the sun and
  (x,y) - plane in its orbital plane ( i.e. equal to the ecliptic). In
  addition, quality flags give information about missing or bad data.
 
  SURVEY file
 
  The SURVEY file of the magnetic field data, measured by the MISCHA -
  experiment on board of the Vega spacecraft, contains the following types
  of data: the position of the spacecraft in heliocentric (solar) ecliptic
  coordinates (H.SE), magnetic field data in a coordinate system centered
  at the spacecraft with x - axis to the sun and (x,y) - plane in its
  orbital plane (i.e. almost equal to the ecliptic). In addition, quality
  flags give information about missing or bad data.
 
 
  Ancillary Data
  ===============
 
  During some short time periods , the spacecraft is spinning about one
  fixed axis (x-axis).  As described in the processing reference, the
  state  of the spacecraft spin are called spinning sessions as outlined
  in the table below (which is also provided in
  /DOCUMENT/OFFSET/B0OFFSET.ASC):
 
  1.  spinning sessions of vega1
  ______________________________
     from                     to                       flatfile
     date, hour           day  date, hour       day
  1  22-12-84 15:16:28  357  23-12-84 09:58:58  358    b01ct1
  2  26-12-84 14:34:44  361  14-01-85 09:54:39   14     ''
  3  18-01-85 09:02:08   18  21-01-85 08:14:39   21     ''
  4  02-02-85 10:12:09   33  18-02-85 13:28:45   49     ''
  5  09-03-85 09:32:42   68  14-03-85 07:26:27   73     ''
 
 
 
  Coordinate Systems
  ==================
 
  The MISCHA data were initially submitted in multiple coordinate systems,
  but after discussions with the submitter, the following three are being
  reviewed.
 
  PSSO - coordinate system: PSuedo SOlar coordinate system: spacecraft
  centered, x-axis only approximately directed to the sun (slight deviation
  possible), z-axis only approximately to ecliptic pole and (xy)- plane
  approximately the ecliptic in case of three-axis-stabilisation; in case
  of one-axis stabilisation of spacecraft no information about z-axis.
 
  SE - coordinate system: Solar Ecliptic system: spacecraft-centered, x-
  axis to sun, (xy)-plane is orbital plane of spacecraft, i.e. nearly equal
  to the ecliptic plane, z-axis nearly to ecliptic pole.
 
  H.SE - coordinate system: Heliocentric Solar Ecliptic system: (xy)-plane
  is the ecliptic, x-axis to vernal equinox 1950, z-axis to ecliptic pole
 
 
  Software
  ========
 
  An initial program, MISCHA, is used to convert the Rohrdata to flat
  files; the MASTERFILE program applies the offset corrections using a
  B0*offse.* file.
 
  The files flattesthed.asc/.dat and example.asc can be used to control
  data conversion from the flat file data format to an other data format.
  The files contain the same data so it is easy to compare the conversion.
 
  For more information about the flat file data format read the Flat File
  Specification document (flatspec.doc)
 
 
  Media/Format
  ============
 
  The standard distribution format for the data is CDROM.

Confidence Level Overview
  =========================
 
   For SE - FLYBY data-files:
 
   1. Additional offset correction was done very carefully with variable
   offsets, in addition to the constant ones already applied Constant
   offsets are for V1: (-10, -5, 40) Example for additional offset
   correction: for 1986-03-05  07:21:15:00:  (Ox, Oy, Oz= -3, 4, 1  in PSSO
   -data) This leads to a substantial difference in B- total after offset-
   correction!
 
   Transformation to SE from PSSO was done with variable matrices,
   according to the S/C position. However, the difference should not be too
   large for times close to the closest approach .
 
   The Z(PSSO) sensor clearly saturates near C/A; there is no mention of
   the digitization/quantization that was performed.
 
 
  Review
  ======
 
  The MISHCA data is part of the Small Bodies Node review in Dec 2002.
 
 
  Data Coverage and Quality
  =========================
 
  During the cruise and flyby phases there are data gaps which are not
  explained.  In addition, for Vega 2, one of the sensors was lost after
  encounter on Mar 9, 1986, so from that time forward, only Bx data was
  available.
 
 
  Limitations
  ===========
 
  Since the SE-data have been better offset-corrected and better
  transformed, it is recommenced to use the SE data as much as possible;
  they should be more reliable.