gfsep_c |

Table of contents## Proceduregfsep_c (GF, angular separation search) void gfsep_c ( ConstSpiceChar * targ1, ConstSpiceChar * shape1, ConstSpiceChar * frame1, ConstSpiceChar * targ2, ConstSpiceChar * shape2, ConstSpiceChar * frame2, ConstSpiceChar * abcorr, ConstSpiceChar * obsrvr, ConstSpiceChar * relate, SpiceDouble refval, SpiceDouble adjust, SpiceDouble step, SpiceInt nintvls, SpiceCell * cnfine, SpiceCell * result ) ## AbstractDetermine time intervals when the angular separation between the position vectors of two target bodies relative to an observer satisfies a numerical relationship. ## Required_ReadingGF NAIF_IDS SPK TIME WINDOWS ## KeywordsEVENT GEOMETRY SEARCH SEPARATION ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- SPICE_GF_CNVTOL P Convergence tolerance. targ1 I Name of first body. shape1 I Name of shape model describing the first body. frame1 I The body-fixed reference frame of the first body. targ2 I Name of second body. shape2 I Name of the shape model describing the second body. frame2 I The body-fixed reference frame of the second body. abcorr I Aberration correction flag. obsrvr I Name of the observing body. relate I Operator that either looks for an extreme value (max, min, local, absolute) or compares the angular separation value and `refval'. refval I Reference value. adjust I Absolute extremum adjustment value. step I Step size in seconds for finding angular separation events. nintvls I Workspace window interval count. cnfine I-O SPICE window to which the search is restricted. result O SPICE window containing results. ## Detailed_Inputtarg1 is the string naming the first body of interest. You can also 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. shape1 is the string naming the geometric model used to represent the shape of the `targ1' body. Models supported by this routine: "SPHERE" Treat the body as a sphere with radius equal to the maximum value of BODYnnn_RADII. "POINT" Treat the body as a point; radius has value zero. The `shape1' string lacks sensitivity to case, leading and trailing blanks. frame1 is the string naming the body-fixed reference frame corresponding to `targ1'. ## 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 SPICE window of intervals, contained within the confinement window `cnfine', on which the specified constraint is satisfied. `result' must be declared and initialized with sufficient size to capture the full set of time intervals within the search region on which the specified condition 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. One technique to handle such a situation, slightly contract `result' using the window routine wncond_c. 3) If the number of intervals `nintvls' is less than 1, the error SPICE(VALUEOUTOFRANGE) is signaled. 4) If result window, `result', is not at least 2 and an even value, the error SPICE(INVALIDDIMENSION) is signaled by a routine in the call tree of this routine. 5) If `result' has insufficient capacity to contain the number of intervals on which the specified distance condition is met, an error is signaled by a routine in the call tree of this routine. 6) If an error (typically cell overflow) occurs during window arithmetic, the error is signaled by a routine in the call tree of this routine. 7) If the relational operator `relate' is not recognized, an error is signaled by a routine in the call tree of this routine. 8) If the aberration correction specifier contains an unrecognized value, an error is signaled by a routine in the call tree of this routine. 9) If `adjust' is negative, an error is signaled by a routine in the call tree of this routine. 10) If either of the input body names, `targ1', `targ2' do not map to NAIF ID codes, an error is signaled by a routine in the call tree of this routine. 11) If either of the input body shape names, `shape1', `shape2', are not recognized by the GF subsystem, an error is signaled by a routine in the call tree of this routine. 12) If either of the input body frame names, `frame1', `frame2', are not recognized by the frame subsystem, an error is signaled by a routine in the call tree of this routine. 13) If either of the input body frames, `frame1', `frame2', are not centered on the corresponding body (`frame1' on `targ1', `frame2' on `targ2'), an error is signaled by a routine in the call tree of this routine. 14) If required ephemerides or other kernel data are not available, an error is signaled by a routine in the call tree of this routine. 15) If any of the `targ1', `shape1', `frame1', `targ2', `shape2', `frame2', `abcorr', `obsrvr' or `relate' input string pointers is null, the error SPICE(NULLPOINTER) is signaled. 16) If any of the `targ1', `shape1', `frame1', `targ2', `shape2', `frame2', `abcorr', `obsrvr' or `relate' input strings has zero length, the error SPICE(EMPTYSTRING) is signaled. 17) If any the `cnfine' or `result' cell arguments has a type other than SpiceDouble, the error SPICE(TYPEMISMATCH) is signaled. 18) If memory cannot be allocated to create the temporary variable required for the execution of the underlying Fortran routine, the error SPICE(MALLOCFAILED) is 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_c. - 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_c. - If non-inertial reference frames are used, then PCK files, frame kernels, C-kernels, and SCLK kernels may be needed. - In some cases the observer's state may be computed at times outside of `cnfine' by as much as 2 seconds; data required to compute this state must be provided by loaded kernels. See -Particulars for details. 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 angular separation 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 for which the angular separation between the two bodies satisfies some defined relationship. 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 angular separation 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 angular separation 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 angular separation (angular separation rate) 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 angular separation rate 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 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" are both times when local extrema are attained and times when the distance function is equal to a reference value. 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 default convergence tolerance used by this routine is set by the parameter SPICE_GF_CNVTOL (defined in SpiceGF.h). 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 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. 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. Certain types of searches require the state of the observer, relative to the solar system barycenter, to be computed at times slightly outside the confinement window `cnfine'. The time window that is actually used is the result of "expanding" `cnfine' by a specified amount "T": each time interval of `cnfine' is expanded by shifting the interval's left endpoint to the left and the right endpoint to the right by T seconds. Any overlapping intervals are merged. (The input argument `cnfine' is not modified.) The window expansions listed below are additive: if both conditions apply, the window expansion amount is the sum of the individual amounts. - If a search uses an equality constraint, the time window over which the state of the observer is computed is expanded by 1 second at both ends of all of the time intervals comprising the window over which the search is conducted. - If a search uses stellar aberration corrections, the time window over which the state of the observer is computed is expanded as described above. When light time corrections are used, expansion of the search window also affects the set of times at which the light time- corrected state of the target is computed. In addition to the possible 2 second expansion of the search window that occurs when both an equality constraint and stellar aberration corrections are used, round-off error should be taken into account when the need for data availability is analyzed. Negative Angular Separation =========================== For those searches using a SPHERE shape identifier for both target bodies, the angular separation function returns a negative value when the bodies overlap (occult), e.g. a search for an ABSMIN of angular separation in a confinement window covering an occultation event will return the time when the apparent center of the occulting body passes closest to the apparent center of the occulted body. Elongation =========================== The angular separation of two targets as seen from an observer where one of those targets is the sun is known as elongation. ## 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) Determine the times of local maxima of the angular separation between the moon and earth as observed from the sun from January 1, 2007 UTC to July 1, 2007 UTC. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: gfsep_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 --------- -------- de421.bsp Planetary ephemeris pck00009.tpc Planet orientation and radii naif0009.tls Leapseconds \begindata KERNELS_TO_LOAD = ( 'de421.bsp', 'pck00009.tpc', 'naif0009.tls' ) \begintext End of meta-kernel Example code begins here. /. Program gfsep_ex1 ./ #include <stdio.h> #include "SpiceUsr.h" int main( ) { #define BODYLN 21 #define MAXWIN 1000 #define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND" #define TIMLEN 41 /. Create the needed windows. Note, one window consists of two values, so the total number of cell values to allocate equals twice the number of windows. ./ 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 targ1 [ BODYLN ]; SpiceChar targ2 [ BODYLN ]; SpiceChar obsrvr [ BODYLN ]; SpiceChar * frame1 = "NULL"; SpiceChar * shape1 = "SPHERE"; SpiceChar * frame2 = "NULL"; SpiceChar * shape2 = "SPHERE"; SpiceChar * abcorr = "NONE"; SpiceChar * relate = "LOCMAX"; SpiceInt count; SpiceInt i; /. Load kernels. ./ furnsh_c( "gfsep_ex1.tm" ); /. Store the time bounds of our search interval in the cnfine confinement window. ./ str2et_c( "2007 JAN 01", &begtim ); str2et_c( "2007 JUL 01", &endtim ); wninsd_c ( begtim, endtim, &cnfine ); /. Prompt for the inputs. ./ prompt_c ( "First body > ", BODYLN, targ1 ); prompt_c ( "Second body > ", BODYLN, targ2 ); prompt_c ( "Observing body > ", BODYLN, obsrvr ); /. Search using a step size of 6 days (in units of seconds). ./ step = 6.*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 angular separation quantity utility package. Callers may need to re-initialize the package after calling this routine. 3) Due to the current logic implemented in zzgfspu, a direct search for zero angular separation of two point targets will always fails, i.e., 'relate' has value "=" 'refval' has value 0. Use 'relate' values of "ABSMIN" or "LOCMIN" to detect such an event(s). ## Literature_ReferencesNone. ## Author_and_InstitutionN.J. Bachman (JPL) J. Diaz del Rio (ODC Space) E.D. Wright (JPL) ## Version-CSPICE Version 1.1.0, 01-NOV-2021 (JDR) (EDW) Added use of ALLOC_CHECK_INTRA to check net null effect on alloc count. Updated header to describe use of expanded confinement window. Edited the header to comply with NAIF standard. Reduced the search interval to limit the length of the solutions and modified the example code to prompt for the required inputs. Updated the description of "nintvls", "cnfine" and "result" arguments. Added entries #4, #5 and #17 in -Exceptions section. -CSPICE Version 1.0.2, 30-JUL-2014 (EDW) Edit to argument I/O 'frame1' and 'frame2' to mention use of "NULL." Edit to header, correct Required Reading entry eliminating ".REQ" suffix. -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. Edited argument descriptions. Removed mention of "ELLIPSOID" shape from 'shape1' and 'shape2' as that option is not yet implemented. Typo corrected in -Version: entry 1.0.1 updated: replaced "gfrr_c" with " ## Index_EntriesGF angular separation search |

Fri Dec 31 18:41:07 2021