gfsntc_c |

## Procedurevoid gfsntc_c ( ConstSpiceChar * target, ConstSpiceChar * fixref, ConstSpiceChar * method, ConstSpiceChar * abcorr, ConstSpiceChar * obsrvr, ConstSpiceChar * dref, ConstSpiceDouble dvec [3], ConstSpiceChar * crdsys, ConstSpiceChar * coord, ConstSpiceChar * relate, SpiceDouble refval, SpiceDouble adjust, SpiceDouble step, SpiceInt nintvls, SpiceCell * cnfine, SpiceCell * result ) ## AbstractDetermine time intervals for which a coordinate of an surface intercept position vector satisfies a numerical constraint. ## Required_ReadingGF SPK CK TIME WINDOWS ## KeywordsSEPARATION GEOMETRY SEARCH EVENT ## Brief_I/OVariable I/O Description -------- --- -------------------------------------------------- SPICE_GF_CNVTOL P Convergence tolerance target I Name of the target body fixref I Body fixed frame associated with 'target' method I Name of method type for surface intercept calculation abcorr I Aberration correction flag obsrvr I Name of the observing body dref I Reference frame of direction vector 'dvec' dvec I Pointing direction vector from 'obsrvr' crdsys I Name of the coordinate system containing COORD coord I Name of the coordinate of interest relate I Operator that either looks for an extreme value (max, min, local, absolute) or compares the coordinate value and refval refval I Reference value adjust I Adjustment value for absolute extrema searches step I Step size used for locating extrema and roots nintvls I Workspace window interval count cnfine I-O SPICE window to which the search is restricted result O SPICE window containing results ## Detailed_Inputtarget the string name of a target body. Optionally, you may supply the integer ID code for the object as an integer string. For example both "MOON" and "301" are legitimate strings that indicate the moon is the target body. On calling ## Detailed_Outputcnfine 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 the SPICE window of intervals, contained within the confinement window cnfine, on which the specified constraint is satisfied. If result is non-empty on input, its contents will be discarded before ## ParametersSPICE_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_CNVTOL has the value 1.0e-6. Units are TDB seconds. ## 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_c can be used to contract the result window. 3) If an error (typically cell overflow) occurs while performing window arithmetic, the error will be diagnosed by a routine in the call tree of this routine. 4) If the relational operator `relate' is not recognized, an error is signaled by a routine in the call tree of this routine. 5) If the aberration correction specifier contains an unrecognized value, an error is signaled by a routine in the call tree of this routine. 6) If `adjust' is negative, an error is signaled by a routine in the call tree of this routine. 7) If either of the input body names do not map to NAIF ID codes, an error is signaled by a routine in the call tree of this routine. 8) If required ephemerides or other kernel data are not available, an error is signaled by a routine in the call tree of this routine. 9) If any input string argument pointer is null, the error SPICE(NULLPOINTER) will be signaled. 10) If any input string argument is empty, the error SPICE(EMPTYSTRING) will be signaled. 11) If the workspace interval count 'nintvls' is less than 1, the error SPICE(VALUEOUTOFRANGE) will be signaled. 12) If the required amount of workspace memory cannot be allocated, the error SPICE(MALLOCFAILURE) will be signaled. ## FilesAppropriate SPK and PCK kernels must be loaded by the calling program before this routine is called. The following data are required: - SPK data: the calling application must load ephemeris data for the targets, observer, and any intermediate objects in a chain connecting the targets and observer that cover the time period specified by the window CNFINE. 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. Typically ephemeris data are made available by loading one or more SPK files using FURNSH. - PCK data: bodies modeled as triaxial ellipsoids must have semi-axis lengths provided by variables in the kernel pool. Typically these data are made available by loading a text PCK file using FURNSH. - If non-inertial reference frames are used, then PCK files, frame kernels, C-kernels, and SCLK kernels may be needed. Such kernel data are normally loaded once per program run, NOT every time this routine is called. ## ParticularsThis routine provides a simpler, but less flexible interface than does the routine gfevnt_c for conducting searches for surface intercept vector coordinate value events. Applications that require support for progress reporting, interrupt handling, non-default step or refinement functions, or non-default convergence tolerance should call gfevnt_c rather than this routine. This routine determines a set of one or more time intervals within the confinement window when the selected coordinate of the surface intercept vector satisfies a caller-specified constraint. The resulting set of intervals 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 ================== Regardless of the type of constraint selected by the caller, this routine starts the search for solutions by determining the time periods, within the confinement window, over which the specified coordinate function is monotone increasing and monotone decreasing. Each of these time periods is represented by a SPICE window. Having found these windows, all of the coordinate function's local extrema within the confinement window are known. Absolute extrema then can be found very easily. Within any interval of these "monotone" windows, there will be at most one solution of any equality constraint. Since the boundary of the solution set for any inequality constraint is contained in the union of - the set of points where an equality constraint is met - the boundary points of the confinement window the solutions of both equality and inequality constraints can be found easily once the monotone windows have been found. Step Size ========= The monotone windows (described above) are found using a two-step search process. 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 sign of the rate of change of coordinate will be sampled. Starting at the left endpoint of an interval, samples will be taken at each step. If a change of sign is found, a root has been bracketed; at that point, the time at which the time derivative of the coordinate is zero can be found by a refinement process, for example, using a binary search. Note that the optimal choice of step size depends on the lengths of the intervals over which the coordinate function is monotone: the step size should be shorter than the shortest of these intervals (within the confinement window). The optimal step size is *not* necessarily related to the lengths of the intervals comprising the result window. For example, if the shortest monotone interval has length 10 days, and if the shortest result window interval has length 5 minutes, a step size of 9.9 days is still adequate to find all of the intervals in the result window. In situations like this, the technique of using monotone windows yields a dramatic efficiency improvement over a state-based search that simply tests at each step whether the specified constraint is satisfied. The latter type of search can miss solution intervals if the step size is longer than the shortest solution interval. 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 ===================== As described above, the root-finding process used by this routine involves first bracketing roots and then using a search process to locate them. "Roots" include times when extrema are attained and times when the geometric quantity function is equal to a reference value or adjusted extremum. All endpoints of the intervals comprising the result window are either endpoints of intervals of the confinement window or roots. 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 limit 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 gfstol_c, e.g. gfstol_c( tolerance value in seconds ) Call gfstol_c prior to calling this routine. All subsequent searches will use the updated tolerance value. Searches over time windows of long duration may require use of larger tolerance values than the default: the tolerance must be large enough so that it, when added to or subtracted from the confinement window's lower and upper bounds, yields distinct time values. 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 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. Practical use of the coordinate search capability would likely consist of searches over multiple coordinate constraints to find time intervals that satisfies the constraints. An effective technique to accomplish such a search is to use the result window from one search as the confinement window of the next. Longitude and Right Ascension ============================= The cyclic nature of the longitude and right ascension coordinates produces branch cuts at +/- 180 degrees longitude and 0-360 longitude. Round-off error may cause solutions near these branches to cross the branch. Use of the SPICE routine wncond_c will contract solution windows by some epsilon, reducing the measure of the windows and eliminating the branch crossing. A one millisecond contraction will in most cases eliminate numerical round-off caused branch crossings. ## ExamplesThe 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. The examples shown below require a "standard" set of SPICE kernels. We list these kernels in a meta kernel named "standard.tm." KPL/MK 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 --------- -------- de414.bsp Planetary ephemeris pck00008.tpc Planet orientation and radii naif0008.tls Leapseconds \begindata KERNELS_TO_LOAD = ( '/kernels/gen/lsk/naif0008.tls' '/kernels/gen/spk/de414.bsp' '/kernels/gen/pck/pck00008.tpc' ) The examples shown below require a frames kernel defining a a dynamic frame, Sun-Earth Motion. The frame defined by the sun-to-earth direction vector as the X axis. The Y axis in the earth orbital plane, and Z completing the right hand system. We name this frames kernel "sem.tf". \begindata FRAME_SEM = 10100000 FRAME_10100000_NAME = 'SEM' FRAME_10100000_CLASS = 5 FRAME_10100000_CLASS_ID = 10100000 FRAME_10100000_CENTER = 10 FRAME_10100000_RELATIVE = 'J2000' FRAME_10100000_DEF_STYLE = 'PARAMETERIZED' FRAME_10100000_FAMILY = 'TWO-VECTOR' FRAME_10100000_PRI_AXIS = 'X' FRAME_10100000_PRI_VECTOR_DEF = 'OBSERVER_TARGET_POSITION' FRAME_10100000_PRI_OBSERVER = 'SUN' FRAME_10100000_PRI_TARGET = 'EARTH' FRAME_10100000_PRI_ABCORR = 'NONE' FRAME_10100000_SEC_AXIS = 'Y' FRAME_10100000_SEC_VECTOR_DEF = 'OBSERVER_TARGET_VELOCITY' FRAME_10100000_SEC_OBSERVER = 'SUN' FRAME_10100000_SEC_TARGET = 'EARTH' FRAME_10100000_SEC_ABCORR = 'NONE' FRAME_10100000_SEC_FRAME = 'J2000' Example(1): Find the time during 2007 for which the latitude of the intercept point of the vector pointing from the sun towards the earth in the IAU_EARTH frame equals zero i.e. the intercept point crosses the equator. #include <stdio.h> #include <stdlib.h> #include <string.h> #include "SpiceUsr.h" #define MAXWIN 1000 #define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND" #define TIMLEN 64 int main( int argc, char **argv ) { /. Create the needed windows. Note, one window consists of two values, so the total number of cell values to allocate is twice the number of intervals. ./ SPICEDOUBLE_CELL ( result, 2*MAXWIN ); SPICEDOUBLE_CELL ( cnfine, 2 ); SpiceDouble begtim; SpiceDouble endtim; SpiceDouble step; SpiceDouble adjust; SpiceDouble refval; SpiceDouble beg; SpiceDouble end; SpiceChar begstr [ TIMLEN ]; SpiceChar endstr [ TIMLEN ]; SpiceChar * relate = "="; SpiceChar * crdsys = "LATITUDINAL"; SpiceChar * coord = "LATITUDE"; SpiceChar * method = "Ellipsoid"; SpiceChar * targ = "EARTH"; SpiceChar * obsrvr = "SUN"; SpiceChar * dref = "SEM"; SpiceDouble dvec[] = { 1, 0, 0 }; SpiceChar * fixref = "IAU_EARTH"; SpiceChar * abcorr = "NONE"; SpiceInt count; SpiceInt i; /. Search for a condition where the latitudinal system coordinate latitude in the IAU_EARTH frame has value zero. In this case, the pointing vector, 'DVEC', defines the vector direction pointing at the earth from the sun. ./ /. Load kernels. ./ furnsh_c( "standard.tm" ); furnsh_c( "sem.tf" ); /. Store the time bounds of our search interval in the cnfine confinement window. ./ str2et_c( "2007 JAN 01", &begtim ); str2et_c( "2008 JAN 01", &endtim ); wninsd_c ( begtim, endtim, &cnfine ); /. The latitude varies relatively slowly, ~46 degrees during the year. The extrema occur approximately every six months. Search using a step size less than half that value (180 days). For this example use ninety days (in units of seconds). ./ step = (90.)*spd_c(); adjust = 0.; refval = 0; /. List the beginning and ending points in each interval if result contains data. ./ ## Restrictions1) The kernel files to be used by this routine must be loaded (normally via the CSPICE routine furnsh_c) before this routine is called. 2) This routine has the side effect of re-initializing the coordinate quantity utility package. Callers may need to re-initialize the package after calling this routine. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.2, 31-JUL-2014 (EDW) Edit to header, replaced ' character with character " to indicate C strings. -CSPICE Version 1.0.1, 28-FEB-2013 (NJB) (EDW) Header was updated to discuss use of gfstol_c. Edit to comments to correct search description. Edits to and corrections of argument descriptions and header. -CSPICE Version 1.0.0, 17-FEB-2009, (EDW) ## Index_EntriesGF surface intercept coordinate search |

Wed Apr 5 17:54:36 2017