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   void fovray_c ( ConstSpiceChar   * inst,
                   ConstSpiceDouble   raydir [3],
                   ConstSpiceChar   * rframe,
                   ConstSpiceChar   * abcorr,
                   ConstSpiceChar   * observer,
                   SpiceDouble      * et,
                   SpiceBoolean     * visible  )


   Determine if a specified ray is within the field-of-view (FOV) of
   a specified instrument at a given time.






   ---------------  ---  ------------------------------------------------  
   inst              I   Name or ID code string of the instrument.
   raydir            I   Ray's direction vector.
   rframe            I   Body-fixed, body-centered frame for target body.
   abcorr            I   Aberration correction flag. 
   observer          I   Name or ID code string of the observer.
   et                I   Time of the observation (seconds past J2000).
   visible           O   Visibility flag (SPICETRUE/SPICEFALSE).


   inst       indicates the name of an instrument, such as a
              spacecraft-mounted framing camera. The field of view
              (FOV) of the instrument will be used to determine if
              the direction from the observer to a target,
              represented as a ray, is visible with respect to the

              The position of the instrument `inst' is considered to
              coincide with that of the ephemeris object `observer' (see
              description below).

              The size of the instrument's FOV is constrained by the
              following: There must be a vector A such that all of
              the instrument's FOV boundary vectors have an angular
              separation from A of less than (pi/2)-MARGIN radians
              (see description below). For FOVs that are circular or
              elliptical, the vector A is the boresight. For FOVs
              that are rectangular or polygonal, the vector A is

              See the header of the CSPICE routine getfov_c for a
              description of the required parameters associated with
              an instrument.

              Both object names and NAIF IDs are accepted. For
              example, both "CASSINI_ISS_NAC" and "-82360" are
              accepted. Case and leading or trailing blanks are not
              significant in the string.

   raydir     is the direction vector associated with a ray
              representing a target. The ray emanates from the
              location of the ephemeris object designated by the
              input argument `observer' and is expressed relative to the
              reference frame designated by `rframe' (see descriptions

   rframe     is the name of the reference frame associated with
              the input ray's direction vector `raydir'. Note: `rframe'
              does not need to be the instrument's reference frame.

              Since light time corrections are not supported for
              rays, the orientation of the frame is always evaluated
              at the epoch associated with the observer, as opposed
              to the epoch associated with the light-time corrected
              position of the frame center.

              Case, leading and trailing blanks are not significant
              in the string.

   abcorr     indicates the aberration corrections to be applied
              when computing the ray's direction.

              The supported aberration correction options are:

                 "NONE"          No correction.
                 "S"             Stellar aberration correction,
                                 reception case.
                 "XS"            Stellar aberration correction,
                                 transmission case.

              For detailed information, see the geometry finder
              required reading, gf.req.

              Case, leading and trailing blanks are not significant
              in the string.

   observer   is the name of the body from which the target
              represented by `raydir' is observed. The instrument
              designated by `inst' is treated as if it were co-located
              with the observer.

              Both object names and NAIF IDs are accepted. For
              example, both "CASSINI" and "-82" are accepted. Case and
              leading or trailing blanks are not significant in the

   et         is the observation time in seconds past the J2000


   visible    is SPICETRUE if the ray is "visible", or in the
              field-of-view, of `inst' at the time `et'. Otherwise,
              `visible' is SPICEFALSE.


   SPICE_GF_MAXVRT     is the maximum number of vertices that may be used
                       to define the boundary of the specified instrument's
                       field of view. See SpiceGF.h for more details.

   MARGIN              is a small positive number used to constrain the
                       orientation of the boundary vectors of polygonal
                       FOVs. Such FOVs must satisfy the following constraints:

                       1)  The boundary vectors must be contained within
                           a right circular cone of angular radius less
                           than than (pi/2) - MARGIN radians; in
                           other words, there must be a vector A such that all
                           boundary vectors have angular separation from
                           A of less than (pi/2)-MARGIN radians.
                       2)  There must be a pair of boundary vectors U, V
                           such that all other boundary vectors lie in
                           the same half space bounded by the plane
                           containing U and V. Furthermore, all other
                           boundary vectors must have orthogonal
                           projections onto a specific plane normal to
                           this plane (the normal plane contains the angle
                           bisector defined by U and V) such that the
                           projections have angular separation of at least
                           2*MARGIN radians from the plane spanned
                           by U and V.
                       MARGIN is currently set to 1.D-6.


   1)  If the observer's name cannot be mapped to a NAIF ID code, the
       error SPICE(IDCODENOTFOUND) is signaled.

   2)  If the aberration correction flag calls for light time
       correction, the error SPICE(INVALIDOPTION) is signaled.

   3)  If the ray's direction vector is zero, the error
       SPICE(ZEROVECTOR) is signaled.

   4)  If the instrument name `inst' does not have corresponding NAIF
       ID code, the error will be diagnosed by a routine in the call
       tree of this routine.

   5)  If the FOV parameters of the instrument are not present in
       the kernel pool, the error will be diagnosed by routines
       in the call tree of this routine.

   6)  If the FOV boundary has more than SPICE_GF_MAXVRT vertices, the error
       will be diagnosed by routines in the call tree of this

   7)  If the instrument FOV shape is a polygon or rectangle, and
       this routine cannot find a ray R emanating from the FOV
       vertex such that maximum angular separation of R and any FOV
       boundary vector is within the limit (pi/2)-MARGIN radians,
       the error will be diagnosed by a routine in the call tree of
       this routine. If the FOV is any other shape, the same error
       check will be applied with the instrument boresight vector
       serving the role of R.

   8)  If the loaded kernels provide insufficient data to compute a
       requested state vector, the error will be diagnosed by a
       routine in the call tree of this routine.

   9)  If an error occurs while reading an SPK or other kernel file,
       the error will be diagnosed by a routine in the call tree
       of this routine.

   10) If any input string argument pointer is null, the error
       SPICE(NULLPOINTER) will be signaled.
   11) If any input string argument other than `rframe' is empty, the
       error SPICE(EMPTYSTRING) will be signaled.


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

   The following data are required:

      - SPK data: ephemeris data for the observer at the time
        `et'. If aberration corrections are used, the state of the
        observer relative to the solar system barycenter
        must be calculable from the available ephemeris data.

      - Data defining the reference frame in which the instrument's
        FOV is defined must be available in the kernel pool.
        Additionally the name `inst' must be associated with an ID

      - IK data: the kernel pool must contain data such that
        the CSPICE routine getfov_c may be called to obtain
        parameters for `inst'.

   The following data may be required:

      - CK data: if the frame in which the instrument's FOV is
        defined is fixed to a spacecraft, at least one CK file will
        be needed to permit transformation of vectors between that
        frame and the J2000 frame.

      - SCLK data: if a CK file is needed, an associated SCLK
        kernel is required to enable conversion between encoded SCLK
        (used to time-tag CK data) and barycentric dynamical time

      - Since the input ray direction may be expressed in any
        frame, additional FKs, CKs, SCLK kernels, PCKs, and SPKs
        may be required to map the direction to the J2000 frame.

   Kernel data are normally loaded via furnsh_c once per program run,
   NOT every time this routine is called.


   To treat the target as an ephemeris object rather than a ray, use
   the higher-level CSPICE routine fovtrg_c. fovtrg_c may be used to
   determine if ephemeris objects such as Saturn are visible in an
   instrument's FOV at a given time.


   1) The Cassini Ultraviolet Imaging Spectrograph (UVIS)
      has been used to measure variations in starlight as
      rings and moons occult Cassini's view of the stars.
      One of these events happened at 2008-054T21:31:55.158 UTC.
      Let's verify that Epsilon CMa (Adhara) was in the
      Cassini UVIS field-of-view at the observation time.


         File name:

         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
           ---------                      --------
           naif0010.tls                   Leapseconds
           cpck26Jan2007.tpc              Satellite orientation and
           cas00145.tsc                   Cassini SCLK
                      Cassini frames
           cas_uvis_v06.ti                Cassini UVIS instrument
           080428R_SCPSE_08045_08067.bsp  Merged spacecraft,
                                          planetary, and satellite
           08052_08057ra.bc               Orientation for Cassini


           KERNELS_TO_LOAD = ( 'cpck26Jan2007.tpc'


      Example code begins here.
         #include <stdio.h>
         #include "SpiceUsr.h"
         #include "SpiceZmc.h"
         int main()
            Local constants 
            #define META  ""
            #define BODLEN 32
            #define TIMLEN 32
            #define FRMLEN 32
            Local variables 
            The variable `time' is the observation time.
            SpiceChar             * time = "2008-054T21:31:55.158";
            SpiceChar               time_output[TIMLEN];
            ConstSpiceChar        * time_format = 
                                    "YYYY-MON-DD HR:MN:SC.###::TDB (TDB)";
            The variables `right_asc' and `dec' are the right ascension
            and declination of Epsilon CMa in degrees.
            SpiceDouble             dec       = -28.972;
            SpiceDouble             et;
            SpiceDouble             raydir [3];
            SpiceDouble             right_asc = 104.656;
            SpiceBoolean            visible;
            Load kernels.
            furnsh_c ( META );
            Convert the observation time to `et'.
            str2et_c ( time, &et );
            Create a unit direction vector pointing from Cassini
            to the specified star. For details on corrections such
            as parallax, please see the example in gfrfov_c.
            radrec_c ( 1.0, right_asc*rpd_c(), dec*rpd_c(), raydir );
            Is the star in the field-of-view of Cassini's UVIS?
            fovray_c ( "CASSINI_UVIS_FUV_OCC", raydir, "J2000",
                       "S", "Cassini", &et, &visible );
            Put the time in a specified format for output and
            report the result.
            timout_c ( et, time_format, TIMLEN, time_output );
            if ( visible ) {
               printf ( "Epsilon CMa was visible from the Cassini\n" );
               printf ( "UVIS instrument at %s\n", time_output );
            return (0);
      When this program was executed on a PC/Linux/gcc platform, the 
      output was: 

         Epsilon CMa was visible from the Cassini
         UVIS instrument at 2008-FEB-23 21:33:00.343 (TDB)






   S.C. Krening  (JPL)
   N.J. Bachman  (JPL) 


   -CSPICE Version 1.0.0, 15-FEB-2012 (SCK) (NJB)


   Ray in instrument FOV at specified time
   Ray in instrument field_of_view at specified time
Wed Apr  5 17:54:35 2017