Index Page gfrfov_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 gfrfov_c ( ConstSpiceChar     * inst,
                       ConstSpiceDouble     raydir [3],
                       ConstSpiceChar     * rframe,
                       ConstSpiceChar     * abcorr,
                       ConstSpiceChar     * obsrvr,
                       SpiceDouble          step,
                       SpiceCell          * cnfine,
                       SpiceCell          * result  )

#### Abstract

       Determine time intervals when a specified ray intersects the
       space bounded by the field-of-view (FOV) of a specified
       instrument.

#### Required_Reading

       CK
       FRAMES
       GF
       KERNEL
       NAIF_IDS
       PCK
       SPK
       TIME
       WINDOWS

#### Keywords

       EVENT
       FOV
       GEOMETRY
       INSTRUMENT
       SEARCH
       WINDOW

#### Brief_I/O

       VARIABLE         I/O  DESCRIPTION
       ---------------  ---  ------------------------------------------------
       SPICE_GF_MARGIN   P   Minimum complement of FOV cone angle.
       SPICE_GF_CNVTOL   P   Convergence tolerance.
       SPICE_GF_MAXVRT   P   Maximum number of FOV boundary vertices.
       inst              I   Name 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 of the observing body.
       step              I   Step size in seconds for finding FOV events.
       cnfine           I-O  SPICE window to which the search is restricted.
       result            O   SPICE window containing results.

#### Detailed_Input

       inst       indicates the name of an instrument, such as a
                  spacecraft-mounted framing camera, the field of view
                  (FOV) of which is to be used for an target intersection
                  search: the direction from the observer to a target
                  is represented by a ray, and times when the specified
                  ray intersects the region of space bounded by the FOV
                  are sought.

                  The position of the instrument designated by `inst' is
                  considered to coincide with that of the ephemeris
                  object designated by the input argument `obsrvr' (see
                  description below).

                  `inst' must have a corresponding NAIF ID and a frame
                  defined, as is normally done in a frame kernel. It
                  must also have an associated reference frame and a FOV
                  shape, boresight and boundary vertices (or reference
                  vector and reference angles) defined, as is usually
                  done in an instrument kernel.

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

       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 `obsrvr' 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'.

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

       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 `abcorr'.

       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.

       step       is the step size to be used in the search. `step' must
                  be shorter than any interval, within the confinement
                  window, over which the specified condition is met. In
                  other words, `step' must be shorter than the shortest
                  visibility event that the user wishes to detect. `step'
                  also must be shorter than the minimum duration
                  separating any two visibility events. However, `step'
                  must not be *too* short, or the search will take an
                  unreasonable amount of time.

                  The choice of `step' affects the completeness but not
                  the precision of solutions found by this routine; the
                  precision is controlled by the convergence tolerance.
                  See the discussion of the parameter SPICE_GF_CNVTOL for
                  details.

                  `step' has units of seconds.

       cnfine     is a SPICE window that confines the time period over
                  which the specified search is conducted. `cnfine' may
                  consist of a single interval or a collection of
                  intervals.

                  The endpoints of the time intervals comprising `cnfine'
                  are interpreted as seconds past J2000 TDB.

                  See the Examples section below for a code example
                  that shows how to create a confinement window.

#### Detailed_Output

       cnfine     is the input confinement window, updated if necessary
                  so the control area of its data array indicates the
                  window's size and cardinality. The window data are
                  unchanged.

       result     is a SPICE window representing the set of time
                  intervals, within the confinement period, when the
                  input ray is "visible"; that is, when the ray is
                  contained in the space bounded by the specified
                  instrument's field of view.

                  The endpoints of the time intervals comprising `result'
                  are interpreted as seconds past J2000 TDB.

                  If `result' is non-empty on input, its contents
                  will be discarded before gfrfov_c conducts its
                  search.

#### Parameters

       All parameters described here are declared in the header file
       SpiceGF.h. See that file for parameter values.

       SPICE_GF_CNVTOL

                  is the convergence tolerance used for finding endpoints
                  of the intervals comprising the result window.
                  SPICE_GF_CNVTOL is used to determine when binary searches
                  for roots should terminate: when a root is bracketed
                  within an interval of length SPICE_GF_CNVTOL, the root is
                  considered to have been found.

                  The accuracy, as opposed to precision, of roots found
                  by this routine depends on the accuracy of the input
                  data. In most cases, the accuracy of solutions will be
                  inferior to their precision.

       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.

#### Exceptions

       1)  In order for this routine to produce correct results,
           the step size must be appropriate for the problem at hand.
           Step sizes that are too large may cause this routine to miss
           roots; step sizes that are too small may cause this routine
           to run unacceptably slowly and in some cases, find spurious
           roots.

           This routine does not diagnose invalid step sizes, except
           that if the step size is non-positive, the error
           SPICE(INVALIDSTEPSIZE) will be signaled.

       2)  Due to numerical errors, in particular,

              - Truncation error in time values
              - Finite tolerance value
              - Errors in computed geometric quantities

           it is *normal* for the condition of interest to not always be
           satisfied near the endpoints of the intervals comprising the
           result window.

           The result window may need to be contracted slightly by the
           caller to achieve desired results. The SPICE window routine
           WNCOND can be used to contract the result window.

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

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

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

       6)  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.

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

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

       9)  If the instrument FOV is polygonal, 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, 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.

       10) 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.

       11) 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.

       12) If the output SPICE window `result' has insufficient capacity
           to contain the number of intervals on which the specified
           visibility condition is met, the error will be diagnosed
           by a routine in the call tree of this routine. If the result
           window has size less than 2, the error SPICE(WINDOWTOOSMALL)
           will be signaled by this routine.

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

       14) 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 the observer for the period
            defined by the confinement window `cnfine' must be loaded.
            If aberration corrections are used, the state of the
            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.

          - Data defining the reference frame associated with the
            instrument designated by `inst' must be available in the kernel
            pool. Additionally the name `inst' must be associated with an
            ID code. Normally these data are  made available by loading a
            frame kernel via furnsh_c.

          - IK data: the kernel pool must contain data such that
            the CSPICE routine getfov_c may be called to obtain
            parameters for `inst'. Normally such data are provided by
            an IK via furnsh_c.

       The following data may be required:

          - CK data: if the instrument frame 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, FKs, CKs, SCLK kernels, PCKs, and SPKs may be
            required to map the direction to the J2000 frame.

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

#### Particulars

       This routine determines a set of one or more time intervals when
       the specified ray in contained within the field of view of a
       specified instrument. We'll use the term "visibility event" to
       designate such an appearance. The set of time intervals resulting
       from the search is returned as a SPICE window.

       This routine provides a simpler, but less flexible, interface
       than does the CSPICE routine gffove_c for conducting searches for
       visibility events. Applications that require support for progress
       reporting, interrupt handling, non-default step or refinement
       functions, or non-default convergence tolerance should call
       gffove_c rather than this routine.

       Below we discuss in greater detail aspects of this routine's
       solution process that are relevant to correct and efficient use
       of this routine in user applications.

       The Search Process
       ==================

       The search for visibility events is treated as a search for state
       transitions: times are sought when the state of the ray
       changes from "not visible" to "visible" or vice versa.

       Step Size
       =========

       Each interval of the confinement window is searched as follows:
       first, the input step size is used to determine the time
       separation at which the visibility state will be sampled.
       Starting at the left endpoint of an interval, samples will be
       taken at each step. If a state change is detected, a root has
       been bracketed; at that point, the "root"--the time at which the
       state change occurs---is found by a refinement process, for
       example, via binary search.

       Note that the optimal choice of step size depends on the lengths
       of the intervals over which the visibility state is constant:
       the step size should be shorter than the shortest visibility event
       duration and the shortest period between visibility events, within
       the confinement window.

       Having some knowledge of the relative geometry of the ray and
       observer can be a valuable aid in picking a reasonable step size.
       In general, the user can compensate for lack of such knowledge by
       picking a very short step size; the cost is increased computation
       time.

       Note that the step size is not related to the precision with which
       the endpoints of the intervals of the result window are computed.
       That precision level is controlled by the convergence tolerance.

       Convergence Tolerance
       =====================

       Once a root has been bracketed, a refinement process is used to
       narrow down the time interval within which the root must lie.
       This refinement process terminates when the location of the root
       has been determined to within an error margin called the
       "convergence tolerance." The convergence tolerance used by this
       routine is set via the parameter SPICE_GF_CNVTOL.

       The value of SPICE_GF_CNVTOL is set to a "tight" value so that the
       tolerance doesn't become the limiting factor in the accuracy of
       solutions found by this routine. In general the accuracy of input
       data will be the limiting factor.

       To use a different tolerance value, a lower-level GF routine such
       as gffove_c  must be called. Making the tolerance tighter than
       SPICE_GF_CNVTOL is unlikely to be useful, since the results are unlikely
       to be more accurate. Making the tolerance looser will speed up
       searches somewhat, since a few convergence steps will be omitted.
       However, in most cases, the step size is likely to have a much
       greater effect on processing time than would the convergence
       tolerance.

       The Confinement Window
       ======================

       The simplest use of the confinement window is to specify a time
       interval within which a solution is sought. However, the confinement
       window can, in some cases, be used to make searches more efficient.
       Sometimes it's possible to do an efficient search to reduce the size
       of the time period over which a relatively slow search of interest
       must be performed. For an example, see the program CASCADE in the GF
       Example Programs chapter of the GF Required Reading, gf.req.

#### 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) This example is an extension of example #1 in the
          header of

             gftfov_c

          The problem statement for that example is

             Search for times when Saturn's satellite Phoebe is within the
             FOV of the Cassini narrow angle camera (CASSINI_ISS_NAC). To
             simplify the problem, restrict the search to a short time
             period where continuous Cassini bus attitude data are
             available.

             Use a step size of 10 seconds to reduce chances of missing
             short visibility events.

          Here we search the same confinement window for times when a
          selected background star is visible. We use the FOV of the
          Cassini ISS wide angle camera (CASSINI_ISS_WAC) to enhance the
          probability of viewing the star.

          The star we'll use has catalog number 6000 in the Hipparcos
          Catalog. The star's J2000 right ascension and declination, proper
          motion, and parallax are taken from that catalog.

          Use the meta-kernel from the gftfov_c example:

             KPL/MK

             File name: gftfov_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
                ---------                     --------
                naif0009.tls                  Leapseconds
                cpck05Mar2004.tpc             Satellite orientation and
                                              radii
                981005_PLTEPH-DE405S.bsp      Planetary ephemeris
                020514_SE_SAT105.bsp          Satellite ephemeris
                030201AP_SK_SM546_T45.bsp     Spacecraft ephemeris
                cas_v37.tf                    Cassini FK
                04135_04171pc_psiv2.bc        Cassini bus CK
                cas00084.tsc                  Cassini SCLK kernel
                cas_iss_v09.ti                Cassini IK

             \begindata

                KERNELS_TO_LOAD = ( 'naif0009.tls',
                                    'cpck05Mar2004.tpc',
                                    '981005_PLTEPH-DE405S.bsp',
                                    '020514_SE_SAT105.bsp',
                                    '030201AP_SK_SM546_T45.bsp',
                                    'cas_v37.tf',
                                    '04135_04171pc_psiv2.bc',
                                    'cas00084.tsc',
                                    'cas_iss_v09.ti'            )
             \begintext

          Example code begins here.

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

             int main()
             {
                /.
                PROGRAM EX1
                ./

                /.
                Local constants
                ./
                #define  AU             149597870.693
                #define  META           "gftfov_ex1.tm"
                #define  TIMFMT         "YYYY-MON-DD HR:MN:SC.######::TDB (TDB)"
                #define  TIMLEN         41
                #define  MAXWIN         10000

                /.
                Local variables
                ./
                SPICEDOUBLE_CELL ( cnfine, MAXWIN );
                SPICEDOUBLE_CELL ( result, MAXWIN );

                SpiceChar             * abcorr;
                SpiceChar             * inst;
                SpiceChar             * obsrvr;
                SpiceChar             * rframe;
                SpiceChar               timstr  [2][ TIMLEN ];

                SpiceDouble             dec;
                SpiceDouble             dec_deg;
                SpiceDouble             dec_deg_0;
                SpiceDouble             dec_epoch;
                SpiceDouble             dec_pm;
                SpiceDouble             dtdec;
                SpiceDouble             dtra;
                SpiceDouble             endpt   [2];
                SpiceDouble             et0;
                SpiceDouble             et1;
                SpiceDouble             lt;
                SpiceDouble             parallax;
                SpiceDouble             parallax_deg;
                SpiceDouble             pos     [3];
                SpiceDouble             ra;
                SpiceDouble             ra_deg;
                SpiceDouble             ra_deg_0;
                SpiceDouble             ra_epoch;
                SpiceDouble             ra_pm;
                SpiceDouble             raydir  [3];
                SpiceDouble             stardist;
                SpiceDouble             starpos [3];
                SpiceDouble             stepsz;
                SpiceDouble             t;

                SpiceInt                catno;
                SpiceInt                i;
                SpiceInt                j;
                SpiceInt                n;

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

                /.
                Insert search time interval bounds into the
                confinement window.
                ./
                str2et_c ( "2004 JUN 11 06:30:00 TDB", &et0 );
                str2et_c ( "2004 JUN 11 12:00:00 TDB", &et1 );

                wninsd_c ( et0, et1, &cnfine );

                /.
                Initialize inputs for the search.
                ./
                inst   = "CASSINI_ISS_WAC";

                /.
                Create a unit direction vector pointing from observer to star.
                We'll assume the direction is constant during the confinement
                window, and we'll use et0 as the epoch at which to compute the
                direction from the spacecraft to the star.

                The data below are for the star with catalog number 6000
                in the Hipparcos catalog. Angular units are degrees; epochs
                have units of Julian years and have a reference epoch of J1950.
                The reference frame is J2000.
                ./
                catno        = 6000;

                parallax_deg = 0.000001056;

                ra_deg_0     = 19.290789927;
                ra_pm        = -0.000000720;
                ra_epoch     = 41.2000;

                dec_deg_0    =  2.015271007;
                dec_pm       =  0.000001814;
                dec_epoch    = 41.1300;

                rframe       = "J2000";

                /.
                Correct the star's direction for proper motion.

                The argument t represents et0 as Julian years past J1950.
                ./
                t         = et0/jyear_c()  +  ( j2000_c()- j1950_c() )/365.25;

                dtra      = t - ra_epoch;
                dtdec     = t - dec_epoch;

                ra_deg    = ra_deg_0  +  dtra  * ra_pm;
                dec_deg   = dec_deg_0 +  dtdec * dec_pm;

                ra        = ra_deg  * rpd_c();
                dec       = dec_deg * rpd_c();

                radrec_c ( 1.0, ra, dec, starpos );

                /.
                Correct star position for parallax applicable at
                the Cassini orbiter's position. (The parallax effect
                is negligible in this case; we're simply demonstrating
                the computation.)
                ./
                parallax = parallax_deg * rpd_c();
                stardist = AU / tan(parallax);

                /.
                Scale the star's direction vector by its distance from
                the solar system barycenter. Subtract off the position
                of the spacecraft relative to the solar system barycenter;
                the result is the ray's direction vector.
                ./
                vscl_c   ( stardist, starpos, starpos );

                spkpos_c ( "cassini", et0, "J2000",  "NONE",
                           "solar system barycenter", pos,  &lt );

                vsub_c   ( starpos, pos, raydir );

                /.
                Correct the star direction for stellar aberration when
                we conduct the search.
                ./
                abcorr = "S";
                obsrvr = "CASSINI";
                stepsz = 10.0;

                printf ( "\n"
                         " Instrument:            %s\n"
                         " Star's catalog number: %d\n"
                         "\n",
                         inst,
                         (int)catno            );

                /.
                Perform the search.
                ./
                gfrfov_c ( inst,   raydir, rframe,  abcorr,
                           obsrvr, stepsz, &cnfine, &result );

                n = wncard_c ( &result );

                if ( n == 0 )
                {
                   printf (  "No FOV intersection found.\n" );
                }
                else
                {
                   printf ( "  Visibility start time              Stop time\n" );

                   for ( i = 0;  i < n;  i++ )
                   {
                      wnfetd_c ( &result, i, endpt, endpt+1 );

                      for ( j = 0;  j < 2;  j++ )
                      {
                         timout_c ( endpt[j], TIMFMT, TIMLEN, timstr[j] );
                      }

                      printf ( "  %s  %s\n",
                               timstr[0],
                               timstr[1]                                   );
                   }
                }

                printf ( "\n" );

                return ( 0 );
             }

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

             Instrument:            CASSINI_ISS_WAC
             Star's catalog number: 6000

              Visibility start time              Stop time
              2004-JUN-11 06:30:00.000000 (TDB)  2004-JUN-11 12:00:00.000000 (TDB)

          The star is visible throughout the confinement window.

#### Restrictions

       The kernel files to be used by gfrfov_c must be loaded (normally via
       the CSPICE routine furnsh_c) before gfrfov_c is called.

#### Literature_References

       None.

#### Author_and_Institution

       N.J. Bachman  (JPL)
       L.S. Elson    (JPL)
       E.D. Wright   (JPL)

#### Version

       -CSPICE Version 1.0.1, 12-JUL-2016 (EDW)

          Edit to example program to use "%d" with explicit casts
          to int for printing SpiceInts with printf.

       -CSPICE Version 1.0.0, 12-FEB-2009 (NJB) (LSE) (EDW)

#### Index_Entries

       GF ray in instrument FOV search

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

    Wed Apr  5 17:54:36 2017