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slalib/pv2ue.f
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167
slalib/pv2ue.f
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SUBROUTINE sla_PV2UE (PV, DATE, PMASS, U, JSTAT)
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*+
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* - - - - - -
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* P V 2 U E
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* - - - - - -
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*
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* Construct a universal element set based on an instantaneous position
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* and velocity.
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*
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* Given:
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* PV d(6) heliocentric x,y,z,xdot,ydot,zdot of date,
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* (AU,AU/s; Note 1)
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* DATE d date (TT Modified Julian Date = JD-2400000.5)
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* PMASS d mass of the planet (Sun=1; Note 2)
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*
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* Returned:
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* U d(13) universal orbital elements (Note 3)
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*
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* (1) combined mass (M+m)
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* (2) total energy of the orbit (alpha)
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* (3) reference (osculating) epoch (t0)
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* (4-6) position at reference epoch (r0)
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* (7-9) velocity at reference epoch (v0)
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* (10) heliocentric distance at reference epoch
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* (11) r0.v0
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* (12) date (t)
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* (13) universal eccentric anomaly (psi) of date, approx
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*
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* JSTAT i status: 0 = OK
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* -1 = illegal PMASS
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* -2 = too close to Sun
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* -3 = too slow
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*
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* Notes
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*
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* 1 The PV 6-vector can be with respect to any chosen inertial frame,
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* and the resulting universal-element set will be with respect to
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* the same frame. A common choice will be mean equator and ecliptic
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* of epoch J2000.
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*
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* 2 The mass, PMASS, is important only for the larger planets. For
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* most purposes (e.g. asteroids) use 0D0. Values less than zero
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* are illegal.
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*
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* 3 The "universal" elements are those which define the orbit for the
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* purposes of the method of universal variables (see reference).
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* They consist of the combined mass of the two bodies, an epoch,
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* and the position and velocity vectors (arbitrary reference frame)
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* at that epoch. The parameter set used here includes also various
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* quantities that can, in fact, be derived from the other
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* information. This approach is taken to avoiding unnecessary
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* computation and loss of accuracy. The supplementary quantities
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* are (i) alpha, which is proportional to the total energy of the
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* orbit, (ii) the heliocentric distance at epoch, (iii) the
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* outwards component of the velocity at the given epoch, (iv) an
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* estimate of psi, the "universal eccentric anomaly" at a given
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* date and (v) that date.
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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 18 March 1999
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*
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* Copyright (C) 1999 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 PV(6),DATE,PMASS,U(13)
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INTEGER JSTAT
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* Gaussian gravitational constant (exact)
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DOUBLE PRECISION GCON
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PARAMETER (GCON=0.01720209895D0)
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* Canonical days to seconds
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DOUBLE PRECISION CD2S
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PARAMETER (CD2S=GCON/86400D0)
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* Minimum allowed distance (AU) and speed (AU per canonical day)
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DOUBLE PRECISION RMIN,VMIN
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PARAMETER (RMIN=1D-3,VMIN=1D-3)
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DOUBLE PRECISION T0,CM,X,Y,Z,XD,YD,ZD,R,V2,V,ALPHA,RDV
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* Reference epoch.
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T0 = DATE
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* Combined mass (mu=M+m).
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IF (PMASS.LT.0D0) GO TO 9010
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CM = 1D0+PMASS
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* Unpack the state vector, expressing velocity in AU per canonical day.
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X = PV(1)
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Y = PV(2)
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Z = PV(3)
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XD = PV(4)/CD2S
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YD = PV(5)/CD2S
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ZD = PV(6)/CD2S
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* Heliocentric distance, and speed.
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R = SQRT(X*X+Y*Y+Z*Z)
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V2 = XD*XD+YD*YD+ZD*ZD
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V = SQRT(V2)
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* Reject unreasonably small values.
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IF (R.LT.RMIN) GO TO 9020
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IF (V.LT.VMIN) GO TO 9030
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* Total energy of the orbit.
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ALPHA = V2-2D0*CM/R
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* Outward component of velocity.
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RDV = X*XD+Y*YD+Z*ZD
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* Construct the universal-element set.
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U(1) = CM
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U(2) = ALPHA
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U(3) = T0
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U(4) = X
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U(5) = Y
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U(6) = Z
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U(7) = XD
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U(8) = YD
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U(9) = ZD
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U(10) = R
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U(11) = RDV
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U(12) = T0
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U(13) = 0D0
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* Exit.
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JSTAT = 0
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GO TO 9999
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* Negative PMASS.
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9010 CONTINUE
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JSTAT = -1
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GO TO 9999
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* Too close.
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9020 CONTINUE
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JSTAT = -2
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GO TO 9999
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* Too slow.
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9030 CONTINUE
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JSTAT = -3
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9999 CONTINUE
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END
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