gfudb_c |

## Procedurevoid gfudb_c ( void ( * udfuns ) ( SpiceDouble et, SpiceDouble * value ), void ( * udfunb ) ( void ( * udfuns ) ( SpiceDouble et, SpiceDouble * value ), SpiceDouble et, SpiceBoolean * xbool ), SpiceDouble step, SpiceCell * cnfine, SpiceCell * result ) ## AbstractPerform a GF search on a user defined boolean quantity. ## Required_ReadingGF WINDOWS ## KeywordsEVENT GEOMETRY SEARCH WINDOW ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- udfuns I Name of the routine that computes a scalar quantity of interest corresponding to an `et'. udfunb I Name of the routine returning the boolean value corresponding to an `et'. step I Step size used for locating extrema and roots. cnfine I-O SPICE window to which the search is restricted. result O SPICE window containing results. ## Detailed_Inputudfuns the name of the external routine that returns the value of the scalar quantity of interest at time `et'. The calling sequence for "udfuns" is: udfuns ( et, &value ) where: et a double precision value representing ephemeris time, expressed as seconds past J2000 TDB at which to evaluate "udfuns." value is the value of the geometric quantity at `et'. udfunb the user defined routine returning a boolean value for an epoch ET. The calling sequence for "udfunb" is: udfunb ( udfuns, et, xbool ) where: udfuns the name of the scalar function as defined above. et a double precision value representing ephemeris time, expressed as seconds past J2000 TDB, at which to evaluate "udfunb." xbool the boolean value at `et'. ## 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 is a SPICE window representing the set of time intervals, within the confinement period, when the specified geometric event occurs. If `result' is non-empty on input, its contents will be discarded before gfuds_c conducts its search. ## 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 is declared in the header file SpiceGF.h. ## 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 required ephemerides or other kernel data are not available, an error is signaled by a routine in the call tree of this routine. 5) If the output SPICE window `result' has insufficient capacity to contain the number of intervals on which the specified geometric 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(INVALIDDIMENSION) will signal. 6) If either input cell has type other than SpiceDouble, the error SPICE(TYPEMISMATCH) will signaled from a routine in the call tree of this routine. ## FilesAppropriate kernels must be loaded by the calling program before this routine is called. If the user defined function requires access to ephemeris data: - SPK data: ephemeris data for any body over the time period defined by the confinement window must be loaded. 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 via furnsh_c. - If non-inertial reference frames are used, then PCK files, frame kernels, C-kernels, and SCLK kernels may be needed. In all cases, 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 within the confinement window when the boolean function evaluates to true. 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. udfuns Default Template ======================= The boolean function includes an argument for an input scalar function. Use of a scalar function during the evaluation of the boolean function is not required. SPICE provides a no-op scalar routine, udf_c, as a dummy argument for instances when the boolean function does not need to call the scalar function. The Search Process ================== The search for boolean events is treated as a search for state transitions: times are sought when the boolean function value changes from true to false 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 boolean function will be sampled. Starting at the left endpoint of the interval, samples of the boolean function 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 boolean function is constant: the step size should be shorter than the shortest such interval and the shortest separation between the intervals, within the confinement window. Having some knowledge of the relative geometry of the targets 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 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 ) Call gfstol_c prior to calling this routine. All subsequent searches will use the updated reference 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 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. The confinement window also can be used to restrict a search to a time window over which required data are known to be available. In some cases, the confinement window can 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. See the "CASCADE" example program in gf.req for a demonstration. ## 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. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: standard.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. \begindata KERNELS_TO_LOAD = ( 'de418.bsp', 'pck00008.tpc', 'naif0009.tls' ) \begintext Example(1): Calculate the time intervals when the position of the moon relative to the earth in the IAU_EARTH frame has a positive value in for the Z position component, with also a positive value for the Vz velocity component. #include <stdio.h> #include <stdlib.h> #include <string.h> #include "SpiceUsr.h" #include "SpiceZfc.h" #include "SpiceZad.h" #define MAXWIN 20000 #define TIMFMT "YYYY-MON-DD HR:MN:SC.###" #define TIMLEN 41 #define NLOOPS 7 void gfq ( void ( * udfunc ) ( SpiceDouble et, SpiceDouble * value ), SpiceDouble et, SpiceBoolean * xbool ); int main( int argc, char **argv ) { /. Create the needed windows. Note, one interval 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 left; SpiceDouble right; SpiceDouble step; SpiceDouble ltime; SpiceDouble state [6]; SpiceChar begstr [ TIMLEN ]; SpiceChar endstr [ TIMLEN ]; SpiceInt count; SpiceInt i; printf( "Compile date %s, %s\n\n", __DATE__, __TIME__ ); /. Load kernels. ./ furnsh_c( "standard.tm" ); /. Store the time bounds of our search interval in the 'cnfine' confinement window. ./ str2et_c ( "Jan 1 2011", &begtim ); str2et_c ( "Jan 1 2012", &endtim ); wninsd_c ( begtim, endtim, &cnfine ); /. The moon orbit about the earth-moon barycenter is twenty-eight days. The event condition occurs during (very) approximately a quarter of the orbit. Use a step of five days. ./ step = 5.0 * spd_c(); ## Restrictions1) Any kernel files required by this routine must be loaded before this routine is called. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.1, 28-JUN-2016 (EDW) Edit to Example code, SpiceInts output as ints using explicit casting. -CSPICE Version 1.0.0, 23-OCT-2013 (EDW) (NJB) ## Index_EntriesGF user defined boolean function search |

Wed Apr 5 17:54:36 2017