MAGSAT MAGNETIC FIELD DATA
                            
                           INTRODUCTION
                                                      
 This directory contains half-second resolution magnetic field vectors from
 NASA's MAGSAT spacecraft (300-600 km altitude, 90 deg inclination) taken
 between November 2, 1979 and May 6,1980. 
                              
                    DESCRIPTION OF THE DATA          
                                
  The MAGSAT project objectives were:
  - to obtain accurate vector measurements of the near-Earth geomagnetic 
     field;
  - to use those measurements to identify and model the main (core) and 
    crustal fields;
  - to study of fields from external sources.       
                                  
                                                                     
  The data consist of position data for the MAGSAT spacecraft,
  and 0.5 s  resolution  observations from the MAGSAT triaxial fluxgate
  magnetometer (Dr. Robert A. Langel,  NASA/GSFC,  Principal  Investigator).
  These data have been extracted from the original 1/16-second CHRONFIN data
  tapes archived at the NSSDC. The geomagnetic field data are given as
  Cartesian components in a NEV (North, East, Vertical) geocentric spherical 
  coordinate system. In this system, the  V component is positive directly
  towards the Earth's center. The N and E components are positive northward
  and eastward. The N and V components  are  defined  relative to geocentric
  circles, and not relative to geodetic ellipsoids relative to which ground-
  observatory-measured geomagnetic BX, BY, and BZ are defined. Use of `BX,BY
  BZ' in this documentation refer to  BN, BE, BV, and not to the (BX,BY,BZ)
  defined relative to an ellipsoid. Spacecraft position data are also given
  relative to a geocentric spherical system rather than to ellipsoidal
  coordinates. See [1] for a discussion of the differencies between the
  coordinate systems as applied to field data and spacecraft position data
  (also see below), and for a discussion of MAGSAT data and of the
  International Geomagnetic Reference Field.

  Fluxgate magnetometers are subject to drift, although inflight calibration
  with the MAGSAT cesium scalar magnetometer largely compensated for these
  drifts. Spacecraft attitude was measured with a combination of star
  trackers and sun sensor. Small shifts in attitude are detectible during
  the field modeling  process. Afte inflight calibration, the vector
  measurements are thought to be accurate to a 1-sigma value of 3 nT. Users
  of the data should note that because of imperfect calibration of the
  attitude determination system, there are numerous small jumps in the
  component data (not in the corresponding magnitude data). The MAGSAT
  position accuracy is believed to be 60  meters radially and 300 meters
  horizontally. Note that MAGSAT moved at about 8 km/s in its orbit. 
                  
            
                              
                                            
                             MAGSAT FORMAT
                                     
  Logical record: 64 bytes + CR/LF for ASCII files.
                                                  
  ==========================================================================
    Item   Description        Binary  ASCII     Units           Notes    
  ==========================================================================
   1   Milliseconds of day      I*4    I8      millisec     
   2   LAMBDA Geocentric        R*4   F8.3     degree     -90.000 -  90.000 
       latitude of spacecraft
   3   PHI Geocentric           R*4   F8.3     degree    -180.000 - 180.000 
       longitude of spacecraft
   4   R Radial distance        R*4   F9.3         km          6670 - 7000 
       from center of the 
       Earth to spacecraft
   5   BX Geocentric            R*4   F8.1         nT
       North component
       of magnetic field
   6   BY Geocentric            R*4   F8.1         nT 
       East component       
       of magnetic field
   7   BZ Geocentric            R*4   F8.1         nT 
       vertical component
       of magnetic field
   8   Atitude processing       I*4   I5                    
       flag [2]
  ==========================================================================
                                          

                              REFERENCES 
                                                
  1. R. A. Langel, International  Geomagnetic  Reference  Field:  The  Sixth 
     Generation, J. Geomag. Geoelectr, 44, 699-702, 1992.
  2. R. Langel, J.Berbert, T.Jennings, and R.Horner, Magsat data processing: 
     a report for investigators, NASA  Technical  Memorandum 82160, November 
     1981, GSFC (Available from NSSDC).