Main Page Geometric Event Finding Hands-On Lesson, using MPO (IDL)
## Table of Contents
       Geometric Event Finding Hands-On Lesson, using MPO (IDL)
          Overview
          Note About HTML Links
          References
             Tutorials
             Required Readings
             The Permuted Index
             Icy API Documentation
          Kernels Used
          Icy Modules Used

       Find View Periods
          Task Statement
          Learning Goals
          Approach
             Solution steps
          Solution
             Solution Meta-Kernel
             Solution Code
             Solution Sample Output

       Find Times when Target is Visible
          Task Statement
          Learning Goals
          Approach
             Solution steps
          Solution
             Solution Meta-Kernel
             Solution Code
             Solution Sample Output

       Extra Credit
          Task statements
          Solutions
Top # Geometric Event Finding Hands-On Lesson, using MPO (IDL) March 01, 2023 Top ## Overview This lesson illustrates how the Geometry Finder (GF) subsystem of the Icy 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 BepiColombo Mercury Planetary Orbiter (MPO) is visible from ESA's deep space station in New Norcia. Possible occultation of the spacecraft by Mercury is to be considered. Top ## 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, if not done already by installing the lessons package under the Toolkit's ``doc/html'' directory, create a subdirectory called ``lessons'' under the ``doc/html'' directory of the ``icy/'' tree and copy this document to that subdirectory before loading it into a Web browser. Top ## References This section lists SPICE documents referred to in this lesson. Of these documents, the ``Tutorials'' contains the highest level descriptions with the least number of details while the ``Required Reading'' documents contain much more detailed specifications. The most complete specifications are provided in the ``API Documentation''. In some cases the lesson explanations also refer to the information provided in the meta-data area of the kernels used in the lesson examples. It is especially true in case of the FK and IK files, which often contain comprehensive descriptions of the frames, instrument FOVs, etc. Since both FK and IK are text kernels, the information provided in them can be viewed using any text editor, while the meta information provided in binary kernels -- SPKs and CKs -- can be viewed using ``commnt'' or ``spacit'' utility programs located in ``icy/exe'' of Toolkit installation tree. Top ### Tutorials The following SPICE tutorials serve as references for the discussions in this lesson:
       Name              Lesson steps/routines 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 Finder (GF) subsystem
       DSK               Detailed Target Shape (Topography) Data
These tutorials are available from the NAIF server at JPL:
       https://naif.jpl.nasa.gov/naif/tutorials.html
Top ### Required Readings The Required Reading documents are provided with the Toolkit and are located under the ``icy/doc'' directory in the Icy installation tree.
       Name             Lesson steps/routines that it describes
       ---------------  -----------------------------------------
       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
       icy.req          The Icy API
Top ### The Permuted Index Another useful document distributed with the Toolkit is the permuted index. It is located under the ``icy/doc'' directory in the IDL installation tree. This text document provides a simple mechanism by which users can discover which Icy procedures perform functions of interest, as well as the names of the source files that contain these procedures.
Top ### Icy API Documentation An Icy routine's specification is found in the HTML API documentation page located under ``icy/doc/html/icy''. For example, the document
       icy/doc/html/icy/cspice_str2et.html
describes the cspice_str2et routine. Top ## Kernels Used The following kernels are used in examples provided in this lesson:
       1.  Solar System Ephemeris SPK, subsetted to cover only the time
           range of interest:

              de432s.bsp

       2.  ESA stations SPK:

              estrack_v04.bsp

       3.  ESA stations frame definitions:

              estrack_v04.tf

       4.  EARTH_FIXED/ITRF93 frame connection:

              earthfixeditrf93.tf

       5.  Binary PCK for Earth:

              earth_070425_370426_predict.bpc

       6.  BepiColombo MPO Spacecraft Trajectory SPK, subsetted to cover
           only the time range of interest:

              bc_mpo_mlt_50037_20260314_20280529_v05.bsp

       7.  Generic LSK:

              naif0012.tls

       8.  Generic PCK:

              pck00011.tpc

       9.  Low-resolution Mercury DSK:

              mercury_lowres.bds
These SPICE kernels are included in the lesson package. Top ## Icy Modules Used This section provides a complete list of the procedures and kernels that are suggested for usage in each of the exercises in this lesson. (You may wish to not look at this list unless/until you ``get stuck'' while working on your own.)
       CHAPTER EXERCISE   FUNCTIONS      NON-VOID       KERNELS
       ------- ---------  -------------  -------------  ----------
          1    viewpr     cspice_furnsh  cspice_rpd     1-8
                          cspice_str2et  cspice_wncard
                          cspice_timout
                          cspice_wninsd
                          cspice_gfposc
                          cspice_wnfetd
                          cspice_kclear

          2    visibl     cspice_furnsh  cspice_rpd     1-9
                          cspice_str2et  cspice_wncard
                          cspice_timout
                          cspice_wninsd
                          cspice_gfposc
                          cspice_gfoclt
                          cspice_wndifd
                          cspice_wnfetd
                          cspice_unload

               extra (*)  cspice_repmc                  1,6-8
                          cspice_repmf
                          cspice_gfdist

          (*) Additional APIs and kernels used in Extra Credit tasks.
Refer to the Icy HTML API documentation pages located under ``icy/doc/html/icy'' for detailed interface specifications of these procedures. Top # Find View Periods Top ## Task Statement Write a program that finds the set of time intervals, within the time range
       2027 JAN 03 TDB
       2027 JAN 06 TDB
when the BepiColombo Mercury Planetary Orbiter (MPO) is visible from ESA's New Norcia station. 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 NEW_NORCIA_TOPO. In this exercise, we ignore the possibility of occultation of the spacecraft by Mercury. 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. Top ## 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. Top ## Approach Top ### Solution steps A possible solution could consist of the following steps: Preparation: `1.` 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. `2.` 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 cspice_furnsh to load the meta-kernel. `2.` Create confinement and output Icy windows using cspice_celld. `3.` Insert the given time bounds into the confinement window using cspice_wninsd. `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 cspice_gfposc to find the window of times, within the confinement window, during which the BepiColombo MPO spacecraft is above the elevation limit as seen from ESA's New Norcia station, in the reference frame NEW_NORCIA_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 cspice_wnfetd 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 cspice_timout. You may find it useful to consult the references listed above. In particular, the header of the SPICE GF procedure cspice_gfposc contains pertinent documentation. Top ## Solution Top ### Solution Meta-Kernel The meta-kernel we created for the solution to this exercise is named 'viewpr.tm'. Its contents follow:
       KPL/MK

          This is the meta-kernel used in the solution of the tasks in the
          Geometric Event Finding Hands On Lesson.

          The names and contents of the kernels referenced by this
          meta-kernel are as follows:

             1.  Solar System Ephemeris SPK, subsetted to cover only the
                 time range of interest:

                    de432s.bsp

             2.  ESA stations SPK:

                    estrack_v04.bsp

             3.  ESA stations frame definitions:

                    estrack_v04.tf

             4.  EARTH_FIXED/ITRF93 frame connection:

                    earthfixeditrf93.tf

             5.  Binary PCK for Earth:

                    earth_070425_370426_predict.bpc

             6.  BepiColombo MPO Spacecraft Trajectory SPK, subsetted to
                 cover only the time range of interest:

                    bc_mpo_mlt_50037_20260314_20280529_v05.bsp

             7.  Generic LSK:

                    naif0012.tls

             8.  Generic PCK:

                   pck00011.tpc

       \begindata

          KERNELS_TO_LOAD = (

             'kernels/spk/de432s.bsp'
             'kernels/spk/estrack_v04.bsp'
             'kernels/fk/estrack_v04.tf'
             'kernels/fk/earthfixeditrf93.tf'
             'kernels/pck/earth_070425_370426_predict.bpc'
             'kernels/lsk/naif0012.tls'
             'kernels/spk/bc_mpo_mlt_50037_20260314_20280529_v05.bsp'
             'kernels/pck/pck00011.tpc'

                            )

       \begintext
Top ### Solution Code The example program below shows one possible solution.
       PRO viewpr

          ;; Find and display the window of times when the BepiColombo
          ;; MPO spacecraft is above a specified elevation limit in the
          ;; topocentric reference frame of ESA's New Norcia station.

          ;;
          ;; The meta-kernel:
          ;;
          META   = 'viewpr.tm'

          ;;
          ;; Maximum number of intervals in any window:
          ;;
          MAXIVL = 1000

          ;;
          ;; Time string length:
          ;;
          TIMLEN = 50

          ;;
          ;; Format string for time output:
          ;;
          TDBFMT = 'YYYY MON DD HR:MN:SC.### (TDB) ::TDB'

          ;;
          ;; Load the meta-kernel.
          ;;
          cspice_furnsh, META

          ;;
          ;; 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 cspice_gfoclt.
          ;;
          srfpt  = 'NEW_NORCIA'
          obsfrm = 'NEW_NORCIA_TOPO'
          target = 'MPO'
          abcorr = 'CN+S'
          start  = '2027 JAN 03 TDB'
          stop   = '2027 JAN 06 TDB'
          elvlim =  6.0d

          ;;
          ;; 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 = cspice_rpd() * 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 = 0.0d

          ;;
          ;; 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 = 300.0d

          ;;
          ;; Display a banner for the output report:
          ;;

          print, ' '
          print, 'Inputs for target visibility search:'
          print, ' '

          print, '   Target                       = '  + target
          print, '   Observation surface location = '  + srfpt
          print, '   Observer''s reference frame   = ' + obsfrm
          print, FORMAT = '(A,F8.6)', $
                 '   Elevation limit (degrees)    = ',   elvlim
          print, '   Aberration correction        = '  + abcorr
          print, FORMAT = '(A,F10.6)', $
                 '   Step size (seconds)          = ',   stepsz

          ;;
          ;; Convert the start and stop times to ET.
          ;;
          cspice_str2et, start, etbeg
          cspice_str2et, stop,  etend

          ;;
          ;; Display the search interval start time and stop
          ;; times using the format shown below.
          ;;
          ;;     2004 MAY 06 20:15:00.000 (TDB)
          ;;
          cspice_timout, etbeg, TDBFMT, TIMLEN, timstr0
          cspice_timout, etend, TDBFMT, TIMLEN, timstr1

          print, '   Start time                   = ' + timstr0
          print, '   Stop time                    = ' + timstr1
          print, ' '

          ;;
          ;; Create the "confinement" window; store in this window
          ;; the interval over which we'll conduct the search.
          ;;
          cnfine = cspice_celld( 2 )

          cspice_wninsd, etbeg, etend, cnfine

          ;;
          ;; Create an empty search result window that can hold MAXIVL
          ;; intervals.
          ;;
          riswin = cspice_celld( MAXIVL )

          ;;
          ;; In the call below, the maximum number of window
          ;; intervals cspice_gfposc 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.
          ;;
          cspice_gfposc, target, obsfrm, abcorr, srfpt,  $
                         crdsys, coord,  relate, revlim, $
                         adjust, stepsz, MAXIVL, cnfine, riswin

          ;;
          ;; The function cspice_wncard returns the number of intervals
          ;; in a SPICE window.
          ;;
          winsiz = cspice_wncard( riswin )

          ;;
          ;; Display the rise and set times.
          ;;

          if  winsiz EQ 0 then begin

             print, 'No events were found.'

          endif else begin

             ;;
             ;;  Display the visibility time periods.
             ;;

             print, 'Visibility times of ' + TARGET + $
                    ' as seen from ' + SRFPT + ':'
             print, ' '

             for i = 0,  winsiz-1  do begin

                ;;
                ;; Fetch the Ith interval from the window.
                ;;
                cspice_wnfetd, riswin, i, intbeg, intend

                ;;
                ;; Convert the rise time to a TDB calendar string.
                ;;
                cspice_timout, intbeg, TDBFMT, TIMLEN, timstr

                ;;
                ;; Write the string to standard output.
                ;;
                if  i EQ 0  then begin

                   line = 'Visibility or window start time:  '

                endif else begin

                   line = 'Visibility start time:            '

                endelse

                print, line + timstr

                ;;
                ;; Convert the set time to a TDB calendar string.
                ;;
                cspice_timout, intend, TDBFMT, TIMLEN, timstr

                ;;
                ;; Write the string to standard output.
                ;;
                if  i EQ (winsiz-1)  then begin

                   line = 'Visibility or window stop time:   '

                endif else begin

                   line = 'Visibility stop time:             '

                endelse

                print, line + timstr

                print, ' '

             endfor

          endelse

          ;;
          ;; Unload kernels so they are not accidentally used by another
          ;; SPICE-based program during the current IDL session.
          ;;
          cspice_kclear

       END
Top ### 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                       = MPO
          Observation surface location = NEW_NORCIA
          Observer's reference frame   = NEW_NORCIA_TOPO
          Elevation limit (degrees)    = 6.000000
          Aberration correction        = CN+S
          Step size (seconds)          = 300.000000
          Start time                   = 2027 JAN 03 00:00:00.000 (TDB)
          Stop time                    = 2027 JAN 06 00:00:00.000 (TDB)

       Visibility times of MPO as seen from NEW_NORCIA:

       Visibility or window start time:  2027 JAN 03 00:00:00.000 (TDB)
       Visibility stop time:             2027 JAN 03 10:58:25.063 (TDB)

       Visibility start time:            2027 JAN 03 21:55:08.488 (TDB)
       Visibility stop time:             2027 JAN 04 11:01:14.279 (TDB)

       Visibility start time:            2027 JAN 04 21:58:41.333 (TDB)
       Visibility stop time:             2027 JAN 05 11:04:00.020 (TDB)

       Visibility start time:            2027 JAN 05 22:02:18.477 (TDB)
       Visibility or window stop time:   2027 JAN 06 00:00:00.000 (TDB)
Top # Find Times when Target is Visible Top ## Task Statement Extend the program of the previous chapter to find times when the BepiColombo MPO orbiter is: `--` Above the elevation limit in the NEW_NORCIA_TOPO topocentric reference frame. `--` and is not occulted by Mercury Finding time intervals that satisfy the second condition requires a search for occultations of the spacecraft by Mercury. Perform this search twice: once using an ellipsoidal shape model for Mercury, and once using a DSK shape model. Compute the final results twice as well, using the results of both occultation searches. For each of the two shape model cases, store the set of time intervals when the spacecraft is visible in an Icy 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. Top ## Learning Goals Familiarity with the GF occultation finding routine cspice_gfoclt. Experience with Digital Shape Kernel (DSK) shape models. Further experience with the Icy window procedures. Top ## Approach Top ### Solution steps A possible solution would consist of the following steps: `1.` Use the meta-kernel from the previous chapter as the starting point. Add more kernels to it as needed. `` Name the meta-kernel 'visibl.tm'. `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.` Your program will need additional windows to capture the results of occultation searches performed using both ellipsoidal and DSK shape models. Additional windows will be needed to compute the set differences of the elevation search (``view period'') window and each of the occultation search windows. Further details are provided below. `` Create additional output Icy windows using cspice_celld. `4.` The remaining steps can be performed twice: once using an ellipsoidal shape model for Mercury, and once using a DSK Mercury shape model. Alternatively, two copies of the entire solution program can be created: one for each shape model. `5.` Search for occultations of the BepiColombo MPO orbiter as seen from New Norcia station using cspice_gfoclt. Use as the confinement window for this search the result window from the elevation search performed by cspice_gfposc. `` Since occultations occur when the apparent BepiColombo MPO spacecraft position is behind the apparent figure of Mercury, light time correction must be performed for the occultation search. To improve accuracy of the occultation state determination, use ``converged Newtonian'' light time correction. `6.` Use the Icy window subtraction routine cspice_wndifd 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 is the final result. `7.` Modify the code to display the contents of the difference window. This completes the assignment. Top ## Solution Top ### Solution Meta-Kernel The meta-kernel we created for the solution to this exercise is named 'visibl.tm'. Its contents follow:
       KPL/MK

          This is the meta-kernel used in the solution of the tasks in the
          Geometric Event Finding Hands On Lesson.

          The names and contents of the kernels referenced by this
          meta-kernel are as follows:

             1.  Solar System Ephemeris SPK, subsetted to cover only the
                 time range of interest:

                    de432s.bsp

             2.  ESA stations SPK:

                    estrack_v04.bsp

             3.  ESA stations frame definitions:

                    estrack_v04.tf

             4.  EARTH_FIXED/ITRF93 frame connection:

                    earthfixeditrf93.tf

             5.  Binary PCK for Earth:

                    earth_070425_370426_predict.bpc

             6.  BepiColombo MPO Spacecraft Trajectory SPK, subsetted to
                 cover only the time range of interest:

                    bc_mpo_mlt_50037_20260314_20280529_v05.bsp

             7.  Generic LSK:

                    naif0012.tls

             8.  Generic PCK:

                   pck00011.tpc

             9. Low-resolution Mercury DSK:

                   mercury_lowres.bds

       \begindata

          KERNELS_TO_LOAD = (

             'kernels/spk/de432s.bsp'
             'kernels/spk/estrack_v04.bsp'
             'kernels/fk/estrack_v04.tf'
             'kernels/fk/earthfixeditrf93.tf'
             'kernels/pck/earth_070425_370426_predict.bpc'
             'kernels/lsk/naif0012.tls'
             'kernels/spk/bc_mpo_mlt_50037_20260314_20280529_v05.bsp'
             'kernels/pck/pck00011.tpc'
             'kernels/dsk/mercury_lowres.bds'

                            )

       \begintext
Top ### Solution Code
       PRO visibl

          ;; Find and display the window of times when the BepiColombo
          ;; MPO spacecraft is above a specified elevation limit in the
          ;; topocentric reference frame of ESA's New Norcia station
          ;; and is not occulted by Mercury.
          ;;
          ;; The meta-kernel:
          ;;
          METAKR = 'visibl.tm'

          ;;
          ;; Maximum number of intervals in any window:
          ;;
          MAXIVL = 1000

          ;;
          ;; Time string length:
          ;;
          TIMLEN = 50

          ;;
          ;; Format string for time output:
          ;;
          TDBFMT = 'YYYY MON DD HR:MN:SC.### TDB ::TDB'

          ;;
          ;; Load the meta-kernel.
          ;;
          cspice_furnsh, 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 cspice_gfoclt.
          ;;
          srfpt  = 'NEW_NORCIA'
          obsfrm = 'NEW_NORCIA_TOPO'
          target = 'MPO'
          abcorr = 'CN+S'
          start  = '2027 JAN 03 TDB'
          stop   = '2027 JAN 06 TDB'
          elvlim =  6.0d

          ;;
          ;; 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 = cspice_rpd() * elvlim

          ;;
          ;; We model the target shape as a point. We either model the
          ;; blocking body's shape as an ellipsoid, or we represent
          ;; its shape using actual topographic data. No body-fixed
          ;; reference frame is required for the target since its
          ;; orientation is not used.
          ;;
          back   = target
          bshape = 'POINT'
          bframe = ' '
          front  = 'MERCURY'
          fshape = 'ELLIPSOID'
          fframe = 'IAU_MERCURY'

          ;;
          ;; 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 = 0.0d

          ;;
          ;; 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 = 300.0d

          ;;
          ;; Display a banner for the output report:
          ;;

          print, ' '
          print, 'Inputs for target visibility search:'
          print, ' '

          print, '   Target                       = '  + target
          print, '   Observation surface location = '  + srfpt
          print, '   Observer''s reference frame   = ' + obsfrm
          print, '   Blocking body                = '  + front
          print, '   Blocker''s reference frame    = ' + fframe
          print, FORMAT = '(A,F8.6)', $
                 '   Elevation limit (degrees)    = ',   elvlim
          print, '   Aberration correction        = '  + abcorr
          print, FORMAT = '(A,F10.6)', $
                 '   Step size (seconds)          = ',   stepsz

          ;;
          ;; Convert the start and stop times to ET.
          ;;
          cspice_str2et, start, etbeg
          cspice_str2et, stop,  etend

          ;;
          ;; Display the search interval start time and stop
          ;; times using the format shown below.
          ;;
          ;;     2004 MAY 06 20:15:00.000 TDB
          ;;
          cspice_timout, etbeg, TDBFMT, TIMLEN, timstr0
          cspice_timout, etend, TDBFMT, TIMLEN, timstr1

          print, '   Start time                   = ' + timstr0
          print, '   Stop time                    = ' + timstr1
          print, ' '

          ;;
          ;; Create the "confinement" window; store in this window
          ;; the interval over which we'll conduct the search.
          ;;
          cnfine = cspice_celld( 2 )

          cspice_wninsd, etbeg, etend, cnfine

          ;;
          ;; Create an empty search result window that can hold MAXIVL
          ;; intervals.
          ;;
          riswin = cspice_celld( MAXIVL )

          ;;
          ;; In the call below, the maximum number of window
          ;; intervals cspice_gfposc 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.
          ;;
          cspice_gfposc, 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 (Mercury). 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.
          ;;
          print, ' Searching using ellipsoid target shape model...'

          fshape = 'ELLIPSOID'

          eocwin = cspice_celld( MAXIVL )

          cspice_gfoclt, occtyp, front,  fshape,  fframe, $
                         back,   bshape, bframe,  abcorr, $
                         srfpt,  stepsz, riswin,  eocwin

          print, ' Done.'

          ;;
          ;; Subtract the occultation window from the view period
          ;; window: this yields the time periods when the target
          ;; is visible.
          ;;
          evswin = cspice_celld( MAXIVL )

          cspice_wndifd, riswin, eocwin, evswin

          ;;
          ;; Repeat the search using low-resolution DSK data
          ;; for the front body.
          ;;
          print, ' Searching using DSK target shape model...'

          fshape = 'DSK/UNPRIORITIZED'

          docwin = cspice_celld( MAXIVL )

          cspice_gfoclt, occtyp, front,  fshape,  fframe, $
                         back,   bshape, bframe,  abcorr, $
                         srfpt,  stepsz, riswin,  docwin

          print, ' Done.'
          print, ' '

          dvswin = cspice_celld( MAXIVL )

          cspice_wndifd, riswin, docwin, dvswin

          ;;
          ;; The function cspice_wncard returns the number of intervals
          ;; in a SPICE window.
          ;;
          winsiz = cspice_wncard( evswin )

          ;;
          ;; Display the rise and set times.
          ;;

          if  winsiz EQ 0 then begin

             print, 'No events were found.'

          endif else begin

             ;;
             ;;  Display the visibility time periods.
             ;;

             print, 'Visibility start and stop times of ' $
                    + TARGET + ' as seen from ' + SRFPT
             print, 'using both ellipsoidal and DSK ' +   $
                    'target shape models:'
             print, ' '

             for i = 0,  winsiz-1  do begin
                ;;
                ;; Fetch the start and stop times of the Ith
                ;; interval from the ellipsoid search result
                ;; window evswin.
                ;;
                cspice_wnfetd, evswin, i, intbeg, intend

                ;;
                ;; Convert the start and stop times to TDB calendar
                ;; strings. Write the results.
                ;;
                cspice_timout, intbeg, TDBFMT, TIMLEN, btmstr
                cspice_timout, intend, TDBFMT, TIMLEN, etmstr

                print, ' Ell: ' + BTMSTR + ' : ' + ETMSTR

                ;;
                ;; Fetch the start and stop times of the Ith
                ;; interval from the DSK search result
                ;; window dvswin.
                ;;
                cspice_wnfetd, dvswin, i, dintbg, dinten

                ;;
                ;; Convert the start and stop times to TDB calendar
                ;; strings. Write the results.
                ;;
                cspice_timout, dintbg, TDBFMT, TIMLEN, btmstr
                cspice_timout, dinten, TDBFMT, TIMLEN, etmstr

                print, ' DSK: ' + BTMSTR + ' : ' + ETMSTR
                print, ' '

             endfor

          endelse

          ;;
          ;; Unload kernels so they're not accidentally used by another
          ;; SPICE-based program during the current IDL session.
          ;;
          cspice_unload, METAKR

       END
Top ### 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                       = MPO
          Observation surface location = NEW_NORCIA
          Observer's reference frame   = NEW_NORCIA_TOPO
          Blocking body                = MERCURY
          Blocker's reference frame    = IAU_MERCURY
          Elevation limit (degrees)    = 6.000000
          Aberration correction        = CN+S
          Step size (seconds)          = 300.000000
          Start time                   = 2027 JAN 03 00:00:00.000 TDB
          Stop time                    = 2027 JAN 06 00:00:00.000 TDB

        Searching using ellipsoid target shape model...
        Done.
        Searching using DSK target shape model...
        Done.

       Visibility start and stop times of MPO as seen from NEW_NORCIA
       using both ellipsoidal and DSK target shape models:

        Ell: 2027 JAN 03 00:00:00.000 TDB : 2027 JAN 03 01:28:03.419 TDB
        DSK: 2027 JAN 03 00:00:00.000 TDB : 2027 JAN 03 01:28:03.202 TDB

        Ell: 2027 JAN 03 02:00:42.993 TDB : 2027 JAN 03 03:49:50.750 TDB
        DSK: 2027 JAN 03 02:00:43.226 TDB : 2027 JAN 03 03:49:50.589 TDB

        Ell: 2027 JAN 03 04:22:20.803 TDB : 2027 JAN 03 06:11:38.050 TDB
        DSK: 2027 JAN 03 04:22:21.167 TDB : 2027 JAN 03 06:11:37.927 TDB

        Ell: 2027 JAN 03 06:43:58.528 TDB : 2027 JAN 03 08:33:25.499 TDB
        DSK: 2027 JAN 03 06:43:58.803 TDB : 2027 JAN 03 08:33:25.452 TDB

        Ell: 2027 JAN 03 09:05:36.070 TDB : 2027 JAN 03 10:55:12.991 TDB
        DSK: 2027 JAN 03 09:05:36.483 TDB : 2027 JAN 03 10:55:13.005 TDB

        Ell: 2027 JAN 03 21:55:08.488 TDB : 2027 JAN 03 22:44:11.490 TDB
        DSK: 2027 JAN 03 21:55:08.488 TDB : 2027 JAN 03 22:44:11.836 TDB

        Ell: 2027 JAN 03 23:15:19.552 TDB : 2027 JAN 04 01:05:59.339 TDB
        DSK: 2027 JAN 03 23:15:20.564 TDB : 2027 JAN 04 01:05:59.788 TDB

        Ell: 2027 JAN 04 01:36:56.572 TDB : 2027 JAN 04 03:27:47.253 TDB
        DSK: 2027 JAN 04 01:36:56.903 TDB : 2027 JAN 04 03:27:47.794 TDB

        Ell: 2027 JAN 04 03:58:33.411 TDB : 2027 JAN 04 05:49:35.238 TDB
        DSK: 2027 JAN 04 03:58:33.685 TDB : 2027 JAN 04 05:49:35.857 TDB

        Ell: 2027 JAN 04 06:20:10.165 TDB : 2027 JAN 04 08:11:23.310 TDB
        DSK: 2027 JAN 04 06:20:10.819 TDB : 2027 JAN 04 08:11:23.843 TDB

        Ell: 2027 JAN 04 08:41:46.813 TDB : 2027 JAN 04 10:33:11.480 TDB
        DSK: 2027 JAN 04 08:41:47.399 TDB : 2027 JAN 04 10:33:12.291 TDB

        Ell: 2027 JAN 04 21:58:41.333 TDB : 2027 JAN 04 22:22:13.911 TDB
        DSK: 2027 JAN 04 21:58:41.333 TDB : 2027 JAN 04 22:22:13.969 TDB

        Ell: 2027 JAN 04 22:51:24.368 TDB : 2027 JAN 05 00:44:02.576 TDB
        DSK: 2027 JAN 04 22:51:24.088 TDB : 2027 JAN 05 00:44:02.498 TDB

        Ell: 2027 JAN 05 01:13:00.256 TDB : 2027 JAN 05 03:05:51.406 TDB
        DSK: 2027 JAN 05 01:13:00.056 TDB : 2027 JAN 05 03:05:51.377 TDB

        Ell: 2027 JAN 05 03:34:36.025 TDB : 2027 JAN 05 05:27:40.260 TDB
        DSK: 2027 JAN 05 03:34:36.194 TDB : 2027 JAN 05 05:27:40.400 TDB

        Ell: 2027 JAN 05 05:56:11.727 TDB : 2027 JAN 05 07:49:29.298 TDB
        DSK: 2027 JAN 05 05:56:11.995 TDB : 2027 JAN 05 07:49:29.743 TDB

        Ell: 2027 JAN 05 08:17:47.213 TDB : 2027 JAN 05 10:11:18.377 TDB
        DSK: 2027 JAN 05 08:17:47.173 TDB : 2027 JAN 05 10:11:18.893 TDB

        Ell: 2027 JAN 05 10:39:22.574 TDB : 2027 JAN 05 11:04:00.020 TDB
        DSK: 2027 JAN 05 10:39:22.690 TDB : 2027 JAN 05 11:04:00.020 TDB

        Ell: 2027 JAN 05 22:27:17.183 TDB : 2027 JAN 06 00:00:00.000 TDB
        DSK: 2027 JAN 05 22:27:17.436 TDB : 2027 JAN 06 00:00:00.000 TDB
Top # Extra Credit In this ``extra credit'' section you will be presented with more complex tasks, aimed at improving your understanding of the geometry event finding subsystem and particularly the cspice_gfposc and cspice_gfdist procedures. These ``extra credit'' tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions to the questions asked in these tasks. Top ## Task statements `1.` Write a program that finds the times, within the time range
             2027 JAN 03 TDB
             2027 JAN 04 TDB
`` when the BepiColombo Mercury Planetary Orbiter (MPO) crosses Mercury' equator. Display the results using TDB calendar dates and millisecond precision. `2.` Write a program that finds the times, within the time range
             2027 JAN 03 TDB
             2027 JAN 04 TDB
`` when the BepiColombo Mercury Planetary Orbiter (MPO) is at periapsis. Display the results using TDB calendar dates and millisecond precision. `3.` Write a program that finds the times, within the time range
             2027 JAN 03 TDB
             2027 JAN 04 TDB
`` when the BepiColombo Mercury Planetary Orbiter (MPO) is at apoapsis. Display the results using TDB calendar dates and millisecond precision. Top ## Solutions `1.` Solution for the equator crossing search, using cspice_gfposc for the BepiColombo Mercury Planetary Orbiter (MPO) spacecraft latitude in the Mercury body-fixed frame equal to 0 degrees:
       Inputs for equator crossing search:

          Target                       = MPO
          Observer                     = MERCURY
          Observer's reference frame   = IAU_MERCURY
          Latitude limit (degrees)     = 0.000000
          Aberration correction        = NONE
          Step size (seconds)          = 300.000000
          Start time                   = 2027 JAN 03 00:00:00.000 (TDB)
          Stop time                    = 2027 JAN 04 00:00:00.000 (TDB)

       MPO MERCURY equator crossing times:

        Equator crossing or start time:  2027 JAN 03 00:21:02.744 (TDB)
        Equator crossing time:           2027 JAN 03 01:34:39.885 (TDB)
        Equator crossing time:           2027 JAN 03 02:42:45.780 (TDB)
        Equator crossing time:           2027 JAN 03 03:56:22.917 (TDB)
        Equator crossing time:           2027 JAN 03 05:04:28.849 (TDB)
        Equator crossing time:           2027 JAN 03 06:18:06.007 (TDB)
        Equator crossing time:           2027 JAN 03 07:26:11.888 (TDB)
        Equator crossing time:           2027 JAN 03 08:39:49.081 (TDB)
        Equator crossing time:           2027 JAN 03 09:47:54.901 (TDB)
        Equator crossing time:           2027 JAN 03 11:01:32.084 (TDB)
        Equator crossing time:           2027 JAN 03 12:09:37.930 (TDB)
        Equator crossing time:           2027 JAN 03 13:23:15.135 (TDB)
        Equator crossing time:           2027 JAN 03 14:31:20.939 (TDB)
        Equator crossing time:           2027 JAN 03 15:44:58.128 (TDB)
        Equator crossing time:           2027 JAN 03 16:53:03.959 (TDB)
        Equator crossing time:           2027 JAN 03 18:06:41.171 (TDB)
        Equator crossing time:           2027 JAN 03 19:14:46.962 (TDB)
        Equator crossing time:           2027 JAN 03 20:28:24.150 (TDB)
        Equator crossing time:           2027 JAN 03 21:36:29.935 (TDB)
        Equator crossing time:           2027 JAN 03 22:50:07.114 (TDB)
        Equator crossing or stop time:   2027 JAN 03 23:58:12.944 (TDB)
`2.` Solution for the periapsis search, using cspice_gfdist for the BepiColombo Mercury Planetary Orbiter (MPO) spacecraft distance from Mercury at a local minimum:
       Inputs for periapsis search:

          Target                       = MPO
          Observer                     = MERCURY
          Aberration correction        = NONE
          Step size (seconds)          = 300.000000
          Start time                   = 2027 JAN 03 00:00:00.000 (TDB)
          Stop time                    = 2027 JAN 04 00:00:00.000 (TDB)

       MPO periapsis times:

        Periapsis or start time:         2027 JAN 03 00:18:33.937 (TDB)
        Periapsis time:                  2027 JAN 03 02:40:16.998 (TDB)
        Periapsis time:                  2027 JAN 03 05:01:59.964 (TDB)
        Periapsis time:                  2027 JAN 03 07:23:43.026 (TDB)
        Periapsis time:                  2027 JAN 03 09:45:25.991 (TDB)
        Periapsis time:                  2027 JAN 03 12:07:09.042 (TDB)
        Periapsis time:                  2027 JAN 03 14:28:52.095 (TDB)
        Periapsis time:                  2027 JAN 03 16:50:35.082 (TDB)
        Periapsis time:                  2027 JAN 03 19:12:18.042 (TDB)
        Periapsis time:                  2027 JAN 03 21:34:01.097 (TDB)
        Periapsis or end time:           2027 JAN 03 23:55:44.079 (TDB)
`3.` Solution for the apoapsis search, using cspice_gfdist for the BepiColombo Mercury Planetary Orbiter (MPO) spacecraft distance from Mercury at a local maximum:
       Inputs for apoapsis search:

          Target                       = MPO
          Observer                     = MERCURY
          Aberration correction        = NONE
          Step size (seconds)          = 300.000000
          Start time                   = 2027 JAN 03 00:00:00.000 (TDB)
          Stop time                    = 2027 JAN 04 00:00:00.000 (TDB)

       MPO apoapsis times:

        Apoapsis or start time:          2027 JAN 03 01:29:25.529 (TDB)
        Apoapsis time:                   2027 JAN 03 03:51:08.495 (TDB)
        Apoapsis time:                   2027 JAN 03 06:12:51.561 (TDB)
        Apoapsis time:                   2027 JAN 03 08:34:34.611 (TDB)
        Apoapsis time:                   2027 JAN 03 10:56:17.595 (TDB)
        Apoapsis time:                   2027 JAN 03 13:18:00.653 (TDB)
        Apoapsis time:                   2027 JAN 03 15:39:43.611 (TDB)
        Apoapsis time:                   2027 JAN 03 18:01:26.677 (TDB)
        Apoapsis time:                   2027 JAN 03 20:23:09.638 (TDB)
        Apoapsis or stop time:           2027 JAN 03 22:44:52.618 (TDB)