sincpt_c

Index Page sincpt_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 sincpt_c ( ConstSpiceChar      * method,
                       ConstSpiceChar      * target,
                       SpiceDouble           et,
                       ConstSpiceChar      * fixref,
                       ConstSpiceChar      * abcorr,
                       ConstSpiceChar      * obsrvr,
                       ConstSpiceChar      * dref,
                       ConstSpiceDouble      dvec   [3],
                       SpiceDouble           spoint [3],
                       SpiceDouble         * trgepc,
                       SpiceDouble           srfvec [3],
                       SpiceBoolean        * found       )
#### Abstract
       Given an observer and a direction vector defining a ray, compute
       the surface intercept of the ray on a target body at a specified
       epoch, optionally corrected for light time and stellar
       aberration.

       This routine supersedes srfxpt_c.
#### 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.
       fixref     I   Body-fixed, body-centered target body frame.
       abcorr     I   Aberration correction.
       obsrvr     I   Name of observing body.
       dref       I   Reference frame of ray's direction vector.
       dvec       I   Ray's direction vector.
       spoint     O   Surface intercept point on the target body.
       trgepc     O   Intercept epoch.
       srfvec     O   Vector from observer to intercept point.
       found      O   Flag indicating whether intercept was found.
#### Detailed_Input
       method      is a short string providing parameters defining
                   the computation method to be used.

                   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.

       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 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
                   intercept 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 intercept point `spoint' and the observer to
                   intercept vector `srfvec' will be expressed relative to
                   this reference frame.

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

                   For remote sensing applications, where the apparent
                   target surface intercept point seen by the observer is
                   desired, normally the correction

                      "CN+S"

                   should be used. This and the other supported options
                   are described below. `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 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 surface intercept
                                 obtained with the "LT" option to account
                                 for the observer's velocity relative to
                                 the solar system barycenter. These
                                 computations yield the apparent surface
                                 intercept 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.

                                 For reception-case applications involving
                                 intercepts near the target body limb, this
                                 option should be used

                   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
                                 "XLT" 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
                                 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. This option produces a
                                 solution that is at least as accurate at
                                 that obtainable with the "XLT+S" option.
                                 Whether the "XCN+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.

                                 For transmission-case applications
                                 involving intercepts near the target body
                                 limb, this option should be used.

                   Case and embedded blanks are not significant in `abcorr'.
                   For example, the string

                      "Cn + s"

                   is valid.

       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
                   "MOON" and "301" are legitimate strings that indicate
                   the Moon is the observer.

       dref        is the name of the reference frame relative to which the
                   ray's 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). The string
                   `dref' is case-insensitive, and leading and trailing
                   blanks in `dref' are not significant.

                   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        Ray direction 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 target body-fixed frame designated by
                   `fixref'. 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 an stellar aberration, `spoint'
                   lies on the ray defined by the observer's location and
                   `dvec'.

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

       trgepc      is the "intercept epoch." `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.

       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.

       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
       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 sincpt_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 sincpt_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 sincpt_c, the error will
           be diagnosed and signaled by a routine in the call tree of
           this routine.

       12) If the reference frame designated by `dref' is not recognized
           by the SPICE frame subsystem, the error SPICE(NOFRAME)
           will be signaled.

       13) If the direction vector `dvec' is the zero vector, the error
           SPICE(ZEROVECTOR) will be signaled.

       14) 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.

       15) 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 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.

       The following data may be required:

          - 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.

          - 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 the J2000 and the target body-fixed frames.

          - 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).

       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, sincpt_c computes the surface intercept point of the ray
       on a specified target body. sincpt_c also determines the vector
       from the observer to 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.

       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.

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

               KPL/MK

               File: mgs_example2.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
                  mgs_moc_v20.ti                MGS MOC instrument
                                                parameters
                  mgs_sclkscet_00061.tsc        MGS SCLK coefficients
                  mgs_sc_ext12.bc               MGS s/c bus attitude
                  mgs_ext12_ipng_mgs95j.bsp     MGS ephemeris

               \begindata

                  KERNELS_TO_LOAD = ( 'de418.bsp',
                                      'pck00008.tpc',
                                      'naif0008.tls',
                                      'mgs_moc_v20.ti',
                                      'mgs_sclkscet_00061.tsc',
                                      'mgs_sc_ext12.bc',
                                      'mgs_ext12_ipng_mgs95j.bsp' )
               \begintext

         Example code begins here.

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

          int main()
          {
             /.
             Local parameters
             ./
             #define META                "mgs_example2.tm"
             #define ABCLEN              20
             #define LNSIZE              81
             #define NAMLEN              33
             #define TIMLEN              51
             #define SHPLEN              81
             #define NCORNR               4

             /.
             Local variables
             ./
             SpiceBoolean            found;

             SpiceChar             * abcorr  = "CN+S";
             SpiceChar             * camera  = "MGS_MOC_NA";
             SpiceChar               dref    [NAMLEN];
             SpiceChar             * fixref  = "IAU_MARS";
             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             radius;
             SpiceDouble             spoint [3];
             SpiceDouble             srfvec [3];
             SpiceDouble             trgepc;

             SpiceInt                camid;
             SpiceInt                i;
             SpiceInt                n;

             /.
             Load kernel files:
             ./
             furnsh_c ( META );

             /.
             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 will return the name of the camera-fixed frame
             in the string `dref', the camera boresight vector in
             the array `bsight', and the FOV corner vectors in the
             array `bounds'.
             ./
             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.
                ./
                sincpt_c ( method,
                           target,  et,     fixref, abcorr,
                           obsrvr,  dref,   dvec,   spoint,
                           &trgepc, srfvec, &found         );

                if ( found )
                {
                   /.
                   Compute range from observer to apparent intercept.
                   ./
                   dist = vnorm_c( srfvec );

                   /.
                   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 was executed on a PC/Linux/gcc platform, the
       output was:

          Surface Intercept Locations for Camera
          FOV Boundary and Boresight Vectors

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

          Corner vector 0

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

            Intercept:

               Radius                   (km)  =   3.3849411359e+03
               Planetocentric Latitude  (deg) =  -4.8477481924e+01
               Planetocentric Longitude (deg) =  -1.2347407905e+02
               Range                    (km)  =   3.8898310366e+02

          Corner vector 1

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

            Intercept:

               Radius                   (km)  =   3.3849396987e+03
               Planetocentric Latitude  (deg) =  -4.8481636340e+01
               Planetocentric Longitude (deg) =  -1.2339882297e+02
               Range                    (km)  =   3.8897512130e+02

          Corner vector 2

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

            Intercept:

               Radius                   (km)  =   3.3849396899e+03
               Planetocentric Latitude  (deg) =  -4.8481661910e+01
               Planetocentric Longitude (deg) =  -1.2339882618e+02
               Range                    (km)  =   3.8897466238e+02

          Corner vector 3

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

            Intercept:

               Radius                   (km)  =   3.3849411271e+03
               Planetocentric Latitude  (deg) =  -4.8477507498e+01
               Planetocentric Longitude (deg) =  -1.2347408220e+02
               Range                    (km)  =   3.8898264472e+02

          Boresight vector

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

            Intercept:

               Radius                   (km)  =   3.3849404102e+03
               Planetocentric Latitude  (deg) =  -4.8479579822e+01
               Planetocentric Longitude (deg) =  -1.2343645396e+02
               Range                    (km)  =   3.8897573572e+02

       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
       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,
       `srfvec' would not be parallel to `dvec'.

       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 1.0.1, 06-FEB-2009 (NJB)

           Typos in the Detailed Input section's description of `dref'
           were corrected. 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 surface intercept point
       find intersection of ray and target body surface
       find intercept of ray on target body surface
#### Link to routine sincpt_c source file sincpt_c.c

    Thu Apr 16 08:59:08 2009