PDS_VERSION_ID       = PDS3
RECORD_TYPE          = STREAM
^TEXT                = DATASET.TXT"
OBJECT               = TEXT
  PUBLICATION.TABE   = 1996-01-01
  NOTE               = DATASET.TXT describes the dataset."
END_OBJECT           = TEXT
END

Dataset Overview:

GO-SS-MAG-4-SUMM-CRUISE-IRC-V1.0 contains data acquired by the Galileo 
Magnetometer during the Interplanetary Cruise to Jupiter. The data sampling 
rate varies with the averaging constant applied by the instrument.
These data have been fully processed to remove instrument response
function characteristics. The data are provided in physical units
(nanoTesla) and in Inertial Rotor (IRC = despun spacecraft) coordinates. 
These files contain some of the data processing parameters from the 
AACS system as well as the sensor zero level for the spin aligned sensor.
The data are also being archived in RTN coordinates which are provided on
this CDROM as a separate dataset. Trajectory have been provided in 
heliographic coordinates also as a separate dataset.

In order to provide a more complete solar wind dataset the high resolution
data from the Earth encounters were averaged down to 16 minute resolution and 
merged with the data from the optimal averager section of the instrument. The 
data inside of the Earth's bowshock is not included. The 16 minute 
averages are derived from the following datasets:

       GO-E-MAG-3-RDR-E1-HIRES-V1.0
       GO-E-MAG-3-RDR-E2-HIRES-V1.0


    Data Columns (IRC):

    sc_clk    S/C clock counter given in the form rim:mod91:mod10:mod8
    time      S/C event time (UT) given in the form yyymmddT<time>Z
    Bx_sc     Magnetic field X component in S/C (IRC) coordinates <nT>
    By_sc     Magnetic field Y component in S/C (IRC) coordinates <nT>
    Bz_sc     Magnetic field Z component in S/C (IRC) coordinates <nT>
    Bmag      |B| Magnitude of B <nT>
    zoff      Offset subtracted from the z-sensor
    avg_con   Averaging Constant (Rims)
    rotattd   Rotor attitude declination (EME-50)
    rotattr   Rotor attitude right ascension
    t         Rotor twist angle (EME-50) minus Rotor spin angle(IRC coordinates)

Data Acquisition:

The data for this dataset were acquired as part of the normal instrument
calibration activities associated with the cruise to Jupiter. As such,
the instrument was commonly configured in modes which required calibration 
even though they may not have been the optimal mode for science data 
acquisition. The Galileo magnetometer has 8 possible acquisition 
configurations (modes). There are two sensor triads mounted 7 and 11 meters 
from the rotor spin axis (inboard and outboard) along the boom. Each of 
the sensor triads has two gain states (high and low). In addition, the 
sensor triads can be "flipped" to move the spacecraft spin-axis
aligned sensor into the spin plane and visa versa. Please see the instrument
description for full details on the instrument, sensors, and geometries. The
combinations of sensor, gain state, and flip direction form modes. 

   _____________________________________________________________________
                             Mode Characteristics
   _____________________________________________________________________
            Mode Name            Acronym         range      quantization
   _____________________________________________________________________
   Inboard,  left,  high range*    ILHR      +/- 16384 nT       8.0   nT
   Inboard,  right, high range*    IRHR      +/- 16384 nT       8.0   nT
   Inboard,  left,  low  range*    ILLR      +/-   512 nT       0.25  nT
   Inboard,  right, low  range*    IRLR      +/-   512 nT       0.25  nT
   Outboard, left,  high range*    ULHR      +/-   512 nT       0.25  nT
   Outboard, right, high range*    URHR      +/-   512 nT       0.25  nT
   Outboard, left,  low  range*    ULLR      +/-    32 nT       0.008 nT
   Outboard, right, low  range*    URLR      +/-    32 nT       0.008 nT


                  __________________________________________
                   s/c clock         date/time          mode
                  __________________________________________
                   00104651:00:0    89-353/20:53        ULLR
                   00120107:00:0    89-364/17:21        URHR
                   00120117:00:0    89-364/17:31        IRHR
                   00120150:00:0    89-364/18:04        IRLR
                   00120178:00:0    89-364/18:33        ILLR
                   00120199:00:0    89-364/18:54        ILHR
                   00120209:00:0    89-364/19:04        URHR
                   00120231:00:0    89-364/19:26        URLR
                   00120283:00:0    89-364/20:19        ULLR
                   00120313:00:0    89-364/20:49        ULHR
                   00572976:00:0    90-040/03:00        ULHR
                   00572976:00:0    90-316/17:00        ULLR
                   00578673:00:0    90-320/17:00        URLR
                   00586204:00:0    90-325/23:55        URHR
                   00592915:00:0    90-330/17:01        ILLR
                   00597439:00:0    90-333/21:15        IRLR
                   00610156:00:0    90-342/19:33        IRHR
                   00610509:00:0    90-343/01:30        IRLR
                   00615701:00:0    90-346/17:00        URLR
                   00618550:00:0    90-348/17:00        URHR
                   00624261:00:0    90-352/17:15        ULHR
                   01003052:72:8    90-253/16:36        ULLR
                   01600269:00:0    92-308/00:48        ULLR
                   01605562:00:0    92-311/18:10        ILLR
                   01629549:00:0    92-328/14:14        IRLR
                   01632087:00:0    92-330/09:00        ULLR
                   01635479:00:0    92-332/18:23        URHR
                   01638675:00:0    92-335/00:01        URLR
                   01639786:00:0    92-335/18:45        IRLR
                   01639974:00:0    92-335/21:55        URLR
                   01640958:00:0    92-336/14:30        IRLR
                   01641285:00:0    92-336/20:40        URHR
                   01641351:00:0    92-336/21:07        IRHR
                   01641407:00:0    92-336/22:15        ILHR
                   01641465:00:0    92-336/23:02        URLR
                   01642481:00:0    92-337/16:22        ULHR
                   01644143:00:0    92-338/20:10        IRLR
                   01646048:00:0    92-340/04:16        ILLR
                   01649533:00:0    92-342/15:00        ULLR
                   01650524:00:0    92-343/07:42        ULHR
                   01650907:00:0    92-343/14:09        ILHR
                   01651026:00:0    92-343/16:10        ULHR
                   01656418:00:0    92-347/11:01        ILLR
                   01659057:00:0    92-349/07:30        ULLR
                   02271339:18:2    94-048/05:34        URLR
                   02378309:54:6    94-123/08:13        ULLR
                   02618987:18:2    94-292/08:04        URLR
                   02702827:18:2    94-351/04:56        ULLR


    * Range is the opposite of gain - high range equals low gain
 
Coordinate Systems:

The data are being archived in two coordinate systems. The first coordinate
system is referred to as inertial rotor coordinates (IRC). This coordinate
system has the Z axis along the rotor spin axis, with the antenna along 
negative Z and the X and Y axes in the rotor spin plane. In a crude sense,
when the spacecraft is far from Earth, +X points south, normal to the
ecliptic plane, positive Y lies in the ecliptic plane in the sense of
Jupiter's orbital motion and positive Z is in the anti-earth direction. The
spacecraft antenna (negative Z direction) is kept earthward pointing to
about +/- 10 deg accuracy.

The second coordinate system is RTN (radial-tangential-normal) coordinates. 
The radial direction is taken along the Sun->S/C line, positive away from the
sun. The tangential direction is found by taking the cross product of the sun
spin axis with the radial (T = Omega x R) direction. Finally, the normal
direction is the cross product of R and T (N = R x T).

Onboard Spacecraft Processing:

The majority of the processing of the interplanetary cruise data is
done onboard the spacecraft. The following procedures take place
onboard.

1) Data are recursively filtered and decimated to 4.5 vectors/second
2) Sensor zero levels are subtracted and gains are applied
3) Data is transformed from a sensor coordinate system to the rotor 
   coordinate system using an orthogonality matrix
4) Data are decimated again to 2/3 second resolution
5) Data are despun into inertial rotor coordinates (IRC) 
6) A recursive filter is applied and data are decimated to a variable
   sample rate

These steps are explained in further detail below.

1) Recursive filter and decimation

   The Galileo magnetometer samples the magnetic field 30 times per second.
   These highest rate samples are recursively filtered and then resampled
   by the instrument at 4.5 vectors per second using a 7,7,6 decimation
   pattern.
   
       Recursive Filter
    
       B(t) = 1/4 Bs(t) + 3/4 B(t-1)
    
       B  = output field
       Bs = input field measured by the sensor
       t  = sample time
   
   The pattern is generated by doubling the spacecraft clock modulo 10 counter
   and then applying the decimation scheme. This gives 3 vectors every
   spacecraft minor frame (about 2/3 second) which are sampled unevenly. The
   first vector in a minor frame is sampled approximately 0.200 seconds
   after the last vector in the preceding minor frame. The other two samples
   are taken approximately 0.233 seconds apart. 

2) The data is processed onboard the spacecraft using a fixed zero level 
   which is provided at the start of each data acquisition interval.
   These zero levels were determined using various methods and were
   periodically updated as significant changes in level were noted.
   Knowledge of the two spin plane sensor zero levels is not critical.
   The data is generally averaged over a long enough interval that any error
   in the zero levels is removed. Knowledge of the spin 
   aligned sensor zero level is critical. However, this can
   be corrected for once the data has been received on the ground
   and is discussed below under Ground Data Processing. 

3) Orthogonality matrix applied: The determination of a calibration matrix 
   is too complex to describe here. The method employed has been well 
   described in "Accurate Determination of Magnetic  Field Gradients
   From Four Point Vector Constraints on Vector Data Obtained from
   a Single Spinning Spacecraft" , E.L. Kepko, K.K. Khurana, M.G. Kivelson,
   R. C Elphic and C. T. Russell (in press).

4) The data is decimated to 2/3 second resolution by keeping 
   only the first minor frame of data.

5) Despinning data to inertial rotor coordinates: The  nominal
   transformation to IRC from SRC is 

      (Bx)   ( cos(theta)   -sin(theta)  0 ) (Bxs)
      (By) = ( sin(theta)    cos(theta)  0 ) (Bys)
      (Bz)   (   0               0       1 ) (Bzs)

   Where s denotes spinning coordinates and theta is the rotor spin angle.

6) Recursive filter and decimation: A recursive filter is applied in
   in the following manner.

   Recursive Filter  
   
       B(t) = 1/C Bs(t) + (1-1/C) B(t-1)
   
       B  = output field 
       Bs = input field measured by the sensor
       t  = sample time
       C  = averaging constant

   The averaging constant is variable and is given at the start of
   each data acquisition period. The data is then decimated. There
   is a one to one correspondence between the averaging constant
   used and the decimation factor as outlined below.

   Time per sample(Rims)       Averaging constant    Decimation Factor
   -------------------------------------------------------------------
          1                        1024                   1
          2                         512                   2
          4                         256                   4
          8                         128                   8
         16                          64                  16
         32                          32                  32
         64                          16                  64
        128                           8                 128
        256                           4                 256
        512                           2                 512
       1024                           1                1024

            
Ground Data Processing:

  Once the data is on the ground there is very little that can be done
  to improve the onboard processing because of the heavy filtering
  and decimation. The following procedure is done on the ground

1) Data is converted to nanoTesla
2) The zero level of the spin aligned sensor is corrected.
3) Timing is set to account for a phase delay
4) Rotation is performed to account for phase delay 
5) Data are transformed to RTN coordinates to produce the RTN dataset.

These steps are outlined in greater detail below.
 
1) Conversion to nanoTesla simply requires dividing the instrument data
   numbers by a constant scale factor. For the inboard high range (low gain)
   mode the scale factor is 2. For the inboard low range and outboard high
   range, the scale factor is 64. The outboard low range data has a scale
   factor of 1024.
 
2) The zero level of the spin axis aligned sensor was determined by several
   means. One method is to take 25 day averages of the data from the
   spin aligned sensor. The basis of this method is that if the
   spacecraft is relatively close to the ecliptic plane, over one solar
   rotation the average of the suns magnetic field in the spacecraft
   z-direction is zero. Therefore whatever remains after averaging is 
   taken to be an offset. The second method takes advantage of the sector 
   structure of the solar wind. In this method we look only at points where 
   heliographic radial component (Y IRC coordinates) crosses from
   positive to negative values. Here we make use of the fact that when a 
   sector boundary is crossed, both the radial and tangential components of 
   the field change sign together. The spin aligned sensor (Z - IRC) measures 
   the tangential component of the IMF. This method then uses 50 day averages 
   of the short intervals about the sector crossings to determine the zero  
   level of the spin aligned sensor. Results of this analysis method 
   are in good agreement with the solar rotation (25 day) averaging mehtod.

   For the majority of the cruise data the instrument was
   configured in the ULLR mode. This mode puts sensor 3 along the spin
   axis. After looking at the results of these two method it became
   apparent that the spin aligned sensor 3 offset had a substantial
   drift that was correlated to the change in temperature. The offset
   determinations  for sensor 3 were fit to temperature. It was found
   that two different fits were necessary. For temperatures greater
   than 272K a straight line fit was used.

     zoff = -17.60926 + .02608*(Temp)

     zoff = Offset for sensor three [nT]
     Temp = Temperature at the outboard sensor [K]


   For temperatures less than 272K a second fit was determined.

     zoff = 22.9016799 + (2511.27490/Temp) - (3176088.25)/(Temp*Temp)

     zoff = Offset for sensor three [nT]
     Temp = Temperature at the outboard sensor [degrees K]
  
   A second instrument configuration commonly used during the
   cruise to Jupiter was the URLR mode. This mode places sensor2
   along the spin aligned axis. For unknown reasons this sensor
   was not as susceptible to changes in temperature and showed
   no strong zero level drifts over time. A fixed offset of 
   2.75574nT was removed from sensor 2.  

3) When the instrument receives and optimal averager on command, the 
   instrument buffers the command until the next rim boundary and then begins 
   filtering the data. The first data sample is taken on the even modulo (%) 
   of the decimation (avg_con) rate. Also, in order to account for the phase  
   delay in the recursive filter, a 1/e factor of the sample rate is removed  
   from the time stamp. This should effectively centers the time stamp in the 
   filtered data. The first sample of a new averaging interval may be under 
   filtered. 

   rim=rim + (avg_con - (rim % avg_con)) - 1/e*avg_con  

   rim     = starting rim of the averaging interval
   avg_con = length of the averaging interval(rims)
   
4) There is another phase delay associated with the Analog to Digital
   converter of the instrument. This means that when the data is 
   despun onboard the spacecraft, the data and the angle used to
   despin it are slightly out of phase. In order to correct this
   delay the data are rotated by a phase angle. The delay is
   equivalent to the rotation outlined below.

   Bx'= cos(delay) * Bx - sin(delay) * By
   By'= sin(delay) * Bx - cos(delay) * By

   delay = -0.045163 radians 
   
To transform the data to RTN coordinates the following step is taken

5) The data are transformed first into Earth Mean Equatorial(equinox 1950)
   coordinates. This system is directly supported by the SPICE software
   provided by the Navigation and Ancillary Information Facility (NAIF)
   at JPL as inertial coordinate system 'FK4'. The angles required for
   this transformation come directly from the Galileo Attitude and
   Articulation Control System (AACS) data.  The transformation matrix
   for this rotation is:


   --                                                                        --
   | (cosTsinDcosR - sinTsinR)  (-sinDsinTcosR - cosTsinR)      cosDcosR      |
   |                                                                          |
   | (cosTsinDsinR + sinTcosR)  (-sinDsinTsinR + cosTcosR)      cosDsinR      |
   |                                                                          |
   | -cosDcosT                  sinTcosD                        sinD          |
   --                                                                        --
 
   where

   R = Rotor-Right Ascension
   D = Rotor-Declination
   T = Rotor-Twist - Rotor-Spin-angle  (despun data)

   Once in an inertial coordinate system, SPICE software provides subroutines 
   which return the transformation matrices to GSE (G_GSETRN), GSM (G_GSMTRN),
   or RTN (G_RTNTRN) coordinate systems for any ephemeris time. These 
   matrices have been used to perform the coordinate system transformations.
   The spacecraft/planet (SPK) , leapsecond (TS), and planetary constants (PCK)
   kernels required for these transformations have been archived in the PDS
   by NAIF. These SPICE kernels are available on the CD_ROM which contains the
   magnetometer data. The SPICE toolkit (software) can be obtained from the
   NAIF node of the PDS for many different platforms and operating systems.
   At the time of this archive, the SPICE toolkit was available via an
   anonymous-ftp site at naif.jpl.nasa.gov
 
Data Quality:

Data quality assessment is a rather vague concept which we will try to address
in a somewhat qualitative manner. Each aspect of the data processing sequence
can be analyzed to determine its maximum possible error contribution. In theory,
these errors could be summed to provide estimates of the maximum error for 
each data point. We have not taken our error analysis to that level. We believe
that our calibrations (sensor geometry and gains) are good enough that they
produce a negligible source of data error. In addition, we believe that the
coordinate system transformations which are derived from the SPICE kernels
and Toolkit are negligible sources of error in the magnetic field vectors. The
sources of error which we feel are the most significant are those associated
with magnetic sources aboard the spacecraft, especially those with temporal
or spacecraft orientation variations. The next greatest contributor of error is
the data from the AACS which affects our knowledge of the spacecraft orientation
and hence rotates the magnetic field vector. Lastly, telemetry or software 
errors which produce "spikes" or bit errors in the data are error sources.

Magnetic sources associated with the spacecraft have been identified.
Over the long averaging intervals of all the interplanetary cruise data
this source of noise is eliminated. There are some time intervals when the 
zero levels of the spin plane sensors show large variations (1-5 nT) on short 
time scales (minutes to hours). After a while (hours to days) the offsets 
return to their nominal levels. The source of these magnetic fields has not yet 
been identified. The method of removing offsets does not remove these rapidly
varying effects.

Errors associated with AACS angles have various effects on the data. The 
rotor right ascension and declination are crucial to the understanding
of the spacecraft orientation. Fortunately, these angles are slowly varying
and can be interpolated to better than 1 degree of accuracy for long (many hour)
time periods except near major spacecraft maneuvers. 

During some time intervals no AACS data is available. As a result, these intervals
can not be transformed from spacecraft coordinates RTN coordinates.

During several intervals of the interplanetary cruise the spacecraft lost
it's sector data. Sector data gives the magnetometer the neccessary information 
to despin the magnetic field data spin plane components (Bx_sc and By_sc in 
spacecraft coordinates). If the magnetic field spin plane components are
not despun they will appear as a sine wave. The next step in the processing
done onboard the spacecraft is averaging. The data that is not despun
will be averaged to zero as you would expect a sine wave to be averaged
over many periods. The intervals where sector data was lost have not
been removed from the processed IRC dataset. The reason for this is that
these intervals can be valuable in helping to determine sensor zero
levels. These intervals have been cut from the processed data in RTN
coordinates. Below is a list of time where sector data was lost.

1990-Jan-26 19:35:48   to   1990-Jan-31 05:17:29
1991-Jan-16 20:24:58   to   1991-Jan-21 17:36:00
1991-Feb-12 18:47:00   to   1991-Feb-25 18:08:00
1993-Jan-27 09:00:40   to   1993-Jan-30 12:30:27


Ancillary data products:

TEMP  - The temperature of the outboard sensor is critical to 
        understanding the zero level of the spin aligned sensor.
 
        Columns in temperature files
        --------------------------------------------------
        name          description
        --------------------------------------------------
        time       S/C event time (UT) given in the form yyymmddT<time>Z
        out_temp   Temperature at the outboard sensor
        in_temp    Temperature at the inboard sensor