spkapo_c |

## Procedurevoid spkapo_c ( SpiceInt targ, SpiceDouble et, ConstSpiceChar * ref, ConstSpiceDouble sobs[6], ConstSpiceChar * abcorr, SpiceDouble ptarg[3], SpiceDouble * lt ) ## AbstractReturn the position of a target body relative to an observer, optionally corrected for light time and stellar aberration. ## Required_ReadingSPK ## KeywordsEPHEMERIS ## Brief_I/OVariable I/O Description -------- --- -------------------------------------------------- targ I Target body. et I Observer epoch. ref I Inertial reference frame of observer's state. sobs I State of observer wrt. solar system barycenter. abcorr I Aberration correction flag. ptarg O Position of target. lt O One way light time between observer and target. ## Detailed_Inputtarg is the NAIF ID code for a target body. The target and observer define a position vector which points from the observer to the target. et is the ephemeris time, expressed as seconds past J2000 TDB, at which the position of the target body relative to the observer is to be computed. 'et' refers to time at the observer's location. ref is the inertial reference frame with respect to which the observer's state 'sobs' is expressed. 'ref' must be recognized by the SPICE Toolkit. The acceptable frames are listed in the Frames Required Reading, as well as in the CSPICE routine chgirf_. Case and blanks are not significant in the string 'ref'. sobs is the geometric (uncorrected) state of the observer relative to the solar system barycenter at epoch et. 'sobs' is a 6-vector: the first three components of 'sobs' represent a Cartesian position vector; the last three components represent the corresponding velocity vector. 'sobs' is expressed relative to the inertial reference frame designated by 'ref'. Units are always km and km/sec. abcorr indicates the aberration corrections to be applied to the position of the target body to account for one-way light time and stellar aberration. See the discussion in the Particulars section for recommendations on how to choose aberration corrections. 'abcorr' may be any of the following: "NONE" Apply no correction. Return the geometric position of the target body relative to the observer. The following values of 'abcorr' apply to the "reception" case in which photons depart from the target's location at the light-time corrected epoch et-lt and *arrive* at the observer's location at et: "LT" Correct for one-way light time (also called "planetary aberration") using a Newtonian formulation. This correction yields the position of the target at the moment it emitted photons arriving at the observer at et. The light time correction involves iterative solution of the light time equation (see Particulars for details). The solution invoked by the "LT" option uses one iteration. "LT+S" Correct for one-way light time and stellar aberration using a Newtonian formulation. This option modifies the position obtained with the "LT" option to account for the observer's velocity relative to the solar system barycenter. The result is the apparent position of the target---the position of the target as seen by the observer. "CN" Converged Newtonian light time correction. In solving the light time equation, the "CN" correction iterates until the solution converges (three iterations on all supported platforms). Whether the "CN+S" solution is substantially more accurate than the "LT" solution depends on the geometry of the participating objects and on the accuracy of the input data. In all cases this routine will execute more slowly when a converged solution is computed. See the Particulars section of spkezr_c for a discussion of precision of light time corrections. "CN+S" Converged Newtonian light time correction and stellar aberration correction. The following values of 'abcorr' apply to the "transmission" case in which photons *depart* from the observer's location at 'et' and arrive at the target's location at the light-time corrected epoch et+lt: "XLT" "Transmission" case: correct for one-way light time using a Newtonian formulation. This correction yields the position of the target at the moment it receives photons emitted from the observer's location at et. "XLT+S" "Transmission" case: correct for one-way light time and stellar aberration using a Newtonian formulation This option modifies the position obtained with the "XLT" option to account for the observer's velocity relative to the solar system barycenter. The target position indicates the direction that photons emitted from the observer's location must be "aimed" to hit the target. "XCN" "Transmission" case: converged Newtonian light time correction. "XCN+S" "Transmission" case: converged Newtonian light time correction and stellar aberration correction. Neither special nor general relativistic effects are accounted for in the aberration corrections applied by this routine. Case and blanks are not significant in the string 'abcorr'. ## Detailed_Outputptarg is a Cartesian 3-vector representing the position of the target body relative to the specified observer. 'ptarg' is corrected for the specified aberrations, and is expressed with respect to the specified inertial reference frame. The components of 'ptarg' represent the x-, y- and z-components of the target's position. Units are always km. The vector 'ptarg' points from the observer's position at 'et' to the aberration-corrected location of the target. Note that the sense of the position vector is independent of the direction of radiation travel implied by the aberration correction. lt is the one-way light time between the observer and target in seconds. If the target position is corrected for aberrations, then 'lt' is the one-way light time between the observer and the light time corrected target location. ## ParametersNone. ## Exceptions1) If the value of 'abcorr' is not recognized, the error SPICE(SPKINVALIDOPTION) is signaled. 2) If the reference frame requested is not a recognized inertial reference frame the error SPICE(BADFRAME) is signaled. 3) If the position of the target relative to the solar system barycenter cannot be computed, the error will be diagnosed by routines in the call tree of this routine. ## FilesThis routine computes positions using SPK files that have been loaded into the SPICE system, normally via the kernel loading interface routine furnsh_c. Application programs typically load kernels once before this routine is called, for example during program initialization; kernels need not be loaded repeatedly. See the routine furnsh_c and the SPK and KERNEL Required Reading for further information on loading (and unloading) kernels. If any of the ephemeris data used to compute 'ptarg' are expressed relative to a non-inertial frame in the SPK files providing those data, additional kernels may be needed to enable the reference frame transformations required to compute 'ptarg'. Normally these additional kernels are PCK files or frame kernels. Any such kernels must already be loaded at the time this routine is called. ## ParticularsIn space science or engineering applications one frequently wishes to know where to point a remote sensing instrument, such as an optical camera or radio antenna, in order to observe or otherwise receive radiation from a target. This pointing problem is complicated by the finite speed of light: one needs to point to where the target appears to be as opposed to where it actually is at the epoch of observation. We use the adjectives "geometric," "uncorrected," or "true" to refer to an actual position or state of a target at a specified epoch. When a geometric position or state vector is modified to reflect how it appears to an observer, we describe that vector by any of the terms "apparent," "corrected," "aberration corrected," or "light time and stellar aberration corrected." The SPICE Toolkit can correct for two phenomena affecting the apparent location of an object: one-way light time (also called "planetary aberration") and stellar aberration. Correcting for one-way light time is done by computing, given an observer and observation epoch, where a target was when the observed photons departed the target's location. The vector from the observer to this computed target location is called a "light time corrected" vector. The light time correction depends on the motion of the target, but it is independent of the velocity of the observer relative to the solar system barycenter. Relativistic effects such as light bending and gravitational delay are not accounted for in the light time correction performed by this routine. The velocity of the observer also affects the apparent location of a target: photons arriving at the observer are subject to a "raindrop effect" whereby their velocity relative to the observer is, using a Newtonian approximation, the photons' velocity relative to the solar system barycenter minus the velocity of the observer relative to the solar system barycenter. This effect is called "stellar aberration." Stellar aberration is independent of the motion of the target. The stellar aberration formula used by this routine is non- relativistic. Stellar aberration corrections are applied after light time corrections: the light time corrected target position vector is used as an input to the stellar aberration correction. When light time and stellar aberration corrections are both applied to a geometric position vector, the resulting position vector indicates where the target "appears to be" from the observer's location. As opposed to computing the apparent position of a target, one may wish to compute the pointing direction required for transmission of photons to the target. This requires correction of the geometric target position for the effects of light time and stellar aberration, but in this case the corrections are computed for radiation traveling from the observer to the target. The "transmission" light time correction yields the target's location as it will be when photons emitted from the observer's location at 'et' arrive at the target. The transmission stellar aberration correction is the inverse of the traditional stellar aberration correction: it indicates the direction in which radiation should be emitted so that, using a Newtonian approximation, the sum of the velocity of the radiation relative to the observer and of the observer's velocity, relative to the solar system barycenter, yields a velocity vector that points in the direction of the light time corrected position of the target. The traditional aberration corrections applicable to observation and those applicable to transmission are related in a simple way: one may picture the geometry of the "transmission" case by imagining the "observation" case running in reverse time order, and vice versa. One may reasonably object to using the term "observer" in the transmission case, in which radiation is emitted from the observer's location. The terminology was retained for consistency with earlier documentation. Below, we indicate the aberration corrections to use for some common applications: 1) Find the apparent direction of a target. This is the most common case for a remote-sensing observation. Use "LT+S" or "CN+S": apply both light time and stellar aberration corrections. Note that using light time corrections alone ("LT") is generally not a good way to obtain an approximation to an apparent target vector: since light time and stellar aberration corrections often partially cancel each other, it may be more accurate to use no correction at all than to use light time alone. 2) Find the corrected pointing direction to radiate a signal to a target. This computation is often applicable for implementing communications sessions. Use "XLT+S" or "XCN+S": apply both light time and stellar aberration corrections for transmission. 3) Compute the apparent position of a target body relative to a star or other distant object. Use one of "LT", "CN", "LT+S", or "CN+S" as needed to match the correction applied to the position of the distant object. For example, if a star position is obtained from a catalog, the position vector may not be corrected for stellar aberration. In this case, to find the angular separation of the star and the limb of a planet, the vector from the observer to the planet should be corrected for light time but not stellar aberration. 4) Obtain an uncorrected state vector derived directly from data in an SPK file. Use "NONE". 5) Use a geometric state vector as a low-accuracy estimate of the apparent state for an application where execution speed is critical. Use "NONE". 6) While this routine cannot perform the relativistic aberration corrections required to compute states with the highest possible accuracy, it can supply the geometric states required as inputs to these computations. Use "NONE", then apply relativistic aberration corrections (not available in the SPICE Toolkit). Below, we discuss in more detail how the aberration corrections applied by this routine are computed. Geometric case ============== ## ExamplesIn the following code fragment, spkssb_c and ## Restrictions1) The ephemeris files to be used by ## Literature_ReferencesSPK Required Reading. ## Author_and_InstitutionN.J. Bachman (JPL) H.A. Neilan (JPL) I.M. Underwood (JPL) W.L. Taber (JPL) ## Version-CSPICE Version 2.0.2, 07-JUL-2014 (NJB) Discussion of light time corrections was updated. Assertions that converged light time corrections are unlikely to be useful were removed. -CSPICE Version 2.0.1, 13-OCT-2003 (EDW) Various minor header changes were made to improve clarity. Added mention that 'lt' returns a value in seconds. -CSPICE Version 2.0.0, 19-DEC-2001 (NJB) Updated to handle aberration corrections for transmission of radiation. Formerly, only the reception case was supported. The header was revised and expanded to explain the functionality of this routine in more detail. -CSPICE Version 1.0.0, 26-JUN-1999 (NJB) (HAN) (IMU) (WLT) ## Index_Entriesapparent position from spk file get apparent position |

Wed Apr 5 17:54:43 2017