/*

-Procedure spkacs_c ( S/P Kernel, aberration corrected state )

-Abstract
 
   Return the state (position and velocity) of a target body 
   relative to an observer, optionally corrected for light time 
   and stellar aberration, expressed relative to an inertial 
   reference frame. 
 
-Disclaimer
 
   THIS SOFTWARE AND ANY RELATED MATERIALS WERE CREATED BY THE 
   CALIFORNIA INSTITUTE OF TECHNOLOGY (CALTECH) UNDER A U.S. 
   GOVERNMENT CONTRACT WITH THE NATIONAL AERONAUTICS AND SPACE 
   ADMINISTRATION (NASA). THE SOFTWARE IS TECHNOLOGY AND SOFTWARE 
   PUBLICLY AVAILABLE UNDER U.S. EXPORT LAWS AND IS PROVIDED "AS-IS" 
   TO THE RECIPIENT WITHOUT WARRANTY OF ANY KIND, INCLUDING ANY 
   WARRANTIES OF PERFORMANCE OR MERCHANTABILITY OR FITNESS FOR A 
   PARTICULAR USE OR PURPOSE (AS SET FORTH IN UNITED STATES UCC 
   SECTIONS 2312-2313) OR FOR ANY PURPOSE WHATSOEVER, FOR THE 
   SOFTWARE AND RELATED MATERIALS, HOWEVER USED. 
 
   IN NO EVENT SHALL CALTECH, ITS JET PROPULSION LABORATORY, OR NASA 
   BE LIABLE FOR ANY DAMAGES AND/OR COSTS, INCLUDING, BUT NOT 
   LIMITED TO, INCIDENTAL OR CONSEQUENTIAL DAMAGES OF ANY KIND, 
   INCLUDING ECONOMIC DAMAGE OR INJURY TO PROPERTY AND LOST PROFITS, 
   REGARDLESS OF WHETHER CALTECH, JPL, OR NASA BE ADVISED, HAVE 
   REASON TO KNOW, OR, IN FACT, SHALL KNOW OF THE POSSIBILITY. 
 
   RECIPIENT BEARS ALL RISK RELATING TO QUALITY AND PERFORMANCE OF 
   THE SOFTWARE AND ANY RELATED MATERIALS, AND AGREES TO INDEMNIFY 
   CALTECH AND NASA FOR ALL THIRD-PARTY CLAIMS RESULTING FROM THE 
   ACTIONS OF RECIPIENT IN THE USE OF THE SOFTWARE. 
 
-Required_Reading
 
   SPK 
 
-Keywords
 
   EPHEMERIS 
 
*/

   #include "SpiceUsr.h"
   #include "SpiceZfc.h"
   #include "SpiceZmc.h"
   #include "SpiceZst.h"

   void spkacs_c ( SpiceInt           targ,
                   SpiceDouble        et,
                   ConstSpiceChar   * ref,
                   ConstSpiceChar   * abcorr,
                   SpiceInt           obs,
                   SpiceDouble        starg[6],
                   SpiceDouble      * lt,
                   SpiceDouble      * dlt      ) 

/*

-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. 
   obs        I   Observer. 
   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 output state `starg' is 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 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). 
 
                             The "CN" correction typically does not 
                             substantially improve accuracy because 
                             the errors made by ignoring 
                             relativistic effects may be larger than 
                             the improvement afforded by obtaining 
                             convergence of the light time solution. 
                             The "CN" correction computation also  
                             requires a significantly greater number 
                             of CPU cycles than does the  
                             one-iteration light time correction. 
 
                  "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+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 and stellar  
                             aberration corrections. 
 
 
   obs         is the NAIF ID code for the observer body. The 
               target and observer define a state vector whose 
               position component points from the observer to the 
               target. 
 
-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 aberrations, 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 aberrations, 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 is 
      diagnosed by a routine in the call tree of this routine.
 
   2) If the reference frame requested is not a recognized
      inertial reference frame, the error SPICE(BADFRAME)  
      is signaled. 
 
   3) 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. 
 
   4) 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. 
 
   6) 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.
      
   7) 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. 
 
   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"

         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             dlt;
            SpiceDouble             et;
            SpiceDouble             et0;
            SpiceDouble             lt;
            SpiceDouble             state  [6];
            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)
               ./
               spkacs_c ( 301, et,    "j2000", "lt+s", 
                          399, 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) The kernel files to be used by spkacs_c must be loaded 
      (normally by the CSPICE kernel loader furnsh_c) before  
      this routine is called. 
 
   2) 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 1.0.0, 11-JAN-2008 (NJB)

-Index_Entries
 
   low-level aberration correction 
   aberration-corrected state from spk file 
   get light time and stellar aberration-corrected state 
 
-&
*/

{ /* Begin spkacs_c */


   /*
   Participate in error tracing.
   */
   chkin_c ( "spkacs_c" );


   /*
   Check the input strings to make sure the pointers are non-null
   and the string lengths are non-zero.
   */
   CHKFSTR ( CHK_STANDARD, "spkacs_c", ref    );
   CHKFSTR ( CHK_STANDARD, "spkacs_c", abcorr );


   spkacs_ ( (integer    *) &targ,
             (doublereal *) &et,
             (char       *) ref,
             (char       *) abcorr,
             (integer    *) &obs,
             (doublereal *) starg,
             (doublereal *) lt,
             (doublereal *) dlt,
             (ftnlen      ) strlen(ref),
             (ftnlen      ) strlen(abcorr) );


   chkout_c ( "spkacs_c" );

} /* End spkacs_c */
