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Diffstat (limited to 'src/slalib/ue2pv.f')
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diff --git a/src/slalib/ue2pv.f b/src/slalib/ue2pv.f new file mode 100644 index 0000000..be7b59f --- /dev/null +++ b/src/slalib/ue2pv.f @@ -0,0 +1,215 @@ + SUBROUTINE sla_UE2PV (DATE, U, PV, JSTAT) +*+ +* - - - - - - +* U E 2 P V +* - - - - - - +* +* Heliocentric position and velocity of a planet, asteroid or comet, +* starting from orbital elements in the "universal variables" form. +* +* Given: +* DATE d date, Modified Julian Date (JD-2400000.5) +* +* Given and returned: +* U d(13) universal orbital elements (updated; Note 1) +* +* given (1) combined mass (M+m) +* " (2) total energy of the orbit (alpha) +* " (3) reference (osculating) epoch (t0) +* " (4-6) position at reference epoch (r0) +* " (7-9) velocity at reference epoch (v0) +* " (10) heliocentric distance at reference epoch +* " (11) r0.v0 +* returned (12) date (t) +* " (13) universal eccentric anomaly (psi) of date +* +* Returned: +* PV d(6) position (AU) and velocity (AU/s) +* JSTAT i status: 0 = OK +* -1 = radius vector zero +* -2 = failed to converge +* +* Notes +* +* 1 The "universal" elements are those which define the orbit for the +* purposes of the method of universal variables (see reference). +* They consist of the combined mass of the two bodies, an epoch, +* and the position and velocity vectors (arbitrary reference frame) +* at that epoch. The parameter set used here includes also various +* quantities that can, in fact, be derived from the other +* information. This approach is taken to avoiding unnecessary +* computation and loss of accuracy. The supplementary quantities +* are (i) alpha, which is proportional to the total energy of the +* orbit, (ii) the heliocentric distance at epoch, (iii) the +* outwards component of the velocity at the given epoch, (iv) an +* estimate of psi, the "universal eccentric anomaly" at a given +* date and (v) that date. +* +* 2 The companion routine is sla_EL2UE. This takes the conventional +* orbital elements and transforms them into the set of numbers +* needed by the present routine. A single prediction requires one +* one call to sla_EL2UE followed by one call to the present routine; +* for convenience, the two calls are packaged as the routine +* sla_PLANEL. Multiple predictions may be made by again +* calling sla_EL2UE once, but then calling the present routine +* multiple times, which is faster than multiple calls to sla_PLANEL. +* +* It is not obligatory to use sla_EL2UE to obtain the parameters. +* However, it should be noted that because sla_EL2UE performs its +* own validation, no checks on the contents of the array U are made +* by the present routine. +* +* 3 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). +* +* 4 The universal elements supplied in the array U are in canonical +* units (solar masses, AU and canonical days). The position and +* velocity are not sensitive to the choice of reference frame. The +* sla_EL2UE routine in fact produces coordinates with respect to the +* J2000 equator and equinox. +* +* 5 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 19 March 1999 +* +* Copyright (C) 1999 Rutherford Appleton Laboratory +*- + + IMPLICIT NONE + + DOUBLE PRECISION DATE,U(13),PV(6) + INTEGER JSTAT + +* Gaussian gravitational constant (exact) + DOUBLE PRECISION GCON + PARAMETER (GCON=0.01720209895D0) + +* Canonical days to seconds + DOUBLE PRECISION CD2S + PARAMETER (CD2S=GCON/86400D0) + +* Test value for solution and maximum number of iterations + DOUBLE PRECISION TEST + INTEGER NITMAX + PARAMETER (TEST=1D-13,NITMAX=20) + + INTEGER I,NIT,N + + DOUBLE PRECISION CM,ALPHA,T0,P0(3),V0(3),R0,SIGMA0,T,PSI,DT,W, + : TOL,PSJ,PSJ2,BETA,S0,S1,S2,S3,FF,R,F,G,FD,GD + + + +* Unpack the parameters. + CM = U(1) + ALPHA = U(2) + T0 = U(3) + DO I=1,3 + P0(I) = U(I+3) + V0(I) = U(I+6) + END DO + R0 = U(10) + SIGMA0 = U(11) + T = U(12) + PSI = U(13) + +* Approximately update the universal eccentric anomaly. + PSI = PSI+(DATE-T)*GCON/R0 + +* Time from reference epoch to date (in Canonical Days: a canonical +* day is 58.1324409... days, defined as 1/GCON). + DT = (DATE-T0)*GCON + +* Refine the universal eccentric anomaly. + NIT = 1 + W = 1D0 + TOL = 0D0 + DO WHILE (ABS(W).GE.TOL) + +* Form half angles until BETA small enough. + N = 0 + PSJ = PSI + PSJ2 = PSJ*PSJ + BETA = ALPHA*PSJ2 + DO WHILE (ABS(BETA).GT.0.7D0) + N = N+1 + BETA = BETA/4D0 + PSJ = PSJ/2D0 + PSJ2 = PSJ2/4D0 + END DO + +* Calculate Universal Variables S0,S1,S2,S3 by nested series. + S3 = PSJ*PSJ2*((((((BETA/210D0+1D0) + : *BETA/156D0+1D0) + : *BETA/110D0+1D0) + : *BETA/72D0+1D0) + : *BETA/42D0+1D0) + : *BETA/20D0+1D0)/6D0 + S2 = PSJ2*((((((BETA/182D0+1D0) + : *BETA/132D0+1D0) + : *BETA/90D0+1D0) + : *BETA/56D0+1D0) + : *BETA/30D0+1D0) + : *BETA/12D0+1D0)/2D0 + S1 = PSJ+ALPHA*S3 + S0 = 1D0+ALPHA*S2 + +* Undo the angle-halving. + TOL = TEST + DO WHILE (N.GT.0) + S3 = 2D0*(S0*S3+PSJ*S2) + S2 = 2D0*S1*S1 + S1 = 2D0*S0*S1 + S0 = 2D0*S0*S0-1D0 + PSJ = PSJ+PSJ + TOL = TOL+TOL + N = N-1 + END DO + +* Improve the approximation to PSI. + FF = R0*S1+SIGMA0*S2+CM*S3-DT + R = R0*S0+SIGMA0*S1+CM*S2 + IF (R.EQ.0D0) GO TO 9010 + W = FF/R + PSI = PSI-W + +* Next iteration, unless too many already. + IF (NIT.GE.NITMAX) GO TO 9020 + NIT = NIT+1 + END DO + +* Project the position and velocity vectors (scaling velocity to AU/s). + W = CM*S2 + F = 1D0-W/R0 + G = DT-CM*S3 + FD = -CM*S1/(R0*R) + GD = 1D0-W/R + DO I=1,3 + PV(I) = P0(I)*F+V0(I)*G + PV(I+3) = CD2S*(P0(I)*FD+V0(I)*GD) + END DO + +* Update the parameters to allow speedy prediction of PSI next time. + U(12) = DATE + U(13) = PSI + +* OK exit. + JSTAT = 0 + GO TO 9999 + +* Null radius vector. + 9010 CONTINUE + JSTAT = -1 + GO TO 9999 + +* Failed to converge. + 9020 CONTINUE + JSTAT = -2 + + 9999 CONTINUE + END |