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C $Id: dsf.f 3304 2011-01-17 15:25:59Z brideout $
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C
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SUBROUTINE DSF(GCLAT,GLON,RKM,ALT,HALT,ISTOP,DS)
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C
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C Calculates an optimum integration step size for geomagnetic
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C field line tracing routine LINTRA, as an empirical function of
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C the geomagnetic dipole coordinates of the starting point. when
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C start and end points are in the same hemisphere and
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C abs(halt-alt)<10000, the empirical formula is modified so that
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C ds is no greater than 1/100 of the altitude difference between
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C start and end points.
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C
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C input:
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C GCLAT - start point geocentric latitude (deg)
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C GLON - start point geocentric longitude (deg)
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C RKM - start point radial distance (km)
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C ALT - starting point geodetic altitude
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C HALT - tuning parameter for altitude test.
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C ISTOP - set to -1 to enable abs(halt-alt)<10000 test
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C output:
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C DS - step size
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C
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C .. Scalar Arguments ..
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DOUBLE PRECISION ALT,DS,GCLAT,GLON,HALT,RKM
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INTEGER ISTOP
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C ..
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C .. Local Scalars ..
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DOUBLE PRECISION RAD,SINGML
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C ..
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C .. Intrinsic Functions ..
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INTRINSIC ABS,DCOS,DSIN
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C ..
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C .. Data statements ..
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DATA RAD/57.2957795D0/
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C ..
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C
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SINGML = .98D0*DSIN(GCLAT/RAD) +
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* .199D0*DCOS(GCLAT/RAD)*DCOS((GLON+69.D0)/RAD)
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DS = .06D0*RKM/(1.D0-SINGML*SINGML) - 370.D0
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IF (DS.GT.3000.D0) DS = 3000.D0
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IF (ISTOP.EQ.-1 .AND. ABS(ALT-HALT).LT.
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* 10000.D0) DS = ABS(ALT-HALT)/100.D0
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RETURN
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C
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END
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