stlabx_c |

Table of contents## Procedurestlabx_c ( Stellar aberration, transmission case ) void stlabx_c ( ConstSpiceDouble pobj [3], ConstSpiceDouble vobs [3], SpiceDouble corpos [3] ) ## AbstractCorrect the position of a target for the stellar aberration effect on radiation transmitted from a specified observer to the target. ## Required_ReadingNone. ## KeywordsEPHEMERIS ## Brief_I/OVARIABLE 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. corpos O Corrected position of the object. ## Detailed_Inputpobj is the cartesian position vector of an object with respect to the observer, possibly corrected for light time. Units are km. vobs is the cartesian velocity vector of the observer with respect to the Solar System barycenter. Units are km/s. ## Detailed_Outputcorpos is the position of the object relative to the observer, corrected for the stellar aberration effect on radiation directed toward the target. This correction is the inverse of the usual stellar aberration correction: the corrected vector indicates the direction in which radiation must be emitted from the observer, as seen in an inertial reference frame having velocity equal to that of the observer, in order to reach the position indicated by the input vector `pobj'. ## ParametersNone. ## Exceptions1) If the velocity of the observer is greater than or equal to the speed of light, an error is signaled by a routine in the call tree of this routine. The outputs are undefined. ## FilesNone. ## ParticularsIn order to transmit radiation from an observer to a specified target, the emission direction must be corrected for one way light time and for the motion of the observer relative to the solar system barycenter. The correction for the observer's motion when transmitting to a target is the inverse of the usual stellar aberration correction applied to the light-time corrected position of the target as seen by the observer. Below is the description of the stellar aberration correction used in the CSPICE routine stelab_c (with the notation changed slightly): 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. This routine applies the inverse correction, so here the rotation about `h' is by -phi radians. ## ExamplesThe numerical results shown for this example may differ across platforms. The results depend on the SPICE kernels used as input, the compiler and supporting libraries, and the machine specific arithmetic implementation. 1) Compute the apparent position of the Moon relative to the Earth, corrected for one way light-time and stellar aberration effect on radiation transmitted from the Earth to the Moon, given the geometric state of the Earth relative to the Solar System Barycenter, and the difference between the stelar aberration corrected and uncorrected position vectors, taking several steps. First, compute the light-time corrected state of the Moon body as seen by the Earth, using its geometric state. Then apply the correction for stellar aberration to the light-time corrected state of the target body, both for the transmission case. The code in this example could be replaced by a single call to spkpos_c: spkpos_c ( "MOON", et, "J2000", "XLT+S", "EARTH", pos, < ); Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: stlabx_ex1.tm This meta-kernel is intended to support operation of SPICE example programs. The kernels shown here should not be assumed to contain adequate or correct versions of data required by SPICE-based user applications. In order for an application to use this meta-kernel, the kernels referenced here must be present in the user's current working directory. The names and contents of the kernels referenced by this meta-kernel are as follows: File name Contents --------- -------- de418.bsp Planetary ephemeris naif0009.tls Leapseconds \begindata KERNELS_TO_LOAD = ( 'de418.bsp', 'naif0009.tls' ) \begintext End of meta-kernel Example code begins here. /. Program stlabx_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main( ) { /. Local variables. ./ SpiceChar * reffrm; SpiceChar * utcstr; SpiceDouble appdif [3]; SpiceDouble et; SpiceDouble lt; SpiceDouble pcorr [3]; SpiceDouble pos [3]; SpiceDouble sobs [6]; SpiceInt idobs; SpiceInt idtarg; /. Assign an observer, Earth, target, Moon, time of interest and reference frame for returned vectors. ./ idobs = 399; idtarg = 301; utcstr = "July 4 2004"; reffrm = "J2000"; /. Load the needed kernels. ./ furnsh_c ( "stlabx_ex1.tm" ); /. Convert the time string to ephemeris time. ./ str2et_c ( utcstr, &et ); /. Get the state of the observer with respect to the solar system barycenter. ./ spkssb_c ( idobs, et, reffrm, sobs ); /. Get the light-time corrected position `pos' of the target body `idtarg' as seen by the observer. Normally we would call spkpos_c to obtain this vector, but we already have the state of the observer relative to the solar system barycenter, so we can avoid looking up that state twice by calling spkapo_c. ./ spkapo_c ( idtarg, et, reffrm, sobs, "XLT", pos, < ); /. Output the uncorrected vector. ./ printf( "Uncorrected position vector\n" ); printf( " %18.6f %18.6f %18.6f\n", pos[0], pos[1], pos[2] ); /. Apply the correction for stellar aberration to the light-time corrected position of the target body. ./ ## RestrictionsNone. ## Literature_References[1] W. Owen, "The Treatment of Aberration in Optical Navigation", JPL IOM #314.8-524, 8 February 1985. ## Author_and_InstitutionJ. Diaz del Rio (ODC Space) ## Version-CSPICE Version 1.0.0, 13-AUG-2021 (JDR) ## Index_Entriesstellar aberration for transmission case |

Fri Dec 31 18:41:13 2021