cspice_spkpos |
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## AbstractCSPICE_SPKPOS returns the position of a target body relative to an observing body, optionally corrected for light time (planetary aberration) and stellar aberration. For important details concerning this module's function, please refer to the CSPICE routine spkpos_c. ## I/OGiven: targ the scalar string name of a target body. Optionally, you may supply the integer ID code for the object as an integer string, i.e. both 'MOON' and '301' are legitimate strings that indicate the Moon is the target body. The target and observer define a position vector whose position component points from the observer to the target. et the scalar or N-array of double precision ephemeris time, expressed as seconds past J2000 TDB, at which position of the target body relative to the observer the is to be computed, 'et' refers to time at the observer's location. ref the scalar string name of the reference frame relative to which the output position vector should be expressed. This may be any frame supported by the SPICE system, including built-in frames (documented in the Frames Required Reading) and frames defined by a loaded frame kernel (FK). When 'ref' designates a non-inertial frame, the orientation of the frame is evaluated at an epoch dependent on the selected aberration correction. abcorr a scalar string that indicates the aberration corrections to apply to the position of the target body to account for one-way light time and stellar aberration. '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 uses an 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 and velocity 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 below 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 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. Both letter case and embedded blanks are not significant in the 'abcorr' string. obs the scalar string name of a observing body. Optionally, you may supply the integer ID code for the object as an integer string, i.e. both 'MOON' and '301' are legitimate strings that indicate the Moon is the observing body. the call: ## ExamplesAny numerical results shown for this example may differ between platforms as the results depend on the SPICE kernels used as input and the machine specific arithmetic implementation. ;; ;; Load a set of kernels: an SPK file, a PCK ;; file and a leapseconds file. Use a meta ;; kernel for convenience. ;; cspice_furnsh, 'standard.tm' ;; ;; Define parameters for a position lookup: ;; ;; Return the position vector of Mars as seen from ;; Earth in the J2000 frame using aberration ;; correction LT+S (light time plus stellar aberration) ;; at the epoch July 4, 2003 11:00 AM PST. ;; target = 'Mars' epoch = 'July 4, 2003 11:00 AM PST' frame = 'J2000' abcorr = 'LT+S' observer = 'Earth' ;; ;; Convert the epoch to ephemeris time. ;; cspice_str2et, epoch, et ;; ;; Look-up the position for the defined parameters. ;; ## ParticularsPlease refer to the Aberation Corrections Required Reading (ABCORR.REQ) for detailed information describing the nature and calculation of the applied corrections. ## Required ReadingICY.REQ ABCORR.REQ SPK.REQ NAIF_IDS.REQ FRAMES.REQ TIME.REQ ## Version-Icy Version 1.1.5, 22-DEC-2015 (EDW) Corrected typo in Version section. The 07-NOV-2013 version entry lacked a version ID. Particulars updated to refer to Aberration Corrections Required Reading document. -Icy Version 1.1.4, 07-JUL-2014 (NJB)(EDW) Discussion of light time corrections was updated. Assertions that converged light time corrections are unlikely to be useful were removed. -Icy Version 1.1.3, 07-NOV-2013 (EDW) Added aberration algorithm explanation to Particulars section. -Icy Version 1.1.2, 22-DEC-2008, EDW (JPL) Header edits performed to improve argument descriptions. These descriptions should now closely match the descriptions in the corresponding CSPICE routine. -Icy Version 1.1.1, 29-SEP-2007, EDW (JPL) Replaced the comment fragment in the I/O section "return with the same order" with "return with the same measure of vectorization" -Icy Version 1.1.0, 12-SEP-2004, EDW (JPL) Added capability to process vector 'et' input returning a matrix 'ptarg' and vector 'ltime' on output. -Icy Version 1.0.0, 16-JUN-2003, EDW (JPL) ## Index_Entriesusing names get target position relative to an observer position relative to observer corrected for aberrations read ephemeris data read trajectory data |

Wed Apr 5 17:58:03 2017