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Abstract
I/O
Examples
Particulars
Required Reading
Version
Index_Entries

Abstract


   CSPICE_SPKEZP 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 spkezp_c.

I/O

   
   Given:
   
      targ      the scalar NAIF integer ID code for a target body.
   
                The target and observer define a position vector 
                whose position component points from the observer 
                to the target.
   
      et        the scalar double precision 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       the scalar string name of the reference frame relative 
                to which the output position vector should be expressed
   
      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 state 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 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 (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 
                              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+lt: 
  
                   '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. 
  
                Both letter case and embedded blanks are not significant 
                in the 'abcorr' string. 
   
      obs       the scalar NAIF integer ID code for an observing body.
   
   the call:
   
      cspice_spkezp, targ, et, ref, abcorr, obs, ptarg, ltime
   
   returns:
   
      ptarg   a double precision Cartesian 3-vector representing
              the position of the target body in kilometers
              relative to the specified observer (the three 
              components of 'ptarg' represent the x-, y- and 
              z-components of the target's position)
   
      ltime   the scalar, double precision one-way light time between 
              the observer and target in seconds; if the target 
              position is corrected for aberrations, then 'ltime' 
              is the one-way light time between the observer and the 
              light time corrected target location
              
              Please note, CSPICE documentation and source code 
              uniformly uses the variable name 'lt' to designate
              the light-time between an observer and target. IDL 
              uses "lt" as the less-than numeric comparison 
              operator and so does not allow "lt" as a variable name. 
              Therefore, Icy documentation uses the name 'ltime' 
              for the light-time value. 
   
   Note: NAIF recommends the use of cspice_spkpos over the use of
   cspice_spkezp.
   

Examples


   Any 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 (499) as seen from
      ;; Earth (399) 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   = 499
      epoch    = 'July 4, 2003 11:00 AM PST'
      frame    = 'J2000'
      abcorr   = 'LT+S'
      observer = 399
      
      ;;
      ;; Convert the epoch to ephemeris time.
      ;;
      cspice_str2et, epoch, et
      
      ;;
      ;; Look-up the position for the defined parameters.
      ;;
      cspice_spkezp, target, et, frame, abcorr, observer, $
                     ptarg, ltime
   
      ;;
      ;; Output...
      ;;
      print, 'The position of    : ', target
      print, 'As observed from   : ', observer
      print, 'In reference frame : ', frame
      print, 'At epoch           : ', epoch 
      print, 'R (kilometers)     : ', ptarg
      print
      print, 'Light time (secs) between observer and target: ', ltime
   
   IDL outputs:
   
      The position of    :      499
      As observed from   :      399
      In reference frame : J2000
      At epoch           : July 4, 2003 11:00 AM PST
      R (kilometers)     :   73822235.  -27127919.  -18741306.
   
      Light time (secs) between observer and target:  269.68988

Particulars


   Please refer to the Aberation Corrections Required Reading (ABCORR.REQ)
   for detailed information describing the nature and calculation of the
   applied corrections.

Required Reading


   ICY.REQ
   ABCORR.REQ
   SPK.REQ
   NAIF_IDS.REQ 
   FRAMES.REQ
   TIME.REQ

Version


   -Icy Version 1.0.3, 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 1.0.2, 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 1.0.1, 07-NOV-2013 (EDW)

      Added aberration algorithm explanation to Particulars section.

   -Icy Version 1.0.0, 16-JUN-2003, EDW (JPL)

Index_Entries

 
   get target position relative to an observer 
   get position relative observer corrected for aberrations 
   read ephemeris data 
   read trajectory data 
 



Wed Apr  5 17:58:03 2017