SXFORM ( State Transformation Matrix )
SUBROUTINE SXFORM ( FROM, TO, ET, XFORM )
Return the state transformation matrix from one frame to
another at a specified epoch.
DOUBLE PRECISION ET
DOUBLE PRECISION XFORM ( 6, 6 )
VARIABLE I/O DESCRIPTION
-------- --- --------------------------------------------------
FROM I Name of the frame to transform from.
TO I Name of the frame to transform to.
ET I Epoch of the state transformation matrix.
XFORM O A state transformation matrix.
FROM is the name of a reference frame in which a state is
TO is the name of a reference frame in which it is
desired to represent the state.
ET is the epoch in ephemeris seconds past the epoch of
J2000 (TDB) at which the state transformation matrix
should be evaluated.
XFORM is the state transformation matrix that transforms
states from the reference frame FROM to the frame TO
at epoch ET. If (x, y, z, dx, dy, dz) is a state
relative to the frame FROM then the vector ( x', y',
z', dx', dy', dz' ) is the same state relative to the
frame TO at epoch ET. Here the vector ( x', y', z',
dx', dy', dz' ) is defined by the equation:
- - - - - -
| x' | | | | x |
| y' | | | | y |
| z' | = | XFORM | | z |
| dx' | | | | dx |
| dy' | | | | dy |
| dz' | | | | dz |
- - - - - -
1) If sufficient information has not been supplied via loaded
SPICE kernels to compute the transformation between the
two frames, the error will be diagnosed by a routine
in the call tree to this routine.
2) If either frame FROM or TO is not recognized the error
SPICE(UNKNOWNFRAME) will be signaled.
This routine provides the user level interface to computing state
transformations from one reference frame to another.
Note that the reference frames may be inertial or non-inertial.
However, the user must take care that sufficient SPICE kernel
information is loaded to provide a complete state transformation
path from the FROM frame to the TO frame.
Suppose that you have geodetic coordinates of a station on
the surface of the earth and that you need the inertial
(J2000) state of this station. The following code fragment
illustrates how to transform the position of the station to
a J2000 state.
CALL BODVRD ( 'EARTH', RADII, 3, N, ABC )
EQUATR = ABC(1)
POLAR = ABC(3)
F = (EQUATR - POLAR) / EQUATR
CALL GEOREC ( LONG, LAT, 0.0D0, EQUATR, F, ESTATE )
ESTATE(4) = 0.0D0
ESTATE(5) = 0.0D0
ESTATE(6) = 0.0D0
CALL SXFORM ( 'IAU_EARTH', 'J2000', ET, XFORM )
CALL MXVG ( XFORM, ESTATE, 6, 6, JSTATE )
The state JSTATE is the desired J2000 state of the station.
C.H. Acton (JPL)
N.J. Bachman (JPL)
B.V. Semenov (JPL)
W.L. Taber (JPL)
SPICELIB Version 1.1.0, 23-SEP-2013 (BVS)
Updated to save the input frame names and POOL state counters
and to do frame name-ID conversions only if the counters have
SPICELIB Version 1.0.3, 27-FEB-2008 (BVS)
Added FRAMES to the Required_Reading section.
SPICELIB Version 1.0.2, 23-OCT-2005 (NJB)
Header example had invalid flattening factor computation;
this was corrected. Reference to BODVAR in header was
replaced with reference to BODVRD.
SPICELIB Version 1.0.1, 29-JUL-2003 (NJB) (CHA)
Minor header changes were made to improve clarity.
SPICELIB Version 1.0.0, 19-SEP-1995 (WLT)