SUBROUTINE sla_PLANEL (DATE, JFORM, EPOCH, ORBINC, ANODE, PERIH, : AORQ, E, AORL, DM, PV, JSTAT) *+ * - - - - - - - * P L A N E L * - - - - - - - * * Heliocentric position and velocity of a planet, asteroid or comet, * starting from orbital elements. * * Given: * DATE d date, Modified Julian Date (JD - 2400000.5, Note 1) * JFORM i choice of element set (1-3; Note 3) * EPOCH d epoch of elements (TT MJD, Note 4) * ORBINC d inclination (radians) * ANODE d longitude of the ascending node (radians) * PERIH d longitude or argument of perihelion (radians) * AORQ d mean distance or perihelion distance (AU) * E d eccentricity * AORL d mean anomaly or longitude (radians, JFORM=1,2 only) * DM d daily motion (radians, JFORM=1 only) * * Returned: * PV d(6) heliocentric x,y,z,xdot,ydot,zdot of date, * J2000 equatorial triad (AU,AU/s) * JSTAT i status: 0 = OK * -1 = illegal JFORM * -2 = illegal E * -3 = illegal AORQ * -4 = illegal DM * -5 = numerical error * * Called: sla_EL2UE, sla_UE2PV * * Notes * * 1 DATE is the instant for which the prediction is required. It is * in the TT timescale (formerly Ephemeris Time, ET) and is a * Modified Julian Date (JD-2400000.5). * * 2 The elements are with respect to the J2000 ecliptic and equinox. * * 3 A choice of three different element-set options is available: * * Option JFORM = 1, suitable for the major planets: * * EPOCH = epoch of elements (TT MJD) * ORBINC = inclination i (radians) * ANODE = longitude of the ascending node, big omega (radians) * PERIH = longitude of perihelion, curly pi (radians) * AORQ = mean distance, a (AU) * E = eccentricity, e (range 0 to <1) * AORL = mean longitude L (radians) * DM = daily motion (radians) * * Option JFORM = 2, suitable for minor planets: * * EPOCH = epoch of elements (TT MJD) * ORBINC = inclination i (radians) * ANODE = longitude of the ascending node, big omega (radians) * PERIH = argument of perihelion, little omega (radians) * AORQ = mean distance, a (AU) * E = eccentricity, e (range 0 to <1) * AORL = mean anomaly M (radians) * * Option JFORM = 3, suitable for comets: * * EPOCH = epoch of elements and perihelion (TT MJD) * ORBINC = inclination i (radians) * ANODE = longitude of the ascending node, big omega (radians) * PERIH = argument of perihelion, little omega (radians) * AORQ = perihelion distance, q (AU) * E = eccentricity, e (range 0 to 10) * * Unused arguments (DM for JFORM=2, AORL and DM for JFORM=3) are not * accessed. * * 4 Each of the three element sets defines an unperturbed heliocentric * orbit. For a given epoch of observation, the position of the body * in its orbit can be predicted from these elements, which are * called "osculating elements", using standard two-body analytical * solutions. However, due to planetary perturbations, a given set * of osculating elements remains usable for only as long as the * unperturbed orbit that it describes is an adequate approximation * to reality. Attached to such a set of elements is a date called * the "osculating epoch", at which the elements are, momentarily, * a perfect representation of the instantaneous position and * velocity of the body. * * Therefore, for any given problem there are up to three different * epochs in play, and it is vital to distinguish clearly between * them: * * . The epoch of observation: the moment in time for which the * position of the body is to be predicted. * * . The epoch defining the position of the body: the moment in time * at which, in the absence of purturbations, the specified * position (mean longitude, mean anomaly, or perihelion) is * reached. * * . The osculating epoch: the moment in time at which the given * elements are correct. * * For the major-planet and minor-planet cases it is usual to make * the epoch that defines the position of the body the same as the * epoch of osculation. Thus, only two different epochs are * involved: the epoch of the elements and the epoch of observation. * * For comets, the epoch of perihelion fixes the position in the * orbit and in general a different epoch of osculation will be * chosen. Thus, all three types of epoch are involved. * * For the present routine: * * . The epoch of observation is the argument DATE. * * . The epoch defining the position of the body is the argument * EPOCH. * * . The osculating epoch is not used and is assumed to be close * enough to the epoch of observation to deliver adequate accuracy. * If not, a preliminary call to sla_PERTEL may be used to update * the element-set (and its associated osculating epoch) by * applying planetary perturbations. * * 5 The reference frame for the result is with respect to the mean * equator and equinox of epoch J2000. * * 6 The algorithm was originally adapted from the EPHSLA program of * D.H.P.Jones (private communication, 1996). The method is based * on Stumpff's Universal Variables. * * Reference: Everhart, E. & Pitkin, E.T., Am.J.Phys. 51, 712, 1983. * * P.T.Wallace Starlink 31 December 2002 * * Copyright (C) 2002 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., 59 Temple Place, Suite 330, * Boston, MA 02111-1307 USA * *- IMPLICIT NONE DOUBLE PRECISION DATE INTEGER JFORM DOUBLE PRECISION EPOCH,ORBINC,ANODE,PERIH,AORQ,E,AORL,DM,PV(6) INTEGER JSTAT DOUBLE PRECISION U(13) INTEGER J * Validate elements and convert to "universal variables" parameters. CALL sla_EL2UE(DATE,JFORM, : EPOCH,ORBINC,ANODE,PERIH,AORQ,E,AORL,DM,U,J) * Determine the position and velocity. IF (J.EQ.0) THEN CALL sla_UE2PV(DATE,U,PV,J) IF (J.NE.0) J=-5 END IF * Wrap up. JSTAT = J END