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.help ue2pv Jun99 "Slalib Package"
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SUBROUTINE slUEPV (DATE, U, PV, JSTAT)
- - - - - -
U E 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 slELUE. 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 slELUE followed by one call to the present routine;
for convenience, the two calls are packaged as the routine
slPLNE. Multiple predictions may be made by again
calling slELUE once, but then calling the present routine
multiple times, which is faster than multiple calls to slPLNE.
It is not obligatory to use slELUE to obtain the parameters.
However, it should be noted that because slELUE 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
slELUE 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
Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
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