 
Remote Sensing Hands-On Lesson (IDL)
===========================================================================
 
     October 14, 2004
 
 
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:
 
        ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Tutorials
 
 
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 functions perform a particular function of interest
     as well as the name of the source module that contains the function.
 
 
Source Code Headers
 
     The most detailed specification of a given ICY function is contained
     in the header section of its source code. The source code is
     distributed with the Toolkit and is located under ``icy/src/cspice''
     in the IDL versions. For example the header of cspice_str2et is
     contained in the file:
 
        icy/src/cspice/str2et_c.c
 
 
Kernels Used
--------------------------------------------------------
 
     The programs that are produced in the course of this lesson will
     compute geometry for the Cassini orbiter. The following CASSINI SPICE
     kernels will be used:
 
        #  FILE NAME                 TYPE  DESCRIPTION
        -- ------------------------- ----  ------------------------
        1  naif0007.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
 
 
ICY Modules Used
--------------------------------------------------------
 
     This section provides a complete summary of the functions, 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 functions 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 functions
     available in the Toolkit. Exposure to source code headers and their
     usage in learning to call functions.
 
 
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/naif0007.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
              function 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/naif0007.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 functions. 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 functions with which you are already familiar. The ``long-way''
     presented above is intended to facilitate the introduction of the
     functions cspice_pxform and cspice_sxfo
 
     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/naif0007.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 functions 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 functions 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/naif0007.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_P
              (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/naif0007.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 intercep
              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 function and another time
     conversion function 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/naif0007.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
 
 
