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100 lines
3.5 KiB
Fortran
100 lines
3.5 KiB
Fortran
DOUBLE PRECISION FUNCTION sla_GMSTA (DATE, UT)
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*+
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* - - - - - -
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* G M S T A
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* - - - - - -
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*
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* Conversion from Universal Time to Greenwich mean sidereal time,
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* with rounding errors minimized.
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*
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* double precision
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*
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* Given:
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* DATE d UT1 date (MJD: integer part of JD-2400000.5))
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* UT d UT1 time (fraction of a day)
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*
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* The result is the Greenwich mean sidereal time (double precision,
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* radians, in the range 0 to 2pi).
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*
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* There is no restriction on how the UT is apportioned between the
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* DATE and UT arguments. Either of the two arguments could, for
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* example, be zero and the entire date+time supplied in the other.
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* However, the routine is designed to deliver maximum accuracy when
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* the DATE argument is a whole number and the UT lies in the range
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* 0 to 1 (or vice versa).
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*
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* The algorithm is based on the IAU 1982 expression (see page S15 of
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* the 1984 Astronomical Almanac). This is always described as giving
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* the GMST at 0 hours UT1. In fact, it gives the difference between
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* the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of
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* ST with respect to UT. When whole days are ignored, the expression
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* happens to equal the GMST at 0 hours UT1 each day. Note that the
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* factor 1.0027379... does not appear explicitly but in the form of
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* the coefficient 8640184.812866, which is 86400x36525x0.0027379...
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*
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* In this routine, the entire UT1 (the sum of the two arguments DATE
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* and UT) is used directly as the argument for the standard formula.
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* The UT1 is then added, but omitting whole days to conserve accuracy.
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*
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* See also the routine sla_GMST, which accepts the UT as a single
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* argument. Compared with sla_GMST, the extra numerical precision
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* delivered by the present routine is unlikely to be important in
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* an absolute sense, but may be useful when critically comparing
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* algorithms and in applications where two sidereal times close
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* together are differenced.
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*
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* Called: sla_DRANRM
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*
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* P.T.Wallace Starlink 14 October 2001
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*
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* Copyright (C) 2001 Rutherford Appleton Laboratory
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*
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* License:
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program (see SLA_CONDITIONS); if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place, Suite 330,
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* Boston, MA 02111-1307 USA
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*
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*-
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IMPLICIT NONE
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DOUBLE PRECISION DATE,UT
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* Seconds of time to radians
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DOUBLE PRECISION S2R
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PARAMETER (S2R=7.272205216643039903848712D-5)
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DOUBLE PRECISION D1,D2,T
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DOUBLE PRECISION sla_DRANRM
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* Julian centuries since J2000.
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IF (DATE.LT.UT) THEN
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D1=DATE
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D2=UT
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ELSE
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D1=UT
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D2=DATE
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END IF
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T=(D1+(D2-51544.5D0))/36525D0
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* GMST at this UT1.
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sla_GMSTA=sla_DRANRM(S2R*(24110.54841D0+
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: (8640184.812866D0+
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: (0.093104D0
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: -6.2D-6*T)*T)*T
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: +86400D0*(MOD(D1,1D0)+MOD(D2,1D0))))
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END
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