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Procedure
Abstract
Required_Reading
Keywords
Brief_I/O
Detailed_Input
Detailed_Output
Parameters
Exceptions
Files
Particulars
Examples
Restrictions
Literature_References
Author_and_Institution
Version
Index_Entries

Procedure

   void spkapo_c ( SpiceInt               targ,
                   SpiceDouble            et,
                   ConstSpiceChar       * ref,
                   ConstSpiceDouble       sobs[6],
                   ConstSpiceChar       * abcorr,
                   SpiceDouble            ptarg[3],
                   SpiceDouble          * lt        ) 

Abstract

 
   Return the position of a target body relative to an observer, 
   optionally corrected for light time and stellar aberration. 
 

Required_Reading

 
   SPK 
 

Keywords

 
   EPHEMERIS 
 

Brief_I/O

 
   Variable  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_Input

 
   targ        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_Output

 
   ptarg       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. 

Parameters

 
   None. 
 

Exceptions

 
   1) 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. 
 

Files

 
 
   This 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. 
 

Particulars

 
   In 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 
   ============== 
 
      spkapo_c begins by computing the geometric position T(et) of the 
      target body relative to the solar system barycenter (SSB). 
      Subtracting the geometric position of the observer O(et) gives 
      the geometric position of the target body relative to the 
      observer. The one-way light time, lt, is given by 
 
                | T(et) - O(et) | 
         lt = ------------------- 
                        c 
 
      The geometric relationship between the observer, target, and 
      solar system barycenter is as shown: 
 
 
         SSB ---> O(et) 
          |      / 
          |     / 
          |    /                            
          |   /  T(et) - O(et)   
          V  V                                   
         T(et) 
 
 
      The returned position vector is 
 
         T(et) - O(et) 
 
 
   Reception case 
   ============== 
 
      When any of the options "LT", "CN", "LT+S", "CN+S" are 
      selected, spkapo_c computes the position of the target body at 
      epoch et-lt, where lt is the one-way light time.  Let T(t) 
      and O(t) represent the positions of the target and observer 
      relative to the solar system barycenter at time t; then lt  
      is the solution of the 
      light-time equation 
 
                | T(et-lt) - O(et) | 
         lt = ------------------------                            (1) 
                         c 
 
      The ratio  
 
          | T(et) - O(et) | 
        ---------------------                                     (2) 
                  c 
 
      is used as a first approximation to lt; inserting (2) into the 
      RHS of the light-time equation (1) yields the "one-iteration" 
      estimate of the one-way light time. Repeating the process 
      until the estimates of lt converge yields the "converged 
      Newtonian" light time estimate. 
      
      Subtracting the geometric position of the observer O(et) gives 
      the position of the target body relative to the observer: 
      T(et-lt) - O(et). 
 
         SSB ---> O(et) 
          | \     | 
          |  \    | 
          |   \   | T(et-lt) - O(et) 
          |    \  | 
          V     V V 
         T(et)  T(et-lt) 
 
       
      The light-time corrected position is the vector 
 
         T(et-lt) - O(et) 
 
      If correction for stellar aberration is requested, the target 
      position is rotated toward the solar system barycenter-relative 
      velocity vector of the observer. The magnitude of the rotation 
      depends on the magnitude of the observer's velocity relative 
      to the solar system barycenter and the angle between  
      this velocity and the observer-target vector.  The rotation 
      is computed as follows: 
 
         Let r be the light time corrected vector from the observer 
         to the object, and v be the velocity of the observer with 
         respect to the solar system barycenter. Let w be the angle 
         between them. The aberration angle phi is given by 
 
            sin(phi) = v sin(w) / c 
 
         Let h be the vector given by the cross product 
 
            h = r X v 
 
         Rotate r by phi radians about h to obtain the apparent 
         position of the object. 
 
 
 
   Transmission case 
   ================== 
 
      When any of the options "XLT", "XCN", "XLT+S", "XCN+S" are 
      selected, spkapo_c computes the position of the target body T at 
      epoch et+lt, where lt is the one-way light time.  lt is the 
      solution of the light-time equation 
 
                | T(et+lt) - O(et) | 
         lt = ------------------------                            (3) 
                          c 
 
      Subtracting the geometric position of the observer, O(et), 
      gives the position of the target body relative to the 
      observer: T(et-lt) - O(et). 
 
                 SSB --> O(et) 
                / |    *  
               /  |  *  T(et+lt) - O(et)   
              /   |*      
             /   *|     
            V  V  V      
        T(et+lt)  T(et)     
 
 
      The light-time corrected position is  
 
         T(et+lt) - O(et) 
 
      If correction for stellar aberration is requested, the target 
      position is rotated away from the solar system barycenter- 
      relative velocity vector of the observer.  The magnitude of the 
      rotation depends on the magnitude of the velocity and the 
      angle between the velocity and the observer-target vector. 
      The rotation is computed as in the reception case, but the 
      sign of the rotation angle is negated. 
 
   Neither special nor general relativistic effects are accounted  
   for in the aberration corrections performed by this routine. 
 

Examples

 
   In the following code fragment, spkssb_c and spkapo_c are used 
   to display the position of Io (body 501) as seen from the  
   Voyager 2 spacecraft (Body -32) at a series of epochs. 
 
   Normally, one would call the high-level reader spkpos_c to obtain 
   position vectors.  The example below illustrates the interface 
   of this routine, but is not intended as a recommendation on 
   how to use the CSPICE SPK subsystem. 
 
   The use of integer ID codes is necessitated by the low-level 
   interface of this routine. 
 
      
      #include <stdio.h>
      #include "SpiceUsr.h"
           .
           .
           .
      #define  IO          501
      #define  VGR2        -32
    
      while ( epoch <= end )
      {
         spkssb_c ( VGR2,  epoch,  "J2000", stvgr2                   );
         spkapo_c ( IO,    epoch,  "J2000", stvgr2, "LT", posio, &lt ); 
         recrad_c ( posio, &range, &ra,     &dec                     );
         
         printf ( "RA = %f   DEC = %f\n", ra*dpr_c(), dec*dpr_c() ); 
 
         epoch +=  delta;
      } 
 

Restrictions

 
   1) The ephemeris files to be used by spkapo_c must be loaded 
      (normally by the CSPICE kernel loader furnsh_c) before  
      this routine is called. 
 
   2) Unlike most other SPK position computation routines, this 
      routine requires that the input state be relative to an 
      inertial reference frame.  Non-inertial frames are not 
      supported by this routine. 
 
   3) In a future version of this routine, the implementation  
      of the aberration corrections may be enhanced to improve 
      accuracy.     
  

Literature_References

 
   SPK Required Reading. 
 

Author_and_Institution

 
   N.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_Entries

 
   apparent position from spk file 
   get apparent position 
 
Wed Apr  5 17:54:43 2017