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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 spkaps_c ( SpiceInt           targ,
                   SpiceDouble        et,
                   ConstSpiceChar   * ref,
                   ConstSpiceChar   * abcorr,
                   ConstSpiceDouble   stobs [6],
                   ConstSpiceDouble   accobs[6],
                   SpiceDouble        starg [6],
                   SpiceDouble      * lt,
                   SpiceDouble      * dlt      )

Abstract

 
   Given the state and acceleration of an observer relative to the 
   solar system barycenter, return the state (position and velocity) 
   of a target body relative to the observer, optionally corrected 
   for light time and stellar aberration. All input and output 
   vectors are expressed relative to an inertial reference frame. 
 
   This routine supersedes spkapp_c. 
 
   SPICE users normally should call the high-level API routines 
   spkezr_c or spkez_c rather than this routine. 
 

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 output state. 
   abcorr     I   Aberration correction flag. 
   stobs      I   State of the observer relative to the SSB. 
   accobs     I   Acceleration of the observer relative to the SSB. 
   starg      O   State of target. 
   lt         O   One way light time between observer and target. 
   dlt        O   Derivative of light time with respect to time. 
 

Detailed_Input

 
   targ        is the NAIF ID code for a target body. The target 
               and observer define a state vector whose position 
               component points from the observer to the target. 
 
   et          is the ephemeris time, expressed as seconds past 
               J2000 TDB, at which the state 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 input state `stobs', the input acceleration `accobs', 
               and the output state `starg' are expressed. `ref' must be 
               recognized by the CSPICE 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'. 
 
   abcorr      indicates the aberration corrections to be applied to
               the state of the target body to account for one-way
               light time and stellar aberration. See the discussion in
               the Particulars section of spkezr_c for recommendations
               on how to choose aberration corrections.
                 
               `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 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 
                             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 blanks are not significant in the string 
               `abcorr'. 
                                
   stobs       is the geometric state of the observer relative 
               to the solar system barycenter at `et'. The 
               target and observer define a state vector whose 
               position component points from the observer to the 
               target. `stobs' is expressed relative to the reference
               frame designated by `ref'.
 
   accobs      is the geometric acceleration of the observer 
               relative to the solar system barycenter at `et'. This 
               is the derivative with respect to time of the 
               velocity portion of STOBS. `accobs' is expressed 
               relative to the reference frame designated by `ref'. 
 
               `accobs' is used for computing stellar aberration 
               corrected velocity. If stellar aberration corrections 
               are not specified by `abcorr', `accobs' is ignored; the 
               caller need not provide a valid input value in this 
               case. 

Detailed_Output

 
   starg       is a Cartesian state vector representing the position 
               and velocity of the target body relative to the 
               specified observer. `starg' is corrected for the 
               specified aberration, and is expressed with respect 
               to the specified inertial reference frame. The first 
               three components of `starg' represent the x-, y- and 
               z-components of the target's position; last three 
               components form the corresponding velocity vector. 
 
               The position component of `starg' points from the 
               observer's location 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. 
 
               Units are always km and km/sec. 
 
   lt          is the one-way light time between the observer and 
               target in seconds. If the target state is corrected 
               for light time, then `lt' is the one-way light time  
               between the observer and the light time-corrected  
               target location. 
 
   dlt         is the derivative with respect to barycentric 
               dynamical time of the one way light time between 
               target and observer: 
 
                  dlt = d(lt)/d(et) 
 
               `dlt' can also be described as the rate of change of  
               one way light time. `dlt' is unitless, since `lt' and 
               `et' both have units of TDB seconds. 
 
               If the observer and target are at the same position, 
               then `dlt' is set to zero. 
  

Parameters

 
   None. 
 

Exceptions

  
   1) If the value of `abcorr' is not recognized, the error 
      the error will be diagnosed by routines in the call tree of this
      routine.

   2) If `abcorr' calls for stellar aberration but not light
        time corrections, the error SPICE(NOTSUPPORTED) is
        signaled.

   3) If `abcorr' calls for relativistic light time corrections, the
      error SPICE(NOTSUPPORTED) is signaled.

   4) If the reference frame requested is not a recognized 
      inertial reference frame, the error SPICE(BADFRAME)  
      is signaled. 
 
   5) If the state 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. 
 
   6) If the observer and target are at the same position, 
      then `dlt' is set to zero. This situation could arise, 
      for example, when the observer is Mars and the target 
      is the Mars barycenter.  

   7) The error SPICE(EMPTYSTRING) is signaled if either of the input
      strings `ref' or `abcorr' do not contain at least one character,
      since such an input string cannot be converted to a Fortran-style
      string.
      
   8) The error SPICE(NULLPOINTER) is signaled if either of the input 
      string pointers `ref' or `abcorr' are null.

Files

 
   This routine computes states 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 `starg' 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 the state. 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

 
   This routine supports higher-level SPK API routines that can 
   perform both light time and stellar aberration corrections. 
 
   User applications normally will not need to call this routine 
   directly. However, this routine can improve run-time efficiency 
   in situations where many targets are observed from the same 
   location at the same time. In such cases, the state and 
   acceleration of the observer relative to the solar system 
   barycenter need be computed only once per look-up epoch. 
 
   When apparent positions, rather than apparent states, are 
   required, consider using the high-level position-only API 
   routines 
 
      spkpos_c 
      spkezp_c 
 
   or the low-level, position-only analog of this routine 
 
      spkapo_c 
 
   In general, the position-only routines are more efficient. 
 
   See the header of the routine spkezr_c for a detailed discussion 
   of aberration corrections. 
 

Examples

   1) Look up a sequence of states of the Moon as seen from the
      Earth. Use light time and stellar aberration corrections.
      Compute the first state for the epoch 2000 JAN 1 12:00:00 TDB;
      compute subsequent states at intervals of 1 hour. For each
      epoch, display the states, the one way light time between
      target and observer, and the rate of change of the one way
      light time.

      Use the following meta-kernel to specify the kernels to
      load:

         KPL/MK

         This meta-kernel is intended to support operation of SPICE
         example programs. The kernels shown here should not be
         assumed to contain adequate or correct versions of data
         required by SPICE-based user applications.

         In order for an application to use this meta-kernel, the
         kernels referenced here must be present in the user's
         current working directory.


         \begindata

            KERNELS_TO_LOAD = ( 'de418.bsp',
                                'pck00008.tpc',
                                'naif0008.tls'  )

         \begintext


      The code example follows:

         #include <stdio.h>
         #include "SpiceUsr.h"
         #include "SpiceZfc.h"

         int main()
         {
            /.
            Local constants

            The meta-kernel name shown here refers to a file whose contents
            are those shown above. This file and the kernels it references
            must exist in your current working directory.
            ./
            #define META                   "example.mk"

            /.
            Use a time step of 1 hour; look up 100 states.
            ./
            #define STEP                   3600.0
            #define MAXITR                 5

            /.
            Local variables
            ./
            SpiceDouble             acc     [3];
            SpiceDouble             dlt;
            SpiceDouble             et;
            SpiceDouble             et0;
            SpiceDouble             lt;
            SpiceDouble             state   [6];
            SpiceDouble             state0  [6];
            SpiceDouble             state2  [6];
            SpiceDouble             stobs   [6];
            SpiceDouble             tdelta;

            SpiceInt                dim;
            SpiceInt                i;

            /.
            Load the SPK and LSK kernels via the meta-kernel.
            ./
            furnsh_c ( META );

            /.
            Convert the start time to seconds past J2000 TDB.
            ./
            str2et_c ( "2000 JAN 1 12:00:00 TDB", &et0 );

            /.
            Step through a series of epochs, looking up a
            state vector at each one.
            ./
            for ( i = 0;  i < MAXITR;  i++ )
            {
               et = et0 + i*STEP;

               /.
               Look up a state vector at epoch ET using the
               following inputs:

                  Target:                 Moon (NAIF ID code 301)
                  Reference frame:        J2000
                  Aberration correction:  Light time and stellar
                                          aberration ('LT+S')
                  Observer:               Earth (NAIF ID code 399)

               Before we can execute this computation, we'll need the
               geometric state and acceleration of the observer relative to
               the solar system barycenter at ET, expressed relative to the
               J2000 reference frame. First find the state:
               ./
               spkssb_c ( 399, et, "j2000", stobs );

               /.
               Next compute the acceleration. We numerically differentiate
               the velocity using a quadratic approximation.
               ./
               tdelta = 1.0;

               spkssb_c ( 399, et-tdelta, "j2000", state0 );
               spkssb_c ( 399, et+tdelta, "j2000", state2 );

               /.
               Note that qderiv_ is an f2c'd Fortran routine, so
               we must pass in the dimension and time delta by
               reference.
               ./
               dim = 3;
               qderiv_ ( &dim, state0+3, state2+3, &tdelta, acc );

               /.
               Now compute the desired state vector:
               ./
               spkaps_c ( 301,   et,  "j2000", "lt+s", 
                          stobs, acc, state,   &lt,    &dlt );

               printf( "et = %20.6f\n",                        et       );
               printf( "J2000 x-position (km):   %20.8f\n",    state[0] );
               printf( "J2000 y-position (km):   %20.8f\n",    state[1] );
               printf( "J2000 z-position (km):   %20.8f\n",    state[2] );
               printf( "J2000 x-velocity (km/s): %20.12f\n",   state[3] );
               printf( "J2000 y-velocity (km/s): %20.12f\n",   state[4] );
               printf( "J2000 z-velocity (km/s): %20.12f\n",   state[5] );
               printf( "One-way light time (s):  %20.12f\n",   lt       );
               printf( "Light time rate:         %20.08e\n\n", dlt      );      
            }
            return ( 0 ); 
         }


      The output produced by this program will vary somewhat as
      a function of the platform on which the program is built and
      executed. On a PC/Linux/gcc platform, the following output
      was produced:

         et =             0.000000
         J2000 x-position (km):       -291584.61369498
         J2000 y-position (km):       -266693.40583163
         J2000 z-position (km):        -76095.65320924
         J2000 x-velocity (km/s):       0.643439157435
         J2000 y-velocity (km/s):      -0.666065873657
         J2000 z-velocity (km/s):      -0.301310063429
         One-way light time (s):        1.342310610325
         Light time rate:               1.07316909e-07

         et =          3600.000000
         J2000 x-position (km):       -289256.45942322
         J2000 y-position (km):       -269080.60545908
         J2000 z-position (km):        -77177.35277130
         J2000 x-velocity (km/s):       0.649970320169
         J2000 y-velocity (km/s):      -0.660148253293
         J2000 z-velocity (km/s):      -0.299630417907
         One-way light time (s):        1.342693954864
         Light time rate:               1.05652599e-07

         et =          7200.000000
         J2000 x-position (km):       -286904.89654240
         J2000 y-position (km):       -271446.41676468
         J2000 z-position (km):        -78252.96553362
         J2000 x-velocity (km/s):       0.656443883155
         J2000 y-velocity (km/s):      -0.654183552046
         J2000 z-velocity (km/s):      -0.297928532945
         One-way light time (s):        1.343071311734
         Light time rate:               1.03990457e-07

         et =         10800.000000
         J2000 x-position (km):       -284530.13302756
         J2000 y-position (km):       -273790.67111559
         J2000 z-position (km):        -79322.41170392
         J2000 x-velocity (km/s):       0.662859504730
         J2000 y-velocity (km/s):      -0.648172246851
         J2000 z-velocity (km/s):      -0.296204558469
         One-way light time (s):        1.343442689069
         Light time rate:               1.02330665e-07

         et =         14400.000000
         J2000 x-position (km):       -282132.37807792
         J2000 y-position (km):       -276113.20159697
         J2000 z-position (km):        -80385.61203056
         J2000 x-velocity (km/s):       0.669216846492
         J2000 y-velocity (km/s):      -0.642114815280
         J2000 z-velocity (km/s):      -0.294458644904
         One-way light time (s):        1.343808095656
         Light time rate:               1.00673404e-07

Restrictions

 
   1) This routine should not be used to compute geometric states. 
      Instead, use spkezr_c, spkez_c, or spkgeo_c. spkgeo_c, which is called 
      by spkezr_c and spkez_c, introduces less round-off error when the 
      observer and target have a common center that is closer to 
      both objects than is the solar system barycenter. 
 
   2) The kernel files to be used by spkaps_c must be loaded 
      (normally by the CSPICE kernel loader furnsh_c) before  
      this routine is called. 
 
   3) Unlike most other SPK state computation routines, this 
      routine requires that the output state be relative to an 
      inertial reference frame.  
 

Literature_References

 
   SPK Required Reading. 
 

Author_and_Institution

 
   N.J. Bachman    (JPL) 
 

Version

 
   -CSPICE Version 3.0.1, 07-JUL-2014 (NJB)

       Descriptions of aberration correction choices that include
       stellar aberration were missing. These have been added.
       Erroneous claim that stellar aberration specifiers (instances
       of "+S") in `abcorr' are ignored was deleted.

       Discussion of light time corrections was updated. Assertions
       that converged light time corrections are unlikely to be
       useful were removed.

   -CSPICE Version 1.0.0, 11-JAN-2008 (NJB)

Index_Entries

 
   low-level aberration-corrected state computation 
   low-level light time and stellar aberration correction 
 
Wed Apr  5 17:54:43 2017