cspice_spkapo |
|||

## AbstractCSPICE_SPKAPO returns the position 3-vector of a target body relative to an observer, optionally corrected for light time and stellar aberration, given the geometric state 6-vector of the observer relative to the Solar System Barycenter. For important details concerning this module's function, please refer to the CSPICE routine spkapo_c. ## I/OGiven: targ scalar integer SPICE ID of the target body (the observed body). et scalar double precision ephemeris time (ET) epoch for the observation of interest. ref scalar string name of the reference frame in which to output the position 3-vector. sobs double precision geometric (uncorrected) state 6-vector of the observer relative to the solar system barycenter at epoch 'et'. abcorr scalar string name of the aberration correction to apply to 'sobs'. 'abcorr' may be any of the following: 'NONE' Apply no correction. Return the geometric state of the target 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-ltime 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 state 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. 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 state obtained with the 'LT' option to account for the observer's velocity relative to the solar system barycenter. The result is the apparent state 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+ltime: 'XLT' "Transmission" case: correct for one-way light time using a Newtonian formulation. This correction yields the state 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 state obtained with the 'XLT' option to account for the observer's velocity relative to the solar system barycenter. The position component of the computed target state 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 leading and trailing blanks are not significant in the string 'abcorr'. 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. ;; ;; Define the needed parameters. ;; MARS = 499 MOON = 301 EPOCH = 'Jan 1 2004 5:00 PM' REF = 'J2000' ABCORR = 'LT+S' ;; ;; Load a meta kernel listing a leapseconds kernel. ;; cspice_furnsh, 'standard.tm' ;; ;; Convert the time string to ephemeris time J2000. ;; cspice_str2et, EPOCH, et ;; ;; Compute the geometric state of the Moon wrt the ;; the Solar System barycenter. ;; cspice_spkssb, MOON, et, REF, state ;; ;; Apply an aberration correction for a position vector ;; of the MOON as seen from Mars. ;; ## 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 1.0.4, 01-APR-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 1.0.3, 10-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 1.0.2, 07-NOV-2013 (EDW) Improvement of I/O section descriptions. Added aberration algorithm explanation to Particulars section. -Icy Version 1.0.1, 23-SEP-2008, EDW (JPL) Eliminated error in English. -Icy Version 1.0.0, 16-JUN-2003, EDW (JPL) ## Index_Entriesapparent position from spk file get apparent position |

Wed Apr 5 17:58:03 2017