 
Remote Sensing Hands-On Lesson (IDL)
===========================================================================
 
   March 14, 2006
 
 
Overview
--------------------------------------------------------
 
   In this lesson you will develop a series of simple programs that
   demonstrate the usage of Icy to compute a variety of different geometric
   quantities applicable to experiments carried out by a remote sensing
   instrument flown on an interplanetary spacecraft. This particular lesson
   focuses on a framing camera flying on the Cassini spacecraft, but many
   of the concepts are easily extended and generalized to other scenarios.
 
 
References
--------------------------------------------------------
 
 
Tutorials
 
   The following SPICE tutorials are referred to by 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
      CK               Spacecraft Orientation Data
 
   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 ``icy/doc'' directory in the IDL installation tree.
 
      Name             Lesson steps/routines that it describes
      ---------------  -----------------------------------------
      time.req         Time Conversion
      sclk.req         SCLK Time Conversion
      spk.req          Obtaining Ephemeris Data
      frames.req       Using Reference Frames
      pck.req          Obtaining Planetary Constants Data
      ck.req           Obtaining Spacecraft Orientation Data
      naif_ids.req     Determining Body ID Codes
 
 
The Permuted Index
 
   Another useful document distributed with the Toolkit is the permuted
   index. This is located under the ``icy/doc'' directory in the IDL
   installation tree. This text document provides a simple mechanism to
   discover what Icy procedures perform a particular function of interest
   as well as the name of the source module that contains the procedure.
 
 
Icy API Documentation
 
   An Icy routine's specification is found in the 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.
 
 
Kernels Used
--------------------------------------------------------
 
   The following kernels are used in examples provided in this lesson:
 
      #  FILE NAME                 TYPE  DESCRIPTION
      -- ------------------------- ----  ------------------------
      1  naif0008.tls              LSK   Generic LSK
      2  cas00084.tsc              SCLK  Cassini SCLK
      3  sat128.bsp                SPK   Saturnian Satellite Ephemeris
      4  981005_PLTEPH-DE405S.bsp  SPK   Solar System Ephemeris
      5  020514_SE_SAT105.bsp      SPK   Saturnian Satellite Ephemeris
      6  030201AP_SK_SM546_T45.bsp SPK   Cassini Spacecraft SPK
      7  cas_v37.tf                FK    Cassini FK
      8  04135_04171pc_psiv2.bc    CK    Cassini Spacecraft CK
      9  cpck05Mar2004.tpc         PCK   Cassini Project PCK
      10 cas_iss_v09.ti            IK    ISS Instrument Kernel
 
   These SPICE kernels are available from the NAIF server at JPL:
 
      ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Lessons/
 
 
Icy Modules Used
--------------------------------------------------------
 
   This section provides a complete summary of the procedures, and the
   kernels 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.)
 
      CHAPTER EXERCISE   FUNCTIONS      NON-VOID       KERNELS
      ------- ---------  -------------  ---------      -------
        1     convtm     cspice_furnsh   1,2
                         cspice_prompt
                         cspice_str2et
                         cspice_etcal
                         cspice_timout
                         cspice_sce2c
                         cspice_sce2s
 
        2     getsta     cspice_furnsh  cspice_vnorm   1,3-6
                         cspice_prompt
                         cspice_str2et
                         cspice_spkezr
                         cspice_spkpos
                         cspice_convrt
 
        3     xform      cspice_furnsh  cspice_vsep    1-9
                         cspice_str2et
                         cspice_spkezr
                         cspice_sxform
                         cspice_mxvg
                         cspice_spkpos
                         cspice_pxform
                         cspice_mxv
                         cspice_convrt
 
        4     subpts     cspice_furnsh                 1,3-6,9
                         cspice_str2et
                         cspice_subpt
                         cspice_subsol
 
        5     fovint     cspice_furnsh  cspice_dpr     1-10
                         cspice_str2et
                         cspice_bodn2c
                         cspice_getfov
                         cspice_srfxpt
                         cspice_reclat
 
        6     angles     cspice_furnsh  cspice_dpr     1-10
                         cspice_str2et
                         cspice_bodn2c
                         cspice_getfov
                         cspice_srfxpt
                         cspice_reclat
                         cspice_illum
 
   Refer to the headers of the various procedures listed above, as detailed
   interface specifications are provided with the source code.
 
 
Time Conversion (convtm)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input UTC time string,
   converts it to the following time systems and output formats:
 
       1.   Ephemeris Time (ET) in seconds past J2000
 
       2.   Calendar Ephemeris Time
 
       3.   Spacecraft Clock Time
 
   and displays the results. Use the program to convert "2004 jun 11
   19:32:00" UTC into these alternate systems.
 
 
Learning Goals
--------------------------------------------------------
 
   Familiarity with the various time conversion and parsing procedures
   available in the Toolkit. Exposure to source code headers and their
   usage in learning to call procedures.
 
 
Approach
--------------------------------------------------------
 
   The solution to the problem can be broken down into a series of simple
   steps:
 
       --   Decide which SPICE kernels are necessary. Prepare a meta-kernel
            listing the kernels and load it into the program.
 
       --   Prompt the user for an input UTC time string.
 
       --   Convert the input time string into ephemeris time expressed as
            seconds past J2000 TDB. Display the result.
 
       --   Convert ephemeris time into a calendar format. Display the
            result.
 
       --   Convert ephemeris time into a spacecraft clock string. Display
            the result.
 
   You may find it useful to consult the permuted index, the headers of
   various source modules, and the ``Time Required Reading'' and ``SCLK
   Required Reading'' documents.
 
   When completing the ``calendar format'' step above, consider using one
   of two possible methods: cspice_etcal or cspice_timout.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'convtm.mk'. Its contents follow:
 
      KPL/MK
 
      This is the meta-kernel used in the solution of the ``Time
      Conversion'' task in the Remote Sensing Hands On Lesson.
 
         \begindata
         KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                             'kernels/sclk/cas00084.tsc' )
         \begintext
 
 
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO convtm
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = "convtm.mk"
         SCLKID = -82
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels his program requires.
         ;; Both the spacecraft clock kernel and a
         ;; leapseconds kernel should be listed in
         ;; the meta-kernel.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT="Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past 2000: ", et
 
         ;;
         ;; Now convert ET to a formal calendar time
         ;; string.  This can be accomplished in two
         ;; ways.
         ;;
         cspice_etcal, et, calet
 
         print, "   Calendar ET (cspice_etcal): ", calet
 
 
         ;;
         ;; Or use cspice_timout for finer control over the
         ;; output format.  The picture below was built
         ;; by examining the header of cspice_timout.
         ;;
         cspice_timout, et   , "YYYY-MON-DDTHR:MN:SC ::TDB", $
                       STRLEN, calet
 
         print, "   Calendar ET (cspice_timout): ", calet
 
         ;;
         ;; Convert ET to spacecraft clock time.
         ;;
         cspice_sce2s, SCLKID, et, sclkst
 
         print, "   Spacecraft Clock Time: ", sclkst
 
         cspice_unload, METAKR
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Converting UTC Time: 2004 jun 11 19:32:00
         ET Seconds Past 2000:    140254384.185
         Calendar ET (cspice_etcal): 2004 JUN 11 19:33:04.184
         Calendar ET (cspice_timout): 2004-JUN-11T19:33:04
         Spacecraft Clock Time: 1/1465674964.105
 
 
Obtaining Target States and Positions (getsta)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input UTC time string,
   computes the following quantities at that epoch:
 
       1.   The apparent state of Phoebe as seen from CASSINI in the J2000
            frame, in kilometers and kilometers/second. This vector itself
            is not of any particular interest, but it is a useful
            intermediate quantity in some geometry calculations.
 
       2.   The apparent position of the Earth as seen from CASSINI in the
            J2000 frame, in kilometers.
 
       3.   The one-way light time between CASSINI and the apparent
            position of Earth, in seconds.
 
       4.   The apparent position of the Sun as seen from Phoebe in the
            J2000 frame (J2000), in kilometers.
 
       5.   The actual (geometric) distance between the Sun and Phoebe, in
            astronomical units.
 
   and displays the results. Use the program to compute these quantities at
   "2004 jun 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
   Understand the anatomy of an cspice_spkezr call. Discover the difference
   between cspice_spkezr and cspice_spkpos. Familiarity with the Toolkit
   utility ``brief''. Exposure to unit conversion with Icy.
 
 
Approach
--------------------------------------------------------
 
   The solution to the problem can be broken down into a series of simple
   steps:
 
       --   Decide which SPICE kernels are necessary. Prepare a meta-kernel
            listing the kernels and load it into the program.
 
       --   Prompt the user for an input time string.
 
       --   Convert the input time string into ephemeris time expressed as
            seconds past J2000 TDB.
 
       --   Compute the state of Phoebe relative to CASSINI in the J2000
            reference frame, corrected for aberrations.
 
       --   Compute the position of Earth relative to CASSINI in the J2000
            reference frame, corrected for aberrations. (The procedure in
            the library that computes this also returns the one-way light
            time between CASSINI and Earth.)
 
       --   Compute the position of the Sun relative to Phoebe in the J2000
            reference frame, corrected for aberrations.
 
       --   Compute the position of the Sun relative to Phoebe without
            correcting for aberration.
 
       --   Compute the length of this vector. This provides the desired
            distance in kilometers.
 
       --   Convert the distance in kilometers into AU.
 
   You may find it useful to consult the permuted index, the headers of
   various source modules, and the ``SPK Required Reading'' document.
 
   When deciding which SPK files to load, the Toolkit utility ``brief'' may
   be of some use.
 
   ``brief'' is located in the ``icy/exe'' directory for IDL toolkits.
   Consult its user's guide available in ``icy/doc/brief.ug'' for details.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'getsta.mk'. Its contents follow:
 
      KPL/MK
 
      This is the meta-kernel used in the solution of the
      ``Obtaining Target States and Positions'' task in the
      Remote Sensing Hands On Lesson.
 
         \begindata
         KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                             'kernels/spk/sat128.bsp'
                             'kernels/spk/981005_PLTEPH-DE405S.bsp',
                             'kernels/spk/020514_SE_SAT105.bsp',
                             'kernels/spk/030201AP_SK_SM546_T45.bsp' )
         \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO getsta
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = "getsta.mk"
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need a leapseconds kernel to convert input
         ;; UTC time strings into ET.  We also will need the
         ;; necessary SPK files with coverage for the bodies
         ;; in which we are interested.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = "Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past 2000: ", et
 
         ;;
         ;; Compute the apparent state of Phoebe as seen from
         ;; CASSINI in the J2000 frame.  All of the ephemeris
         ;; readers return states in units of kilometers and
         ;; kilometers per second.
         ;;
         cspice_spkezr, "PHOEBE" , et   , "J2000", "LT+S", $
                        "CASSINI", state, ltime
 
         print, "   Apparent State of Phoebe as seen " +$
                "from CASSINI in the J2000 "
         print, "      frame (km, km/s): "
         print, FORMAT="(A,F16.3)", "      X = ", state[0]
         print, FORMAT="(A,F16.3)", "      Y = ", state[1]
         print, FORMAT="(A,F16.3)", "      Z = ", state[2]
         print, FORMAT="(A,F16.3)", "     VX = ", state[3]
         print, FORMAT="(A,F16.3)", "     VY = ", state[4]
         print, FORMAT="(A,F16.3)", "     VZ = ", state[5]
 
 
         ;;
         ;; Compute the apparent position of Earth as seen from
         ;; CASSINI in the J2000 frame.  Note: We could have
         ;; continued using cspice_spkezr and simply ignored the
         ;; velocity components.
         ;;
         cspice_spkpos, "EARTH"  , et , "J2000", "LT+S", $
                        "CASSINI", pos, ltime
 
         print, "   Apparent position of Earth as "  +$
                     "seen from CASSINI in the J2000 "
         print, "      frame (kilometers):  "
         print, FORMAT="(A,F16.3)", "      X = ", pos[0]
         print, FORMAT="(A,F16.3)", "      Y = ", pos[1]
         print, FORMAT="(A,F16.3)", "      Z = ", pos[2]
 
         ;;
         ;; We need only display LT, as it is precisely the
         ;; light time in which we are interested.
         ;;
         print, "   One way light time between CASSINI and " +$
                "the apparent position"
         print, FORMAT="(A,F16.3)", "      of Earth (seconds): ", $
                ltime
 
         ;;
         ;; Compute the apparent position of the Sun as seen
         ;; from Phoebe in the J2000 frame.
         ;;
         cspice_spkpos, "SUN"   , et , "J2000", "LT+S", $
                        "PHOEBE", pos, ltime
 
         print, "   Apparent position of Sun as seen " +$
                     "from Phoebe in the "
         print, "      J2000 frame (kilometers): "
         print, FORMAT="(A,F16.3)", "      X = ", pos[0]
         print, FORMAT="(A,F16.3)", "      Y = ", pos[1]
         print, FORMAT="(A,F16.3)", "      Z = ", pos[2]
 
         ;;
         ;; Now we need to compute the actual distance between
         ;; the Sun and Phoebe.  The above SPKPOS call gives us
         ;; the apparent distance, so we need to adjust our
         ;; aberration correction appropriately.
         ;;
         cspice_spkpos, "SUN"   , et , "J2000", "NONE", $
                        "PHOEBE", pos, ltime
 
         ;;
         ;; Compute the distance between the body centers in
         ;; kilometers.
         ;;
         dist = cspice_vnorm ( pos )
 
         ;;
         ;; Convert this value to AU using cspice_convrt.
         ;; Recall, cspice_convrt cannot overwrite the
         ;; input with the output. Use 'dist_au' for the
         ;; output value.
         ;;
         cspice_convrt, dist, "KM", "AU", dist_au
 
         print, "   Actual distance between Sun and Phoebe"
         print, FORMAT="(A,F16.3)", "      (AU): ", dist_au
 
         cspice_unload, METAKR
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Converting UTC Time: 2004 jun 11 19:32:00
         ET Seconds Past 2000:    140254384.185
         Apparent State of Phoebe as seen from CASSINI in the J2000
            frame (km, km/s):
            X =         -119.921
            Y =         2194.139
            Z =          -57.639
           VX =           -5.980
           VY =           -2.119
           VZ =           -0.295
         Apparent position of Earth as seen from CASSINI in the J2000
            frame (kilometers):
            X =    353019393.123
            Y =  -1328180352.140
            Z =   -568134171.697
         One way light time between CASSINI and the apparent position
            of Earth (seconds):         4960.427
         Apparent position of Sun as seen from Phoebe in the
            J2000 frame (kilometers):
            X =    376551465.272
            Y =  -1190495630.303
            Z =   -508438699.110
         Actual distance between Sun and Phoebe
            (AU):            9.012
 
 
Spacecraft Orientation and Reference Frames (xform)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input time string, computes
   the following at the epoch of interest:
 
       1.   The apparent state of Phoebe as seen from CASSINI in the
            IAU_PHOEBE body-fixed frame. This vector itself is not of any
            particular interest, but it is a useful intermediate quantity
            in some geometry calculations.
 
       2.   The angular separation between the apparent position of Earth
            as seen from CASSINI and the nominal boresight of the CASSINI
            high gain antenna.
 
   and displays the results. Use the program to compute these quantities at
   the epoch "2004 jun 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
   Familiarity with the different types of kernels involved in chaining
   reference frames together, both inertial and non-inertial. Discover some
   of the matrix and vector math procedures. Understand the difference
   between cspice_pxform and cspice_sxform.
 
 
Approach
--------------------------------------------------------
 
   The solution to the problem can be broken down into a series of simple
   steps:
 
       --   Decide which SPICE kernels are necessary. Prepare a meta-kernel
            listing the kernels and load it into the program.
 
       --   Prompt the user for an input time string.
 
       --   Convert the input time string into ephemeris time expressed as
            seconds past J2000 TDB.
 
       --   Compute the state of Phoebe relative to CASSINI in the J2000
            reference frame, corrected for aberrations.
 
       --   Compute the state transformation matrix from J2000 to
            IAU_PHOEBE at the epoch, adjusted for light time.
 
       --   Multiply the state of Phoebe relative to CASSINI in the J2000
            reference frame by the state transformation matrix computed in
            the previous step.
 
       --   Compute the position of Earth relative to CASSINI in the J2000
            reference frame, corrected for aberrations.
 
       --   Determine what the nominal boresight of the CASSINI high gain
            antenna is by examining the frame kernel's content.
 
       --   Compute the rotation matrix from the CASSINI high gain antenna
            frame to J2000.
 
       --   Multiply the nominal boresight expressed in the CASSINI high
            gain antenna frame by the rotation matrix from the previous
            step.
 
       --   Compute the separation between the result of the previous step
            and the apparent position of the Earth relative to CASSINI in
            the J2000 frame.
 
   HINT: Several of the steps above may be compressed into a single using
   Icy procedures with which you are already familiar. The ``long-way''
   presented above is intended to facilitate the introduction of the
   procedures cspice_pxform and cspice_sxform.
 
   You may find it useful to consult the permuted index, the headers of
   various source modules, and the following toolkit documentation:
 
       1.   Frames Required Reading
 
       2.   PCK Required Reading
 
       3.   SPK Required Reading
 
       4.   CK Required Reading
 
   This particular example makes use of many of the different types of
   SPICE kernels. You should spend a few moments thinking about which
   kernels you will need and what data they provide.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'xform.mk'. Its contents follow:
 
      KPL/MK
 
      This is the meta-kernel used in the solution of the ``Spacecraft
      Orientation and Reference Frames'' task in the Remote Sensing
      Hands On Lesson.
 
         \begindata
         KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                             'kernels/sclk/cas00084.tsc',
                             'kernels/spk/sat128.bsp'
                             'kernels/spk/981005_PLTEPH-DE405S.bsp',
                             'kernels/spk/020514_SE_SAT105.bsp',
                             'kernels/spk/030201AP_SK_SM546_T45.bsp',
                             'kernels/fk/cas_v37.tf',
                             'kernels/ck/04135_04171pc_psiv2.bc',
                             'kernels/pck/cpck05Mar2004.tpc' )
         \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO xform
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = "xform.mk"
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need:
         ;;
         ;;    A leapseconds kernel
         ;;    A spacecraft clock kernel for CASSINI
         ;;    The necessary ephemerides
         ;;    A planetary constants file (PCK)
         ;;    A spacecraft orientation kernel for CASSINI (CK)
         ;;    A frame kernel (TF)
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = "Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past 2000: ", et
 
         ;;
         ;; Compute the apparent state of Phoebe as seen from
         ;; CASSINI in the J2000 frame.  All of the ephemeris
         ;; readers return states in units of kilometers and
         ;; kilometers per second.
         ;;
         cspice_spkezr, "PHOEBE" , et   , "J2000", "LT+S", $
                        "CASSINI", state, ltime
 
         ;;
         ;; Now obtain the transformation from the inertial
         ;; J2000 frame to the non-inertial body-fixed IAU_PHOEBE
         ;; frame.  Since we want the apparent position, we
         ;; need to subtract ltime from et.
         ;;
         cspice_sxform, "J2000", "IAU_PHOEBE", et-ltime, sform
 
         ;;
         ;; Now rotate the apparent J2000 state into IAU_PHOEBE
         ;; with the following matrix multiplication:
         ;;
         bfixst = transpose(sform) # state
 
         ;;
         ;; Display the results.
         ;;
         print, "   Apparent state of Phoebe as seen " +$
                     "from CASSINI in the IAU_PHOEBE"
         print, "      body-fixed frame (kilometers "  +$
                     "and kilometers per second):"
         print, FORMAT="(A,F19.6)", "      X = ", bfixst[0]
         print, FORMAT="(A,F19.6)", "      Y = ", bfixst[1]
         print, FORMAT="(A,F19.6)", "      Z = ", bfixst[2]
         print, FORMAT="(A,F19.6)", "     VX = ", bfixst[3]
         print, FORMAT="(A,F19.6)", "     VY = ", bfixst[4]
         print, FORMAT="(A,F19.6)", "     VZ = ", bfixst[5]
 
         ;;
         ;; It is worth pointing out, all of the above could
         ;; have been done with a single use of cspice_spkezr:
         ;;
         ;;
         cspice_spkezr, "PHOEBE" , et    , "IAU_PHOEBE", "LT+S", $
                        "CASSINI",  state, ltime
 
         ;;
         ;; Display the results.
         ;;
         print, "   Apparent state of Phoebe as seen " +$
                     "from CASSINI in the IAU_PHOEBE"
         print, "      body-fixed frame (kilometers "  +$
                     "and kilometers per"
         print, "      second) obtained using " +$
                     "cspice_spkezr directly:"
         print, FORMAT="(A,F19.6)", "      X = ", state[0]
         print, FORMAT="(A,F19.6)", "      Y = ", state[1]
         print, FORMAT="(A,F19.6)", "      Z = ", state[2]
         print, FORMAT="(A,F19.6)", "     VX = ", state[3]
         print, FORMAT="(A,F19.6)", "     VY = ", state[4]
         print, FORMAT="(A,F19.6)", "     VZ = ", state[5]
 
         ;;
         ;; Now we are to compute the angular separation between
         ;; the apparent position of the Earth as seen from the
         ;; orbiter and the nominal boresight of the high gain
         ;; antenna.  First, compute the apparent position of
         ;; the Earth as seen from CASSINI in the J2000 frame.
         ;;
         cspice_spkpos, "EARTH"  , et,  "J2000", "LT+S", $
                        "CASSINI", pos,  ltime
 
         ;;
         ;; Now compute the location of the antenna boresight
         ;; at this same epoch.  From reading the frame kernel
         ;; we know that the antenna boresight is nominally the
         ;; +Z axis of the CASSINI_HGA frame defined there.
         ;;
         bsight = [ 0.D0, 0.D0, 1.D0]
 
         ;;
         ;; Now compute the rotation matrix from CASSINI_HGA into
         ;; J2000.
         ;;
         cspice_pxform, "CASSINI_HGA", "J2000", et, pform
 
         ;;
         ;; And multiply the result to obtain the nominal
         ;; antenna boresight in the J2000 reference frame.
         ;;
         cspice_mxv, pform, bsight, bsight
 
         ;;
         ;; Lastly compute the angular separation.
         ;;
         cspice_convrt, cspice_vsep(bsight, pos), "RADIANS", $
                       "DEGREES", sep
 
         print, "   Angular separation between the " +$
                     "apparent position of"
         print, "      Earth and the CASSINI high "  +$
                     "gain antenna boresight (degrees):"
         print, FORMAT="(A,F16.3)", "      ", sep
 
         ;;
         ;; Or alternatively we can work in the antenna
         ;; frame directly.
         ;;
         cspice_spkpos, "EARTH"  , et , "CASSINI_HGA", "LT+S", $
                        "CASSINI", pos, ltime
 
         ;;
         ;; The antenna boresight is the Z-axis in the
         ;; CASSINI_HGA frame.
         ;;
         bsight = [ 0.D0, 0.D0, 1.D0]
 
         ;;
         ;; Lastly compute the angular separation.
         ;;
         cspice_convrt, cspice_vsep(bsight, pos), "RADIANS", $
                       "DEGREES", sep
 
         print, "   Angular separation between the "      +$
                     "apparent position of"
         print, "      Earth and the CASSINI high "       +$
                     "gain antenna boresight computed"
         print, "      using vectors in the CASSINI_HGA " +$
                     "frame (degrees):"
         print, FORMAT="(A,F16.3)", "      ", sep
 
         cspice_unload, METAKR
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Converting UTC Time: 2004 jun 11 19:32:00
         ET Seconds Past 2000:    140254384.185
         Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
            body-fixed frame (kilometers and kilometers per second):
            X =        -1982.639762
            Y =         -934.530471
            Z =         -166.562595
           VX =            3.970729
           VY =           -3.812531
           VZ =           -2.371665
         Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
            body-fixed frame (kilometers and kilometers per
            second) obtained using cspice_spkezr directly:
            X =        -1982.639762
            Y =         -934.530471
            Z =         -166.562595
           VX =            3.970729
           VY =           -3.812531
           VZ =           -2.371665
         Angular separation between the apparent position of
            Earth and the CASSINI high gain antenna boresight (degrees):
                      71.924
         Angular separation between the apparent position of
            Earth and the CASSINI high gain antenna boresight computed
            using vectors in the CASSINI_HGA frame (degrees):
                      71.924
 
 
Computing Sub-spacecraft and Sub-solar Points (subpts)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input UTC time string,
   computes the following quantities at that epoch:
 
       1.   The apparent sub-observer point of CASSINI on Phoebe in the
            body fixed frame IAU_PHOEBE in kilometers.
 
       2.   The apparent sub-solar point on Phoebe as seen from CASSINI in
            the body fixed frame IAU_PHOEBE in kilometers.
 
   and displays the results. Use the program to compute these quantities at
   "2004 jun 11 19:32:00" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
   Discover higher level geometry calculation procedures in Icy and their
   usage as it relates to CASSINI.
 
 
Approach
--------------------------------------------------------
 
   This particular problem is more of an exercise in searching the permuted
   index to find the appropriate procedures and then reading their headers
   to understand how to call them.
 
   One point worth considering: Which method do you want to use to compute
   the sub-solar (or sub-observer) point?
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'subpts.mk'. Its contents follow:
 
      KPL/MK
 
      This is the meta-kernel used in the solution of the
      ``Computing Sub-spacecraft and Sub-solar Points'' task
      in the Remote Sensing Hands On Lesson.
 
         \begindata
         KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                             'kernels/spk/sat128.bsp'
                             'kernels/spk/981005_PLTEPH-DE405S.bsp',
                             'kernels/spk/020514_SE_SAT105.bsp',
                             'kernels/spk/030201AP_SK_SM546_T45.bsp',
                             'kernels/pck/cpck05Mar2004.tpc' )
         \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO subpt
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = "subpts.mk"
         STRLEN = 50
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need:
         ;;
         ;;    A leapseconds kernel
         ;;    The necessary ephemerides
         ;;    A planetary constants file (PCK)
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = "Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past 2000: ", et
 
         ;;
         ;; Compute the apparent sub-observer point of CASSINI
         ;; on Phoebe.
         ;;
         cspice_subpt, "NEAR POINT", "PHOEBE", et,  "LT+S", $
                       "CASSINI"   , spoint  , alt
 
         print, "   Apparent Sub-Observer point of CASSINI " +$
                     "on Phoebe in IAU_PHOEBE"
         print, "      (kilometers):"
         print, FORMAT="(A,F16.3)", "      X = ", spoint[0]
         print, FORMAT="(A,F16.3)", "      Y = ", spoint[1]
         print, FORMAT="(A,F16.3)", "      Z = ", spoint[2]
         print, FORMAT="(A,F16.3)", "    ALT = ", alt
 
         ;;
         ;; Compute the apparent sub-solar point on Phoebe
         ;; as seen from CASSINI.
         ;;
         cspice_subsol, "NEAR POINT", "PHOEBE", et, "LT+S", $
                        "CASSINI"   , spoint
 
         print, "   Apparent Sub-Solar point on Phoebe " +$
                     "as seen from CASSINI in IAU_PHOEBE"
         print, "      (kilometers):"
         print, FORMAT="(A,F16.3)", "      X = ", spoint[0]
         print, FORMAT="(A,F16.3)", "      Y = ", spoint[1]
         print, FORMAT="(A,F16.3)", "      Z = ", spoint[2]
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Converting UTC Time: 2004 jun 11 19:32:00
         ET Seconds Past 2000:    140254384.185
         Apparent Sub-Observer point of CASSINI on Phoebe in IAU_PHOEBE
            (kilometers):
            X =          104.498
            Y =           45.269
            Z =            7.383
          ALT =         2084.116
         Apparent Sub-Solar point on Phoebe as seen from CASSINI in
      IAU_PHOEBE
            (kilometers):
            X =           78.681
            Y =           76.879
            Z =          -21.885
 
 
Intersecting Vectors with a Triaxial Ellipsoid (fovint)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input UTC time string and
   computes the intersection of the CASSINI ISS NAC camera boresight with
   the surface of Phoebe and presents it in the following coordinates:
 
       1.   Cartesian vector in the IAU_PHOEBE frame
 
       2.   Planetocentric (latitudinal)
 
   If this intersection is found, the program displays the results of the
   above computations, otherwise it indicates no intersection has occurred.
   Use this program to compute values at the following epochs:
 
       1.   2004 jun 11 19:32:00 UTC
 
 
Learning Goals
--------------------------------------------------------
 
   Understand how field of view parameters are retrieved from instrument
   kernels. Learn how various standard planetary constants are retrieved
   from text PCKs. Discover how to compute the intersection of field of
   view vectors with triaxial ellipsoidal target bodies.
 
 
Approach
--------------------------------------------------------
 
   This problem can be broken down into several simple, small steps:
 
       --   Decide which SPICE kernels are necessary. Prepare a meta-kernel
            listing the kernels and load it into the program. Remember, you
            will need to find a kernel with information about the CASSINI
            NAC camera.
 
       --   Prompt the user for an input time string.
 
       --   Convert the input time string into ephemeris time expressed as
            seconds past J2000 TDB.
 
       --   Retrieve the field of view configuration for the CASSINI ISS
            NAC camera.
 
       --   Determine if an intercept of the camera boresight and Phoebe
            exists.
 
       --   Convert the position vector of the intercept into
            planetocentric coordinates.
 
   It may be useful to consult the CASSINI ISS instrument kernel to
   determine the name of the NAC camera as well as its configuration. This
   exercise may make use of some of the concepts and (loosely) code from
   the ``Spacecraft Orientation and Reference Frames'' task.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'fovint.mk'. Its contents follow:
 
      KPL/MK
 
      This is the meta-kernel used in the solution of the
      ``Intersecting Vectors with a Triaxial Ellipsoid'' task
      in the Remote Sensing Hands On Lesson.
 
         \begindata
         KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                             'kernels/sclk/cas00084.tsc',
                             'kernels/spk/sat128.bsp'
                             'kernels/spk/981005_PLTEPH-DE405S.bsp',
                             'kernels/spk/020514_SE_SAT105.bsp',
                             'kernels/spk/030201AP_SK_SM546_T45.bsp',
                             'kernels/fk/cas_v37.tf',
                             'kernels/ck/04135_04171pc_psiv2.bc',
                             'kernels/pck/cpck05Mar2004.tpc',
                             'kernels/ik/cas_iss_v09.ti' )
         \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO fovint
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR = "fovint.mk"
         STRLEN = 50
         BCVLEN = 4
         utctim = ''
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need:
         ;;
         ;;    A leapseconds kernel.
         ;;    A SCLK kernel for CASSINI.
         ;;    Any necessary ephemerides.
         ;;    The CASSINI frame kernel.
         ;;    A CASSINI C-kernel.
         ;;    A PCK file with Phoebe constants.
         ;;    The CASSINI ISS I-kernel.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = "Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et, utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past 2000: ", et
 
         ;;
         ;; Now we need to obtain the FOV configuration of
         ;; the ISS NAC camera. To do this we will need the
         ;; ID code for CASSINI_ISS_NAC.
         ;;
         cspice_bodn2c, "CASSINI_ISS_NAC", nacid, found
 
         ;;
         ;; Stop the program if the code was not found.
         ;;
         if ( NOT found ) then begin
            print, "Unable to locate the ID code for " +$
                        "CASSINI_ISS_NAC."
            return
         endif
 
         ;;
         ;; Now retrieve the field of view parameters.
         ;;
         cspice_getfov, nacid, BCVLEN, shape, frame, bsight, $
                        bounds
 
         ;;
         ;; Call srfxpt to determine coordinates of the
         ;; intersection of this vector with the surface
         ;; of Phoebe.
         ;;
         cspice_srfxpt, "Ellipsoid", "PHOEBE",  et, "LT+S", $
                       "CASSINI", frame, bsight, point,     $
                       dist, trgepc, obspos, found
 
         ;;
         ;; Check the found flag.  Display a message if the
         ;; point of intersection was not found and stop.
         ;;
         if ( NOT found ) then begin
            print, "No intersection point found at this epoch."
            return
         endif
 
         ;;
         ;; Now, we have discovered a point of intersection.
         ;; Start by displaying the position vector in the
         ;; IAU_PHOEBE frame of the intersection.
         ;;
         print, "   Position vector of CASSINI NAC camera " +$
                     "boresight surface intercept"
         print, "      in the IAU_PHOEBE frame "            +$
                     "(kilometers):"
         print, FORMAT="(A,F16.3)", "      X = ", point[0]
         print, FORMAT="(A,F16.3)", "      Y = ", point[1]
         print, FORMAT="(A,F16.3)", "      Z = ", point[2]
 
         ;;
         ;; Now express the coordinates of this point in
         ;; planetocentric latitude and longitude.
         ;;
         cspice_reclat, point, radius, lon, lat
 
         ;;
         ;; Convert the angles to degrees for displaying.
         ;;
         print, "   Planetocentric coordinates of the " +$
                     "intercept (degrees):"
         print, FORMAT="(A,F16.3)", "      LAT = ", lat * cspice_dpr()
         print, FORMAT="(A,F16.3)", "      LON = ", lon * cspice_dpr()
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Converting UTC Time: 2004 jun 11 19:32:00
         ET Seconds Past 2000:    140254384.185
         Position vector of CASSINI NAC camera boresight surface intercept
            in the IAU_PHOEBE frame (kilometers):
            X =           86.390
            Y =           72.089
            Z =            8.255
         Planetocentric coordinates of the intercept (degrees):
            LAT =            4.196
            LON =           39.844
 
 
Computing Illumination Angles and Local Time (angles)
===========================================================================
 
 
Task Statement
--------------------------------------------------------
 
   Write a program that prompts the user for an input time string and
   computes the intersection of the CASSINI NAC camera boresight and field
   of view boundary vectors with the surface of Phoebe. At these points of
   intersection, if they exist, compute the following:
 
       1.   Phase angle
 
       2.   Solar incidence angle
 
       3.   Emission angle
 
   Additionally compute the local solar time at the intercept of the camera
   boresight with the surface of Phoebe.
 
   Display the results of the above computations if an intersection occurs,
   otherwise indicate the absence of an intersection. Use this program to
   compute values at the epoch "2004-01-12T4:15.24.000" UTC.
 
 
Learning Goals
--------------------------------------------------------
 
   Discover another high level geometry procedure and another time
   conversion procedure in Icy. Reinforce the concepts introduced in the
   previous task.
 
 
Approach
--------------------------------------------------------
 
   Making use of the code you wrote for the previous task is probably the
   fastest means to an end. A significant percentage of the task is devoted
   to similar computations.
 
   This problem can be broken down into several steps:
 
       --   Decide which SPICE kernels are necessary. Prepare a meta-kernel
            listing these kernels and load it into the program.
 
       --   Prompt the user for an input time string.
 
       --   Convert the input time string into ephemeris time expressed as
            seconds past J2000 TDB.
 
       --   Retrieve the FOV (field of view) configuration for the CASSINI
            NAC camera.
 
   For each vector in the set of boundary corner vectors, and for the
   boresight vector, perform the following operations:
 
       --   Compute the intercept of the vector with Phoebe.
 
       --   If this intercept is found, then compute the phase, solar
            incidence, and emission angles. Otherwise indicate to the user
            no intercept was found for this vector.
 
   At this point, if a boresight intercept was located, then proceed.
 
       --   Compute the planetocentric longitude of the boresight
            intercept.
 
 
Solution
--------------------------------------------------------
 
 
Solution Meta-Kernel
 
   The meta-kernel we created for the solution to this exercise is named
   'angles.mk'. Its contents follow:
 
      KPL/MK
      This is the meta-kernel used in the solution of the
      ``Computing Illumination Angles and Local Time'' task
      in the Remote Sensing Hands On Lesson.
 
         \begindata
         KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
                             'kernels/sclk/cas00084.tsc',
                             'kernels/spk/sat128.bsp'
                             'kernels/spk/981005_PLTEPH-DE405S.bsp',
                             'kernels/spk/020514_SE_SAT105.bsp',
                             'kernels/spk/030201AP_SK_SM546_T45.bsp',
                             'kernels/fk/cas_v37.tf',
                             'kernels/ck/04135_04171pc_psiv2.bc',
                             'kernels/pck/cpck05Mar2004.tpc',
                             'kernels/ik/cas_iss_v09.ti' )
         \begintext
 
 
Solution Source Code
 
   A sample solution to the problem follows:
 
      PRO angles
 
         ;;
         ;; Local Parameters
         ;;
 
         METAKR     = "angles.mk"
         STRLEN     = 50
         BCVLEN     = 5
         utctim     = ''
         scan_vecs = dblarr( 3, BCVLEN )
 
         vecnam  = ["Boundary Corner 1", $
                    "Boundary Corner 2", $
                    "Boundary Corner 3", $
                    "Boundary Corner 4", $
                    "Boresight" ]
 
         ;;
         ;; Load the kernels that this program requires.  We
         ;; will need:
         ;;
         ;;    A leapseconds kernel.
         ;;    A SCLK kernel for CASSINI.
         ;;    Any necessary ephemerides.
         ;;    The CASSINI frame kernel.
         ;;    A CASSINI C-kernel.
         ;;    A PCK file with Phoebe constants.
         ;;    The CASSINI ISS I-kernel.
         ;;
         cspice_furnsh, METAKR
 
         ;;
         ;; Prompt the user for the input time string.
         ;;
         read, utctim, PROMPT = "Input UTC Time: "
 
         print, "Converting UTC Time: ", utctim
 
         ;;
         ;; Convert utctim to et.
         ;;
         cspice_str2et,  utctim, et
 
         print, FORMAT="(A,F16.3)", "   ET Seconds Past 2000: ", et
 
         ;;
         ;; Now we need to obtain the FOV configuration of
         ;; the ISS NAC camera.  To do this we will need the
         ;;ID code for CASSINI_ISS_NAC.
         ;;
         cspice_bodn2c, "CASSINI_ISS_NAC", nacid, found
 
         ;;
         ;; Stop the program if the code was not found.
         ;;
         if ( NOT found ) then begin
            print, "Unable to locate the ID code for " +$
                   "CASSINI_ISS_NAC"
            return
         endif
 
         ;;
         ;; Now retrieve the field of view parameters.
         ;;
         cspice_getfov, nacid, BCVLEN, shape, frame, bsight, bounds
 
         ;;
         ;; Rather than treat 'bsight' as a separate vector,
         ;; copy it and 'bounds to 'scan_vecs'.
         ;;
         scan_vecs[ 0:11] = bounds[0:11]
         scan_vecs[12:14] = bsight[0:2]
 
         ;;
         ;; Now perform the same set of calculations for each
         ;; vector listed in the 'bounds' array.
         ;;
         for i=0, 4 do begin
 
            ;;
            ;; Call srfxpt to determine coordinates of the
            ;; intersection of this vector with the surface
            ;; of Phoebe.
            ;;
            cspice_srfxpt, "Ellipsoid", "PHOEBE",  et, "LT+S", $
                          "CASSINI", frame, scan_vecs[*,i],    $
                          point, dist, trgepc, obspos, found
 
            ;;
            ;; Check the found flag.  Display a message if
            ;; the point of intersection was not found,
            ;; otherwise continue with the calculations.
            ;;
            print, "Vector: ", vecnam[i]
 
            if ( NOT found ) then begin
               print, "No intersection point found at "  +$
                           "this epoch for this vector."
            endif else begin
 
               ;;
               ;; Display the planetocentric latitude and longitude
               ;; of the intercept.
               ;;
               cspice_reclat, point, radius, lon, lat
 
               print, "   Planetocentric coordinates of " +$
                      "the intercept (degrees):"
               print, FORMAT="(A,F16.3)", "    LAT = ", $
                      lat * cspice_dpr()
               print, FORMAT="(A,F16.3)", "    LON = ", $
                      lon * cspice_dpr()
 
               ;;
               ;; Compute the illumination angles at this
               ;; point.
               ;;
               cspice_illum, "PHOEBE", et, "LT+S", "CASSINI", $
                              point, phase, solar, emissn
 
               print, FORMAT="(A,F16.3)", $
                      "   Phase angle (degrees):           ", $
                      phase * cspice_dpr()
               print, FORMAT="(A,F16.3)", $
                      "   Solar incidence angle (degrees): ", $
                      solar * cspice_dpr()
               print, FORMAT="(A,F16.3)", $
                      "   Emission angle (degrees):        ", $
                      emissn * cspice_dpr()
 
               endelse
 
            print
 
         endfor
 
         cspice_unload, METAKR
 
      END
 
 
Solution Sample Output
 
   After compiling the program, execute it:
 
      Converting UTC Time: 2004 jun 11 19:32:00
         ET Seconds Past 2000:    140254384.185
      Vector: Boundary Corner 1
         Planetocentric coordinates of the intercept (degrees):
          LAT =            1.028
          LON =           36.433
         Phase angle (degrees):                     28.110
         Solar incidence angle (degrees):           16.121
         Emission angle (degrees):                  14.627
 
      Vector: Boundary Corner 2
         Planetocentric coordinates of the intercept (degrees):
          LAT =            7.492
          LON =           36.556
         Phase angle (degrees):                     27.894
         Solar incidence angle (degrees):           22.894
         Emission angle (degrees):                  14.988
 
      Vector: Boundary Corner 3
         Planetocentric coordinates of the intercept (degrees):
          LAT =            7.373
          LON =           43.430
         Phase angle (degrees):                     28.171
         Solar incidence angle (degrees):           21.315
         Emission angle (degrees):                  21.977
 
      Vector: Boundary Corner 4
         Planetocentric coordinates of the intercept (degrees):
          LAT =            0.865
          LON =           43.239
         Phase angle (degrees):                     28.385
         Solar incidence angle (degrees):           13.882
         Emission angle (degrees):                  21.763
 
      Vector: Boresight
         Planetocentric coordinates of the intercept (degrees):
          LAT =            4.196
          LON =           39.844
         Phase angle (degrees):                     28.140
         Solar incidence angle (degrees):           18.247
         Emission angle (degrees):                  17.858
 
 
