 
Geometric Event Finding Hands-On Lesson (C)
===========================================================================
 
   May 12, 2009
 
 
Overview
--------------------------------------------------------
 
   This lesson illustrates how the Geometry Finder (GF) subsystem of the
   CSPICE Toolkit can be used to find time intervals when specified
   geometric conditions are satisfied.
 
   In this lesson the student is asked to construct a program that finds
   the time intervals, within a specified time range, when the Mars Express
   Orbiter (MEX) is visible from the DSN station DSS-14. Possible
   occultation of the spacecraft by Mars is to be considered.
 
 
Note About HTML Links
--------------------------------------------------------
 
   The HTML version of this lesson contains links pointing to various HTML
   documents provided with the Toolkit. All of these links are relative
   and, in order to function, require this document to be in a certain
   location in the Toolkit HTML documentation directory tree.
 
   In order for the links to be resolved, create a subdirectory called
   ``lessons'' under the ``doc/html'' directory of the ``cspice/'' tree and
   copy this document to that subdirectory before loading it into a Web
   browser.
 
 
References
--------------------------------------------------------
 
 
Tutorials
 
   The following SPICE tutorials serve as references for the discussions in
   this lesson:
 
 
      Name             Lesson steps/functions it describes
      ---------------  -----------------------------------------
      Time             Time Conversion
      SCLK and LSK     Time Conversion
      SPK              Obtaining Ephemeris Data
      Frames           Reference Frames
      Using Frames     Reference Frames
      PCK              Planetary Constants Data
      Lunar-Earth PCK  Lunar and Earth Orientation Data
      GF               The SPICE Geometry Ginder (GF) subsystem
 
   These tutorials are available from the NAIF ftp server at JPL:
 
      http://naif.jpl.nasa.gov/naif/tutorials.html
 
 
Required Readings
 
   The Required Reading documents are provided with the Toolkit and are
   located under the ``cspice/doc'' directory in the C installation tree.
 
      Name             Lesson steps/functions that it describes
      ---------------  -----------------------------------------
      cells.req        Cell/window initialization
      frames.req       Using reference frames
      gf.req           The SPICE geometry finder (GF) subsystem
      kernel.req       Loading SPICE kernels
      naif_ids.req     Body and reference frame names
      pck.req          Obtaining planetary constants data
      spk.req          Computing positions and velocities
      time.req         UTC to ET time conversion
      windows.req      The SPICE window data type
 
 
The Permuted Index
 
   Another useful document distributed with the Toolkit is the permuted
   index. This is located under the ``cspice/doc'' directory in the C
   installation tree.
 
   This text document provides a simple mechanism by which users can
   discover which CSPICE routines perform functions of interest, as well as
   the names of the source files that contain these routines.
 
 
Source Code Header Comments
 
   The most detailed specification of a given SPICE C routine is contained
   in the header section of its source code. The source code is distributed
   with the Toolkit and is located under the ``cspice/src/cspice'' path.
 
   For example path of the source code of the str2et_c routine is
 
      cspice/src/cspice/str2et_c.c
 
 
Kernels Used
--------------------------------------------------------
 
   The following kernels are used in examples provided in this lesson:
 
      File Name                       Type  Description
      -----------------------         ----  --------------------------
      de405xs.bsp                     SPK   Planetary ephemeris SPK,
                                            subsetted to cover only
                                            time range of interest
      earthstns_itrf93_050714.bsp     SPK   DSN station SPK
      earth_topo_050714.tf            FK    DSN station frame definitions
      earth_000101_060525_060303.bpc  PCK   Binary PCK for Earth
      ORMM__040501000000_00076XS.BSP  SPK   MEX Orbiter trajectory SPK,
                                            subsetted to cover only
                                            time range of interest
      naif0008.tls                    LSK   Generic LSK
      pck00008.tpc                    PCK   Generic PCK
 
   These SPICE kernels are included in the lesson package available from
   the NAIF server at JPL:
 
      ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Lessons/
 
 
CSPICE Routines Used
--------------------------------------------------------
 
   This section provides a summary of the functions that are suggested for
   usage in each of the exercises in this tutorial. (You may wish to not
   look at this list unless/until you ``get stuck'' while working on your
   own.)
 
      Name        Function that it performs
      ----------  ---------------------------------------------------
      furnsh_c    Loads kernels, individually or listed in meta-kernel
      gfoclt_c    Solve for times of occultation or transit
      gfposc_c    Solve for times when a position vector coordinate
                  constraint is met
      str2et_c    Converts a time string to ET seconds past J2000
      timout_c    Format a time string for output
      wncard_c    Return cardinality of a SPICE window
      wndifd_c    Find the difference of two d.p. windows
      wnfetd_c    Fetch a specified interval from a d.p. window
      wninsd_c    Insert an interval into a d.p. window
 
   Refer to the headers of the various functions listed above, as detailed
   interface specifications are provided with the source code.
 
 
Find View Periods
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that finds the set of time intervals, within the time
   range
 
      2004 MAY 2 TDB
      2004 MAY 6 TDB
 
   when the Mars Express Orbiter (MEX) is visible from the DSN station
   DSS-14. These time intervals are frequently called ``view periods.''
 
   The spacecraft is considered visible if its apparent position (that is,
   its position corrected for light time and stellar aberration) has
   elevation of at least 6 degrees in the topocentric reference frame
   DSS-14_TOPO. In this exercise, we ignore the possibility of occultation
   of the spacecraft by Mars.
 
   Use a search step size that ensures that no view periods of duration 5
   minutes or longer will be missed by the search.
 
   Display the start and stop times of these intervals using TDB calendar
   dates and millisecond precision.
 
 
Learning Goals
--------------------------------------------------------
 
   Exposure to SPICE GF event finding routines. Familiarity with SPICE
   windows and routines that manipulate them. Exposure to SPICE time
   parsing and output formatting routines.
 
 
Approach
--------------------------------------------------------
 
 
Solution steps
 
   A possible solution could consist of the following steps:
 
   Preparation:
 
       1.   Review the CSPICE cell and window Required Reading.
 
       2.   Decide what SPICE kernels are necessary. Use the SPICE summary
            tool BRIEF to examine the coverage of the binary kernels and
            verify the availability of required data.
 
       3.   Create a meta-kernel listing the SPICE kernels to be loaded.
            (Hint: consult a programming example tutorial, or the
            Introduction to Kernels tutorial, for a reminder of how to do
            this.)
 
            Name the meta-kernel 'viewpr.tm'.
 
   Next, write a program that performs the following steps:
 
       1.   Use furnsh_c to load the meta-kernel.
 
       2.   Declare a CSPICE window ```cnfine''' to hold the time period
            within which the search is to be conducted, and a CSPICE window
            `riswin' ("rise/set window") to hold the view periods found by
            the search.
 
       3.   Insert the given time bounds into the confinement window using
            wninsd_c.
 
       4.   Select a step size for searching for visibility state
            transitions: in this case, each target rise or set event is a
            state transition.
 
            The step size must be large enough so the search proceeds with
            reasonable speed, but small enough so that no visibility
            transition events---that is, target rise or set events---are
            missed.
 
       5.   Use the GF routine gfposc_c to find the window of times, within
            the confinement window `cnfine', during which the MEX
            spacecraft is above the elevation limit as seen from DSN
            station DSS-14, in the the reference frame DSS-14_TOPO.
 
            Use light time and stellar aberration corrections for the
            apparent position of the spacecraft as seen from the station.
 
       6.   Fetch and display the contents of the result window. Use
            wnfetd_c to extract from the result window the start and stop
            times of each time interval. Display each of the intervals in
            the result window as a pair of start and stop times. Express
            each time as a TDB calendar date using the routine timout_c.
 
   You may find it useful to consult the references listed above. In
   particular, the header of the SPICE GF function gfposc_c contains
   pertinent documentation.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'viewpr.tm'. Its contents follow:
 
 
      KPL/MK
 
         Example meta-kernel for geometric event finding hands-on
         coding lesson.
 
            Version 2.0.0 15-MAR-2008 (NJB)
 
 
         Identify names of kernels to load:
 
      \begindata
 
         KERNELS_TO_LOAD = (
 
                 'kernels/spk/de405xs.bsp'
                 'kernels/spk/earthstns_itrf93_050714.bsp'
                 'kernels/fk/earth_topo_050714.tf'
                 'kernels/pck/earth_000101_060525_060303.bpc'
                 'kernels/lsk/naif0008.tls'
                 'kernels/spk/ORMM__040501000000_00076XS.BSP'
                 'kernels/pck/pck00008.tpc'
                           )
 
      \begintext
 
 
 
Solution Code
 
   The example program below shows one possible solution.
 
 
      #include <string.h>
      #include <stdio.h>
      #include "SpiceUsr.h"
 
         /*
         PROGRAM VIEWPR
 
         Find and display the window of times when the MEX
         spacecraft is above a specified elevation limit in the
         topocentric reference frame of DSN station DSS-14.
         */
 
      int main()
      {
         /*
         Local constants
         */
 
         /*
         The  meta-kernel:
         */
         #define METAKR          "viewpr.tm"
 
         /*
         Maximum number of intervals in any window:
         */
         #define MAXIVL          1000
 
         /*
         Maximum result window size:
         */
         #define MAXWIN          ( 2 * MAXIVL )
 
         /*
         Format string for time output:
         */
         #define TDBFMT          "YYYY MON DD HR:MN:SC.### (TDB) ::TDB"
 
         /*
         String lengths and other bounds:
         */
         #define LNSIZE          201
         #define TIMLEN          51
 
         /*
         Local variables
         */
 
         /*
         Confinement window used to store interval to be searched:
         */
         SPICEDOUBLE_CELL        ( cnfine, MAXWIN );
 
         /*
         Window to hold sets of times when
         target is above the elevation limit:
         */
         SPICEDOUBLE_CELL        ( riswin, MAXWIN );
 
         SpiceChar             * abcorr;
         SpiceChar             * coord;
         SpiceChar             * crdsys;
         SpiceChar             * obsfrm;
         SpiceChar             * relate;
         SpiceChar             * srfpt;
         SpiceChar             * start;
         SpiceChar             * stop;
         SpiceChar             * target;
         SpiceChar               timstr  [ TIMLEN ];
 
         SpiceDouble             adjust;
         SpiceDouble             elvlim;
         SpiceDouble             etbeg;
         SpiceDouble             etend;
         SpiceDouble             intbeg;
         SpiceDouble             intend;
         SpiceDouble             revlim;
 
         /*
         stepsz is the step size, measured in seconds, used to search
         for times bracketing a state transition.
         */
         SpiceDouble             stepsz;
 
         SpiceInt                i;
         SpiceInt                winsiz;
 
 
         /*
         Load the meta-kernel.
         */
         furnsh_c ( METAKR );
 
         /*
         Assign the inputs for our search.
 
         Since we're interested in the apparent location of the
         target, we use light time and stellar aberration
         corrections. We use the "converged Newtonian" form
         of the light time correction because this choice may
         increase the accuracy of the occultation times we'll
         compute using gfoclt_c.
         */
         srfpt  = "DSS-14";
         obsfrm = "DSS-14_TOPO";
         target = "MEX";
         abcorr = "CN+S";
         start  = "2004 MAY 2 TDB";
         stop   = "2004 MAY 6 TDB";
         elvlim =  6.0;
 
         /*
         The elevation limit above has units of degrees; we convert
         this value to radians for computation using SPICE routines.
         We'll store the equivalent value in radians in REVLIM.
         */
         revlim = rpd_c() * elvlim;
 
         /*
         Since SPICE doesn't directly support the AZ/EL coordinate
         system, we use the equivalent constraint
 
            latitude > REVLIM
 
         in the latitudinal coordinate system, where the reference
         frame is topocentric and is centered at the viewing location.
         */
         crdsys = "LATITUDINAL";
         coord  = "LATITUDE";
         relate = ">";
 
         /*
         The adjustment value only applies to absolute extrema
         searches; simply give it an initial value of zero
         for this inequality search.
         */
         adjust = 0.0;
 
         /*
         STEPSZ is the step size, measured in seconds, used to search
         for times bracketing a state transition. Since we don't expect
         any events of interest to be shorter than five minutes, and
         since the separation between events is well over 5 minutes,
         we'll use this value as our step size. Units are seconds.
         */
         stepsz = 300.0;
 
         /*
         Display a banner for the output report:
         */
         printf ( "\n%s\n\n",
                  "Inputs for target visibility search:" );
 
         printf ( "   Target                       = %s\n", target );
         printf ( "   Observation surface location = %s\n", srfpt  );
         printf ( "   Observer's reference frame   = %s\n", obsfrm );
         printf ( "   Elevation limit (degrees)    = %f\n", elvlim );
         printf ( "   Aberration correction        = %s\n", abcorr );
         printf ( "   Step size (seconds)          = %f\n", stepsz );
 
         /*
         Convert the start and stop times to ET.
         */
         str2et_c ( start, &etbeg );
         str2et_c ( stop,  &etend );
 
         /*
         Display the search interval start and stop times
         using the format shown below.
 
            2004 MAY 06 20:15:00.000 (TDB)
 
         */
         timout_c ( etbeg, TDBFMT, TIMLEN, timstr );
         printf ( "   Start time                   = %s\n", timstr  );
 
         timout_c ( etend, TDBFMT, TIMLEN, timstr );
         printf ( "   Stop time                    = %s\n", timstr  );
 
         printf ( "\n" );
 
         /*
         Initialize the "confinement" window with the interval
         over which we'll conduct the search.
         */
         wninsd_c ( etbeg, etend, &cnfine );
 
         /*
         In the call below, the maximum number of window
         intervals gfposc_c can store internally is set to MAXIVL.
 
         Now search for the time period, within our confinement
         window, during which the apparent target has elevation
         at least equal to the elevation limit.
         */
         gfposc_c ( target, obsfrm, abcorr, srfpt,
                    crdsys, coord,  relate, revlim,
                    adjust, stepsz, MAXIVL, &cnfine, &riswin );
 
         /*
         The function wncard_c returns the number of intervals
         in a SPICE window.
         */
         winsiz = wncard_c( &riswin );
 
         if ( winsiz == 0 )
         {
            printf ( "No events were found.\n" );
         }
         else
         {
            /*
            Display the visibility time periods.
            */
            printf ( "Visibility times of %s as seen from %s:"
                     "\n\n",
                     target, srfpt                               );
 
            for (  i = 0;   i < winsiz;   i++  )
            {
               /*
               Fetch the start and stop times of the Ith interval
               from the search result window viswin.
               */
               wnfetd_c ( &riswin, i, &intbeg, &intend );
 
               /*
               Convert the rise time to a TDB calendar string.
               */
               timout_c ( intbeg, TDBFMT, TIMLEN, timstr );
 
               /*
               Write the string to standard output.
               */
               if ( i == 0 )
               {
                  printf ( "Visibility or window start time:  %s\n",
                            timstr                                  );
               }
               else
               {
                  printf ( "Visibility start time:            %s\n",
                            timstr                                  );
               }
 
               /*
               Convert the set time to a TDB calendar string.
               */
               timout_c ( intend, TDBFMT, TIMLEN, timstr );
 
               /*
               Write the string to standard output.
               */
               if ( i == winsiz-1 )
               {
                  printf ( "Visibility or window stop time:   %s\n",
                            timstr                                  );
               }
               else
               {
                  printf ( "Visibility stop time:             %s\n",
                            timstr                                  );
               }
 
               printf ( "\n" );
            }
         }
 
         return ( 0 );
      }
 
 
Solution Sample Output
 
   Numerical results shown for this example may differ across platforms
   since the results depend on the SPICE kernels used as input and on the
   host platform's arithmetic implementation.
 
   After compiling the program, execute it. The output is:
 
 
 
      Inputs for target visibility search:
 
         Target                       = MEX
         Observation surface location = DSS-14
         Observer's reference frame   = DSS-14_TOPO
         Elevation limit (degrees)    = 6.000000
         Aberration correction        = CN+S
         Step size (seconds)          = 300.000000
         Start time                   = 2004 MAY 02 00:00:00.000 (TDB)
         Stop time                    = 2004 MAY 06 00:00:00.000 (TDB)
 
      Visibility times of MEX as seen from DSS-14:
 
      Visibility or window start time:  2004 MAY 02 00:00:00.000 (TDB)
      Visibility stop time:             2004 MAY 02 05:35:03.096 (TDB)
 
      Visibility start time:            2004 MAY 02 16:09:14.078 (TDB)
      Visibility stop time:             2004 MAY 03 05:33:57.257 (TDB)
 
      Visibility start time:            2004 MAY 03 16:08:02.279 (TDB)
      Visibility stop time:             2004 MAY 04 05:32:50.765 (TDB)
 
      Visibility start time:            2004 MAY 04 16:06:51.259 (TDB)
      Visibility stop time:             2004 MAY 05 05:31:43.600 (TDB)
 
      Visibility start time:            2004 MAY 05 16:05:40.994 (TDB)
      Visibility or window stop time:   2004 MAY 06 00:00:00.000 (TDB)
 
 
 
Find Times when Target is Visible
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Extend the program of the previous chapter to find times when the MEX
   orbiter is:
 
       --   Above the elevation limit in the DSS-14_TOPO topocentric
            reference frame.
 
       --   and is not occulted by Mars
 
   Store the set of time intervals when the spacecraft is visible in a
   CSPICE window. We'll call this the ``result window.''
 
   Display each of the intervals in the result window as a pair of start
   and stop times. Express each time as a TDB calendar date using the same
   format as in the previous program.
 
 
Learning Goals
--------------------------------------------------------
 
   Familiarity with the GF occultation finding routine gfoclt_c. Further
   experience with the CSPICE window functions.
 
 
Approach
--------------------------------------------------------
 
 
Solution steps
 
   A possible solution would consist of the following steps:
 
       1.   Use the meta-kernel of the previous lesson.
 
       2.   Include the code from the program of the previous chapter in a
            new source file; modify this code to create the new program.
 
       3.   Declare a CSPICE window `occwin' to hold the results of the
            occultation search. Also declare a second CSPICE window
            `viswin' to hold the final result.
 
       4.   Search for occultations of the MEX orbiter as seen from DSS-14
            using gfoclt_c. Use as the confinement window for this search
            the result window from the elevation search performed by
            gfposc_c.
 
            Since occultations occur when the apparent MEX spacecraft
            position is behind the apparent figure of Mars, light time
            correction must be performed for the occultation search. To
            improve accuracy of the occultation state determination, use
            ``converged Newtonian'' light time correction.
 
       5.   Use the CSPICE window subtraction routine wndifd_c to subtract
            the window of times when the spacecraft is occulted from the
            window of times when the spacecraft is above the elevation
            limit. The difference window `viswin' is the final result.
 
       6.   Modify the code to display the contents of the window `viswin'.
 
   This completes the assignment.
 
 
Solution
--------------------------------------------------------
 
 
Solution Code
 
 
      #include <string.h>
      #include <stdio.h>
      #include "SpiceUsr.h"
 
         /*
         PROGRAM VISIBL
 
         Find and display the window of times when the MEX
         spacecraft is above a specified elevation limit in the
         topocentric reference frame of DSN station DSS-14
         and is not occulted by Mars.
         */
 
      int main()
      {
         /*
         Local constants
         */
 
         /*
         The  meta-kernel:
         */
         #define METAKR          "viewpr.tm"
 
         /*
         Maximum number of intervals in any window:
         */
         #define MAXIVL          1000
 
         /*
         Maximum result window size:
         */
         #define MAXWIN          ( 2 * MAXIVL )
 
         /*
         Format string for time output:
         */
         #define TDBFMT          "YYYY MON DD HR:MN:SC.### (TDB) ::TDB"
 
         /*
         String lengths and other bounds:
         */
         #define TIMLEN          51
 
         /*
         Local variables
         */
 
         /*
         Confinement window used to store interval to be searched:
         */
         SPICEDOUBLE_CELL        ( cnfine, MAXWIN );
 
         /*
         Windows to hold sets of times when
 
            - target is occulted
            - target is above the elevation limit
            - target is visible
         */
         SPICEDOUBLE_CELL        ( occwin, MAXWIN );
         SPICEDOUBLE_CELL        ( riswin, MAXWIN );
         SPICEDOUBLE_CELL        ( viswin, MAXWIN );
 
         SpiceChar             * abcorr;
         SpiceChar             * back;
         SpiceChar             * bframe;
         SpiceChar             * bshape;
         SpiceChar             * coord;
         SpiceChar             * crdsys;
         SpiceChar             * fframe;
         SpiceChar             * front;
         SpiceChar             * fshape;
         SpiceChar             * obsfrm;
         SpiceChar             * occtyp;
         SpiceChar             * relate;
         SpiceChar             * srfpt;
         SpiceChar             * start;
         SpiceChar             * stop;
         SpiceChar             * target;
         SpiceChar               timstr  [ TIMLEN ];
 
         SpiceDouble             adjust;
         SpiceDouble             elvlim;
         SpiceDouble             etbeg;
         SpiceDouble             etend;
         SpiceDouble             intbeg;
         SpiceDouble             intend;
         SpiceDouble             revlim;
 
         /*
         stepsz is the step size, measured in seconds, used to search
         for times bracketing a state transition.
         */
         SpiceDouble             stepsz;
 
         SpiceInt                i;
         SpiceInt                winsiz;
 
 
         /*
         Load the meta-kernel.
         */
         furnsh_c ( METAKR );
 
         /*
         Assign the inputs for our search.
 
         Since we're interested in the apparent location of the
         target, we use light time and stellar aberration
         corrections. We use the "converged Newtonian" form
         of the light time correction because this choice may
         increase the accuracy of the occultation times we'll
         compute using gfoclt_c.
         */
         srfpt  = "DSS-14";
         obsfrm = "DSS-14_TOPO";
         target = "MEX";
         abcorr = "CN+S";
         start  = "2004 MAY 2 TDB";
         stop   = "2004 MAY 6 TDB";
         elvlim =  6.0;
 
         /*
         The elevation limit above has units of degrees; we convert
         this value to radians for computation using SPICE routines.
         We'll store the equivalent value in radians in REVLIM.
         */
         revlim = rpd_c() * elvlim;
 
         /*
         We model the target shape as a point and the blocking body's
         shape as an ellipsoid. No body-fixed reference frame is
         required for the target since its orientation is not used.
         */
         back   = target;
         bshape = "POINT";
         bframe = " ";
         front  = "MARS";
         fshape = "ELLIPSOID";
         fframe = "IAU_MARS";
 
         /*
         The occultation type should be set to "ANY" for a point
         target.
         */
         occtyp = "any";
 
         /*
         Since SPICE doesn't directly support the AZ/EL coordinate
         system, we use the equivalent constraint
 
            latitude > REVLIM
 
         in the latitudinal coordinate system, where the reference
         frame is topocentric and is centered at the viewing location.
         */
         crdsys = "LATITUDINAL";
         coord  = "LATITUDE";
         relate = ">";
 
         /*
         The adjustment value only applies to absolute extrema
         searches; simply give it an initial value of zero
         for this inequality search.
         */
         adjust = 0.0;
 
         /*
         STEPSZ is the step size, measured in seconds, used to search
         for times bracketing a state transition. Since we don't expect
         any events of interest to be shorter than five minutes, and
         since the separation between events is well over 5 minutes,
         we'll use this value as our step size. Units are seconds.
         */
         stepsz = 300.0;
 
         /*
         Display a banner for the output report:
         */
         printf ( "\n%s\n\n",
                  "Inputs for target visibility search:" );
 
         printf ( "   Target                       = %s\n", target );
         printf ( "   Observation surface location = %s\n", srfpt  );
         printf ( "   Observer's reference frame   = %s\n", obsfrm );
         printf ( "   Blocking body                = %s\n", front  );
         printf ( "   Blocker's reference frame    = %s\n", fframe );
         printf ( "   Elevation limit (degrees)    = %f\n", elvlim );
         printf ( "   Aberration correction        = %s\n", abcorr );
         printf ( "   Step size (seconds)          = %f\n", stepsz );
 
         /*
         Convert the start and stop times to ET.
         */
         str2et_c ( start, &etbeg );
         str2et_c ( stop,  &etend );
 
         /*
         Display the search interval start and stop times
         using the format shown below.
 
            2004 MAY 06 20:15:00.000 (TDB)
         */
         timout_c ( etbeg, TDBFMT, TIMLEN, timstr );
         printf ( "   Start time                   = %s\n", timstr  );
 
         timout_c ( etend, TDBFMT, TIMLEN, timstr );
         printf ( "   Stop time                    = %s\n", timstr  );
 
         printf ( "\n" );
 
         /*
         Initialize the "confinement" window with the interval
         over which we'll conduct the search.
         */
         wninsd_c ( etbeg, etend, &cnfine );
 
         /*
         In the call below, the maximum number of window
         intervals gfposc_c can store internally is set to MAXIVL.
 
         Now search for the time period, within our confinement
         window, during which the apparent target has elevation
         at least equal to the elevation limit.
         */
         gfposc_c ( target, obsfrm, abcorr, srfpt,
                    crdsys, coord,  relate, revlim,
                    adjust, stepsz, MAXIVL, &cnfine, &riswin );
 
         /*
         Now find the times when the apparent target is above
         the elevation limit and is not occulted by the
         blocking body (Mars). We'll find the window of times when
         the target is above the elevation limit and *is* occulted,
         then subtract that window from the view period window
         riswin found above.
 
         For this occultation search, we can use riswin as
         the confinement window because we're not interested in
         occultations that occur when the target is below the
         elevation limit.
 
         Find occultations within the view period window.
         */
         gfoclt_c ( occtyp, front,  fshape,  fframe,
                    back,   bshape, bframe,  abcorr,
                    srfpt,  stepsz, &riswin, &occwin );
 
         /*
         Subtract the occultation window from the view period
         window: this yields the time periods when the target
         is visible.
         */
         wndifd_c ( &riswin, &occwin, &viswin );
 
         /*
         The function wncard_c returns the number of intervals
         in a SPICE window.
         */
         winsiz = wncard_c( &viswin );
 
         if ( winsiz == 0 )
         {
            printf ( "No events were found.\n" );
         }
         else
         {
            /*
            Display the visibility time periods.
            */
            printf ( "Visibility times of %s as seen from %s:"
                     "\n\n",
                     target, srfpt                               );
 
            for (  i = 0;   i < winsiz;   i++  )
            {
               /*
               Fetch the start and stop times of the Ith interval
               from the search result window viswin.
               */
               wnfetd_c ( &viswin, i, &intbeg, &intend );
 
               /*
               Convert the rise time to a TDB calendar string.
               */
               timout_c ( intbeg, TDBFMT, TIMLEN, timstr );
 
               /*
               Write the string to standard output.
               */
               if ( i == 0 )
               {
                  printf ( "Visibility or window start time:  %s\n",
                            timstr                                  );
               }
               else
               {
                  printf ( "Visibility start time:            %s\n",
                            timstr                                  );
               }
 
               /*
               Convert the set time to a TDB calendar string.
               */
               timout_c ( intend, TDBFMT, TIMLEN, timstr );
 
               /*
               Write the string to standard output.
               */
               if ( i == winsiz-1 )
               {
                  printf ( "Visibility or window stop time:   %s\n",
                            timstr                                  );
               }
               else
               {
                  printf ( "Visibility stop time:             %s\n",
                            timstr                                  );
               }
 
               printf ( "\n" );
            }
         }
 
         return ( 0 );
      }
 
 
Solution Sample Output
 
   Numerical results shown for this example may differ across platforms
   since the results depend on the SPICE kernels used as input and on the
   host platform's arithmetic implementation.
 
   After compiling the program, execute it. The output is:
 
 
      Inputs for target visibility search:
 
         Target                       = MEX
         Observation surface location = DSS-14
         Observer's reference frame   = DSS-14_TOPO
         Blocking body                = MARS
         Blocker's reference frame    = IAU_MARS
         Elevation limit (degrees)    = 6.000000
         Aberration correction        = CN+S
         Step size (seconds)          = 300.000000
         Start time                   = 2004 MAY 02 00:00:00.000 (TDB)
         Stop time                    = 2004 MAY 06 00:00:00.000 (TDB)
 
      Visibility times of MEX as seen from DSS-14:
 
      Visibility or window start time:  2004 MAY 02 00:00:00.000 (TDB)
      Visibility stop time:             2004 MAY 02 04:49:30.827 (TDB)
 
      Visibility start time:            2004 MAY 02 16:09:14.078 (TDB)
      Visibility stop time:             2004 MAY 02 20:00:22.514 (TDB)
 
      Visibility start time:            2004 MAY 02 21:01:38.222 (TDB)
      Visibility stop time:             2004 MAY 03 03:35:42.256 (TDB)
 
      Visibility start time:            2004 MAY 03 04:36:42.484 (TDB)
      Visibility stop time:             2004 MAY 03 05:33:57.257 (TDB)
 
      Visibility start time:            2004 MAY 03 16:08:02.279 (TDB)
      Visibility stop time:             2004 MAY 03 18:46:26.013 (TDB)
 
      Visibility start time:            2004 MAY 03 19:46:54.618 (TDB)
      Visibility stop time:             2004 MAY 04 02:21:44.562 (TDB)
 
      Visibility start time:            2004 MAY 04 03:21:56.347 (TDB)
      Visibility stop time:             2004 MAY 04 05:32:50.765 (TDB)
 
      Visibility start time:            2004 MAY 04 16:06:51.259 (TDB)
      Visibility stop time:             2004 MAY 04 17:32:25.809 (TDB)
 
      Visibility start time:            2004 MAY 04 18:32:05.975 (TDB)
      Visibility stop time:             2004 MAY 05 01:07:48.264 (TDB)
 
      Visibility start time:            2004 MAY 05 02:07:11.601 (TDB)
      Visibility stop time:             2004 MAY 05 05:31:43.600 (TDB)
 
      Visibility start time:            2004 MAY 05 16:05:40.994 (TDB)
      Visibility stop time:             2004 MAY 05 16:18:35.560 (TDB)
 
      Visibility start time:            2004 MAY 05 17:17:27.717 (TDB)
      Visibility or window stop time:   2004 MAY 05 23:54:04.672 (TDB)
 
 
