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Algorithm

   subroutine orbit
    read_parameters    # get parameters from param file
    OAL_odfInfo        # get StartTime, EndTime of file

    open_outfile       # open FITS output file
    addTable           # create new table
    foreach column     # create 10 new columns: time, x, y, z, Vx, Vy, Vz
     addColumn
    next

    for start_time to stop_time, step sampling_interval

     OAL_getPosition   # return position and velocity vectors from ODF

     foreach column    # time, x, y, z, Vx, Vy, Vz
      fill_column_temp_pointer
      if (column_temp_pointer(i) not defined)
       column_temp_pointer(i) = HUGE
      end if
      convert_GEI_to_GSE(time,gei_x,gei_y,gei_z,gse_x,gse_y,gse_z)
    ! gei_x,y,z : celestial x,y,z of XMM in km.
    ! gse_x,y,z : geo solar ecliptic x,y,z of XMM in km.
    ! lambda = ecliptic long of sun  (radians)
    !    eta = obliquity of ecliptic (radians)
    ! The Sun's ecliptic longitude (lambdaO) can be calculated using the 
    ! series of formulae:
    !
    !  meanAnom = 357.528 + 35999.050T0 
    !  Lambda = 280.460 + 36000.772T0 
    !  lambdaO = Lambda + (1.915 * sinM) + 0.020 sin2M
    !
    ! where T0 is the time in Julian centuries from 12:00 UT on 1 January 2000
    ! to the midnight Universal Time (UT) preceding the time of interest and 
    ! H is the time in hours since that preceding UT midnight. Formulae 
    ! derived from the Almanac for Computers. In the intermediate formulae, 
    ! meanAnom is the Sun's mean anomaly and Lambda its mean longitude.
    !
    ! M = 357.528+35999.050*Tzero+0.04107*UT; Sun's mean anomaly
    ! L = 280.460+36000.772*Tzero+0.04107*UT; Sun's mean longitude

     next

    next  

    OAL_proposalInfo   # get basic info to write as keywords
    write_attributes_to_outfile     # add proposal info as keywords
    release Table      # close FITS extension
    release Set        # close FITS file
    OAL_releaseMemory  # close ODF files
    close_outfile
   end subroutine orbit