Index Page gfudb_c

A B C D E F G H I J K L M N O P Q R S T U V W X

Procedure Abstract Required_Reading Keywords Brief_I/O Detailed_Input | Detailed_Output Parameters Exceptions Files Particulars Examples | Restrictions Literature_References Author_and_Institution Version Index_Entries

#### Procedure

       void 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 )

#### Abstract

       Perform a GF search on a user defined boolean quantity.

#### Required_Reading

       GF
       WINDOWS

#### Keywords

       EVENT
       GEOMETRY
       SEARCH
       WINDOW

#### Brief_I/O

       VARIABLE  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_Input

       udfuns     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'.

                  gfudb_c will correctly operate only for boolean
                  functions with true conditions defining non zero
                  measure time intervals.

                  Note, "udfunb" need not call "udfuns." The use of "udfuns"
                  is determined by the needs of the calculation and
                  the user's design.

       step       the double precision time step size to use in
                  the search.

                  `step' must be shorter than any interval, within the
                  confinement window, over which the user defined boolean
                  function is met. In other words, `step' must be shorter
                  than the shortest time interval for which the boolean
                  function is true; `step' must also be shorter than the
                  shortest time interval between two boolean function true events
                  occurring within the confinement window (see below).
                  However, `step' must not be *too* short, or the search
                  will take an unreasonable amount of time.

                  The choice of `step' affects the completeness but not
                  the precision of solutions found by this routine; the
                  precision is controlled by the convergence tolerance.
                  See the discussion of the parameter SPICE_GF_CNVTOL for
                  details.

                  `step' has units of TDB seconds.

       cnfine     a double precision SPICE window that confines the time
                  period over which the specified search is conducted.
                  cnfine may consist of a single interval or a collection
                  of intervals.

                  In some cases the confinement window can be used to
                  greatly reduce the time period that must be searched
                  for the desired solution. See the Particulars section
                  below for further discussion.

                  See the Examples section below for a code example
                  that shows how to create a confinement window.

#### Detailed_Output

       cnfine     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.

#### Parameters

       SPICE_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.

#### Exceptions

       1)  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.

#### Files

       Appropriate 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.

#### Particulars

       This 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.

#### Examples

       The 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();

             gfudb_c ( udf_c,
                       gfq,
                       step,
                       &cnfine,
                       &result );

             count = wncard_c( &result );

             /.
             Display the results.
             ./
             if (count == 0 )
                {
                printf ( "Result window is empty.\n\n" );
                }
             else
                {
                for ( i = 0;  i < count;  i++ )
                   {

                   /.
                   Fetch the endpoints of the Ith interval
                   of the result window.
                   ./
                   wnfetd_c ( &result, i, &left, &right );

                   printf ( "Interval %d\n", (int)i );

                   timout_c ( left, TIMFMT, TIMLEN, begstr );
                   printf   ( "   Interval start: %s \n", begstr );
                   spkez_c  ( 301, left, "IAU_EARTH", "NONE", 399, state, &ltime);
                   printf   ( "                Z= %.12g \n", state[2] );
                   printf   ( "               Vz= %.12g \n", state[5] );

                   timout_c ( right, TIMFMT, TIMLEN, endstr );
                   printf   ( "   Interval end  : %s \n", endstr );
                   spkez_c  ( 301, right, "IAU_EARTH", "NONE", 399, state, &ltime);
                   printf   ( "                Z= %.12g \n",   state[2] );
                   printf   ( "               Vz= %.12g \n\n", state[5] );
                   }

                }

             kclear_c();
             return( 0 );
             }

          /.
          The user defined functions required by gfudb_c.

             udf_c   for udfuns
             gfq     for udfunb
          ./

          /.
          -Procedure Procedure gfq
          ./

          void gfq ( void ( * udfuns ) ( SpiceDouble    et,
                                         SpiceDouble  * value ),
                     SpiceDouble et,
                     SpiceBoolean * xbool )

          /.
          -Abstract

            User defined geometric boolean function:

               Z >= 0 with dZ/dt > 0.

          ./
             {

             /.
             Initialization. Retrieve the vector from the earth to
             the moon in the IAU_EARTH frame, without aberration
             correction.
             ./
             SpiceInt             targ   = 301;
             SpiceInt             obs    = 399;

             SpiceChar          * ref    = "IAU_EARTH";
             SpiceChar          * abcorr = "NONE";

             SpiceDouble          state [6];
             SpiceDouble          lt;

             /.
             Evaluate the state of TARG from OBS at ET with
             correction ABCORR.
             ./
             spkez_c ( targ, et, ref, abcorr, obs, state, &lt );

             /.
             Calculate the boolean value.
             ./

             *xbool = (state[2] >= 0.0) && (state[5] > 0.0);

             return;
             }

       The program outputs:

          Interval 0
             Interval start: 2011-JAN-09 15:24:23.415
                          Z= -3.67969050785e-08
                         Vz= 0.396984084929
             Interval end  : 2011-JAN-16 16:08:28.563
                          Z= 156247.488202
                         Vz= 3.76859567858e-13

          Interval 1
             Interval start: 2011-FEB-05 23:17:57.359
                          Z= -3.98442807636e-08
                         Vz= 0.396781283223
             Interval end  : 2011-FEB-13 01:38:28.425
                          Z= 157016.055162
                         Vz= 3.2238816651e-13

          Interval 2
             Interval start: 2011-MAR-05 06:08:17.667
                          Z= -1.16190221888e-08
                         Vz= 0.393990253999
             Interval end  : 2011-MAR-12 10:27:45.188
                          Z= 157503.773934
                         Vz= -3.41879302646e-13

                            ...

          Interval 11
             Interval start: 2011-NOV-05 18:43:39.742
                          Z= -1.80199890565e-08
                         Vz= 0.373937629543
             Interval end  : 2011-NOV-13 03:50:17.153
                          Z= 153172.086618
                         Vz= -3.62962481251e-13

          Interval 12
             Interval start: 2011-DEC-03 01:16:40.817
                          Z= 1.30391470066e-07
                         Vz= 0.374257845032
             Interval end  : 2011-DEC-10 09:51:07.718
                          Z= 152511.720377
                         Vz= 2.11386680729e-13

          Interval 13
             Interval start: 2011-DEC-30 09:48:57.409
                          Z= 9.79434844339e-09
                         Vz= 0.377333201453
             Interval end  : 2012-JAN-01 00:00:00.000
                          Z= 50793.0833127
                         Vz= 0.354549969268

       Example(2):

       Calculate the time intervals when the Z component of the earth
       to moon position vector in the IAU_EARTH frame has value
       between -1000 km and 1000 km (e.g. above and below the equatorial
       plane).

          Code:

          #include <stdio.h>
          #include <stdlib.h>
          #include <string.h>

          #include "SpiceUsr.h"

          #define       MAXWIN    20000
          #define       TIMFMT    "YYYY-MON-DD HR:MN:SC.###"
          #define       TIMLEN    41
          #define       NLOOPS    7

          void gfq ( SpiceDouble    et,
                     SpiceDouble  * value );

          void gfb ( void ( * udfuns ) ( 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 duration of the event is approximately ninety minutes.
             Use a step of one hour.
             ./

             step = 60.*60.;

             gfudb_c ( gfq,
                       gfb,
                       step,
                       &cnfine,
                       &result );

             count = wncard_c( &result );

             /.
             Display the results.
             ./
             if (count == 0 )
                {
                printf ( "Result window is empty.\n\n" );
                }
             else
                {
                for ( i = 0;  i < count;  i++ )
                   {

                   /.
                   Fetch the endpoints of the Ith interval
                   of the result window.
                   ./
                   wnfetd_c ( &result, i, &left, &right );

                   printf ( "Interval %d\n", (int)i );

                   timout_c ( left, TIMFMT, TIMLEN, begstr );
                   printf   ( "   Interval start: %s \n", begstr );
                   spkez_c  ( 301, left, "IAU_EARTH", "NONE", 399, state, &ltime);
                   printf   ( "                Z= %.12g \n", state[2] );

                   timout_c ( right, TIMFMT, TIMLEN, endstr );
                   printf   ( "   Interval end  : %s \n", endstr );
                   spkez_c  ( 301, right, "IAU_EARTH", "NONE", 399, state, &ltime);
                   printf   ( "                Z= %.12g \n",   state[2] );
                   }

                }

             kclear_c();
             return( 0 );
             }

          /.
          The user defined functions required by gfudb_c.

             gfq     for udfuns
             gfb     for udfunb
          ./

          /.
          -Procedure Procedure gfq
          ./

          void gfq ( SpiceDouble    et,
                     SpiceDouble  * value )

          /.
          -Abstract

            User defined scalar function:

          ./
             {

             /.
             Initialization. Retrieve the vector from the earth to
             the moon in the IAU_EARTH frame, without aberration
             correction.
             ./
             SpiceInt             targ   = 301;
             SpiceInt             obs    = 399;

             SpiceChar          * ref    = "IAU_EARTH";
             SpiceChar          * abcorr = "NONE";

             SpiceDouble          pos [3];
             SpiceDouble          lt;

             /.
             Evaluate the state of TARG from OBS at ET with
             correction ABCORR.
             ./
             spkezp_c ( targ, et, ref, abcorr, obs, pos, &lt );

             *value = pos[2];

             return;
             }

          /.
          -Procedure Procedure gfb
          ./

          void gfb ( void ( * udfuns ) ( SpiceDouble    et,
                                         SpiceDouble  * value ),
                     SpiceDouble    et,
                     SpiceBoolean * xbool )

          /.
          -Abstract

            User defined boolean function:

          ./
             {
             SpiceDouble          value;
             SpiceDouble          lim1   = -1000.;
             SpiceDouble          lim2   = 1000.;

             udfuns( et, &value );

             /.
             Calculate the boolean value.
             ./

             *xbool = (value >= lim1) && (value <= lim2);

             return;
             }

       The program outputs:

          Interval 0
             Interval start: 2011-JAN-09 14:42:24.484
                          Z= -999.999999903
             Interval end  : 2011-JAN-09 16:06:22.502
                          Z= 1000.00000009
          Interval 1
             Interval start: 2011-JAN-23 04:07:44.455
                          Z= 1000.00000012
             Interval end  : 2011-JAN-23 05:23:06.243
                          Z= -1000.00000011
          Interval 2
             Interval start: 2011-FEB-05 22:35:57.156
                          Z= -999.999999975
             Interval end  : 2011-FEB-05 23:59:57.748
                          Z= 999.999999891

                            ...

          Interval 24
             Interval start: 2011-DEC-03 00:32:08.820
                          Z= -999.99999988
             Interval end  : 2011-DEC-03 02:01:12.769
                          Z= 999.999999876
          Interval 25
             Interval start: 2011-DEC-17 10:17:24.039
                          Z= 1000.00000008
             Interval end  : 2011-DEC-17 11:40:37.223
                          Z= -999.999999975
          Interval 26
             Interval start: 2011-DEC-30 09:04:47.275
                          Z= -1000.00000005
             Interval end  : 2011-DEC-30 10:33:07.670
                          Z= 999.999999868

       Recall the default convergence tolerance for the GF system has
       value 10^-6 seconds.

#### Restrictions

       1) Any kernel files required by this routine must be loaded
          before this routine is called.

#### Literature_References

       None.

#### Author_and_Institution

       N.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_Entries

       GF user defined boolean function search

#### Link to routine gfudb_c source file gfudb_c.c

    Wed Apr  5 17:54:36 2017