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SUBROUTINE CGM_TO_ALTITUDE(R_HEIGHT_IN,R_LAT_IN,R_LAT_ADJ,I_ERR64)
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c ccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
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c Original version writen by RADEX, INC. for use on VAX/VMS systems.
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c The 1990 version of sfc_convert_geo_coord used a subroutine
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c called 'cg_alt_dip'. This subroutine has been replaced
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c by two subroutines, cgm_to_altitude, and altitude_to_cgm.
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c
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c Initial version for POSIX compliant systems made by KBB
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c at the Johns Hopkins Univ. Applied Physics Laboratory.
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c These revisions have been managed using the Revision Control
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c System (RCS). The log of revisions follows:
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c
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c cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
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c
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c $Log: not supported by cvs2svn $
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c Revision 1.1 1996/03/11 19:24:31 baker
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c Initial revision
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c
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c
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c cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
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C
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C Purpose:
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C
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C Computes Latitude Adjustment for R_HEIGHT_IN for use with the
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C Spherical Harmonic expansion computation.
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C
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C Except for low R_HEIGHT_IN, the corrected geomagnetic latitude
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C is discontinuous in the vicinity of the magnetic equator. This
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C continuity would result in poor convergence of a spherical
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C harmonic expansion. To avoid this problem, it was necessary to
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C perform the spherical harmonic fit and expansion with respect
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C to dipole coordinates at the table R_HEIGHT_INs, and to compute
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C an appropriate latitude adjustment. This routine computes the
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C required adjustment.
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C
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C
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C Input Arguments:
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C
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C R_HEIGHT_IN - Single Precision - Input height in km above
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C earth mean radius
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C
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C R_LAT_IN - Single Precision - Corrected Geomagnetic
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C Latitude
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C
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C Output Arguments:
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C
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C R_LAT_ADJ - Single Precision
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C Height Adjusted Corrected Dipole Coordinates
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C corresponding to R_LAT_IN
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C
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C I_ERR64 - Logical - Error Flag
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C
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C = .false. normal return
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C
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C = .true. the input R_LAT value for
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C the IFLAG = 2 option is invalid for the
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C input altitude R_HEIGHT_IN.
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C
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C Local Variables:
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C
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C RA, R0 - Single Precision - for intermediate results
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C
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C Constants:
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C
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C DEGRAD - Double Precision - Conversion Factor
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C degrees to radians
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C
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C ERADIUS - Single Precision - Earth Radius in km
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C
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C UNIM - Single Precision - limit parameter, used to
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C avoid computational singularities
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C
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C Revision History
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C
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C Written by Radex, Inc., 3 Preston Court, Bedford, MA 01730 12/94
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C
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C
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C .. Parameters ..
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DOUBLE PRECISION DEGRAD
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PARAMETER (DEGRAD=1.745329251994330D-2)
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C ..
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C .. Scalar Arguments ..
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DOUBLE PRECISION R_HEIGHT_IN,R_LAT_ADJ,R_LAT_IN
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LOGICAL I_ERR64
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C ..
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C .. Local Scalars ..
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DOUBLE PRECISION ERADIUS,RA,UNIM
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C ..
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C .. Intrinsic Functions ..
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INTRINSIC ACOS,COS,SIGN,SQRT
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C ..
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C .. Save statement ..
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SAVE
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C ..
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C .. Data statements ..
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C
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C
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C
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DATA ERADIUS/6371.2D0/
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DATA UNIM/1.0D0/
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C ..
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C
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I_ERR64 = .false.
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C
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C Compute the corresponding altitude adjusted dipole latitude.
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C
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RA = (1.0D0+R_HEIGHT_IN/ERADIUS)*(COS(DEGRAD*R_LAT_IN))**2
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IF (RA.GT.UNIM) THEN
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RA = UNIM
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I_ERR64 = .TRUE.
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END IF
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C
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R_LAT_ADJ = SIGN(ACOS(SQRT(RA)),R_LAT_IN)/DEGRAD
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C
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RETURN
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END
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