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184 lines
6.9 KiB
Fortran
184 lines
6.9 KiB
Fortran
SUBROUTINE sla_PLANEL (DATE, JFORM, EPOCH, ORBINC, ANODE, PERIH,
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: AORQ, E, AORL, DM, PV, JSTAT)
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*+
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* - - - - - - -
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* P L A N E L
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* - - - - - - -
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*
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* Heliocentric position and velocity of a planet, asteroid or comet,
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* starting from orbital elements.
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*
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* Given:
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* DATE d date, Modified Julian Date (JD - 2400000.5, Note 1)
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* JFORM i choice of element set (1-3; Note 3)
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* EPOCH d epoch of elements (TT MJD, Note 4)
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* ORBINC d inclination (radians)
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* ANODE d longitude of the ascending node (radians)
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* PERIH d longitude or argument of perihelion (radians)
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* AORQ d mean distance or perihelion distance (AU)
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* E d eccentricity
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* AORL d mean anomaly or longitude (radians, JFORM=1,2 only)
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* DM d daily motion (radians, JFORM=1 only)
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*
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* Returned:
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* PV d(6) heliocentric x,y,z,xdot,ydot,zdot of date,
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* J2000 equatorial triad (AU,AU/s)
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* JSTAT i status: 0 = OK
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* -1 = illegal JFORM
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* -2 = illegal E
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* -3 = illegal AORQ
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* -4 = illegal DM
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* -5 = numerical error
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*
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* Called: sla_EL2UE, sla_UE2PV
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*
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* Notes
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*
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* 1 DATE is the instant for which the prediction is required. It is
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* in the TT timescale (formerly Ephemeris Time, ET) and is a
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* Modified Julian Date (JD-2400000.5).
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*
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* 2 The elements are with respect to the J2000 ecliptic and equinox.
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*
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* 3 A choice of three different element-set options is available:
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*
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* Option JFORM = 1, suitable for the major planets:
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*
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* EPOCH = epoch of elements (TT MJD)
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* ORBINC = inclination i (radians)
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* ANODE = longitude of the ascending node, big omega (radians)
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* PERIH = longitude of perihelion, curly pi (radians)
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* AORQ = mean distance, a (AU)
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* E = eccentricity, e (range 0 to <1)
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* AORL = mean longitude L (radians)
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* DM = daily motion (radians)
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*
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* Option JFORM = 2, suitable for minor planets:
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*
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* EPOCH = epoch of elements (TT MJD)
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* ORBINC = inclination i (radians)
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* ANODE = longitude of the ascending node, big omega (radians)
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* PERIH = argument of perihelion, little omega (radians)
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* AORQ = mean distance, a (AU)
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* E = eccentricity, e (range 0 to <1)
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* AORL = mean anomaly M (radians)
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*
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* Option JFORM = 3, suitable for comets:
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*
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* EPOCH = epoch of elements and perihelion (TT MJD)
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* ORBINC = inclination i (radians)
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* ANODE = longitude of the ascending node, big omega (radians)
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* PERIH = argument of perihelion, little omega (radians)
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* AORQ = perihelion distance, q (AU)
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* E = eccentricity, e (range 0 to 10)
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*
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* Unused arguments (DM for JFORM=2, AORL and DM for JFORM=3) are not
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* accessed.
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*
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* 4 Each of the three element sets defines an unperturbed heliocentric
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* orbit. For a given epoch of observation, the position of the body
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* in its orbit can be predicted from these elements, which are
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* called "osculating elements", using standard two-body analytical
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* solutions. However, due to planetary perturbations, a given set
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* of osculating elements remains usable for only as long as the
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* unperturbed orbit that it describes is an adequate approximation
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* to reality. Attached to such a set of elements is a date called
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* the "osculating epoch", at which the elements are, momentarily,
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* a perfect representation of the instantaneous position and
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* velocity of the body.
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*
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* Therefore, for any given problem there are up to three different
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* epochs in play, and it is vital to distinguish clearly between
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* them:
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*
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* . The epoch of observation: the moment in time for which the
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* position of the body is to be predicted.
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*
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* . The epoch defining the position of the body: the moment in time
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* at which, in the absence of purturbations, the specified
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* position (mean longitude, mean anomaly, or perihelion) is
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* reached.
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*
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* . The osculating epoch: the moment in time at which the given
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* elements are correct.
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*
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* For the major-planet and minor-planet cases it is usual to make
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* the epoch that defines the position of the body the same as the
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* epoch of osculation. Thus, only two different epochs are
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* involved: the epoch of the elements and the epoch of observation.
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*
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* For comets, the epoch of perihelion fixes the position in the
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* orbit and in general a different epoch of osculation will be
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* chosen. Thus, all three types of epoch are involved.
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*
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* For the present routine:
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*
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* . The epoch of observation is the argument DATE.
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*
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* . The epoch defining the position of the body is the argument
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* EPOCH.
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*
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* . The osculating epoch is not used and is assumed to be close
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* enough to the epoch of observation to deliver adequate accuracy.
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* If not, a preliminary call to sla_PERTEL may be used to update
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* the element-set (and its associated osculating epoch) by
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* applying planetary perturbations.
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*
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* 5 The reference frame for the result is with respect to the mean
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* equator and equinox of epoch J2000.
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*
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* 6 The algorithm was originally adapted from the EPHSLA program of
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* D.H.P.Jones (private communication, 1996). The method is based
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* on Stumpff's Universal Variables.
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*
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* Reference: Everhart, E. & Pitkin, E.T., Am.J.Phys. 51, 712, 1983.
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*
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* P.T.Wallace Starlink 31 December 2002
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*
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* Copyright (C) 2002 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
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INTEGER JFORM
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DOUBLE PRECISION EPOCH,ORBINC,ANODE,PERIH,AORQ,E,AORL,DM,PV(6)
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INTEGER JSTAT
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DOUBLE PRECISION U(13)
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INTEGER J
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* Validate elements and convert to "universal variables" parameters.
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CALL sla_EL2UE(DATE,JFORM,
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: EPOCH,ORBINC,ANODE,PERIH,AORQ,E,AORL,DM,U,J)
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* Determine the position and velocity.
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IF (J.EQ.0) THEN
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CALL sla_UE2PV(DATE,U,PV,J)
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IF (J.NE.0) J=-5
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END IF
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* Wrap up.
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JSTAT = J
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END
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