gfrfov |

Table of contents## ProcedureGFRFOV ( GF, is ray in FOV? ) SUBROUTINE GFRFOV ( INST, RAYDIR, RFRAME, ABCORR, . OBSRVR, STEP, CNFINE, RESULT ) ## AbstractDetermine time intervals when a specified ray intersects the space bounded by the field-of-view (FOV) of a specified instrument. ## Required_ReadingCK FRAMES GF KERNEL NAIF_IDS PCK SPK TIME WINDOWS ## KeywordsEVENT FOV GEOMETRY INSTRUMENT SEARCH WINDOW ## DeclarationsIMPLICIT NONE INCLUDE 'gf.inc' INCLUDE 'zzholdd.inc' INTEGER LBCELL PARAMETER ( LBCELL = -5 ) CHARACTER*(*) INST DOUBLE PRECISION RAYDIR ( 3 ) CHARACTER*(*) RFRAME CHARACTER*(*) ABCORR CHARACTER*(*) OBSRVR DOUBLE PRECISION STEP DOUBLE PRECISION CNFINE ( LBCELL : * ) DOUBLE PRECISION RESULT ( LBCELL : * ) ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- MARGIN P Minimum complement of FOV cone angle. LBCELL P SPICE Cell lower bound. CNVTOL P Convergence tolerance. MAXVRT P Maximum number of FOV boundary vertices. ZZGET P ZZHOLDD retrieves a stored DP value. GF_TOL P ZZHOLDD acts on the GF subsystem tolerance. 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 SPICE window to which the search is restricted. RESULT I-O SPICE window containing results. ## Detailed_InputINST 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 SPICELIB routine GETFOV 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. Optionally, you may supply the integer NAIF ID code for the body as a string. Case and leading or trailing blanks are not significant in the string OBSRVR. 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 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. CNFINE must be initialized by the caller via the SPICELIB routine SSIZED. RESULT is a double precision SPICE window which will contain the search results. RESULT must be declared and initialized with sufficient size to capture the full set of time intervals within the search region on which the specified condition is satisfied. RESULT must be initialized by the caller via the SPICELIB routine SSIZED. If RESULT is non-empty on input, its contents will be discarded before ## Detailed_OutputRESULT 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 no times within the confinement window satisfy the search criteria, RESULT will be returned with a cardinality of zero. ## ParametersLBCELL is the lower bound for SPICE cell arrays. CNVTOL is the convergence tolerance used for finding endpoints of the intervals comprising the result window. CNVTOL is used to determine when binary searches for roots should terminate: when a root is bracketed within an interval of length 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. MAXVRT is the maximum number of vertices that may be used to define the boundary of the specified instrument's field of view. 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-12. See INCLUDE file gf.inc for declarations and descriptions of parameters used throughout the GF system. ## Exceptions1) 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, an error is signaled by a routine in the call tree of this routine. 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, an error is signaled by a routine in the call tree of this routine. 4) If the aberration correction flag calls for light time correction, an error is signaled by a routine in the call tree of this routine. 5) If the ray's direction vector is zero, an error is signaled by a routine in the call tree of this routine. 6) 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. 7) 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. 8) If the FOV boundary has more than MAXVRT vertices, an error is signaled by a routine 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)-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. 10) 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. 11) 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. 12) If the output SPICE window RESULT has size less than 2, the error SPICE(WINDOWTOOSMALL) is signaled. 13) If the output SPICE window RESULT has insufficient capacity to contain the number of intervals on which the specified visibility condition is met, an error is signaled by a routine in the call tree of this routine. ## FilesAppropriate 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. - 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. - IK data: the kernel pool must contain data such that the SPICELIB routine GETFOV may be called to obtain parameters for INST. Normally such data are provided by an IK via FURNSH. 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. ## ParticularsThis 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 SPICELIB routine GFFOVE 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 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 default convergence tolerance used by this routine is set by the parameter CNVTOL (defined in gf.inc). The value of 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. The user may change the convergence tolerance from the default CNVTOL value by calling the routine GFSTOL, e.g. CALL GFSTOL( tolerance value ) Call GFSTOL prior to calling this routine. All subsequent searches will use the updated tolerance value. Setting the tolerance tighter than 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. ## ExamplesThe 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) This example is an extension of example #1 in the header of GFTFOV 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 shown below to load the required SPICE kernels. KPL/MK File name: gfrfov_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 ----------------------------- ---------------------- naif0012.tls Leapseconds 041014R_SCPSE_01066_04199.bsp CASSINI, planetary and Saturn satellite ephemeris cas_v40.tf Cassini FK 04161_04164ra.bc Cassini bus CK cas00071.tsc Cassini SCLK kernel cas_iss_v10.ti Cassini IK \begindata KERNELS_TO_LOAD = ( 'naif0012.tls', '041014R_SCPSE_01066_04199.bsp', 'cas_v40.tf', '04161_04164ra.bc', 'cas00071.tsc', 'cas_iss_v10.ti' ) \begintext End of meta-kernel Example code begins here. PROGRAM GFRFOV_EX1 IMPLICIT NONE C C SPICELIB functions C DOUBLE PRECISION J1950 DOUBLE PRECISION J2000 DOUBLE PRECISION JYEAR DOUBLE PRECISION RPD INTEGER WNCARD C C Local parameters C CHARACTER*(*) META PARAMETER ( META = 'gfrfov_ex1.tm' ) CHARACTER*(*) TIMFMT PARAMETER ( TIMFMT = . 'YYYY-MON-DD HR:MN:SC.######::TDB' ) DOUBLE PRECISION AU PARAMETER ( AU = 149597870.693D0 ) INTEGER LBCELL PARAMETER ( LBCELL = -5 ) INTEGER MAXWIN PARAMETER ( MAXWIN = 10000 ) INTEGER CORLEN PARAMETER ( CORLEN = 10 ) INTEGER BDNMLN PARAMETER ( BDNMLN = 36 ) INTEGER FRNMLN PARAMETER ( FRNMLN = 32 ) INTEGER TIMLEN PARAMETER ( TIMLEN = 35 ) INTEGER LNSIZE PARAMETER ( LNSIZE = 80 ) C C Local variables C CHARACTER*(CORLEN) ABCORR CHARACTER*(BDNMLN) INST CHARACTER*(LNSIZE) LINE CHARACTER*(BDNMLN) OBSRVR CHARACTER*(FRNMLN) RFRAME CHARACTER*(TIMLEN) TIMSTR ( 2 ) DOUBLE PRECISION CNFINE ( LBCELL : 2 ) DOUBLE PRECISION DEC DOUBLE PRECISION DECEPC DOUBLE PRECISION DECPM DOUBLE PRECISION DECDEG DOUBLE PRECISION DECDG0 DOUBLE PRECISION DTDEC DOUBLE PRECISION DTRA DOUBLE PRECISION ENDPT ( 2 ) DOUBLE PRECISION ET0 DOUBLE PRECISION ET1 DOUBLE PRECISION LT DOUBLE PRECISION PARLAX DOUBLE PRECISION PLXDEG DOUBLE PRECISION POS ( 3 ) DOUBLE PRECISION PSTAR ( 3 ) DOUBLE PRECISION RA DOUBLE PRECISION RADEG DOUBLE PRECISION RADEG0 DOUBLE PRECISION RAEPC DOUBLE PRECISION RAPM DOUBLE PRECISION RAYDIR ( 3 ) DOUBLE PRECISION RESULT ( LBCELL : MAXWIN ) DOUBLE PRECISION RSTAR DOUBLE PRECISION STEPSZ DOUBLE PRECISION T INTEGER CATNO INTEGER I INTEGER J INTEGER N C C Saved variables C C The confinement and result windows CNFINE and RESULT are C saved because this practice helps to prevent stack C overflow. C SAVE CNFINE SAVE RESULT C C Load kernels. C CALL FURNSH ( META ) C C Initialize windows. C CALL SSIZED ( 2, CNFINE ) CALL SSIZED ( MAXWIN, RESULT ) C C Insert search time interval bounds into the C confinement window. C CALL STR2ET ( '2004 JUN 11 06:30:00 TDB', ET0 ) CALL STR2ET ( '2004 JUN 11 12:00:00 TDB', ET1 ) CALL WNINSD ( ET0, ET1, CNFINE ) C C Initialize inputs for the search. C INST = 'CASSINI_ISS_WAC' C C Create a unit direction vector pointing from c observer to star. We'll assume the direction C is constant during the confinement window, and C we'll use et0 as the epoch at which to compute the C direction from the spacecraft to the star. C C The data below are for the star with catalog C number 6000 in the Hipparcos catalog. Angular C units are degrees; epochs have units of Julian C years and have a reference epoch of J1950. C The reference frame is J2000. C CATNO = 6000 PLXDEG = 0.000001056D0 RADEG0 = 19.290789927D0 RAPM = -0.000000720D0 RAEPC = 41.2000D0 DECDG0 = 2.015271007D0 DECPM = 0.000001814D0 DECEPC = 41.1300D0 RFRAME = 'J2000' C C Correct the star's direction for proper motion. C C The argument t represents et0 as Julian years C past J1950. C T = ET0/JYEAR() . + ( J2000()- J1950() ) / 365.25D0 DTRA = T - RAEPC DTDEC = T - DECEPC RADEG = RADEG0 + DTRA * RAPM DECDEG = DECDG0 + DTDEC * DECPM RA = RADEG * RPD() DEC = DECDEG * RPD() CALL RADREC ( 1.D0, RA, DEC, PSTAR ) C C Correct star position for parallax applicable at C the Cassini orbiter's position. (The parallax effect C is negligible in this case; we're simply demonstrating C the computation.) C PARLAX = PLXDEG * RPD() RSTAR = AU / TAN(PARLAX) C C Scale the star's direction vector by its distance from C the solar system barycenter. Subtract off the position C of the spacecraft relative to the solar system C barycenter; the result is the ray's direction vector. C CALL VSCLIP ( RSTAR, PSTAR ) CALL SPKPOS ( 'CASSINI', ET0, 'J2000', 'NONE', . 'SOLAR SYSTEM BARYCENTER', POS, LT ) CALL VSUB ( PSTAR, POS, RAYDIR ) C C Correct the star direction for stellar aberration when C we conduct the search. C ABCORR = 'S' OBSRVR = 'CASSINI' STEPSZ = 10.D0 WRITE (*,*) ' ' WRITE (*,*) 'Instrument: '//INST WRITE (*,*) 'Star''s catalog number: ', CATNO WRITE (*,*) ' ' C C Perform the search. C CALL ## Restrictions1) The kernel files to be used by ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) L.S. Elson (JPL) E.D. Wright (JPL) ## VersionSPICELIB Version 1.1.1, 06-AUG-2021 (JDR) Edited the header to comply with NAIF standard. Modified code examples' output to comply with maximum line length of header comments. Updated Example's kernels set to use PDS archived data. Added SAVE statements for CNFINE and RESULT variables in code example. Updated description of RESULT argument in $Brief_I/O, $Detailed_Input and $Detailed_Output. SPICELIB Version 1.1.0, 28-FEB-2012 (EDW) Implemented use of ZZHOLDD to allow user to alter convergence tolerance. Removed the STEP > 0 error check. The GFSSTP call includes the check. SPICELIB Version 1.0.0, 15-APR-2009 (NJB) (LSE) (EDW) |

Fri Dec 31 18:36:25 2021