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C $Id: gdran.f 3304 2011-01-17 15:25:59Z brideout $
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C
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DOUBLE PRECISION FUNCTION GDRAN(SR,SLATGC,SLON,AZ,EL,ALT)
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C
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C GDRAN uses a half-interval (binary) search technique to determine
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C the geodetic range to a point of observation given in terms of
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C azimuth, elevation, and geodetic altitude.
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C
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C Input:
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C SR - radial distance of station from center of earth
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C SLATGC - station geocentric latitude
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C SLON - station longitude
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C AZ - radar azimuth
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C EL - radar elevation
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C ALT - geodetic altitude
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C
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C harris fortran 77
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C rgm - 8/85
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C
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C .. Scalar Arguments ..
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DOUBLE PRECISION ALT,AZ,EL,SLATGC,SLON,SR
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C ..
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C .. Local Scalars ..
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DOUBLE PRECISION ALT1,ALT2,GCLAT,GDALT,GDLAT,GLONG,PR,R1,R2,RFRST,
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* TINTER
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C ..
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C .. External Functions ..
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DOUBLE PRECISION RFUN
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EXTERNAL RFUN
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C ..
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C .. External Subroutines ..
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EXTERNAL CONVRT,POINT
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C ..
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C .. Intrinsic Functions ..
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INTRINSIC ABS,SIGN
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C ..
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C
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C determine two ranges which bound the actual range. the first
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C range is obtained by making a spherical earth range calculation
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C using the supplied station radius. this range will be on one
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C side or the other of the actual range and will be added to or
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C subtracted from to determine the second range until the second
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C range is on the opposite side of the actual range as the first
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C range (whew!).
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C
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R1 = RFUN(SR,EL,ALT)
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R2 = R1
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CALL POINT(SR,SLATGC,SLON,AZ,EL,R2,PR,GCLAT,GLONG)
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CALL CONVRT(2,GDLAT,ALT1,GCLAT,PR)
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10 CONTINUE
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R2 = R2 + SIGN(1.D0,ALT-ALT1)
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CALL POINT(SR,SLATGC,SLON,AZ,EL,R2,PR,GCLAT,GLONG)
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CALL CONVRT(2,GDLAT,ALT2,GCLAT,PR)
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IF (SIGN(1.D0,ALT-ALT1).EQ.SIGN(1.D0,ALT-ALT2)) GO TO 10
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C
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C use the center of the range boundary as a first guess. the
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C half-interval is initially 1/4 of the total range boundary.
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C
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RFRST = (R1+R2)/2.D0
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TINTER = ABS(R2-R1)/4.D0
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C
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C add to or subtract from the current range guess the current
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C half-interval, halving the half-interval on each iteration until
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C the actual range is approached to within a tolerance of .01 km.
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C
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20 CONTINUE
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CALL POINT(SR,SLATGC,SLON,AZ,EL,RFRST,PR,GCLAT,GLONG)
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CALL CONVRT(2,GDLAT,GDALT,GCLAT,PR)
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RFRST = RFRST + SIGN(TINTER,ALT-GDALT)
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TINTER = TINTER/2.D0
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IF (ABS(GDALT-ALT).GE.1.D-2) GO TO 20
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GDRAN = RFRST
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RETURN
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END
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