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DOUBLE PRECISION FUNCTION slGMSA (DATE, UT)
*+
* - - - - - -
* G M S A
* - - - - - -
*
* Conversion from Universal Time to Greenwich mean sidereal time,
* with rounding errors minimized.
*
* double precision
*
* Given:
* DATE d UT1 date (MJD: integer part of JD-2400000.5))
* UT d UT1 time (fraction of a day)
*
* The result is the Greenwich mean sidereal time (double precision,
* radians, in the range 0 to 2pi).
*
* There is no restriction on how the UT is apportioned between the
* DATE and UT arguments. Either of the two arguments could, for
* example, be zero and the entire date+time supplied in the other.
* However, the routine is designed to deliver maximum accuracy when
* the DATE argument is a whole number and the UT lies in the range
* 0 to 1 (or vice versa).
*
* The algorithm is based on the IAU 1982 expression (see page S15 of
* the 1984 Astronomical Almanac). This is always described as giving
* the GMST at 0 hours UT1. In fact, it gives the difference between
* the GMST and the UT, the steady 4-minutes-per-day drawing-ahead of
* ST with respect to UT. When whole days are ignored, the expression
* happens to equal the GMST at 0 hours UT1 each day. Note that the
* factor 1.0027379... does not appear explicitly but in the form of
* the coefficient 8640184.812866, which is 86400x36525x0.0027379...
*
* In this routine, the entire UT1 (the sum of the two arguments DATE
* and UT) is used directly as the argument for the standard formula.
* The UT1 is then added, but omitting whole days to conserve accuracy.
*
* See also the routine slGMST, which accepts the UT as a single
* argument. Compared with slGMST, the extra numerical precision
* delivered by the present routine is unlikely to be important in
* an absolute sense, but may be useful when critically comparing
* algorithms and in applications where two sidereal times close
* together are differenced.
*
* Called: slDA2P
*
* P.T.Wallace Starlink 14 October 2001
*
* Copyright (C) 2001 Rutherford Appleton Laboratory
*
* License:
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program (see SLA_CONDITIONS); if not, write to the
* Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
* Boston, MA 02110-1301 USA
*
* Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
*-
IMPLICIT NONE
DOUBLE PRECISION DATE,UT
* Seconds of time to radians
DOUBLE PRECISION S2R
PARAMETER (S2R=7.272205216643039903848712D-5)
DOUBLE PRECISION D1,D2,T
DOUBLE PRECISION slDA2P
* Julian centuries since J2000.
IF (DATE.LT.UT) THEN
D1=DATE
D2=UT
ELSE
D1=UT
D2=DATE
END IF
T=(D1+(D2-51544.5D0))/36525D0
* GMST at this UT1.
slGMSA=slDA2P(S2R*(24110.54841D0+
: (8640184.812866D0+
: (0.093104D0
: -6.2D-6*T)*T)*T
: +86400D0*(MOD(D1,1D0)+MOD(D2,1D0))))
END
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