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Index Page fovray_c

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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

       fovray_c ( Is ray in FOV at time? )

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

#### Abstract

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

#### Required_Reading

       CK
       FRAMES
       KERNEL
       NAIF_IDS
       PCK
       SPK
       TIME

#### Keywords

       EVENT
       FOV
       GEOMETRY
       INSTRUMENT

#### Brief_I/O

       VARIABLE  I/O  DESCRIPTION
       --------  ---  -------------------------------------------------
       inst       I   Name or ID code string of the instrument.
       raydir     I   Ray's direction vector.
       rframe     I   Reference frame of ray's direction vector.
       abcorr     I   Aberration correction flag.
       obsrvr     I   Name or ID code string of the observer.
       et         I   Time of the observation (seconds past J2000).
       visibl     O   Visibility flag (SPICETRUE/SPICEFALSE).

#### Detailed_Input

       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
                   instrument.

                   The position of the instrument `inst' is considered to
                   coincide with that of the ephemeris object `obsrvr' (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)-SPICE_GF_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
                   calculated.

                   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 defining a ray of interest.
                   The ray emanates from the location of the ephemeris
                   object designated by the input argument `obsrvr' and
                   is expressed relative to the reference frame
                   designated by `rframe' (see description below).

       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.

       obsrvr      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 string.

       et          is the observation time in seconds past the J2000
                   epoch.

#### Detailed_Output

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

#### Parameters

       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.

       SPICE_GF_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) - SPICE_GF_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)-SPICE_GF_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*SPICE_GF_MARGIN radians
                                from the plane spanned by U and V.

                         SPICE_GF_MARGIN is currently set to 1.e-12.

       See header file SpiceGF.h for declarations and descriptions of
       parameters used throughout the GF system.

#### Exceptions

       1)  If the observer's name cannot be mapped to a NAIF ID code, the
           error SPICE(IDCODENOTFOUND) is signaled by a routine in the
           call tree of this routine.

       2)  If the aberration correction flag calls for light time
           correction, the error SPICE(INVALIDOPTION) is signaled by a
           routine in the call tree of this routine.

       3)  If the ray's direction vector is zero, the error
           SPICE(ZEROVECTOR) is signaled by a routine in the call tree of
           this routine.

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

       6)  If the FOV boundary has more than SPICE_GF_MAXVRT vertices, an error
           is signaled by a routine in the call tree of this
           routine.

       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)-SPICE_GF_MARGIN radians, an error
           is signaled 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, an error is signaled 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 is signaled by a routine in the call tree
           of this routine.

       10) If any of the `inst', `abcorr', `obsrvr' or `rframe' input
           string pointers is null, the error SPICE(NULLPOINTER) is
           signaled.

       11) If any of the `inst', `abcorr' or `obsrvr' input strings has
           zero length, the error SPICE(EMPTYSTRING) is signaled.

#### Files

       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
          code.

       -  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
          (TDB).

       -  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.

#### Particulars

       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.

#### 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 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.

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

             KPL/MK

             File name: fovray_ex1.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
               ---------                      --------
               naif0010.tls                   Leapseconds
               cpck26Jan2007.tpc              Satellite orientation and
                                              radii
               cas00145.tsc                   Cassini SCLK
               cas_v40.tf                     Cassini frames
               cas_uvis_v06.ti                Cassini UVIS instrument
               080428R_SCPSE_08045_08067.bsp  Merged spacecraft,
                                              planetary, and satellite
                                              ephemeris
               08052_08057ra.bc               Orientation for Cassini

             \begindata

               KERNELS_TO_LOAD = ( 'cpck26Jan2007.tpc'
                                   'naif0010.tls'
                                   'cas00145.tsc'
                                   'cas_v40.tf'
                                   'cas_uvis_v06.ti'
                                   '080428R_SCPSE_08045_08067.bsp'
                                   '08052_08057ra.bc')

             \begintext

             End of meta-kernel

          Example code begins here.

          /.
             Program fovray_ex1
          ./

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

          int main()
          {

             /.
             Local parameters
             ./
             #define META   "fovray_ex1.tm"
             #define BODLEN 32
             #define TIMLEN 32
             #define FRMLEN 32
             #define TIMFMT "YYYY-MON-DD HR:MN:SC.###::TDB (TDB)"

             /.
             This is the UTC time of the observation.
             ./
             #define TIME   "2008-054T21:31:55.158"

             /.
             Local variables
             ./
             SpiceChar               timstr[TIMLEN];

             SpiceDouble             dec;
             SpiceDouble             et;
             SpiceDouble             ra;
             SpiceDouble             raydir [3];

             SpiceBoolean            visibl;

             /.
             The variables `ra' and `dec' are the right ascension
             and declination of Epsilon CMa in degrees.
             ./
             ra  = 104.656;
             dec = -28.972;

             /.
             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, ra*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, &visibl );

             /.
             Put the time in a specified format for output and
             report the result.
             ./
             timout_c ( et, TIMFMT, TIMLEN, timstr );

             if ( visibl )
             {
                printf ( "Epsilon CMa was visible from the Cassini\n" );
                printf ( "UVIS instrument at %s\n", timstr );
             }

             return ( 0 );
          }

          When this program was executed on a Mac/Intel/cc/64-bit
          platform, the output was:

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

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman        (JPL)
       J. Diaz del Rio     (ODC Space)
       S.C. Krening        (JPL)

#### Version

       -CSPICE Version 1.0.1, 02-AUG-2021 (JDR)

           Edited header to comply with NAIF standard. Changed the
           parameter name "MARGIN" to "SPICE_GF_MARGIN".

           Changed the argument names 'observer' and 'visible' to 'obsrvr'
           and 'visibl' for consistency with other routines.

           Changed parameter and variable names in code example to comply
           with NAIF standard.

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

#### Index_Entries

       Ray in instrument FOV at specified time
       Ray in instrument field_of_view at specified time

#### Link to routine fovray_c source file fovray_c.c

    Fri Dec 31 18:41:06 2021