cspice_gfrfov |
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## AbstractCSPICE_GFRFOV determine the time intervals when a specified ray intersects the space bounded by the field-of-view (FOV) of a specified instrument. For important details concerning this module's function, please refer to the CSPICE routine gfrfov_c. ## I/OGiven: 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 (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 plane normal to this plane such that the projections have angular separation of at least 2*SPICE_GF_MARGIN radians from the plane spanned by U and V. Arguments- inst the scalar string naming the 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 a double precision 3-vector describing a ray pointing toward 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 the scalar string naming 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 the scalar string indicating the aberration corrections to apply when computing the 'raydir' 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 the scalar string naming 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 ID code of the object as an integer string. For example, both 'EARTH' and '399' are legitimate strings to supply to indicate the observer is Earth. step the scalar double precision time step size to use in the search. 'step' must be short enough for a search using this step size to locate the time intervals where coordinate function of the surface intercept vector is monotone increasing or decreasing. 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. 'step' has units of TDB seconds. cnfine a scalar double precision 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. In some cases the confinement window can be used to greatly reduce the time period that must be searched for the desired solution. See the Particulars section below for further discussion. See the Examples section below for a code example that shows how to create a confinement window. the call: ## ExamplesAny numerical results shown for this example may differ between platforms as the results depend on the SPICE kernels used as input and the machine specific arithmetic implementation. This example is an extension of the example in the header of cspice_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 from the cspice_gftfov 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 MAXWIN = 1000 TIMFMT = 'YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND' TIMLEN = 41 AU = 149597870.693D ;; ;; Load kernels. ;; cspice_furnsh, 'gftfov_ex1.tm' ;; ;; Store the time bounds of our search interval in ;; the cnfine confinement window. ;; cspice_str2et, [ '2004 JUN 11 06:30:00 TDB', $ '2004 JUN 11 12:00:00 TDB' ], et cnfine = cspice_celld( 2 ) cspice_wninsd, et[0], et[1], 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 'et[0]' 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. ;; parallax_deg = 0.000001056D ra_deg_0 = 19.290789927D ra_pm = -0.000000720D ra_epoch = 41.2000D dec_deg_0 = 2.015271007D dec_pm = 0.000001814D dec_epoch = 41.1300D rframe = 'J2000' result = cspice_celld( MAXWIN*2) ;; ;; Correct the star's direction for proper motion. ;; ;; The argument 't' represents 'et[0]' as Julian years past J1950. ;; t = et[0]/cspice_jyear() $ + ( cspice_j2000()- cspice_j1950() )/365.25D dtra = t - ra_epoch dtdec = t - dec_epoch ra_deg = ra_deg_0 + dtra * ra_pm dec_deg = dec_deg_0 + dtra * dec_pm ra = ra_deg * cspice_rpd() dec = dec_deg * cspice_rpd() cspice_radrec, 1.D, 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 * cspice_rpd() 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. ;; starpos = stardist * starpos cspice_spkpos, 'cassini', et[0], 'J2000', 'NONE', $ 'solar system barycenter', pos, ltime raydir = starpos - pos ;; ;; Correct the star direction for stellar aberration when ;; we conduct the search. ;; abcorr = 'S' obsrvr = 'CASSINI' stepsz = 10.D0 ## 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. 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, by a 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 by 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. The user may change the convergence tolerance from the default SPICE_GF_CNVTOL value by calling the routine cspice_gfstol, e.g. cspice_gfstol, tolerance value in seconds Call cspice_gfstol prior to calling this routine. All subsequent searches will use the updated tolerance value. Setting 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 affect 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. ## Required ReadingICY.REQ CK.REQ FRAMES.REQ GF.REQ KERNEL.REQ NAIF_IDS.REQ PCK.REQ SPK.REQ TIME.REQ WINDOWS.REQ ## Version-Icy Version 1.0.1, 14-MAY-2012, EDW (JPL) Minor edit to code comments eliminating typo. Header updated to describe use of cspice_gfstol. -Icy Version 1.0.0, 15-APR-2009, EDW (JPL) ## Index_EntriesGF ray in instrument FOV search |

Wed Apr 5 17:58:01 2017