Index Page subpnt_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 subpnt_c ( ConstSpiceChar       * method,
                       ConstSpiceChar       * target,
                       SpiceDouble            et,
                       ConstSpiceChar       * fixref,
                       ConstSpiceChar       * abcorr,
                       ConstSpiceChar       * obsrvr,
                       SpiceDouble            spoint [3],
                       SpiceDouble          * trgepc,
                       SpiceDouble            srfvec [3] )
#### Abstract
       Compute the rectangular coordinates of the sub-observer point on
       a target body at a specified epoch, optionally corrected for
       light time and stellar aberration.

       This routine supersedes subpt_c.
#### Required_Reading
       frame_c
       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.
       fixref     I   Body-fixed, body-centered target body frame.
       abcorr     I   Aberration correction.
       obsrvr     I   Name of observing body.
       spoint     O   Sub-observer point on the target body.
       trgepc     O   Sub-observer point epoch.
       srfvec     O   Vector from observer to sub-observer point.
#### Detailed_Input
       method      is a short string providing parameters defining
                   the computation method to be used.

                   The supported values of `method' are listed below.
                   Please note that the colon is a required delimiter;
                   using a blank will not work.

                      "Near point: ellipsoid"   The sub-observer point
                                                computation uses a
                                                triaxial ellipsoid to
                                                model the surface of the
                                                target body. The
                                                sub-observer point is
                                                defined as the nearest
                                                point on the target
                                                relative to the
                                                observer.

                      "Intercept: ellipsoid"    The sub-observer point
                                                computation uses a
                                                triaxial ellipsoid to
                                                model the surface of the
                                                target body. The
                                                sub-observer point is
                                                defined as the target
                                                surface intercept of the
                                                line containing the
                                                observer and the
                                                target's center.

                   Neither case nor white space are significant in
                   `method'. For example, the string

                     " nearpoint:ELLIPSOID "

                   is valid.

       target      is the name of the target body. The target body is
                   an ephemeris object (its trajectory is given by
                   SPK data), and is an extended object.

                   The string `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 position and orientation of
                   the target body are computed.

                   When aberration corrections are used, 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
                   sub-observer point and the observer, and the sign
                   applied to `lt' depends on the selected correction. See
                   the description of `abcorr' below for details.

       fixref      is the name of the body-fixed, body-centered reference
                   frame associated with the target body. The output
                   sub-observer point `spoint' will be expressed relative
                   to this reference frame. The string `fixref' is
                   case-insensitive, and leading and trailing blanks in
                   `fixref' are not significant.

       abcorr      indicates the aberration corrections to be applied when
                   computing the target's position and orientation.

                   For remote sensing applications, where the apparent
                   sub-observer point seen by the observer is desired,
                   normally either of the corrections

                      "LT+S"
                      "CN+S"

                   should be used. These and the other supported options
                   are described below. `abcorr' may be any of the
                   following:

                      "NONE"     Apply no correction. Return the
                                 geometric sub-observer point on the
                                 target body.

                   Let `lt' represent the one-way light time between the
                   observer and the sub-observer 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
                   sub-observer 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 sub-observer
                                 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 position 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 sub-observer
                                 point obtained with the "LT" option to
                                 account for the observer's velocity
                                 relative to the solar system barycenter.
                                 These corrections yield the apparent
                                 sub-observer point.

                      "CN"       Converged Newtonian light time
                                 correction. In solving the light time
                                 equation, the "CN" correction iterates
                                 until the solution converges. Both the
                                 position and rotation of the target
                                 body are corrected for light time.

                      "CN+S"     Converged Newtonian light time and
                                 stellar aberration corrections. This
                                 option produces a solution that is at
                                 least as accurate at that obtainable
                                 with the "LT+S" option. Whether the "CN+S"
                                 solution is substantially more accurate
                                 depends on the geometry of the
                                 participating objects and on the
                                 accuracy of the input data. In all
                                 cases this routine will execute more
                                 slowly when a converged solution is
                                 computed.

                   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
                   sub-observer 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
                                 sub-observer 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 position 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
                                 sub-observer point 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.

                    Neither case nor white space are significant in
                    `abcorr'. For example, the string

                      'Lt + s'

                    is valid.

       obsrvr      is the name of the observing body. The observing body
                   is an ephemeris object: it typically is 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 "MOON" and "301" are
                   legitimate strings that indicate the Moon is the
                   observer.
#### Detailed_Output
       spoint      is the sub-observer point on the target body.

                   The sub-observer point is defined either as the point
                   on the target body that is closest to the observer,
                   or the target surface intercept of the line from the
                   observer to the target's center; the input argument
                   `method' selects the definition to be used.

                   `spoint' is expressed in Cartesian coordinates,
                   relative to the body-fixed target frame designated by
                   `fixref'. The body-fixed target frame is evaluated at
                   the sub-observer 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 aberration corrections are used, `spoint' is
                   computed using target body position and orientation
                   that have been adjusted for the corrections
                   applicable to `spoint' itself rather than to the target
                   body's center. In particular, if the stellar
                   aberration correction applicable to `spoint' is
                   represented by a shift vector `s', then the light-time
                   corrected position of the target is shifted by `s'
                   before the sub-observer point is computed.

                   The components of `spoint' have units of km.

       trgepc      is the "sub-observer point epoch." `trgepc' is defined
                   as follows: letting `lt' be the one-way light time
                   between the observer and the sub-observer 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.

       srfvec      is the vector from the observer's position at `et' to
                   the aberration-corrected (or optionally, geometric)
                   position of `spoint', where the aberration corrections
                   are specified by `abcorr'. `srfvec' is expressed in the
                   target body-fixed reference frame designated by
                   `fixref', evaluated at `trgepc'.

                   The components of `srfvec' are given in units of km.

                   One can use the CSPICE function vnorm_c to obtain the
                   distance between the observer and `spoint':

                      dist = vnorm_c ( srfvec );

                   The observer's position `obspos', relative to the
                   target body's center, where the center's position is
                   corrected for aberration effects as indicated by
                   `abcorr', can be computed via the call:

                      vsub_c ( spoint, srfvec, obspos );

                   To transform the vector `srfvec' to a time-dependent
                   reference frame `ref' at `et', a sequence of two frame
                   transformations is required. For example, let `mfix'
                   and `mref' be 3x3 matrices respectively describing the
                   target body-fixed to J2000 frame transformation at
                   `trgepc' and the J2000 to (time-dependent frame) `ref'
                   transformation at `et', and let `xform' be the 3x3 matrix
                   representing the composition of `mref' with `mfix'. Then
                   `srfvec' can be transformed to the result `refvec' as
                   follows:

                       pxform_c ( fixref,  "j2000", trgepc, mfix   );
                       pxform_c ( "j2000", ref,     et,     mref   );
                       mxm_c    ( mref,    mfix,            xform  );
                       mxv_c    ( xform,   srfvec,          refvec );

                   The second example in the Examples header section
                   below presents a complete program that demonstrates
                   this procedure.
#### Parameters
       None.
#### Exceptions
       1)  If the specified aberration correction is relativistic or
           calls for stellar aberration but not light time correction,
           the error SPICE(NOTSUPPORTED) is signaled. If the specified
           aberration correction is any other unrecognized value, the
           error will be diagnosed and signaled by a routine in the call
           tree of this routine.

       2)  If either the target or observer input strings cannot be
           converted to an integer ID code, the error SPICE(IDCODENOTFOUND)
           is signaled.

       3)  If `obsrvr' and `target' map to the same NAIF integer ID code,
           the error SPICE(BODIESNOTDISTINCT) is signaled.

       4)  If the input target body-fixed frame `fixref' is not recognized,
           the error SPICE(NOFRAME) is signaled. A frame name may fail
           to be recognized because a required frame specification kernel
           has not been loaded; another cause is a misspelling of the
           frame name.

       5)  If the input frame `fixref' is not centered at the target body,
           the error SPICE(INVALIDFRAME) is signaled.

       6)  If the input argument `method' is not recognized, the error
           SPICE(INVALIDMETHOD) is signaled.

       7)  If the target and observer have distinct identities but are
           at the same location (for example, the target is Mars and
           the observer is the Mars barycenter), the error
           SPICE(NOSEPARATION) is signaled.

       8)  If insufficient ephemeris data have been loaded prior to
           calling subpnt_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.

       9)  If the computation method specifies an ellipsoidal target shape
           and triaxial radii of the target body have not been loaded
           into the kernel pool prior to calling subpnt_c, the error will
           be diagnosed and signaled by a routine in the call tree of
           this routine.

       10) 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.

       11) If PCK data specifying the target body-fixed frame orientation
           have not been loaded prior to calling subpnt_c, the error will
           be diagnosed and signaled by a routine in the call tree of
           this routine.

       12)  The error SPICE(EMPTYSTRING) is signaled if any input string
           argument does not contain at least one character, since the
           input string cannot be converted to a Fortran-style string in
           this case.

       13) The error SPICE(NULLPOINTER) is signaled if any input
           string argument pointer is null.
#### 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 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 target body shape is modeled as an
            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. Typically the definition is supplied by loading a
            frame kernel via furnsh_c.

       In all cases, kernel data are normally loaded once per program
       run, NOT every time this routine is called.
#### Particulars
       There are two different popular ways to define the sub-observer
       point: "nearest point on the target to the observer" or "target
       surface intercept of the line containing observer and target."
       These coincide when the target is spherical and generally are
       distinct otherwise.

       This routine computes light time corrections using light time
       between the observer and the sub-observer 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
       sub-observer 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 sub-observer point, in particular when the
       observer to sub-observer point distance is much less than the
       observer to target center distance.

       The definition of the aberration-corrected sub-observer point is
       implicit: `spoint' is defined by an equation of the form

          spoint = f ( spoint )

       Because of the contraction properties of both light time and
       stellar aberration corrections---that is, the difference in the
       corrections for two vectors is much smaller than the difference
       between the vectors themselves---it's easy to solve this equation
       accurately and fairly quickly.

       When comparing sub-observer 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 these examples 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) Find the sub-Earth point on Mars for a specified time. Perform
          the computation twice, using both the "intercept" and "near
          point" options. Display the location of both the Earth and the
          sub-Earth point using both planetocentric and planetographic
          coordinates.

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

             KPL/MK

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

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

             \begindata

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

             \begintext

         Example code begins here.

         /.
         Program EX1
         ./

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

         int main()
         {
            /.
            Local parameters
            ./
            #define META            "example.tm"

            /.
            Local variables
            ./
            static SpiceChar      * method[2] =
                                    {
                                       "Intercept:  ellipsoid",
                                       "Near point: ellipsoid"
                                    };

            SpiceDouble             et;
            SpiceDouble             f;
            SpiceDouble             obspos [3];
            SpiceDouble             odist;
            SpiceDouble             opclat;
            SpiceDouble             opclon;
            SpiceDouble             opcrad;
            SpiceDouble             opgalt;
            SpiceDouble             opglat;
            SpiceDouble             opglon;
            SpiceDouble             radii  [3];
            SpiceDouble             re;
            SpiceDouble             rp;
            SpiceDouble             spclat;
            SpiceDouble             spclon;
            SpiceDouble             spcrad;
            SpiceDouble             spgalt;
            SpiceDouble             spglat;
            SpiceDouble             spglon;
            SpiceDouble             spoint [3];
            SpiceDouble             srfvec [3];
            SpiceDouble             trgepc;

            SpiceInt                i;
            SpiceInt                n;

            /.
            Load kernel files via the meta-kernel.
            ./
            furnsh_c ( META );

            /.
            Convert the UTC request time string to seconds past
            J2000, TDB.
            ./
            str2et_c ( "2008 aug 11 00:00:00", &et );

            /.
            Look up the target body's radii. We'll use these to
            convert Cartesian to planetographic coordinates. Use
            the radii to compute the flattening coefficient of
            the reference ellipsoid.
            ./
            bodvrd_c ( "MARS", "RADII", 3, &n, radii );

            /.
            Let `re and `rp' be, respectively, the equatorial and
            polar radii of the target.
            ./
            re = radii[0];
            rp = radii[2];

            f  = ( re - rp ) / re;

            /.
            Compute sub-observer point using light time and stellar
            aberration corrections. Use the "target surface intercept"
            definition of the sub-observer point on the first loop
            iteration, and use the "near point" definition on the
            second.
            ./

            for ( i = 0;  i < 2;  i++ )
            {
               subpnt_c ( method[i],
                          "mars",  et,     "iau_mars", "lt+s",
                          "earth", spoint, &trgepc,    srfvec );
               /.
               Compute the observer's distance from SPOINT.
               ./
               odist = vnorm_c ( srfvec );

               /.
               Convert the sub-observer point's rectangular coordinates
               to planetographic longitude, latitude and altitude.
               Convert radians to degrees.
               ./
               recpgr_c ( "mars",  spoint,  re,     f,
                          &spglon, &spglat, &spgalt   );

               spglon *= dpr_c();
               spglat *= dpr_c();

               /.
               Convert sub-observer point's rectangular coordinates to
               planetocentric radius, longitude, and latitude. Convert
               radians to degrees.
               ./
               reclat_c ( spoint, &spcrad, &spclon, &spclat );

               spclon *= dpr_c();
               spclat *= dpr_c();

               /.
               Compute the observer's position relative to the center
               of the target, where the center's location has been
               adjusted using the aberration corrections applicable
               to the sub-point. Express the observer's location in
               planetographic coordinates.
               ./
               vsub_c ( spoint, srfvec, obspos );

               recpgr_c ( "mars",  obspos,  re,    f,
                          &opglon, &opglat, &opgalt   );

               opglon *= dpr_c ();
               opglat *= dpr_c ();

               /.
               Convert the observer's rectangular coordinates to
               planetocentric radius, longitude, and latitude.
               Convert radians to degrees.
               ./
               reclat_c ( obspos, &opcrad, &opclon, &opclat );

               opclon *= dpr_c();
               opclat *= dpr_c();

               /.
               Write the results.
               ./
               printf ( "\n"
                        " Computation method = %s\n\n"
                        "  Observer altitude                      (km) = %21.9f\n"
                        "  Length of SRFVEC                       (km) = %21.9f\n"
                        "  Sub-observer point altitude            (km) = %21.9f\n"
                        "  Sub-observer planetographic longitude (deg) = %21.9f\n"
                        "  Observer planetographic longitude     (deg) = %21.9f\n"
                        "  Sub-observer planetographic latitude  (deg) = %21.9f\n"
                        "  Observer planetographic latitude      (deg) = %21.9f\n"
                        "  Sub-observer planetocentric longitude (deg) = %21.9f\n"
                        "  Observer planetocentric longitude     (deg) = %21.9f\n"
                        "  Sub-observer planetocentric latitude  (deg) = %21.9f\n"
                        "  Observer planetocentric latitude      (deg) = %21.9f\n"
                        "\n",
                        method[i],
                        opgalt,
                        odist,
                        spgalt,
                        spglon,
                        opglon,
                        spglat,
                        opglat,
                        spclon,
                        opclon,
                        spclat,
                        opclat      );
            }

            return ( 0 );
         }

       When this program was executed on a PC/Linux/gcc platform, the
       output was:

          Computation method = Intercept:  ellipsoid

           Observer altitude                      (km) =   349199089.542324722
           Length of SRFVEC                       (km) =   349199089.579020321
           Sub-observer point altitude            (km) =           0.000000000
           Sub-observer planetographic longitude (deg) =         199.302305055
           Observer planetographic longitude     (deg) =         199.302305055
           Sub-observer planetographic latitude  (deg) =          26.262401212
           Observer planetographic latitude      (deg) =          25.994936725
           Sub-observer planetocentric longitude (deg) =         160.697694945
           Observer planetocentric longitude     (deg) =         160.697694945
           Sub-observer planetocentric latitude  (deg) =          25.994934146
           Observer planetocentric latitude      (deg) =          25.994934146

          Computation method = Near point: ellipsoid

           Observer altitude                      (km) =   349199089.542316437
           Length of SRFVEC                       (km) =   349199089.542316437
           Sub-observer point altitude            (km) =          -0.000000000
           Sub-observer planetographic longitude (deg) =         199.302305055
           Observer planetographic longitude     (deg) =         199.302305055
           Sub-observer planetographic latitude  (deg) =          25.994936725
           Observer planetographic latitude      (deg) =          25.994936725
           Sub-observer planetocentric longitude (deg) =         160.697694945
           Observer planetocentric longitude     (deg) =         160.697694945
           Sub-observer planetocentric latitude  (deg) =          25.729407202
           Observer planetocentric latitude      (deg) =          25.994934146

       2) Use subpnt_c to find the sub-spacecraft point on Mars for the
          Mars Reconnaissance Orbiter spacecraft (MRO) at a specified
          time, using the "near point: ellipsoid" computation method.
          Use both LT+S and CN+S aberration corrections to illustrate
          the differences.

          Convert the spacecraft to sub-observer point vector obtained
          from subpnt_c into the MRO_HIRISE_LOOK_DIRECTION reference frame
          at the observation time. Perform a consistency check with this
          vector: compare the Mars surface intercept of the ray
          emanating from the spacecraft and pointed along this vector
          with the sub-observer point.

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

             KPL/MK

             File: mro_example.tm

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

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

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

                File name                     Contents
                ---------                     --------
                de418.bsp                     Planetary ephemeris
                pck00008.tpc                  Planet orientation and
                                                   radii
                naif0008.tls                  Leapseconds
                mro_psp4_ssd_mro95a.bsp       MRO ephemeris
                mro_v11.tf                    MRO frame specifications
                mro_sclkscet_00022_65536.tsc  MRO SCLK coefficients and
                                                   parameters
                mro_sc_psp_070925_071001.bc   MRO attitude

             \begindata

                KERNELS_TO_LOAD = ( 'de418.bsp',
                                    'pck00008.tpc',
                                    'naif0008.tls',
                                    'mro_psp4_ssd_mro95a.bsp',
                                    'mro_v11.tf',
                                    'mro_sclkscet_00022_65536.tsc',
                                    'mro_sc_psp_070925_071001.bc'  )
             \begintext

         Example code begins here.

         /.
         Program EX2
         ./
         #include <stdio.h>
         #include "SpiceUsr.h"

         int main()
         {
            /.
            Local constants
            ./
            #define META            "mro_example.tm"
            #define NCORR           2

            /.
            Local variables
            ./
            SpiceBoolean            found;

            static SpiceChar      * abcorr[NCORR] =
                                    {
                                       "LT+S", "CN+S"
                                    };

            static SpiceChar      * hiref;
            static SpiceChar      * method;

            SpiceDouble             alt;
            SpiceDouble             et;
            SpiceDouble             lat;
            SpiceDouble             lon;
            SpiceDouble             mrovec [3];
            SpiceDouble             r1     [3][3];
            SpiceDouble             r2     [3][3];
            SpiceDouble             radius;
            SpiceDouble             spoint [3];
            SpiceDouble             srfvec [3];
            SpiceDouble             trgepc;
            SpiceDouble             xepoch;
            SpiceDouble             xform  [3][3];
            SpiceDouble             xpoint [3];
            SpiceDouble             xvec   [3];

            SpiceInt                i;

            /.
            Load kernel files via the meta-kernel.
            ./
            furnsh_c ( META );

            /.
            Convert the TDB request time string to seconds past
            J2000, TDB.
            ./
            str2et_c ( "2007 SEP 30 00:00:00 TDB", &et );

            /.
            Compute the sub-spacecraft point using the
            "NEAR POINT: ELLIPSOID" definition.
            Compute the results using both LT+S and CN+S
            aberration corrections.
            ./
            method = "Near point: ellipsoid";

            printf ( "\nComputation method = %s\n", method );

            for ( i = 0;  i < 2;  i++ )
            {
               subpnt_c ( method,
                          "mars", et,     "iau_mars", abcorr[i],
                          "mro",  spoint, &trgepc,    srfvec    );

               /.
               Compute the observer's altitude above `spoint'.
               ./
               alt = vnorm_c ( srfvec );

               /.
               Express `srfvec' in the MRO_HIRISE_LOOK_DIRECTION
               reference frame at epoch `et'. Since `srfvec' is expressed
               relative to the IAU_MARS frame at `trgepc', we must
               compose two transformations: that from IAU_MARS to
               J2000 at `trgepc', followed by the transformation from
               J2000 to MRO_HIRISE_LOOK_DIRECTION at `et'.
               (We could use any other inertial frame in place
               of J2000; the result would be the same.)

               To make code formatting a little easier, we'll store
               the long MRO reference frame name in a variable:
               ./
               hiref = "MRO_HIRISE_LOOK_DIRECTION";

               pxform_c ( "iau_mars", "j2000", trgepc, r1 );
               pxform_c ( "j2000",    hiref,   et,     r2 );

               mxm_c ( r2,    r1,     xform  );
               mxv_c ( xform, srfvec, mrovec );

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

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

               /.
               Write the results.
               ./
               printf ( "\n"
                        "Aberration correction = %s\n\n"
                        "  MRO-to-sub-observer vector in\n"
                        "  MRO HIRISE look direction frame\n"
                        "     X-component             (km) = %21.9f\n"
                        "     Y-component             (km) = %21.9f\n"
                        "     Z-component             (km) = %21.9f\n"
                        "  Sub-observer point radius  (km) = %21.9f\n"
                        "  Planetocentric latitude   (deg) = %21.9f\n"
                        "  Planetocentric longitude  (deg) = %21.9f\n"
                        "  Observer altitude          (km) = %21.9f\n",
                        abcorr[i],
                        mrovec[0],
                        mrovec[1],
                        mrovec[2],
                        radius,
                        lat,
                        lon,
                        alt         );

               /.
               Consistency check: find the surface intercept on
               Mars of the ray emanating from the spacecraft and having
               direction vector MROVEC in the MRO HIRISE look direction
               reference frame at ET. Call the intercept point
               XPOINT. XPOINT should coincide with SPOINT, up to a
               small round-off error.
               ./
               sincpt_c ( "ellipsoid", "mars",  et,    "iau_mars",
                          abcorr[i],   "mro",   hiref, mrovec,
                          xpoint,      &xepoch, xvec,  &found     );

               if ( !found )
               {
                  printf ( "Bug: no intercept\n" );
               }
               else
               {
                  /.
                  Report the distance between XPOINT and SPOINT.
                  ./
                  printf ( "  Intercept comparison error (km) = %21.9f\n\n",
                           vdist_c( xpoint, spoint )                        );
               }
            }

            return ( 0 );
         }

       When this program was executed on a PC/Linux/gcc platform, the
       output was:

          Computation method = Near point: ellipsoid

          Aberration correction = LT+S

            MRO-to-sub-observer vector in
            MRO HIRISE look direction frame
               X-component             (km) =           0.286931987
               Y-component             (km) =          -0.260417167
               Z-component             (km) =         253.816284981
            Sub-observer point radius  (km) =        3388.299078207
            Planetocentric latitude   (deg) =         -38.799836879
            Planetocentric longitude  (deg) =        -114.995294746
            Observer altitude          (km) =         253.816580760
            Intercept comparison error (km) =           0.000002144

          Aberration correction = CN+S

            MRO-to-sub-observer vector in
            MRO HIRISE look direction frame
               X-component             (km) =           0.286931866
               Y-component             (km) =          -0.260417914
               Z-component             (km) =         253.816274506
            Sub-observer point radius  (km) =        3388.299078205
            Planetocentric latitude   (deg) =         -38.799836883
            Planetocentric longitude  (deg) =        -114.995294968
            Observer altitude          (km) =         253.816570285
            Intercept comparison error (km) =           0.000000001
#### Restrictions
      None.
#### Literature_References
       None.
#### Author_and_Institution
       N.J. Bachman   (JPL)
#### Version
       -CSPICE Version 1.0.1, 06-FEB-2009 (NJB)

           Incorrect frame name fixfrm was changed to fixref in
           documentation.

           In the header examples, meta-kernel names were updated to use
           the suffix

              ".tm"

       -CSPICE Version 1.0.0, 02-MAR-2008 (NJB)
#### Index_Entries
       find sub-observer point on target body
       find sub-spacecraft point on target body
       find nearest point to observer on target body
#### Link to routine subpnt_c source file subpnt_c.c

    Wed Jun  9 13:05:31 2010