srfxpt_c

Index Page srfxpt_c
A B C D E F G H I J K L M N O P Q R S T U V W X
Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries
#### Procedure
       void 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      )
#### 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       Deprecated: This routine has been superseded by the CSPICE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.
#### Required_Reading
       FRAMES
       NAIF_IDS
       PCK
       SPK
       TIME
#### Keywords
       GEOMETRY
#### 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
#### Link to routine srfxpt_c source file srfxpt_c.c

    Thu Apr 16 08:59:18 2009