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

#### 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   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).

#### 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 `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
                  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 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
                  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.

       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
                  string.

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

#### Detailed_Output

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

#### Exceptions

       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
           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)-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.

#### 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

       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.

             KPL/MK

             File name: fovray_ex.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

          Example code begins here.

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

             int main()
             {

                /.
                Local constants
                ./
                #define META  "fovray_ex.tm"
                #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)

#### Restrictions

       None.

#### Literature_References

       None.

#### Author_and_Institution

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

#### Version

       -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

    Wed Apr  5 17:54:35 2017