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## AbstractCSPICE_SPKCVO returns the state of a specified target relative to an "observer," where the observer has constant velocity in a specified reference frame. The observer's state is provided by the calling program rather than by loaded SPK files. For important details concerning this module's function, please refer to the CSPICE routine spkcvo_c. ## I/OGiven: target scalar string name of a target body. Optionally, you may supply the ID code of the object as an integer string. For example, both 'EARTH' and '399' are legitimate strings to supply to indicate the target is earth. Case and leading and trailing blanks are not significant in the string 'target'. et scalar double precision ephemeris time at which the state of the target relative to the observer is to be computed. 'et' is expressed as seconds past J2000 TDB. 'et' refers to time at the observer's location. 'et' is independent of the observer epoch 'obsepc'. outref scalar string name of the reference frame with respect to which the output state is expressed. When 'outref' is time-dependent (non-inertial), its orientation relative to the J2000 frame is evaluated in the manner commanded by the input argument 'refloc' (see description below). Case and leading and trailing blanks are not significant in the string 'outref'. refloc scalar string name indicating the output reference frame evaluation locus: this is the location associated with the epoch at which this routine is to evaluate the orientation, relative to the J2000 frame, of the output frame 'outref'. The values and meanings of 'refloc' are: 'OBSERVER' Evaluate 'outref' at the observer's epoch 'et'. Normally the locus 'OBSERVER' should be selected when 'outref' is centered at the observer. 'TARGET' Evaluate 'outref' at the target epoch; letting 'ltime' be the one-way light time between the target and observer, the target epoch is et-ltime if reception aberration corrections are used et+ltime if transmission aberration corrections are used et if no aberration corrections are used Normally the locus 'TARGET' should be selected when 'outref' is centered at the target object. 'CENTER' Evaluate the frame 'outref' at the epoch associated its center. This epoch, which we'll call 'etctr', is determined as follows: Let 'ltctr' be the one-way light time between the observer and the center of 'outref'. Then 'etctr' is et-ltctr if reception aberration corrections are used et+ltctr if transmission aberration corrections are used et if no aberration corrections are used The locus 'CENTER' should be selected when the user intends to obtain results compatible with those produced by cspice_spkezr. When 'outref' is inertial, all choices of 'refloc' yield the same results. Case and leading and trailing blanks are not significant in the string 'refloc'. abcorr scalar string name indicating the aberration corrections to be applied to the observer-target state to account for one-way light time and stellar aberration. '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-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 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. 'CN+S' Converged Newtonian light time and stellar aberration corrections. 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 and stellar aberration corrections. 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'. obssta double precision 6-vector defining the geometric state of an observer moving at constant velocity relative to its center of motion 'obsctr', expressed in the reference frame 'obsref', at the epoch 'obsepc'. 'obssta' is a six-dimensional vector representing position and velocity in Cartesian coordinates: the first three components represent the position of an observer relative to its center of motion; the last three components represent the velocity of the observer. Units are always km and km/sec. obsepc scalar double precision epoch, expressed as seconds past J2000 TDB, at which the observer state 'obssta' is applicable. For other epochs, the position of the observer relative to its center of motion is linearly extrapolated using the velocity component of 'obssta'. 'obsepc' is independent of the epoch 'et' at which the state of the target relative to the observer is to be computed. obsctr scalar string name of the center of motion of 'obssta'. The ephemeris of 'obsctr' is provided by loaded SPK files. Optionally, you may supply the integer ID code for the object as an integer string. For example both 'MOON' and '301' are legitimate strings that indicate the moon is the center of motion. Case and leading and trailing blanks are not significant in the string 'obsctr'. obsref scalar string name of the reference frame relative to which the input state 'obssta' is expressed. The observer has constant velocity relative to its center of motion in this reference frame. Case and leading and trailing blanks are not significant in the string 'obsref'. the call: ## ExamplesThe numerical results shown for these examples 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. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: spkcvo.tm This is the meta-kernel file for the header code example for the subroutine ## ParticularsThis routine computes observer-target states for observers whose trajectories are not provided by SPK files. Observers supported by this routine must have constant velocity with respect to a specified center of motion, expressed in a caller-specified reference frame. The state of the center of motion relative to the target must be computable using loaded SPK data. For applications in which the observer has zero velocity relative to its center of motion, the CSPICE routine cspice_spkcpo { SPK, constant position observer } can be used. cspice_spkcpo has a simpler interface than that of ## Required ReadingFRAMES.REQ PCK.REQ SPK.REQ TIME.REQ ## Version-Icy Version 1.0.0, 09-APR-2012 (EDW) ## Index_Entriesstate relative to constant_velocity_observer state relative to constant_velocity surface_point state relative to surface_point on extended_object state relative to landmark on extended_object |

Wed Apr 5 17:58:03 2017