gfdist_c |

## Procedurevoid gfdist_c ( ConstSpiceChar * target, ConstSpiceChar * abcorr, ConstSpiceChar * obsrvr, ConstSpiceChar * relate, SpiceDouble refval, SpiceDouble adjust, SpiceDouble step, SpiceInt nintvls, SpiceCell * cnfine, SpiceCell * result ) ## AbstractReturn the time window over which a specified constraint on observer-target distance is met. ## Required_ReadingGF NAIF_IDS SPK TIME WINDOWS ## KeywordsEPHEMERIS EVENT GEOMETRY SEARCH WINDOW ## Brief_I/OVariable I/O Description --------------- --- ------------------------------------------------ SPICE_GF_CNVTOL P Convergence tolerance target I Name of the target body. abcorr I Aberration correction flag. obsrvr I Name of the observing body. relate I Relational operator. 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 confined. result O SPICE window containing results. ## Detailed_Inputtarget is the name of a target body. Optionally, you may supply a string containing the integer ID code for the object. For example both "MOON" and "301" are legitimate strings that indicate the Moon is the target body. The target and observer define a position vector which points from the observer to the target; the length of this vector is the "distance" that serves as the subject of the search performed by this routine. Case and leading or trailing blanks are not significant in the string `target'. abcorr indicates the aberration corrections to be applied to the observer-target position vector to account for one-way light time and stellar aberration. Any aberration correction accepted by the SPICE routine spkezr_c is accepted here. See the header of spkezr_c for a detailed description of the aberration correction options. For convenience, the options are listed below: "NONE" Apply no correction. "LT" "Reception" case: correct for one-way light time using a Newtonian formulation. "LT+S" "Reception" case: correct for one-way light time and stellar aberration using a Newtonian formulation. "CN" "Reception" case: converged Newtonian light time correction. "CN+S" "Reception" case: converged Newtonian light time and stellar aberration corrections. "XLT" "Transmission" case: correct for one-way light time using a Newtonian formulation. "XLT+S" "Transmission" case: correct for one-way light time and stellar aberration using a Newtonian formulation. "XCN" "Transmission" case: converged Newtonian light time correction. "XCN+S" "Transmission" case: converged Newtonian light time and stellar aberration corrections. Case and leading or trailing blanks are not significant in the string `abcorr'. obsrvr is the name of the observing body. Optionally, you may supply a string containing the integer ID code for the object. For example both "MOON" and "301" are legitimate strings that indicate the Moon is the observer. Case and leading or trailing blanks are not significant in the string `obsrvr'. relate is a relational operator used to define a constraint on the observer-target distance. The result window found by this routine indicates the time intervals where the constraint is satisfied. Supported values of `relate' and corresponding meanings are shown below: ">" Distance is greater than the reference value `refval'. "=" Distance is equal to the reference value `refval'. "<" Distance is less than the reference value `refval'. "ABSMAX" Distance is at an absolute maximum. "ABSMIN" Distance is at an absolute minimum. "LOCMAX" Distance is at a local maximum. "LOCMIN" Distance is at a local minimum. `relate' may be used to specify an "adjusted" absolute extremum constraint: this requires the distance to be within a specified offset relative to an absolute extremum. The argument `adjust' (described below) is used to specify this offset. Local extrema are considered to exist only in the interiors of the intervals comprising the confinement window: a local extremum cannot exist at a boundary point of the confinement window. Case and leading or trailing blanks are not significant in the string `relate'. `refval' is the reference value used together with the argument `relate' to define an equality or inequality to be satisfied by the distance between the specified target and observer. See the discussion of `relate' above for further information. The units of `refval' are km. adjust is a parameter used to modify searches for absolute extrema: when `relate' is set to "ABSMAX" or "ABSMIN" and `adjust' is set to a positive value, ## 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 the window of intervals, contained within the confinement window `cnfine', on which the specified distance constraint is satisfied. The endpoints of the time intervals comprising `result' are interpreted as seconds past J2000 TDB. 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 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 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 the workspace interval count is less than 1, the error SPICE(VALUEOUTOFRANGE) will be signaled. 10) If the required amount of workspace memory cannot be allocated, the error SPICE(MALLOCFAILURE) will be signaled. 11) If the output SPICE window `result' has insufficient capacity to contain the number of intervals on which the specified distance 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 be signaled by this routine. 12) If any input string argument pointer is null, the error SPICE(NULLPOINTER) will be signaled. 13) If any input string argument is empty, the error SPICE(EMPTYSTRING) will be signaled. 14) If either input cell has type other than SpiceDouble, the error SPICE(TYPEMISMATCH) is signaled. ## 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 target and observer for 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 by the SPK files, then PCK files, frame kernels, C-kernels, and SCLK kernels may be needed. 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 distance between the specified target and observer 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 distance 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 distance 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. With these solutions in hand, solutions of inequalities are easily found as well. Step Size ========= The monotone windows (described above) are found via a two-step search process. Each interval of the confinement window is searched as follows: first, the input step size is the time separation at which the sign of the rate of change of distance ("range rate") is sampled. Starting at the left endpoint of the 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 range rate is zero can be 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 distance 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 distance 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 ) Call gfstol_c prior to calling this routine. All subsequent searches will use the updated tolerance value. To use a different tolerance value, a lower-level GF routine such as gfevnt_c must be called. Making 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. 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. 1) Find times during the first three months of the year 2007 when the Earth-Moon distance is greater than 400000 km. Display the start and stop times of the time intervals over which this constraint is met, along with the Earth-Moon distance at each interval endpoint. We expect the Earth-Moon distance to be an oscillatory function with extrema roughly two weeks apart. Using a step size of one day will guarantee that the GF system will find all distance extrema. (Recall that a search for distance extrema is an intermediate step in the GF search process.) 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. The names and contents of the kernels referenced by this meta-kernel are as follows: File name Contents --------- -------- de421.bsp Planetary ephemeris pck00008.tpc Planet orientation and radii naif0009.tls Leapseconds \begindata KERNELS_TO_LOAD = ( 'de421.bsp', 'pck00008.tpc', 'naif0009.tls' ) \begintext End of meta-kernel Example code begins here. #include <stdio.h> #include "SpiceUsr.h" int main() { /. Constants ./ #define TIMFMT "YYYY MON DD HR:MN:SC.###" #define MAXWIN 200 #define NINTVL 100 #define TIMLEN 41 /. Local variables ./ SpiceChar begstr [ TIMLEN ]; SpiceChar endstr [ TIMLEN ]; SPICEDOUBLE_CELL ( cnfine, MAXWIN ); SPICEDOUBLE_CELL ( result, MAXWIN ); SpiceDouble adjust; SpiceDouble dist; SpiceDouble et0; SpiceDouble et1; SpiceDouble lt; SpiceDouble pos [3]; SpiceDouble refval; SpiceDouble start; SpiceDouble step; SpiceDouble stop; SpiceInt i; /. Load kernels. ./ furnsh_c ( "standard.tm" ); /. Store the time bounds of our search interval in the confinement window. ./ str2et_c ( "2007 JAN 1", &et0 ); str2et_c ( "2007 APR 1", &et1 ); wninsd_c ( et0, et1, &cnfine ); /. Search using a step size of 1 day (in units of seconds). The reference value is 400000 km. We're not using the adjustment feature, so we set `adjust' to zero. ./ step = spd_c(); refval = 4.e5; adjust = 0.0; /. Perform the search. The set of times when the constraint is met will be stored in the SPICE window `result'. ./ ## 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 distance quantity utility package. ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) E.D. Wright (JPL) ## Version-CSPICE Version 1.0.1, 28-FEB-2013 (NJB) (EDW) Header was updated to discuss use of gfstol_c. A header typo was corrected. -CSPICE Version 1.0.0, 15-APR-2009 (NJB) (EDW) ## Index_EntriesGF distance search |

Wed Apr 5 17:54:35 2017