Index Page fovtrg_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 fovtrg_c ( ConstSpiceChar   * inst,
                       ConstSpiceChar   * target,
                       ConstSpiceChar   * tshape,
                       ConstSpiceChar   * tframe,
                       ConstSpiceChar   * abcorr,
                       ConstSpiceChar   * obsrvr,
                       SpiceDouble      * et,
                       SpiceBoolean     * visible  )

#### Abstract

       Determine if a specified ephemeris object 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.
       target            I   Name or ID code string of the target.
       tshape            I   Type of shape model used for the target.
       tframe            I   Body-fixed, body-centered frame for target body.
       abcorr            I   Aberration correction flag.
       obsrvr            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 target 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)-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.

       target     is the name of the target body. This routine determines
                  if the target body appears in the instrument's field of
                  view.

                  Both object names and NAIF IDs are accepted. For
                  example, both "Moon" and "301" are accepted. Case and
                  leading or trailing blanks are not significant in the
                  string.

       tshape     is a string indicating the geometric model used to
                  represent the shape of the target body. The supported
                  options are:

                     'ELLIPSOID'     Use a triaxial ellipsoid model,
                                     with radius values provided via the
                                     kernel pool. A kernel variable
                                     having a name of the form

                                        'BODYnnn_RADII'

                                     where nnn represents the NAIF
                                     integer code associated with the
                                     body, must be present in the kernel
                                     pool. This variable must be
                                     associated with three numeric
                                     values giving the lengths of the
                                     ellipsoid's X, Y, and Z semi-axes.

                     'POINT'         Treat the body as a single point.

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

       tframe     is the name of the body-fixed, body-centered reference
                  frame associated with the target body. Examples of
                  such names are 'IAU_SATURN' (for Saturn) and 'ITRF93'
                  (for the Earth).

                  If the target body is modeled as a point, `tframe'
                  is ignored and should be left blank. (Ex: " ").

                  Case and leading or trailing blanks bracketing a
                  non-blank frame name are not significant in the string.

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

                  For remote sensing applications, where the apparent
                  position and orientation of the target seen by the
                  observer are desired, normally either of the
                  corrections:

                     "LT+S"
                     "CN+S"

                  should be used. These and the other supported options
                  are described below.

                  Supported aberration correction options for
                  observation (the case where radiation is received by
                  observer at `et') are:

                     "NONE"         No correction.
                     "LT"           Light time only
                     "LT+S"         Light time and stellar aberration.
                     "CN"           Converged Newtonian (CN) light time.
                     "CN+S"         CN light time and stellar aberration.

                  Supported aberration correction options for
                  transmission (the case where radiation is emitted from
                  observer at `et') are:

                     "XLT"          Light time only.
                     "XLT+S"        Light time and stellar aberration.
                     "XCN"          Converged Newtonian (CN) light time.
                     "XCN+S"        CN light time and stellar aberration.

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

       obsrvr     is the name of the body from which the target is
                  observed. The instrument `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 `target' is fully or partially 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 name of either the target or observer cannot be
           translated to a NAIF ID code, the error will be diagnosed by
           a routine in the call tree of this routine.

       2)  If the specified aberration correction is an unrecognized
           value, the error will be diagnosed and signaled by a routine
           in the call tree of this routine.

       3)  If the radii of a target body modeled as an ellipsoid cannot
           be determined by searching the kernel pool for a kernel
           variable having a name of the form

              'BODYnnn_RADII'

           where nnn represents the NAIF integer code associated with
           the body, the error will be diagnosed by a routine in the
           call tree of this routine.

       4)  If the target and observer bodies are the same, the error will
           be diagnosed by a routine in the call tree of this routine.

       5)  If the body model specifier `tshape' is invalid, the error will
           be diagnosed either here or by a routine in the call tree of
           this routine.

       6)  If a target body-fixed reference frame associated with a
           non-point target is not recognized, the error will be
           diagnosed by a routine in the call tree of this routine.

       7)  If a target body-fixed reference frame is not centered at
           the corresponding target body, the error will be
           diagnosed by a routine in the call tree of this routine.

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

       9)  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.

       10) 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.

       11) 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.

       12) 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.

       13) 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.

       14) If any input string argument pointer is null, the error
           SPICE(NULLPOINTER) will be signaled.

       15) If any input string argument other than `tframe' 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 target and observer that
            describe the ephemerides of these objects at the time `et'.
            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.

          - Frame data: if a frame definition is required to convert
            the observer and target states to the body-fixed frame of
            the target, that definition must be available in the kernel
            pool. Typically the definitions of frames not already
            built-in to SPICE are supplied by loading a frame kernel.

          - 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:

          - PCK data: bodies modeled as triaxial ellipsoids must have
            orientation data provided by variables in the kernel pool.

            Bodies modeled as triaxial ellipsoids must have radii
            lengths provided by variables in the kernel pool.

          - 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 both J2000 and the target body-fixed 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).

       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 a ray rather than as an ephemeris object,
       use the higher-level CSPICE routine fovray_c. fovray_c may be used
       to determine if distant target objects such as stars are visible
       in an instrument's FOV at a given time, as long as the direction
       from the observer to the target can be modeled as a ray.

#### Examples

       The numerical results shown for these examples may differ across
       platforms. The results depend on the SPICE kernels used as
       input, the compiler and supporting libraries, and the machine
       specific arithmetic implementation.

       1) A spectacular picture was taken by Cassini's
          narrow-angle camera on Oct. 6, 2010 that shows
          six of Saturn's moons. Let's verify that the moons
          in the picture are Epimetheus, Atlas, Daphnis, Pan,
          Janus, and Enceladus.

          To see this picture, visit:
          http://photojournal.jpl.nasa.gov/catalog/PIA12741
          or go to the PDS Image Node's Image Atlas at
          http://pds-imaging.jpl.nasa.gov/search/search.html.
          Select Cassini as the mission, ISS as the instrument,
          and enter 1_N1665078907.122 as the Product ID in the
          Product tab. Note: these directions may change as the
          PDS Imaging Node changes.

          Use the meta-kernel shown below to load the required SPICE
          kernels. For project meta-kernels similar to the one shown
          below, please see the PDS section of the NAIF FTP server.
          For example, look at the following path for Cassini
          meta-kernels: ftp://naif.jpl.nasa.gov//pub/naif/pds/data/
          co-s_j_e_v-spice-6-v1.0/cosp_1000/extras/mk

             KPL/MK

             File name: fovtrg_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
                cpck*.tpc                     Satellite orientation and
                                              radii
                pck00010.tpc                  Planet orientation and
                                              radii
                cas_rocks_v18.tf              FK for small satellites
                                              around Saturn
                cas_v40.tf                    Cassini FK
                cas_iss_v10.ti                Cassini ISS IK
                cas00149.tsc                  Cassini SCLK
                *.bsp                         Ephemeris for Cassini,
                                              planets, and satellites
                10279_10284ra.bc              Orientation for Cassini

             \begindata

                KERNELS_TO_LOAD = ( 'naif0010.tls'
                                    'cpck14Oct2010.tpc'
                                    'cpck_rock_21Jan2011_merged.tpc'
                                    'pck00010.tpc'
                                    'cas_rocks_v18.tf'
                                    'cas_v40.tf'
                                    'cas_iss_v10.ti'
                                    'cas00149.tsc'
                                    '110317AP_RE_90165_18018.bsp'
                                    '110120BP_IRRE_00256_25017.bsp'
                                    '101210R_SCPSE_10256_10302.bsp'
                                    '10279_10284ra.bc'              )

             \begintext

          Example code begins here.

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

             int main()
             {

                /.
                Local constants
                ./
                #define META  "fovtrg_ex.tm"
                #define BODLEN 32
                #define TIMLEN 32
                #define FRMLEN 32

                /.
                Local variables

                The variable `sclk' is the spacecraft clock time of the image.
                ./

                SpiceChar               body[BODLEN];
                SpiceChar               frame_name[FRMLEN];
                SpiceChar             * sclk = "1665078907.122";
                SpiceChar               time[TIMLEN];
                ConstSpiceChar        * time_format =
                                        "YYYY-MON-DD HR:MN:SC.###::TDB (TDB)";

                SpiceDouble             et;

                SpiceInt                body_id;
                SpiceInt                cassini_id;
                SpiceInt                frame_code;

                SpiceBoolean            found;
                SpiceBoolean            visible;

                /.
                Load kernels.
                ./
                furnsh_c ( META );

                /.
                Retrieve Cassini's NAIF ID.
                ./
                bodn2c_c ( "Cassini", &cassini_id, &found );

                if ( !found ) {
                   printf ( "Could not find ID code for Cassini." );
                   return (1);
                }

                /.
                Convert the image tag SCLK to ET.
                ./
                scs2e_c ( cassini_id, sclk, &et );

                /.
                Convert the ET to a string format for the output.
                ./
                timout_c ( et, time_format, TIMLEN, time );

                /.
                Search through all of Saturn's moons to see if each
                satellite was in the ISS NAC's field-of-view at
                the image time. We're going to take advantage of the
                fact that all Saturn's moons have a NAIF ID of 6xx.
                ./
                printf ( "At time %s the following were\n", time );
                printf ( "in the field of view of CASSINI_ISS_NAC\n" );

                for ( body_id = 600; body_id < 700; body_id++ ) {

                   /.
                   Check to see if the `body_id' has a translation.
                   ./
                   bodc2n_c ( body_id, BODLEN, body, &found );

                   if ( found ) {
                      /.
                      Check to see if a body-fixed frame for this ID exists.
                      If the frame is not in the kernel pool, we cannot
                      perform the visibility test. The main cause of a
                      failure is a missing kernel.
                      ./
                      cidfrm_c ( body_id, FRMLEN, &frame_code, frame_name,
                                 &found );

                      if ( found ) {
                         /.
                         Is this body in the field-of-view of Cassini's
                         ISS narrow-angle camera?
                         ./
                         fovtrg_c ( "CASSINI_ISS_NAC", body, "Ellipsoid",
                                     frame_name, "CN+S", "Cassini", &et,
                                     &visible );
                         if ( visible ) {
                            printf ( "  %s\n", body );
                         }
                      }
                   }
                }
                return (0);
             }

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

             At time 2010-OCT-06 17:09:45.346 (TDB) the following were
             in the field of view of CASSINI_ISS_NAC
               ENCELADUS
               JANUS
               EPIMETHEUS
               ATLAS
               PAN
               DAPHNIS
               ANTHE

          Note: there were actually 7 of Saturn's satellites in the
          field-of-view of Cassini's narrow-angle camera. However, Anthe
          is very small and was probably obscured by other objects or
          shadow.

#### Restrictions

       The reference frame associated with `inst' must be centered at the
       observer or must be inertial. No check is done to ensure this.

#### 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

       Target in instrument FOV at specified time
       Target in instrument field_of_view at specified time

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

    Wed Apr  5 17:54:35 2017