gfposc_c |

Table of contents## Proceduregfposc_c (GF, observer-target vector coordinate search) void gfposc_c ( ConstSpiceChar * target, ConstSpiceChar * frame, ConstSpiceChar * abcorr, ConstSpiceChar * obsrvr, 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 observer-target position vector satisfies a numerical constraint. ## Required_ReadingGF NAIF_IDS SPK TIME WINDOWS ## KeywordsEVENT GEOMETRY SEARCH SEPARATION ## Brief_I/OVARIABLE I/O DESCRIPTION -------- --- -------------------------------------------------- SPICE_GF_CNVTOL P Convergence tolerance. target I Name of the target body. frame I Name of the reference frame for coordinate calculations. abcorr I Aberration correction flag. obsrvr I Name of the observing body. crdsys I Name of the coordinate system containing `coord'. coord I Name of the coordinate of interest. 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 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. The target and observer define a position vector that points from the observer to the target. frame is the name of the reference frame in which to perform state look-ups and coordinate calculations. The SPICE frame subsystem must recognize the `frame' name. abcorr is the description of the aberration corrections to apply to the state evaluations to account for one-way light time and stellar aberration. This routine accepts the same aberration corrections as does the SPICE routine spkezr_c. 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. Returns the "true" geometric state. "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. The `abcorr' string lacks sensitivity to case, leading and trailing blanks. obsrvr is the name of an observing body. Optionally, you may supply the ID code of the object as an integer string. For example, both "EARTH" and "399" are legitimate strings to indicate the observer is the Earth. crdsys is the name of the coordinate system for which the coordinate of interest is a member. coord is the string name of the coordinate of interest in `crdsys'. The supported coordinate systems and coordinate names: crdsys coord Range ---------------- ----------------- ------------ "RECTANGULAR" "X" "Y" "Z" "LATITUDINAL" "RADIUS" "LONGITUDE" (-Pi,Pi] "LATITUDE" [-Pi/2,Pi/2] "RA/DEC" "RANGE" "RIGHT ASCENSION" [0,2Pi) "DECLINATION" [-Pi/2,Pi/2] "SPHERICAL" "RADIUS" "COLATITUDE" [0,Pi] "LONGITUDE" (-Pi,Pi] "CYLINDRICAL" "RADIUS" "LONGITUDE" [0,2Pi) "Z" "GEODETIC" "LONGITUDE" (-Pi,Pi] "LATITUDE" [-Pi/2,Pi/2] "ALTITUDE" "PLANETOGRAPHIC" "LONGITUDE" [0,2Pi) "LATITUDE" [-Pi/2,Pi/2] "ALTITUDE" The "ALTITUDE" coordinates have a constant value of zero +/- roundoff for ellipsoid targets. Limit searches for coordinate events in the "GEODETIC" and "PLANETOGRAPHIC" coordinate systems to `target' bodies with axial symmetry in the equatorial plane, i.e. equality of the body X and Y radii (oblate or prolate spheroids). Searches on "GEODETIC" or "PLANETOGRAPHIC" coordinates requires body shape data, and in the case of "PLANETOGRAPHIC" coordinates, body rotation data. The body associated with "GEODETIC" or "PLANETOGRAPHIC" coordinates is the center of the frame `frame'. relate is the relational operator used to define a constraint on the selected coordinate of the observer-target vector. 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: ">" The coordinate value is greater than the reference value `refval'. "=" The coordinate value is equal to the reference value `refval'. "<" The coordinate value is less than the reference value `refval'. "ABSMAX" The coordinate value is at an absolute maximum. "ABSMIN" The coordinate value is at an absolute minimum. "LOCMAX" The coordinate value is at a local maximum. "LOCMIN" The coordinate value is at a local minimum. `relate' may be used to specify an "adjusted" absolute extremum constraint: this requires the quantity 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. The `relate' string lacks sensitivity to case, leading and trailing blanks. refval is the double precision reference value used together with the argument `relate' to define an equality or inequality to satisfy by the selected coordinate of the observer- target vector. See the discussion of `relate' above for further information. The units of `refval' correspond to the type as defined by `coord', radians for angular measures, kilometers for distance measures. adjust is a double precision value 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 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 the window size of `result' is less than 2, the error SPICE(INVALIDDIMENSION) 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 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 do not map to NAIF ID codes, an error is signaled by a routine in the call tree of this routine. 11) If required ephemerides or other kernel data are not available, an error is signaled by a routine in the call tree of this routine. 12) If the search uses GEODETIC or PLANETOGRAPHIC coordinates, and the center body of the reference frame has unequal equatorial radii, an error is signaled by a routine in the call tree of this routine. 13) If any of the `target', `frame', `abcorr', `obsrvr', `crdsys', `coord' or `relate' input string pointers is null, the error SPICE(NULLPOINTER) is signaled. 14) If any of the `target', `frame', `abcorr', `obsrvr', `crdsys', `coord' or `relate' input strings has zero length, the error SPICE(EMPTYSTRING) is signaled. 15) If any the `cnfine' or `result' cell arguments has a type other than SpiceDouble, the error SPICE(TYPEMISMATCH) is signaled. 16) 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. - 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 observer-target position 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 observer-target position 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" are both times when local extrema are attained and times when the coordinate 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. 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. 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. Longitude and Right Ascension ============================= The cyclic nature of the longitude and right ascension coordinates produces branch cuts at +/- 180 degrees longitude and 0-360 right ascension. 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. 1) Find the time during 2007 for which the latitude of the Earth-Sun vector in IAU_EARTH frame has the maximum value, i.e. the latitude of the Tropic of Cancer. Use the meta-kernel shown below to load the required SPICE kernels. KPL/MK File name: gfposc_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 gfposc_ex1 ./ #include <stdio.h> #include <stdlib.h> #include <string.h> #include "SpiceUsr.h" #define MAXWIN 750 #define TIMFMT "YYYY-MON-DD HR:MN:SC.###### (TDB) ::TDB ::RND" #define TIMLEN 41 int main( ) { /. 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 = "ABSMAX"; SpiceChar * crdsys = "LATITUDINAL"; SpiceChar * coord = "LATITUDE"; SpiceChar * targ = "SUN"; SpiceChar * obsrvr = "EARTH"; SpiceChar * frame = "IAU_EARTH"; SpiceChar * abcorr = "NONE"; SpiceInt count; SpiceInt i; /. Load kernels. ./ furnsh_c( "gfposc_ex1.tm" ); /. 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) J. Diaz del Rio (ODC Space) E.D. Wright (JPL) ## Version-CSPICE Version 1.1.0, 03-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. Added entries #3, #4, #11 and #14 in -Exceptions section. Updated the description of "nintvls", "cnfine" and "result" arguments. -CSPICE Version 1.0.2, 31-JUL-2014 (EDW) Edit to header, replaced ' character with character " to indicate C strings. 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. Edits to Example section, proper description of "standard.tm" meta kernel. -CSPICE Version 1.0.1, 26-AUG-2009 (EDW) Correction of several typos. -CSPICE Version 1.0.0, 10-FEB-2009 (NJB) (EDW) ## Index_EntriesGF position coordinate search |

Fri Dec 31 18:41:07 2021