/*

-Procedure srfxpt_c ( Surface intercept point )

-Abstract
  
   Deprecated: This routine has been superseded by the CSPICE
   routine sincpt_c. This routine is supported for purposes of
   backward compatibility only.

   Given an observer and a direction vector defining a ray, compute the
   surface intercept point of the ray on a target body at a specified
   epoch, optionally corrected for light time and stellar aberration.
 
-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
   NAIF_IDS 
   PCK 
   SPK 
   TIME 
 
-Keywords
 
   GEOMETRY 
 
*/

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


   void srfxpt_c ( ConstSpiceChar      * method,
                   ConstSpiceChar      * target,
                   SpiceDouble           et,
                   ConstSpiceChar      * abcorr,
                   ConstSpiceChar      * obsrvr,
                   ConstSpiceChar      * dref,
                   ConstSpiceDouble      dvec   [3],
                   SpiceDouble           spoint [3],
                   SpiceDouble         * dist,
                   SpiceDouble         * trgepc,
                   SpiceDouble           obspos [3],
                   SpiceBoolean        * found      )

/*

-Brief_I/O
 
   Variable  I/O  Description 
   --------  ---  -------------------------------------------------- 
   method     I   Computation method. 
   target     I   Name of target body. 
   et         I   Epoch in ephemeris seconds past J2000 TDB. 
   abcorr     I   Aberration correction. 
   obsrvr     I   Name of observing body. 
   dref       I   Reference frame of input direction vector. 
   dvec       I   Ray's direction vector. 
   spoint     O   Surface intercept point on the target body. 
   dist       O   Distance from the observer to the intercept point. 
   trgepc     O   Intercept epoch. 
   obspos     O   Observer position relative to target center. 
   found      O   Flag indicating whether intercept was found. 
 
-Detailed_Input
 
   method      is a short string providing parameters defining 
               the computation method to be used.  Parameters 
               include, but are not limited to, the shape model 
               used to represent the surface of the target body. 
 
               The only choice currently supported is 
 
                  "Ellipsoid"        The intercept computation uses 
                                     a triaxial ellipsoid to model 
                                     the surface of the target body. 
                                     The ellipsoid's radii must be 
                                     available in the kernel pool. 
 
               Neither case nor white space are significant in  
               `method'.  For example, the string " eLLipsoid " is  
               valid.                  
 
               In a later Toolkit release, this argument will be 
               used to invoke a wider range of surface 
               representations. For example, it will be possible to 
               represent the target body's surface using a digital 
               model. 
 
 
   target      is the name of the target body.  `target' is
               case-insensitive, and leading and trailing blanks in
               `target' are not significant. Optionally, you may supply
               a string containing the integer ID code for the object.
               For example both "MOON" and "301" are legitimate strings
               that indicate the moon is the target body.
 
               When the target body's surface is represented by a
               tri-axial ellipsoid, this routine assumes that a kernel
               variable representing the ellipsoid's radii is present
               in the kernel pool.  Normally the kernel variable would
               be defined by loading a PCK file.
 
 
   et          is the epoch of participation of the observer, expressed
               as ephemeris seconds past J2000 TDB: `et' is the epoch
               at which the observer's state is computed.
 
               When aberration corrections are not used, `et' is also
               the epoch at which the state and orientation of the
               target body are computed.
 
               When aberration corrections are used, `et' is the epoch
               at which the observer's state relative to the solar
               system barycenter is computed; in this case the position
               and orientation of the target body are computed at et-lt
               or et+lt, where `lt' is the one-way light time between
               the intercept point and the observer, and the sign
               applied to lt depends on the selected correction. See
               the description of `abcorr' below for details.

 
   abcorr      indicates the aberration correction to be applied 
               when computing the observer-target state and the 
               orientation of the target body.  `abcorr' may be any of 
               the following. 
 
                  "NONE"     Apply no correction. Return the  
                             geometric surface intercept point on the 
                             target body. 
 
               Let `lt' represent the one-way light time between the 
               observer and the surface intercept point (note: NOT 
               between the observer and the target body's center). 
               The following values of `abcorr' apply to the 
               "reception" case in which photons depart from the 
               intercept point'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 location of the surface 
                             intercept point 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. 
 
                             Both the target state as seen by the 
                             observer, and rotation of the target 
                             body, are corrected for light time. 
 
                  "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 
                             surface intercept point 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. Both the 
                             state and rotation of the target body 
                             are corrected for light time. 
 
                  "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 
               intercept point 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 
                             intercept location at the moment it 
                             receives photons emitted from the 
                             observer's location 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. 
 
                             Both the target state as seen by the 
                             observer, and rotation of the target 
                             body, are corrected for light time. 
 
                  "XLT+S"    "Transmission" case:  correct for 
                             one-way light time and stellar 
                             aberration using a Newtonian 
                             formulation  This option modifies the 
                             intercept obtained with the "XLT" 
                             option to account for the observer's 
                             velocity relative to the solar system 
                             barycenter. 
 
                  "XCN"      Converged Newtonian light time 
                             correction.  This is the same as "XLT" 
                             correction but with further iterations 
                             to a converged Newtonian light time 
                             solution.  
 
                  "XCN+S"    "Transmission" case:  converged  
                             Newtonian light time and stellar  
                             aberration corrections. 
 
 
   obsrvr      is the name of the observing body.  This is typically 
               a spacecraft, the earth, or a surface point on the 
               earth.  `obsrvr' is case-insensitive, and leading and 
               trailing blanks in `obsrvr' are not significant. 
               Optionally, you may supply a string containing the 
               integer ID code for the object.  For example both 
               "EARTH" and "399" are legitimate strings that indicate 
               the earth is the observer. 
 
 
   dref        is the name of the reference frame relative to which the
               input direction vector is expressed. This may be any
               frame supported by the SPICE system, including built-in
               frames (documented in the Frames Required Reading) and
               frames defined by a loaded frame kernel (FK).
 
               When `dref' designates a non-inertial frame, the 
               orientation of the frame is evaluated at an epoch 
               dependent on the frame's center and, if the center is 
               not the observer, on the selected aberration 
               correction. See the description of the direction 
               vector `dvec' for details. 
 
 
   dvec        Pointing vector emanating from the observer.  The 
               intercept with the target body's surface of the ray 
               defined by the observer and `dvec' is sought. 
 
               `dvec' is specified relative to the reference frame 
               designated by `dref'. 
 
               Non-inertial reference frames are treated as follows: 
               if the center of the frame is at the observer's 
               location, the frame is evaluated at `et'.  If the 
               frame's center is located elsewhere, then letting 
               `ltcent' be the one-way light time between the observer 
               and the central body associated with the frame, the 
               orientation of the frame is evaluated at et-ltcent, 
               et+ltcent, or `et' depending on whether the requested 
               aberration correction is, respectively, for received 
               radiation, transmitted radiation, or is omitted. 
               `ltcent' is computed using the method indicated by 
               `abcorr'. 
 
-Detailed_Output
 
 
   spoint      is the surface intercept point on the target body of 
               the ray defined by the observer and the direction 
               vector. If the ray intersects the target body in 
               multiple points, the selected intersection point is 
               the one closest to the observer.  The output 
               argument `found' (see below) indicates whether an 
               intercept was found. 
 
               `spoint' is expressed in Cartesian coordinates, 
               relative to the body-fixed frame associated with the 
               target body.  The body-fixed target frame is  
               evaluated at the intercept epoch `trgepc' (see 
               description below). 
 
               When light time correction is used, the duration of 
               light travel between `spoint' to the observer is 
               considered to be the one way light time.  When both 
               light time and stellar aberration corrections are 
               used, `spoint' is selected such that, when `spoint' is 
               corrected for light time and the vector from the 
               observer to the light-time corrected location of 
               `spoint' is corrected for stellar aberration, the 
               resulting vector is parallel to the ray defined by 
               the observer's location and `dvec'. 
 
               The components of `spoint' are given in units of km. 
 
 
   dist        is the distance between the observer and the surface 
               intercept on the target body.  `dist' is given in units
               of km.
 
 
   trgepc      is the "intercept epoch."  This is the epoch at which 
               the ray defined by `obsrvr' and `dvec' intercepts the 
               target surface at `spoint'.  `trgepc' is defined as 
               follows: letting `lt' be the one-way light time between 
               the observer and the intercept point, `trgepc' is the 
               epoch et-lt, et+lt, or `et' depending on whether the 
               requested aberration correction is, respectively, for 
               received radiation, transmitted radiation, or 
               omitted. `lt' is computed using the method indicated by 
               `abcorr'. 
 
               `trgepc' is expressed as seconds past J2000 TDB.


   obspos      is the vector from the center of the target body at 
               epoch `trgepc' to the observer at epoch `et'.  `obspos' is 
               expressed in the target body-fixed reference frame 
               evaluated at `trgepc'.  (This is the frame relative to 
               which `spoint' is given.) 
 
               `obspos' is returned to simplify various related 
               computations that would otherwise be cumbersome.  For 
               example, the vector `xvec' from the observer to `spoint' 
               can be calculated via the call 
 
                  vsub_c ( spoint, obspos, xvec );
 
               The components of `obspos' are given in units of km. 

 
   found       A logical flag indicating whether or not the ray 
               intersects the target.  If an intersection exists 
               `found' will be returned as SPICETRUE.  If the ray misses 
               the target, `found' will be returned as SPICEFALSE.
 
-Parameters
 
   None. 
 
-Exceptions
 
   If any of the listed errors occur, the output arguments are  
   left unchanged. 
  
 
   1)  If the input argument `method' is not recognized, the error 
       SPICE(INVALIDMETHOD) is signaled. 
 
   2)  If `obsrvr' and `target' map to the same NAIF integer ID codes, 
       the error SPICE(BODIESNOTDISTINCT) is signaled. 
 
   3)  If frame definition data enabling the evaluation of the state  
       of the target relative to the observer in target body-fixed 
       coordinates have not been loaded prior to calling srfxpt_c, the 
       error will be diagnosed and signaled by a routine in the call 
       tree of this routine.    
 
   4)  If the specified aberration correction is not recognized, the 
       error will be diagnosed and signaled by a routine in the call 
       tree of this routine.  
 
   5)  If insufficient ephemeris data have been loaded prior to 
       calling srfxpt_c, the error will be diagnosed and signaled by a 
       routine in the call tree of this routine.  Note that when 
       light time correction is used, sufficient ephemeris data 
       must be available to propagate the states of both observer 
       and target to the solar system barycenter. 
 
   6)  If the computation method has been specified as "Ellipsoid" 
       and triaxial radii of the target body have not been loaded 
       into the kernel pool prior to calling srfxpt_c, the error will 
       be diagnosed and signaled by a routine in the call tree of 
       this routine. 

   7)  The target must be an extended body:  if any of the radii of  
       the target body are non-positive, the error will be diagnosed 
       and signaled by routines in the call tree of this routine. 
  
   8)  If PCK data supplying a rotation model for the target body 
       have not been loaded prior to calling srfxpt_c, the error will 
       be diagnosed and signaled by a routine in the call tree of 
       this routine. 
 
   9)  If the reference frame designated by `dref' is not recognized, 
       the error SPICE(NOTSUPPORTED) will be signaled. 
 
   10) If the direction vector `dvec' is the zero vector, the error 
       SPICE(ZEROVECTOR) will be signaled. 

   11) If any of the input string pointers `method', `target',
       `abcorr', `obsrvr', or `dref' are null, the error
       SPICE(NULLPOINTER) will be signaled.

   12) If any of the input strings referred to by `method', `target',
       `abcorr', `obsrvr', or `dref' contain no data characters, the
       error SPICE(EMPTYSTRING) will be signaled.

-Files
 
   Appropriate SPK, PCK, and frame kernels must be loaded by the 
   calling program before this routine is called.  CK, SCLK, and
   IK kernels may be required as well.
 
   The following data are required: 
 
      - SPK data:  ephemeris data for target and observer must be 
        loaded.  If aberration corrections are used, the states of 
        target 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 via furnsh_c. 
 
      - PCK data:  if the computation method is specified as 
        "Ellipsoid," triaxial radii for the target body must be  
        loaded into the kernel pool.  Typically this is done by 
        loading a text PCK file via furnsh_c. 
 
      - Further PCK data:  rotation data for the target body must 
        be loaded.  These may be provided in a text or binary PCK 
        file.  
 
      - Frame data:  if a frame definition is required to convert 
        the observer and target states to the body-fixed frame of 
        the target, that definition must be available in the kernel 
        pool. Similarly, the frame definition required to map 
        between the frame designated by `dref' and the target 
        body-fixed frame must be available. Typically the 
        definitions of frames not already built-in to SPICE are 
        supplied by loading a frame kernel. 

   The following data may be required: 
 
      - CK data:  if the frame to which `dref' refers is fixed to 
        a spacecraft instrument or structure, at least one CK file will
        be needed to permit transformation of vectors between that
        frame and both J2000 and the target body-fixed frame.
 
      - SCLK data:  if a CK file is needed, an associated SCLK kernel
        is required to enable conversion between encoded SCLK 
        (used to time-tag CK data) and barycentric dynamical time
        (TDB).

      - IK data:  one or more I-kernels may be required to 
        enable transformation of vectors from an instrument-fixed
        frame to a spacecraft-fixed frame whose attitude is given
        by a C-kernel.


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

   Given a ray defined by a direction vector and the location of an
   observer, srfxpt_c computes the surface intercept point of the ray
   on a specified target body. srfxpt_c also determines the distance
   between the observer and the surface intercept point.

   When aberration corrections are used, this routine finds the 
   value of `spoint' such that, if `spoint' is regarded as an ephemeris 
   object, after the selected aberration corrections are applied to 
   the vector from the observer to `spoint', the resulting vector is 
   parallel to the direction vector `dvec'. 
 
   This routine computes light time corrections using light time 
   between the observer and the surface intercept point, as opposed 
   to the center of the target.  Similarly, stellar aberration 
   corrections done by this routine are based on the direction of 
   the vector from the observer to the light-time corrected 
   intercept point, not to the target center.  This technique avoids 
   errors due to the differential between aberration corrections 
   across the target body. Therefore it's valid to use aberration 
   corrections with this routine even when the observer is very 
   close to the intercept point, in particular when the 
   observer-intercept point distance is much less than the 
   observer-target center distance.  It's also valid to use stellar 
   aberration corrections even when the intercept point is near or 
   on the limb (as may occur in occultation computations using a 
   point target). 
 
   When comparing surface intercept point computations with results 
   from sources other than SPICE, it's essential to make sure the 
   same geometric definitions are used. 
    
-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.  
 

   Example 1
   ---------

   The following program computes surface intercept points on 
   Mars for the boresight and FOV boundary vectors of the  
   MGS MOC narrow angle camera.  The intercepts are computed 
   for a single observation epoch.  Light time and stellar 
   aberration corrections are used.  For simplicity, camera 
   distortion is ignored. 
    

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

      int main()
      {

         /.
         Local parameters 
         ./

         #define ABCLEN              20
         #define LNSIZE              81
         #define METLEN              41
         #define NAMLEN              33
         #define TIMLEN              51
         #define SHPLEN              81
         #define NCORNR               4


         /. 
         Local variables 
         ./
         SpiceBoolean            found;

         SpiceChar             * abcorr  = "LT+S";
         SpiceChar             * camera  = "MGS_MOC_NA";
         SpiceChar               dref    [NAMLEN];
         SpiceChar             * method  = "Ellipsoid";
         SpiceChar             * obsrvr  = "MGS";
         SpiceChar               shape   [ SHPLEN ];
         SpiceChar             * target  = "Mars";
         SpiceChar               title   [ LNSIZE ];
         SpiceChar             * utc     = "2003 OCT 13 06:00:00 UTC";

         SpiceDouble             bounds  [NCORNR][3];
         SpiceDouble             bsight  [3];
         SpiceDouble             dist;
         SpiceDouble             dvec    [3];
         SpiceDouble             et;
         SpiceDouble             lat;
         SpiceDouble             lon;
         SpiceDouble             obspos [3];
         SpiceDouble             radius;
         SpiceDouble             spoint [3];
         SpiceDouble             trgepc;

         SpiceInt                camid;
         SpiceInt                i;
         SpiceInt                n;


         /.
         Load kernel files:

            - Leapseconds kernel
            - MGS SCLK kernel
            - Text PCK file
            - Planetary SPK file
            - MGS I-kernel
            - MGS spacecraft bus C-kernel
            - MGS SPK file
         ./
         furnsh_c ( "naif0007.tls"           );
         furnsh_c ( "mgs_sclkscet_00052.tsc" ); 
         furnsh_c ( "mars_iau2000_v0.tpc"    );
         furnsh_c ( "de405s.bsp"             );
         furnsh_c ( "mgs_moc_v20.ti"         );
         furnsh_c ( "mgs_sc_ext12.bc"        );
         furnsh_c ( "mgs_ext12.bsp"          );

         /.
         Convert the UTC request time to ET (seconds past
         J2000, TDB). 
         ./ 
         str2et_c ( utc, &et );

         /.
         Get the MGS MOC Narrow angle camera (MGS_MOC_NA) 
         ID code.  Then look up the field of view (FOV) 
         parameters. 
         ./ 
         bodn2c_c ( camera, &camid, &found );

         if ( !found )
         {
             setmsg_c ( "Could not find ID code for "
                        "instrument #."               );
             errch_c  ( "#", camera                   );
             sigerr_c ( "SPICE(NOTRANSLATION)"        );
         }

         getfov_c ( camid, NCORNR, SHPLEN, NAMLEN,
                    shape, dref,   bsight, &n,     bounds );


         printf ( "\n"
                  "Surface Intercept Locations for Camera\n" 
                  "FOV Boundary and Boresight Vectors\n" 
                  "\n" 
                  "   Instrument:             %s\n" 
                  "   Epoch:                  %s\n" 
                  "   Aberration correction:  %s\n" 
                  "\n",
                  camera, utc, abcorr                             ); 

         /. 
         Now compute and display the surface intercepts for the 
         boresight and all of the FOV boundary vectors. 
         ./ 

         for ( i = 0;  i <= NCORNR;  i++ )
         {
            if ( i < NCORNR )
            {
               sprintf ( title, "Corner vector %ld", i );

               vequ_c ( bounds[i], dvec );
            }
            else
            {
               strcpy ( title,  "Boresight vector" );

               vequ_c ( bsight, dvec );
            }

            /.  
            Compute the surface intercept point using 
            the specified aberration corrections. 

            srfxpt_c will signal an error if required kernel 
            data are unavailable.  See example (2) below for 
            a suggestion on detecting absence of C-kernel
            data prior to calling srfxpt_c.
            ./
            srfxpt_c ( method, 
                       target,  et,    abcorr, 
                       obsrvr,  dref,  dvec, 
                       spoint,  &dist, &trgepc,  obspos, &found );

            if ( found )
            {
               /.
               Convert rectangular coordinates to planetocentric 
               latitude and longitude.  Convert radians to degrees. 
               ./ 
               reclat_c ( spoint, &radius, &lon, &lat );

               lon *= dpr_c ();
               lat *= dpr_c ();

               /.
               Display the results. 
               ./

               printf ( "\n"
                        "%s\n", title );

               sprintf ( title, "  Vector in %s frame = ", dref );

               printf ( "\n"
                        "%s\n", title );

               if ( i < NCORNR )
               {
                   printf ( "   %18.10e %18.10e %18.10e\n", 
                            bounds[i][0], bounds[i][1], bounds[i][2] );
               }
               else
               {
                  printf ( "   %18.10e %18.10e %18.10e\n", 
                           bsight[0],    bsight[1],    bsight[2]    );
               }

               printf ( "\n"
                        "  Intercept:\n"
                        "\n"                     
                        "     Radius                   (km)  = %18.10e\n"
                        "     Planetocentric Latitude  (deg) = %18.10e\n"
                        "     Planetocentric Longitude (deg) = %18.10e\n"
                        "     Range                    (km)  = %18.10e\n"
                        "\n",
                        radius,  lat,  lon,  dist                          );
            } 
            else 
            { 
                printf ( "\n"
                         "Intercept not found.\n"
                         "\n"                     );
            }

         }
         return ( 0 );
      }
 

   When this program is executed, the output will be:  
 

      Surface Intercept Locations for Camera
      FOV Boundary and Boresight Vectors

         Instrument:             MGS_MOC_NA
         Epoch:                  2003 OCT 13 06:00:00 UTC
         Aberration correction:  LT+S


      Corner vector 0

        Vector in MGS_MOC_NA frame =
           1.8571383810e-06  -3.8015622659e-03   9.9999277403e-01

        Intercept:

           Radius                   (km)  =   3.3849412615e+03
           Planetocentric Latitude  (deg) =  -4.8477118861e+01
           Planetocentric Longitude (deg) =  -1.2347365507e+02
           Range                    (km)  =   3.8898362745e+02


      Corner vector 1

        Vector in MGS_MOC_NA frame =
           1.8571383810e-06   3.8015622659e-03   9.9999277403e-01

        Intercept:

           Radius                   (km)  =   3.3849398244e+03
           Planetocentric Latitude  (deg) =  -4.8481272936e+01
           Planetocentric Longitude (deg) =  -1.2339839939e+02
           Range                    (km)  =   3.8897565851e+02


      Corner vector 2

        Vector in MGS_MOC_NA frame =
          -1.8571383810e-06   3.8015622659e-03   9.9999277403e-01

        Intercept:

           Radius                   (km)  =   3.3849398156e+03
           Planetocentric Latitude  (deg) =  -4.8481298506e+01
           Planetocentric Longitude (deg) =  -1.2339840260e+02
           Range                    (km)  =   3.8897519958e+02


      Corner vector 3

        Vector in MGS_MOC_NA frame =
          -1.8571383810e-06  -3.8015622659e-03   9.9999277403e-01

        Intercept:

           Radius                   (km)  =   3.3849412527e+03
           Planetocentric Latitude  (deg) =  -4.8477144435e+01
           Planetocentric Longitude (deg) =  -1.2347365823e+02
           Range                    (km)  =   3.8898316850e+02


      Boresight vector

        Vector in MGS_MOC_NA frame =
           0.0000000000e+00   0.0000000000e+00   1.0000000000e+00

        Intercept:

           Radius                   (km)  =   3.3849405358e+03
           Planetocentric Latitude  (deg) =  -4.8479216591e+01
           Planetocentric Longitude (deg) =  -1.2343603019e+02
           Range                    (km)  =   3.8897626607e+02


 
   Example 2
   ---------
 
   srfxpt_c will signal an error if required kernel data are
   unavailable:  for example, in the program of Example 1, if the
   C-kernel containing data for the MGS bus had a gap at epoch `et',
   srfxpt_c would be unable to transform the direction vector `dvec'
   from the reference frame fixed to the camera to the reference
   frame fixed to the target body.

   We could modify the code of Example 1 as shown below to test for
   the availability of C-kernel data.  We would add the declarations
   shown, and we'd call the C-kernel reader ckgp_c to find whether the
   desired pointing was available.  Depending on the value of the
   `found' flag returned by ckgp_c, we'd go on to compute the surface
   intercept point or respond to the error condition.
 
 
                         .
                         .
                         .
      /.
      Local parameters
      ./                          
      #define BUSID            ( -94000 )
      #define MGS              ( -94 )
                         .
                         .
                         .

      /. 
      Local variables 
      ./
      SpiceDouble             clkout;
      SpiceDouble             cmat    [3][3];
      SpiceDouble             sclkdp;
  
                         .
                         .
                         .
      /.
      Look up the transformation from the J2000 frame to the 
      MGS spacecraft frame.  To do this, we'll need to represent 
      our observation epoch in terms of MGS encoded SCLK. 
      ./
      sce2c_c ( MGS, et, &sclkdp );

      /.
      Look up the spacecraft attitude from the C-kernel. 
      ./
      ckgp_c ( BUSID, sclkdp,  0.,    "J2000", 
               cmat,  &clkout, &found         );

      if ( found )
      {
 
         [Proceed to compute intercept point]
      }
      else
      {
 
         [Handle case where pointing is unavailable
          for the epoch of interest]
      }
                         .
                         .
                         .
      

-Restrictions
 
   A cautionary note:  if aberration corrections are used, and
   if `dref' is the target body-fixed frame, the epoch at which that
   frame is evaluated is offset from `et' by the light time between the
   observer and the *center* of the target body. This light time
   normally will differ from the light time between the observer and
   intercept point.  Consequently the orientation of the target
   body-fixed frame at `trgepc' will not match that of the target
   body-fixed frame at the epoch associated with `dref'.  As a result,
   various derived quantities may not be as expected:  for example,
   `obspos' would not be the inverse of the aberration-corrected
   position of the target as seen by the observer.
 
   In many applications the errors arising from this frame
   discrepancy may be insignificant; however a safe approach is to
   always use as `dref' a frame other than the target body-fixed
   frame.
 
-Literature_References
 
   None. 
 
-Author_and_Institution
 
   N.J. Bachman   (JPL) 
 
-Version
 
   -CSPICE Version 1.0.2, 07-FEB-2008 (NJB)

        Abstract now states that this routine is deprecated.

        Header typo was corrected; reference to vminus_c was replaced
        with reference to vsub_c.

   -CSPICE Version 1.0.1, 22-JUL-2004 (NJB)

       Made trivial change to description of `obsrvr' in 
       Detailed Input header section.

   -CSPICE Version 1.0.0, 27-FEB-2004 (NJB)

-Index_Entries
 
   surface intercept point 
 
-&
*/

{ /* Begin srfxpt_c */

   /*
   Local variables
   */
   logical                 fnd;


   /*
   Participate in error tracing.
   */
   if ( return_c() )
   {
      return;
   }
   chkin_c ( "srfxpt_c" );

   /*
   Check the input string arguments:

      method
      target
      abcorr
      obsrvr 
      dref

   Make sure each pointer is non-null and each string contains
   at least one data character:  that is, one character 
   preceding the null terminator.
   */
   CHKFSTR ( CHK_STANDARD, "srfxpt_c", method );
   CHKFSTR ( CHK_STANDARD, "srfxpt_c", target );
   CHKFSTR ( CHK_STANDARD, "srfxpt_c", abcorr );
   CHKFSTR ( CHK_STANDARD, "srfxpt_c", obsrvr );
   CHKFSTR ( CHK_STANDARD, "srfxpt_c", dref   );
   
   /*
   Call the f2c'd SPICELIB function.
   */
   srfxpt_ ( (char       *) method,
             (char       *) target,
             (doublereal *) &et,
             (char       *) abcorr,
             (char       *) obsrvr,
             (char       *) dref,
             (doublereal *) dvec,
             (doublereal *) spoint,
             (doublereal *) dist,
             (doublereal *) trgepc,
             (doublereal *) obspos,
             (logical    *) &fnd,
             (ftnlen      ) strlen(method),
             (ftnlen      ) strlen(target),
             (ftnlen      ) strlen(abcorr),
             (ftnlen      ) strlen(obsrvr),
             (ftnlen      ) strlen(dref)    );

   /*
   Move the found flag into a variable of type SpiceBoolean.
   The SpiceBoolean type may have a different size than
   the logical type.
   */
   
   *found = fnd;

   chkout_c ( "srfxpt_c" );

} /* End srfxpt_c */
