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C $Id: invlat.f 3304 2011-01-17 15:25:59Z brideout $
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C
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SUBROUTINE INVLAT(TM,GDLAT,GLON,GDALT,RL,RLATI)
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C .. Scalar Arguments ..
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DOUBLE PRECISION GDALT,GDLAT,GLON,RL,RLATI,TM
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C ..
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C .. Local Scalars ..
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DOUBLE PRECISION BB,DTR,GCALT,GCLAT,R,RKM,ST,T
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C ..
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C .. External Subroutines ..
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EXTERNAL CONVRT,INVAR
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C ..
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C .. Intrinsic Functions ..
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INTRINSIC ACOS,DMAX1,SIN,SQRT
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C ..
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C .. Data statements ..
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C
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C INVLAT CALCULATES THE L SHELL PARAMETER AND INVARIANT LATITUDE
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C OF A POINT SPECIFIED BY ITS GEODETIC LATITUDE, LONGITUDE AND
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C ALTITUDE
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C
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C INPUT
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C TM - TIME IN YEARS (E.G. 1975.2)
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C GDLAT - GEODETIC LATITUDE OF OBSERVATION POINT
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C GLON - LONGITUDE OF OBSERVATION POINT
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C GDALT - ALTITUDE ABOVE SPHEROID OF OBSERVATION POINT
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C OUTPUT
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C RLATI - INVARIANT LATITUDE
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C RL - MAGNETIC L PARAMETER
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C
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DATA DTR/.0174532925199D0/
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C ..
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C
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C .....CALCULATE GEOCENTRIC COORDINATES OF POINT.....
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CALL CONVRT(1,GDLAT,GDALT,GCLAT,RKM)
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T = DTR*(90.D0-GCLAT)
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ST = SIN(T)
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C
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C .....CALCULATE L-SHELL PARAMETER.....
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GCALT = RKM - 6378.16D0/SQRT(1.D0+.0067397D0*ST*ST)
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CALL INVAR(TM,GCLAT,GLON,GCALT,.01D0,BB,RL)
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C
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C .....CALCULATE INVARIANT LATITUDE.....
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R = RKM/(RKM-GCALT)
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RLATI = ACOS(SQRT(R/DMAX1(RL,R)))/DTR
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RETURN
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C
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END
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