/*

-Procedure spkcpt_c ( SPK, constant position target state )

-Abstract

   Return the state, relative to a specified observer, of a target
   having constant position in a specified reference frame. The
   target's position is provided by the calling program rather than
   by loaded SPK files.

-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

   FRAMES
   PCK
   SPK
   TIME

-Keywords

   EPHEMERIS

*/

   #include "SpiceUsr.h"
   #include "SpiceZfc.h"
   #include "SpiceZmc.h"
   #undef   spkcpt_c

   void spkcpt_c ( ConstSpiceDouble       trgpos [3],
                   ConstSpiceChar       * trgctr,
                   ConstSpiceChar       * trgref,
                   SpiceDouble            et,
                   ConstSpiceChar       * outref,
                   ConstSpiceChar       * refloc,
                   ConstSpiceChar       * abcorr,
                   ConstSpiceChar       * obsrvr,
                   SpiceDouble            state  [6],
                   SpiceDouble          * lt          )

/*

-Brief_I/O

   VARIABLE  I/O  DESCRIPTION
   --------  ---  --------------------------------------------------
   trgpos     I   Target position relative to center of motion.
   trgctr     I   Center of motion of target.
   trgref     I   Frame of target position.
   et         I   Observation epoch.
   outref     I   Reference frame of output state.
   refloc     I   Output reference frame evaluation locus.
   abcorr     I   Aberration correction.
   obsrvr     I   Name of observing ephemeris object.
   state      O   State of target with respect to observer.
   lt         O   One way light time between target and
                  observer.

-Detailed_Input

   trgpos      is the fixed (constant) position of a target relative to
               its "center of motion" `trgctr', expressed in the
               reference frame `trgref'.

               Units are always km.


   trgctr      is the name of the center of motion of `trgpos'. The
               ephemeris of `trgctr' 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 `trgctr'.


   trgref      is the name of the reference frame relative to which
               the input position `trgpos' is expressed. The target has
               constant position relative to its center of motion
               in this reference frame.

               Case and leading and trailing blanks are not
               significant in the string `trgref'.


   et          is the 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 target epoch `trgepc'.

   outref      is the 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      is a string 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 `lt' be the one-way light time
                              between the target and observer, the
                              target epoch is

                                 et-lt  if reception aberration
                                        corrections are used

                                 et+lt  if transmission aberration
                                        corrections are used

                                 et     if no aberration corrections
                                        are used

                              Normally the locus "TARGET" should
                              be selected when `outref' is `trgref',
                              the frame in which the target position
                              is specified.


                  "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 spkezr_c.

               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      indicates 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+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.


               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'.


   obsrvr      is the name of an observing 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
               observer is Earth.

               Case and leading and trailing blanks are not
               significant in the string `obsrvr'.

-Detailed_Output

   state       is a Cartesian state vector representing the position
               and velocity of the target relative to the specified
               observer. `state' is corrected for the specified
               aberrations and is expressed with respect to the
               reference frame specified by `outref'. The first three
               components of `state' represent the x-, y- and
               z-components of the target's position; the last three
               components form the corresponding velocity vector.

               The position component of `state' 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.

               The velocity component of `state' is the derivative
               with respect to time of the position component of
               `state'.

               Units are always km and km/sec.

               When `state' is expressed in a time-dependent
               (non-inertial) output frame, the orientation of that
               frame relative to the J2000 frame is evaluated in the
               manner indicated by the input argument `refloc' (see
               description above).


   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.

-Parameters

   None.

-Exceptions

   1)  If either the name of the center of motion or the observer
       cannot be translated to its NAIF ID code, an error is signaled
       by a routine in the call tree of this routine.

   2)  If the reference frame `outref' is unrecognized, an error
       is signaled by a routine in the call tree of this
       routine.

   3)  If the reference frame `trgref' is unrecognized, an error is
       signaled by a routine in the call tree of this routine.

   4)  If the frame evaluation locus `refloc' is not recognized, an
       error is signaled by a routine in the call tree of this
       routine.

   5)  If the loaded kernels provide insufficient data to compute
       the requested state vector, an error is signaled
       by a routine in the call tree of this routine.

   6)  If an error occurs while reading an SPK or other kernel file,
       the error is signaled by a routine in the call tree of
       this routine.

   7)  If the aberration correction `abcorr' is not recognized, an
       error is signaled by a routine in the call tree of this
       routine.

   8)  If any of the `trgctr', `trgref', `outref', `refloc',
       `abcorr', `obsrvr' or `trgpos' input string pointers is null,
       the error SPICE(NULLPOINTER) is signaled.

   9)  If any of the `trgctr', `trgref', `outref', `refloc', `abcorr'
       or `obsrvr' input strings has zero length, the error
       SPICE(EMPTYSTRING) is signaled.

-Files

   Appropriate kernels must be loaded by the calling program before
   this routine is called.

   The following data are required:

   -  SPK data: ephemeris data for target center and observer
      must be loaded. If aberration corrections are used, the
      states of target center and observer relative to the solar
      system barycenter must be calculable from the available
      ephemeris data. Typically ephemeris data are made available
      by loading one or more SPK files using furnsh_c.

   The following data may be required:

   -  PCK data: if the target frame is a PCK frame, rotation data
      for the target frame must be loaded. These may be provided
      in a text or binary PCK file.

   -  Frame data: if a frame definition not built into SPICE is
      required, for example to convert the observer-target state
      to the output frame, that definition must be available in
      the kernel pool. Typically frame definitions are supplied
      by loading a frame kernel using furnsh_c.

   -  Additional kernels: if any frame used in this routine's
      state computation is a CK frame, then at least one CK and
      corresponding SCLK kernel is required. If dynamic frames
      are used, additional SPK, PCK, CK, or SCLK kernels may be
      required.

   In all cases, kernel data are normally loaded once per program
   run, NOT every time this routine is called.

-Particulars

   This routine computes observer-target states for targets whose
   trajectories are not provided by SPK files.

   Targets supported by this routine must have constant position
   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 observer must be computable using
   loaded SPK data.

   For applications in which the target has non-zero, constant
   velocity relative to its center of motion, the CSPICE routine

      spkcvt_c     { SPK, constant velocity target }

   can be used.

   This routine is suitable for computing states of landmarks on the
   surface of an extended object, as seen by a specified observer,
   in cases where no SPK data are available for those landmarks.

   This routine's treatment of the output reference frame differs
   from that of the principal SPK API routines

      spkezr_c
      spkez_c
      spkpos_c
      spkezp_c

   which require both observer and target ephemerides to be provided
   by loaded SPK files:

      The SPK API routines listed above evaluate the orientation of
      the output reference frame (with respect to the J2000 frame)
      at an epoch corrected for one-way light time between the
      observer and the center of the output frame. When the center
      of the output frame is not the target (for example, when the
      target is on the surface of Mars and the output frame is
      centered at Mars' center), the epoch of evaluation may not
      closely match the light-time corrected epoch associated with
      the target itself.

      This routine allows the caller to dictate how the orientation
      of the output reference frame is to be evaluated. The caller
      passes to this routine an input string called the output
      frame's evaluation "locus." This string specifies the location
      associated with the output frame's evaluation epoch. The three
      possible values of the locus are

         "TARGET"
         "OBSERVER"
         "CENTER"

      The choice of locus has an effect when aberration corrections
      are used and the output frame is non-inertial.

      When the locus is "TARGET" and light time corrections are used,
      the orientation of the output frame is evaluated at the epoch
      obtained by correcting the observation epoch `et' for one-way
      observer-target light time `lt'. The evaluation epoch will be
      either et-lt or et+lt for reception or transmission corrections
      respectively.

      For remote sensing applications where the target is a surface
      point on an extended object, and the orientation of that
      object should be evaluated at the emission time, the locus
      "TARGET" should be used.

      When the output frame's orientation should be evaluated at
      the observation epoch `et', which is the case when the
      output frame is centered at the observer, the locus
      "OBSERVER" should be used.

      The locus option "CENTER" is provided for compatibility
      with existing SPK state computation APIs such as spkezr_c.

      Note that the output frame evaluation locus does not affect
      the computation of light time between the target and
      observer.


   The SPK routines that compute observer-target states for
   combinations of objects having ephemerides provided by SPK files and
   objects having constant position or constant velocity are

      spkcpo_c {SPK, Constant position observer}
      spkcpt_c {SPK, Constant position target}
      spkcvo_c {SPK, Constant velocity observer}
      spkcvt_c {SPK, Constant velocity target}

-Examples

   The numerical results shown for this example 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.

   1) Demonstrate use of this routine; in particular demonstrate
      applications of the output frame evaluation locus.

      The following program is not necessarily realistic: for
      brevity, it combines several unrelated computations.

      Task Description
      ================

      Find the state of a given surface point on earth, corrected
      for light time and stellar aberration, relative to the Mars
      Global Surveyor spacecraft, expressed in the earth fixed
      reference frame ITRF93. The selected point is the position
      of the DSN station DSS-14.

      Contrast the states computed by setting the output frame
      evaluation locus to "TARGET" and to "CENTER". Show that the
      latter choice produces results very close to those that
      can be obtained using spkezr_c.

      Also compute the state of a selected Mars surface point as
      seen from MGS. The point we'll use is the narrow angle MOC
      boresight surface intercept corresponding to the chosen
      observation time. Express the state in a spacecraft-centered
      reference frame. Use the output frame evaluation locus
      "OBSERVER" for this computation.

      The observation epoch is 2003 OCT 13 06:00:00 UTC.


      Kernels
      =======

      Use the meta-kernel shown below to load the required SPICE
      kernels.


         KPL/MK

         File name: spkcpt_ex1.tm

         This is the meta-kernel file for the header code example for
         the subroutine spkcpt_c. The kernel files referenced by this
         meta-kernel can be found on the NAIF website.

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

         The names and contents of the kernels referenced
         by this meta-kernel are as follows:

            File name                        Contents
            ---------                        --------
            de421.bsp                        Planetary ephemeris
            pck00010.tpc                     Planet orientation and
                                             radii
            naif0010.tls                     Leapseconds
            earth_720101_070426.bpc          Earth historical
                                             binary PCK
            earthstns_itrf93_050714.bsp      DSN station SPK
            mgs_moc_v20.ti                   MGS MOC instrument
                                             parameters
            mgs_sclkscet_00061.tsc           MGS SCLK coefficients
            mgs_sc_ext12.bc                  MGS s/c bus attitude
            mgs_ext12_ipng_mgs95j.bsp        MGS ephemeris

         \begindata

         KERNELS_TO_LOAD = ( 'de421.bsp',
                             'pck00010.tpc',
                             'naif0010.tls',
                             'earth_720101_070426.bpc',
                             'earthstns_itrf93_050714.bsp',
                             'mgs_moc_v20.ti',
                             'mgs_sclkscet_00061.tsc',
                             'mgs_sc_ext12.bc',
                             'mgs_ext12_ipng_mgs95j.bsp'  )

         \begintext

         End of meta-kernel.


      Example code begins here.


      /.
         Program spkcpt_ex1

         This program demonstrates the use of spkcpt_c.
         Computations are performed using all three possible
         values of the output frame evaluation locus `refloc':

            "TARGET"
            "OBSERVER"
            "CENTER"

         Several unrelated computations are performed in this
         program. In particular, computations involving a surface
         point on Mars are included simply to demonstrate use of
         the "OBSERVER" option.
      ./

      #include <stdio.h>
      #include <string.h>
      #include <stdlib.h>
      #include "SpiceUsr.h"

      int main()
      {
         /.
         Local constants
         ./

         #define  CAMERA   "MGS_MOC_NA"
         #define  MAXBND   100
         #define  META     "spkcpt_ex1.tm"
         #define  FRNMLN   33
         #define  SHAPLN   33
         #define  TIMFMT   "YYYY MON DD HR:MN:SC.###### UTC"
         #define  TIMLEN   41

         /.
         Local variables
         ./
         SpiceBoolean            found;

         SpiceChar             * abcorr;
         SpiceChar               camref  [ FRNMLN ];
         SpiceChar               emitim  [ TIMLEN ];
         SpiceChar             * refloc;
         SpiceChar             * obsrvr;
         SpiceChar             * obstim;
         SpiceChar             * outref;
         SpiceChar               shape   [ SHAPLN ];
         SpiceChar             * target;
         SpiceChar             * trgctr;
         SpiceChar             * trgref;

         SpiceDouble             bounds [ MAXBND ] [ 3 ];
         SpiceDouble             bsight [ 3 ];
         SpiceDouble             et;
         SpiceDouble             lt0;
         SpiceDouble             lt1;
         SpiceDouble             lt2;
         SpiceDouble             lt3;
         SpiceDouble             spoint [ 3 ];
         SpiceDouble             srfvec [ 3 ];
         SpiceDouble             state0 [ 6 ];
         SpiceDouble             state1 [ 6 ];
         SpiceDouble             state2 [ 6 ];
         SpiceDouble             state3 [ 6 ];
         SpiceDouble             trgepc ;
         SpiceDouble             trgpos [ 3 ];

         SpiceInt                camid;
         SpiceInt                n;

         /.
         Load SPICE kernels.
         ./
         furnsh_c ( META );

         /.
         Convert the observation time to seconds past J2000 TDB.
         ./
         obstim = "2003 OCT 13 06:00:00.000000 UTC";

         str2et_c ( obstim, &et );

         /.
         Set the observer, target center, and target frame.
         ./
         obsrvr = "MGS";
         trgctr = "EARTH";
         trgref = "ITRF93";

         /.
         Set the state of DSS-14 relative to the earth's
         center at the J2000 epoch, expressed in the
         ITRF93 reference frame. Values come from the
         earth station SPK specified in the meta-kernel.

         The actual station velocity is non-zero due
         to tectonic plate motion; we ignore the motion
         in this example. See the routine spkcvt_c for an
         example in which the plate motion is accounted for.
         ./
         trgpos[0] =  -2353.6213656676991;
         trgpos[1] =  -4641.3414911499403;
         trgpos[2] =   3677.0523293197439;

         /.
         Find the apparent state of the station relative
         to the spacecraft in the ITRF93 reference frame.
         Evaluate the earth's orientation, that is the
         orientation of the ITRF93 frame relative to the
         J2000 frame, at the epoch obtained by correcting
         the observation time for one-way light time. This
         correction is obtained by setting `refloc' to "TARGET".
         ./
         outref = "ITRF93";
         abcorr = "CN+S";

         refloc = "TARGET";

         /.
         Compute the observer-target state.
         ./
         spkcpt_c ( trgpos, trgctr, trgref,
                    et,     outref, refloc, abcorr,
                    obsrvr, state0, &lt0            );

         /.
         Display the computed state and light time.
         ./
         timout_c ( et-lt0, TIMFMT, TIMLEN, emitim );

         printf ( "\n"
                  " Frame evaluation locus:   %s\n"
                  "\n"
                  " Observer:                 %s\n"
                  " Observation time:         %s\n"
                  " Target center:            %s\n"
                  " Target frame:             %s\n"
                  " Emission time:            %s\n"
                  " Output reference frame:   %s\n"
                  " Aberration correction:    %s\n"
                  "\n"
                  " Observer-target position (km):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Observer-target velocity (km/s):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Light time (s):   %20.8f\n",

                  refloc,    obsrvr,    obstim,    trgctr,
                  trgref,    emitim,    outref,    abcorr,
                  state0[0], state0[1], state0[2],
                  state0[3], state0[4], state0[5], lt0   );

         /.
         Repeat the computation, this time evaluating the
         earth's orientation at the epoch obtained by
         subtracting from the observation time the one way
         light time from the earth's center.

         This is equivalent to looking up the observer-target
         state using spkezr_c.
         ./
         refloc = "CENTER";

         spkcpt_c ( trgpos, trgctr, trgref,
                    et,     outref, refloc, abcorr,
                    obsrvr, state1, &lt1            );

         /.
         Display the computed state and light time.
         ./
         timout_c ( et-lt1, TIMFMT, TIMLEN, emitim );

         printf ( "\n\n"
                  " Frame evaluation locus:   %s\n"
                  "\n"
                  " Observer:                 %s\n"
                  " Observation time:         %s\n"
                  " Target center:            %s\n"
                  " Target frame:             %s\n"
                  " Emission time:            %s\n"
                  " Output reference frame:   %s\n"
                  " Aberration correction:    %s\n"
                  "\n"
                  " Observer-target position (km):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Observer-target velocity (km/s):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Light time (s):   %20.8f\n",

                  refloc,    obsrvr,    obstim,    trgctr,
                  trgref,    emitim,    outref,    abcorr,
                  state1[0], state1[1], state1[2],
                  state1[3], state1[4], state1[5], lt0   );

         printf ( "\n"
                  " Distance between above positions (km):  "
                  "  %20.8f\n"
                  " Velocity difference magnitude  (km/s):  "
                  "  %20.8f\n",
                  vdist_c( state0,   state1   ),
                  vdist_c( state0+3, state1+3 )                     );

         /.
         Check: compare the state computed directly above
         to one produced by spkezr_c:
         ./
         target = "DSS-14";

         spkezr_c ( target,  et,      outref,  abcorr,
                    obsrvr,  state2,  &lt2            );

         printf ( "\n\n"
                  " State computed using spkezr_c:\n"
                  "\n"
                  " Observer:               %s\n"
                  " Observation time:       %s\n"
                  " Target:                 %s\n"
                  " Output reference frame: %s\n"
                  " Aberration correction:  %s\n"
                  "\n"
                  " Observer-target position (km):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Observer-target velocity (km/s):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Light time (s): %20.8f\n",

                  obsrvr,    obstim,    target,
                  outref,    abcorr,
                  state2[0], state2[1], state2[2],
                  state2[3], state2[4], state2[5], lt2   );

         printf ( "\n"
                  " Distance between last two "
                  "positions (km): %20.8f\n"
                  " Velocity difference magnitude    "
                  " (km/s): %20.8f\n",
                  vdist_c( state1,   state2   ),
                  vdist_c( state1+3, state2+3 )          );

         /.
         Finally, compute an observer-target state in
         a frame centered at the observer.
         The reference frame will be that of the
         MGS MOC NA camera.

         In this case we'll use as the target the surface
         intercept on Mars of the camera boresight. This
         allows us to easily verify the correctness of
         the results returned by spkcpt_c.

         Get camera frame and FOV parameters. We'll need
         the camera ID code first.
         ./
         bodn2c_c ( CAMERA, &camid, &found );

         if ( !found )
         {
            printf ( "Camera name could not be mapped "
                     "to an ID code.\n"              );
            exit( 1 );
         }

         /.
         getfov_c will return the name of the camera-fixed frame
         in the string `camref', the camera boresight vector in
         the array `bsight', and the FOV corner vectors in the
         array `bounds'. All we're going to use are the camera
         frame name and camera boresight.
         ./
         getfov_c ( camid, MAXBND, SHAPLN, FRNMLN,
                    shape, camref, bsight, &n,     bounds );

         /.
         Find the camera boresight surface intercept.
         ./

         trgctr = "MARS";
         trgref = "IAU_MARS";

         sincpt_c ( "Ellipsoid",  trgctr,  et,     trgref,
                    abcorr,       obsrvr,  camref, bsight,
                    spoint,       &trgepc, srfvec, &found  );

         outref = camref;

         refloc = "OBSERVER";

         spkcpt_c ( spoint, trgctr, trgref,
                    et,     outref, refloc, abcorr,
                    obsrvr, state3, &lt3           );

         /.
         Convert the emission time and the target state
         evaluation epoch to strings for output.
         ./
         timout_c ( et-lt3, TIMFMT, TIMLEN, emitim );

         printf ( "\n\n"
                  " Frame evaluation locus:   %s\n"
                  "\n"
                  " Observer:                 %s\n"
                  " Observation time:         %s\n"
                  " Target center:            %s\n"
                  " Target frame:             %s\n"
                  " Emission time:            %s\n"
                  " Output reference frame:   %s\n"
                  " Aberration correction:    %s\n"
                  "\n"
                  " Observer-target position (km):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Observer-target velocity (km/s):\n"
                  " %20.8f %20.8f %20.8f\n"
                  " Light time (s): %20.8f\n"
                  " Target range from sincpt_c (km): "
                  "         %20.8f\n",

                  refloc,    obsrvr,    obstim,    trgctr,
                  trgref,    emitim,    outref,    abcorr,
                  state3[0], state3[1], state3[2],
                  state3[3], state3[4], state3[5], lt3,
                  vnorm_c( srfvec )                          );

         return ( 0 );
      }


      When this program was executed on a Mac/Intel/cc/64-bit
      platform, the output was:


       Frame evaluation locus:   TARGET

       Observer:                 MGS
       Observation time:         2003 OCT 13 06:00:00.000000 UTC
       Target center:            EARTH
       Target frame:             ITRF93
       Emission time:            2003 OCT 13 05:55:44.232914 UTC
       Output reference frame:   ITRF93
       Aberration correction:    CN+S

       Observer-target position (km):
          52746468.84243592    52367725.79653772    18836142.68957234
       Observer-target velocity (km/s):
              3823.39593314       -3840.60002121           2.21337692
       Light time (s):           255.76708533


       Frame evaluation locus:   CENTER

       Observer:                 MGS
       Observation time:         2003 OCT 13 06:00:00.000000 UTC
       Target center:            EARTH
       Target frame:             ITRF93
       Emission time:            2003 OCT 13 05:55:44.232914 UTC
       Output reference frame:   ITRF93
       Aberration correction:    CN+S

       Observer-target position (km):
          52746419.34648802    52367775.65036674    18836142.68969753
       Observer-target velocity (km/s):
              3823.40103499       -3840.59789000           2.21337692
       Light time (s):           255.76708533

       Distance between above positions (km):             70.25135676
       Velocity difference magnitude  (km/s):              0.00552910


       State computed using spkezr_c:

       Observer:               MGS
       Observation time:       2003 OCT 13 06:00:00.000000 UTC
       Target:                 DSS-14
       Output reference frame: ITRF93
       Aberration correction:  CN+S

       Observer-target position (km):
          52746419.34641990    52367775.65039122    18836142.68968301
       Observer-target velocity (km/s):
              3823.40103499       -3840.59789000           2.21337692
       Light time (s):         255.76708533

       Distance between last two positions (km):           0.00007383
       Velocity difference magnitude     (km/s):           0.00000000


       Frame evaluation locus:   OBSERVER

       Observer:                 MGS
       Observation time:         2003 OCT 13 06:00:00.000000 UTC
       Target center:            MARS
       Target frame:             IAU_MARS
       Emission time:            2003 OCT 13 05:59:59.998702 UTC
       Output reference frame:   MGS_MOC_NA
       Aberration correction:    CN+S

       Observer-target position (km):
                 0.00000001          -0.00000001         388.97573572
       Observer-target velocity (km/s):
                 2.91968665           0.15140014           0.92363513
       Light time (s):           0.00129748
       Target range from sincpt_c (km):                  388.97573572


-Restrictions

   1)  This routine may not be suitable for work with stars or other
       objects having large distances from the observer, due to loss
       of precision in position vectors.

-Literature_References

   None.

-Author_and_Institution

   N.J. Bachman        (JPL)
   J. Diaz del Rio     (ODC Space)
   S.C. Krening        (JPL)
   B.V. Semenov        (JPL)

-Version

   -CSPICE Version 1.0.2, 10-AUG-2021 (JDR)

       Edited the header to comply with NAIF standard.
       Corrected -Index_Entries.

       Modified code example output format for the solutions to fit
       within the -Examples section without modifications. Removed
       unnecessary code from examples.

   -CSPICE Version 1.0.1, 09-SEP-2015 (NJB)

       The -Exceptions section of the header was updated
       to mention exceptions involving null pointers and
       empty input strings.

   -CSPICE Version 1.0.0, 27-MAR-2012 (NJB) (SCK) (BVS)

-Index_Entries

   state of constant_position_target
   state of fixed_position_target
   state of surface_point on extended_object
   state of landmark on extended_object

-&
*/

{ /* Begin spkcpt_c */


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

   /*
   Check the input state pointer.
   */
   CHKPTR ( CHK_STANDARD, "spkcpt_c", trgpos );

   /*
   Check the input strings.
   */
   CHKFSTR ( CHK_STANDARD, "spkcpt_c", trgctr );
   CHKFSTR ( CHK_STANDARD, "spkcpt_c", trgref );
   CHKFSTR ( CHK_STANDARD, "spkcpt_c", outref );
   CHKFSTR ( CHK_STANDARD, "spkcpt_c", refloc );
   CHKFSTR ( CHK_STANDARD, "spkcpt_c", abcorr );
   CHKFSTR ( CHK_STANDARD, "spkcpt_c", obsrvr );

   /*
   Check the output pointers.
   */
   CHKPTR ( CHK_STANDARD, "spkcpt_c", state );
   CHKPTR ( CHK_STANDARD, "spkcpt_c", lt    );

   /*
   Let the f2c'd routine do the work.
   */
   spkcpt_ ( ( doublereal * ) trgpos,
             ( char       * ) trgctr,
             ( char       * ) trgref,
             ( doublereal * ) &et,
             ( char       * ) outref,
             ( char       * ) refloc,
             ( char       * ) abcorr,
             ( char       * ) obsrvr,
             ( doublereal * ) state,
             ( doublereal * ) lt,
             ( ftnlen       ) strlen(trgctr),
             ( ftnlen       ) strlen(trgref),
             ( ftnlen       ) strlen(outref),
             ( ftnlen       ) strlen(refloc),
             ( ftnlen       ) strlen(abcorr),
             ( ftnlen       ) strlen(obsrvr)  );

   chkout_c ( "spkcpt_c" );

} /* End spkcpt_c */
