Correct the apparent position of an object for stellar aberration.
VARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- pobj I Position of an object with respect to the observer. vobs I Velocity of the observer with respect to the Solar System barycenter. appobj O Apparent position of the object with respect to the observer, corrected for stellar aberration.
pobj is the position (x, y, z, km) of an object with respect to the observer, possibly corrected for light time. vobs is the velocity (dx/dt, dy/dt, dz/dt, km/sec) of the observer with respect to the Solar System barycenter.
appobj is the apparent position of the object relative to the observer, corrected for stellar aberration. appobj may overwrite pobj.
Let r be the vector from the observer to the object, and v be - - the velocity of the observer with respect to the Solar System barycenter. Let w be the angle between them. The aberration angle phi is given by sin(phi) = v sin(w) / c Let h be the vector given by the cross product - h = r X v - - - Rotate r by phi radians about h to obtain the apparent position - - of the object.
In the following example, stelab_c is used to correct the position of a target body for stellar aberration. /. (Previous subroutine calls have loaded the SPK file and the leapseconds kernel file.) ./ /. Get the state of the observer with respect to the solar system barycenter. ./ spkssb_c ( idobs, et, "J2000", sobs ); /. Get the light-time corrected state of the target body as seen by the observer. ./ spkapp_c ( idtarg, et, "J2000", sobs, "LT", starg, < ); /. Apply the correction for stellar aberration to the light-time corrected state of the target body. ./ stelab_c ( starg, starg+3, starg ) Note that this example is somewhat contrived. The correction flag "LT+S" could have been used (in place of "LT") to do the correction automatically in spkapp_c.
1) If the velocity of the observer is greater than or equal to the speed of light, the error SPICE(VALUEOUTOFRANGE) is signalled.
N.J. Bachman (JPL) H.A. Neilan (JPL) W.L. Taber (JPL) I.M. Underwood (JPL)
1) W.M. Owen, Jr., JPL IOM #314.8-524, "The Treatment of Aberration in Optical Navigation", 8 February 1985.
-CSPICE Version 1.0.0, 22-OCT-1998 (NJB) Based on SPICELIB Version 1.0.2, 10-MAR-1992 (WLT)