cspice_gftfov |
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## AbstractCSPICE_GFTFOV determines time intervals when a specified ephemeris object intersects the space bounded by the field-of-view (FOV) of a specified instrument. ## 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 string naming the instrument, such as a spacecraft-mounted framing camera, the field of view (FOV) of which is to be used for a target intersection search: times when the specified target intersects the region of space corresponding to 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 Mice routine cspice_getfov for a description of the required parameters associated with an instrument. [1,c1] = size(inst); char = class(inst) target the string naming the 'target' body, the appearances of which in the specified instrument's field of view are sought. The body must be an ephemeris object. Optionally, you may supply the integer NAIF ID code for the body as a string. For example both 'MOON' and '301' are legitimate strings that designate the Moon. The 'target' string lacks sensitivity to case, and to leading and trailing blanks. [1,c2] = size(target); char = class(target) tshape the string naming 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. The 'tshape' string lacks sensitivity to case, leading and trailing blanks. [1,c3] = size(tshape); char = class(tshape) tframe the string naming 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. The 'tframe' string lacks sensitivity to case, and to leading and trailing blanks. [1,c4] = size(tframe); char = class(tframe) abcorr the string indicating the aberration corrections to apply to the state evaluations to account for one-way light time and stellar aberration. 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. 'NONE' Apply no correction. Supported aberration correction options for reception case (radiation is received by observer at ET) are: 'LT' Correct for one-way light time using a Newtonian formulation. 'LT+S' Correct for one-way light time and stellar aberration using a Newtonian formulation. 'CN' Correct for one-way light time using a converged Newtonian light time correction. 'CN+S' Correct for one-way light time and stellar aberration using a converged Newtonian light time and stellar aberration corrections. Supported aberration correction options for transmission case (radiation is emitted from observer at ET) are: 'XLT' Correct for one-way light time using a Newtonian formulation. 'XLT+S' Correct for one-way light time and stellar aberration using a Newtonian formulation. 'XCN' Correct for one-way light time using a converged Newtonian light time correction. 'XCN+S' Correct for one-way light time and stellar aberration using a converged Newtonian light time and stellar aberration corrections. For detailed information, see the geometry finder required reading, gf.req. The 'abcorr' string lacks sensitivity to case, and to leading and trailing blanks. [1,c5] = size(abcorr); char = class(abcorr) obsrvr the string naming the body from which the target 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. [1,c6] = size(abcorr); char = class(abcorr) step the step size to use in the search to use in the search. 'step' must be short enough for a search using step to locate the time intervals where the specified angular separation function 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. See the discussion of the parameter SPICE_GF_CNVTOL for details. 'step' has units of TDB seconds. [1,1] = size(step); double = class(step) cnfine the 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. 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. [2m,1] = size(cnfine); double = class(cnfine) room the maximum number of intervals to return in 'result'. Note: this value should equal at least the number of expected intervals. Recall two double precision values define an interval. [1,1] = size(room); int32 = class(room) the call: result = ## 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. 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. Use the meta-kernel listed below to load the required SPICE kernels. 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'; % % Load kernels. % cspice_furnsh( 'gftfov_ex1.tm' ) % % Store the time bounds of our search interval in % the cnfine confinement window. % et = cspice_str2et( { '2004 JUN 11 06:30:00 TDB', ... '2004 JUN 11 12:00:00 TDB' } ); cnfine = cspice_wninsd( et(1), et(2) ); % %Initialize inputs for the search. % inst = 'CASSINI_ISS_NAC'; target = 'PHOEBE'; tshape = 'ELLIPSOID'; tframe = 'IAU_PHOEBE'; abcorr = 'LT+S'; obsrvr = 'CASSINI'; step = 10.; room = MAXWIN; result = ## ParticularsThis routine determines a set of one or more time intervals within the confinement window when any portion of a specified target body appears 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. 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 target body 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 target 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 ReadingFor important details concerning this module's function, please refer to the CSPICE routine gftfov_c. MICE.REQ CK.REQ FRAMES.REQ GF.REQ KERNEL.REQ NAIF_IDS.REQ PCK.REQ SPK.REQ TIME.REQ WINDOWS.REQ ## Version-Mice Version 1.1.0, 12-MAY-2012, EDW (JPL) Corrected minor typo in header. Renamed the argument 'size' to 'room'. "size" is a Matlab function name and it's seriously dumb to use a function name word as an argument name. Edited I/O section to conform to NAIF standard for Mice documentation. Header updated to describe use of cspice_gfstol. -Mice Version 1.0.0, 15-APR-2009, EDW (JPL) ## Index_EntriesGF target in instrument FOV search |

Wed Apr 5 18:00:32 2017