 
In-situ Sensing Hands-On Lesson (IDL)
===========================================================================
 
   March 14, 2006
 
 
Overview
--------------------------------------------------------
 
   In this lesson you will develop a simple program illustrating how SPICE
   can be used to compute various kinds of geometry information applicable
   to the experiments carried out by an in-situ instrument flown on an
   interplanetary spacecraft.
 
 
References
--------------------------------------------------------
 
   This section provides a list of SPICE documents that are referred to in
   this lesson.
 
   Of these documents, the ``Tutorials'' contains the highest level
   descriptions with the least number of details while the ``Required
   Reading'' documents contain much more detailed specifications. The most
   complete specifications are provided in the ``Headers'' -- the comments
   in the top section of the source file.
 
   In some cases the lesson explanations also refer to the information
   provided in the meta-data area of the kernels used in the lesson
   examples. It is especially true in case of the FK and IK files, which
   often contain comprehensive descriptions of the frames, instrument FOVs,
   etc. Since both FK and IK are text kernels, the information provided in
   them can be viewed using any text editor, while the meta information
   provided in binary kernels -- SPKs and CKs -- can be viewed using
   ``commnt'' or ``spacit'' utility programs located in ``icy/exe'' of
   Toolkit installation tree.
 
 
Tutorials
 
   The following SPICE tutorials are referred to by the explanation
   provided in this lesson:
 
      Name             Lesson steps/routines that it describes
      ---------------  -----------------------------------------
      Time             UTC to ET and SCLK to ET
      Loading Kernels  Loading SPICE kernels
      SCLK             SCLK to ET time conversion
      SPK              Computing positions and velocities
      Frames           Computing transformations between frames
 
   These tutorials are available in printed form and as MS Office or PDF
   files from NAIF server at JPL:
 
      http://naif.jpl.nasa.gov/naif/tutorials.html
 
 
Required Reading Documents
 
   The Required Reading documents are provided with the Toolkit and are
   located in ``icy/doc'' directory of the Toolkit installation trees.
 
      Name             Lesson steps/routines that it describes
      ---------------  -----------------------------------------
      time.req         UTC to ET time conversion
      kernel.req       Loading SPICE kernels
      sclk.req         SCLK to ET time conversion
      naif_ids.req     Body and reference frame names
      spk.req          Computing positions and velocities
 
   Another very useful document, also distributed with the Toolkit, is
   ``Permuted Index'', called ``spicelib.idx'' for FORTRAN or
   ``cspice.idx'' for C and IDL located under ``doc'' directory in the
   Toolkit installation tree.
 
   This text document provides an easy way to find what SPICE routine(s)
   performs a particular function of interest and the name of the source
   file that contains this function (this is especially useful for FORTRAN
   because some of the routines are entry points and, therefore, their name
   is different from the name of the source file in which they are
   located.)
 
 
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
      ------------------------- ---- --------------------------
      naif0008.tls              LSK  Generic LSK
      cpck05Mar2004.tpc         PCK  Cassini project PCK
      cas00084.tsc              SCLK Cassini SCLK
      020514_SE_SAT105.bsp      SPK  Saturnian Satellite Ephemeris SPK
      030201AP_SK_SM546_T45.bsp SPK  Cassini Spacecraft SPK
      981005_PLTEPH-DE405S.bsp  SPK  Planetary Ephemeris SPK
      sat128.bsp                SPK  Saturnian Satellite Ephemeris SPK
      04135_04171pc_psiv2.bc    CK   Cassini Spacecraft CK
      cas_v37.tf                FK   Cassini FK
 
   These SPICE kernels are available from the NAIF server at JPL:
 
      ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Lessons/
 
 
Icy Routines Used
--------------------------------------------------------
 
   The example provided in this lesson uses the following Icy routines:
 
      Name             Function that it performs
      ----------       ----------------------------------------------
      cspice_furnsh    Loads kernels, individually or listed in
                       meta-kernel
      cspice_str2et    Converts UTC to ET
      cspice_scs2e     Converts SCLK to ET
      cspice_spkezr    Computes states (position & velocity)
      cspice_spkpos    Computes positions
      cspice_vhat      Find unit vector along a 3d vector
      cspice_subpt     Computes body-fixed coordinates of sub-observer
                       point
      cspice_reclat    Converts rectangular coordinated to latitudinal
      cspice_vsub      Subtracts 3d vectors
      cspice_pxform    Computes 3x3 matrix rotating vectors between
                       frames
      cspice_mxv       Multiplies 3d vector by 3x3 matrix
 
 
Step-1: ``UTC to ET''
===========================================================================
 
 
``UTC to ET'' Task Statement
--------------------------------------------------------
 
   Write a program that computes and prints Ephemeris Time (ET), expressed
   as the number of ephemeris seconds past J2000, that corresponds to
   ``2004-06-11T19:32:00'' UTC.
 
 
``UTC to ET'' Hints
--------------------------------------------------------
 
   Find out what SPICE kernel(s) is(are) needed to support this conversion.
   Look at the ``time.req'' and/or ``Time'' tutorial.
 
   Find necessary kernel(s) on the NAIF's FTP site.
 
   Find out what routine should be called to load necessary kernel(s). Look
   at the ``kernel.req'' and/or ``Loading Kernels'' tutorial.
 
   Find the ``loader'' routine calling sequence specification. Look at the
   ``time.req'' and that routine's source code header. This routine may be
   an entry point, in which case there will be no source file with the same
   name. To find out in which source file this entry point is, search for
   its name in the ``Permuted Index''.
 
   Find the routine(s) used to convert time between UTC and ET. Look at the
   ``time.req'' and/or ``Time'' tutorial.
 
   Find the ``converter'' routine(s) calling sequence specification. Look
   in the ``time.req'' and the routine's source code header.
 
   Put all calls together in a program, add variable declarations (the
   routine header's ``Declarations'' and ``Examples'' sections are a good
   place to look for declaration specification and examples) and output
   print statements. Compile it and link it against Icy.
 
 
``UTC to ET'' Solution Steps
--------------------------------------------------------
 
   Only one kernel file is needed to support this conversion -- an LSK file
   ``naif0008.tls''.
 
   As any other SPICE kernel this file can be loaded by the cspice_furnsh
   routine. For that, the name of the file can put be provided as a sole
   argument of this routine:
 
      ...
      lskfle =  'naif0008.tls'
 
      cspice_furnsh, lskfle
 
   or it can be listed in a meta-kernel:
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           )
      \begintext
 
   the name of which, let's call it ``spice_example.mk'', can be then
   provided as a sole argument of the cspice_furnsh routine:
 
      ...
      mkfile = 'spice_example.mk'
 
      cspice_furnsh, mkfile
 
   While the second option seems to involve a bit more work -- it requires
   making an extra file -- it is a much better way to go if you plan to
   load more kernels as you extend the program. With the meta-kernel
   approach simply adding more kernels to the list in KERNEL_TO_LOAD
   without changing the program code will accomplish that.
 
   The highest level Icy time routine converting UTC to ET is cspice_str2et
   (``icy/doc/html/icy/cspice_str2et.html'').
 
   It has two arguments -- input time string representing UTC in a variety
   of formats (see cspice_str2et header's section ``Particulars'' for the
   complete description of input time formats) and output DP number of ET
   seconds past J2000. A call to cspice_str2et converting a given UTC to ET
   could look like this:
 
      utc = '2004-06-11T19:32:00'
      ...
      cspice_str2et, utc, et
 
   By combining cspice_furnsh and cspice_str2et calls and required
   declarations and by adding a simple print statement, one would get a
   complete program that prints ET for the given UTC epoch.
 
   The program's source code then needs to be compiled and linked against
   Icy. Assuming that the program was saved in a file called
   "spice_example.pro", this can be done with the following IDL command
   (the command below assumes that the library file(s) ``icy.so'' and the
   IDL dlm file ``icy.dlm'' are located in current directory, which may not
   be the case):
 
      $ idl
 
      IDL> .compile spice_example.pro
 
   When you run the compiled script, ``spice_example'', it will produce the
   following output (the output below was generated by this script compiled
   on a OS X; your output may differ slightly in its format and numeric
   precision):
 
      IDL > spice_example
      utc  = 2004-06-11T19:32:00
      et   =     140254384.184625
      IDL >
 
 
``UTC to ET'' Code
--------------------------------------------------------
 
   Program ``spice_example.pro'':
 
      PRO spice_example
 
         mkfile  =  'spice_example.mk'
         cspice_furnsh, mkfile
 
         utc     =  '2004-06-11T19:32:00'
         cspice_str2et, utc, et
 
         print,                      "utc  = ", utc
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         cspice_unload, mkfile
 
      END
 
   Meta-kernel file ``spice_example.mk'':
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           )
      \begintext
 
 
Step-2: ``SCLK to ET''
===========================================================================
 
 
``SCLK to ET'' Task Statement
--------------------------------------------------------
 
   Extend the program from Step-1 to compute and print ET for the following
   CASSINI on-board clock epoch ``1465674964.105''.
 
 
``SCLK to ET'' Hints
--------------------------------------------------------
 
   Find out what additional (to those already loaded in Step-1) SPICE
   kernel(s) is(are) needed to support SCLK to ET conversion. Look at the
   ``sclk.req'' and/or ``SCLK'' tutorial.
 
   Find necessary kernel(s) on the NAIF's FTP site.
 
   Modify the program or meta-kernel to load this(these) kernels.
 
   Find the routine(s) that is(are) used to convert time between SCLK and
   ET. Look at the ``sclk.req'' and/or ``Time'' and ``SCLK'' tutorials.
 
   Find the ``converter'' routine's calling sequence specification. Look in
   the ``sclk.req'' and the routine's source code header.
 
   Look at ``naif_ids.req'' and the comments in the additional kernel(s)
   that you have loaded for information on proper values of input arguments
   of this routine.
 
   Add calls to the ``converter'' routine(s), necessary variable
   declarations (the routine header's ``Declarations'' and ``Examples''
   sections are a good place to look for declaration specification and
   examples), and output print statements to the program. Re-compile and
   re-link it against Icy.
 
 
``SCLK to ET'' Solution Steps
--------------------------------------------------------
 
   A CASSINI SCLK file is needed additionally to the LSK file loaded in the
   Step-1 to support this conversion.
 
   No code change is needed in the loading portion of the program if a
   meta-kernel approach was used in the Step-1. The program will load the
   file if it will be added to the list of kernels in the KERNELS_TO_LOAD
   variable:
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           )
      \begintext
 
   The highest level Icy routine converting SCLK to ET is cspice_scs2e
   (``icy/doc/html/icy/cspice_scs2e.html'').
 
   It has three arguments -- NAIF ID for CASSINI s/c (-82 as described by
   ``naif_ids.req'' document), input time string representing CASSINI SCLK,
   and output DP number of ET seconds past J2000. A call to cspice_str2et
   converting given SCLK to ET could look like this:
 
      scid   = -82
      sclk   = '1465674964.105'
      ...
      cspice_scs2e, scid, sclk, et
 
   By adding the cspice_scs2e call, required declarations and a simple
   print statement, one would get a complete program that prints ET for the
   given SCLK epoch.
 
   The program's source code then needs to be re-compiled and re-linked
   against Icy. It can be done using the same compile command as in Step-1:
 
      $ idl
 
      IDL> .compile spice_example.pro
 
   When you run the compiled script, ``spice_example'', it will produce the
   following output (the output below was generated by this script compiled
   on a OS X; your output may differ slightly in its format and numeric
   precision):
 
      IDL > spice_example
      utc  = 2004-06-11T19:32:00
      et   =     140254384.184625
      sclk = 1465674964.105
      et   =     140254384.183426
      IDL >
 
 
``SCLK to ET'' Code
--------------------------------------------------------
 
   Program ``spice_example.pro'':
 
      PRO spice_example
 
         mkfile  =  'spice_example.mk'
         cspice_furnsh, mkfile
 
         utc     =  '2004-06-11T19:32:00'
         cspice_str2et, utc, et
 
         print,                      "utc  = ", utc
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         scid   = -82
         sclk   = '1465674964.105'
         cspice_scs2e, scid, sclk, et
 
         print,                      "sclk = ", sclk
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         cspice_unload, mkfile
 
      END
 
   Meta-kernel file ``spice_example.mk'':
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           )
      \begintext
 
 
Step-3: ``Spacecraft State''
===========================================================================
 
 
``Spacecraft State'' Task Statement
--------------------------------------------------------
 
   Extend the program from Step-2 to compute geometric state -- position
   and velocity -- of the CASSINI spacecraft with respect to the Sun in
   Ecliptic frame at the epoch specified by SCLK time from Step-2.
 
 
``Spacecraft State'' Hints
--------------------------------------------------------
 
   Find out what additional (to those already loaded in Steps-1&2) SPICE
   kernel(s) is(are) needed to support state computation. Look at the
   ``spk.req'' and/or ``SPK'' tutorial.
 
   Find necessary kernel(s) on the NAIF's FTP site.
 
   Verify that the kernels contain enough data to compute the state of
   interest. Use ``brief'' utility program located under ``toolkit/exe''
   directory for that.
 
   Modify the meta-kernel to load this(these) kernels.
 
   Find the routine(s) that is(are) used to compute states. Look at the
   ``spk.req'' and/or ``SPK'' tutorial presentation.
 
   Find the the routine(s) calling sequence specification. Look in the
   ``spk.req'' and the routine's source code header.
 
   Look at the ``naif_ids.req'' and ``frames.req'' and the routine(s)
   header ``Inputs'' and ``Particulars'' sections to determine proper
   values of the input arguments of this routine.
 
   Add calls to the routine(s), necessary variable declarations and output
   print statements to the program. Re-compile and re-link it against Icy.
 
 
``Spacecraft State'' Solution Steps
--------------------------------------------------------
 
   A CASSINI spacecraft trajectory SPK and generic planetary ephemeris SPK
   files are needed to support computation of the state of interest.
 
   The file names can be added to the meta-kernel to get them loaded into
   the program:
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           )
      \begintext
 
   The highest level Icy routine computing states is cspice_spkezr
   (``icy/doc/html/icy/cspice_spkezr.html'').
 
   We are interested in computing CASSINI position and velocity with
   respect to the Sun, therefore the target and observer names should be
   set to 'CASSINI' and 'Sun' (both names can be found in
   ``naif_ids.req'').
 
   The state should be in ecliptic frame, therefore the name of the frame
   in which the state should be computed is 'ECLIPJ2000' (see
   ``frames.req'' document.)
 
   Since the geometric position is sought for, the abcorr argument of the
   routine should be set to 'NONE' (see aberration correction discussion in
   the (``icy/src/cspice/spkezr_c.c''). header).
 
   Putting it all together, we get:
 
      target = 'CASSINI'
      frame  = 'ECLIPJ2000'
      corrtn = 'NONE'
      observ = 'SUN'
 
      cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
 
   The updated program with added calls, required declarations and a simple
   print statements produces the following output (the output below was
   generated by this script compiled on a OS X; your output may differ
   slightly in its format and numeric precision):
 
      IDL> spice_example
      utc  = 2004-06-11T19:32:00
      et   =     140254384.184625
      sclk = 1465674964.105
      et   =     140254384.183426
      state =   -3.7659906e+08   1.2944878e+09      -7064853.1
            -5.1642262   0.80171891     0.040603057
      IDL >
 
 
``Spacecraft State'' Code
--------------------------------------------------------
 
   Program
 
      PRO spice_example
 
         mkfile  =  'spice_example.mk'
         cspice_furnsh, mkfile
 
         utc     =  '2004-06-11T19:32:00'
         cspice_str2et, utc, et
 
         print,                      "utc  = ", utc
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         scid   = -82
         sclk   = '1465674964.105'
         cspice_scs2e, scid, sclk, et
 
         print,                      "sclk = ", sclk
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         target = 'CASSINI'
         frame  = 'ECLIPJ2000'
         corrtn = 'NONE'
         observ = 'SUN'
 
         cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
 
         print, 'state = ', state
 
         cspice_unload, mkfile
 
      END
 
   Meta-kernel file ``spice_example.mk'':
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           )
      \begintext
 
 
Step-4: ``Sun Direction''
===========================================================================
 
 
``Sun Direction'' Task Statement
--------------------------------------------------------
 
   Extend the program from Step-3 to compute apparent direction of the Sun
   in the INMS frame at the epoch specified by SCLK time from Step-2.
 
 
``Sun Direction'' Hints
--------------------------------------------------------
 
   Find out what additional (to those already loaded in previous steps)
   SPICE kernel(s) is(are) needed to support the direction computation,
   knowing that they should provide the s/c and instrument frame
   orientation. Get these kernels from the NAIF's FTP site.
 
   Verify that the orientation data in the kernels have adequate coverage
   to support computation of the direction of interest. Use ``ckbrief''
   utility program located under ``toolkit/exe'' directory for that.
 
   Modify the meta-kernel to load this(these) kernels.
 
   Determine which input arguments should used in the call to cspice_spkpos
   in order to compute this direction (which is the position portion of the
   output state). Look through the Frames Kernel find the name of the frame
   to used.
 
   Add calls to the routine(s), necessary variable declarations and output
   print statements to the program. Re-compile and re-link it against Icy.
 
 
``Sun Direction'' Solution Steps
--------------------------------------------------------
 
   A CASSINI spacecraft orientation CK file, providing s/c orientation with
   respect to an inertial frame, and CASSINI FK file, providing orientation
   of the INMS frame with respect to the s/ frame, are needed additionally
   to already loaded kernels to support computation of this direction.
 
   The file names can be added to the meta-kernel to get them loaded into
   the program:
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           'kernels/ck/04135_04171pc_psiv2.bc'
                           'kernels/fk/cas_v37.tf'
                           )
      \begintext
 
   The same highest level Icy routine computing positions, cspice_spkpos,
   can be used to compute this direction.
 
   Since this is the direction of the Sun as seen from the s/c, the target
   argument should be set to 'Sun' and the observer argument should be set
   to 'CASSINI'. The name of the INMS frame is 'CASSINI_INMS', the
   definition and description of this frame are provided in the CASSINI FK
   file, ``cassini_v02.tf''.
 
   Since the apparent, or ``as seen'', position is sought for, the abcorr
   argument of the routine should be set to 'LT+S' (see aberration
   correction discussion in the (``icy/src/cspice/spkpos_c.c'') header).
 
   If desired, the position can then be turned into a unit vector using
   cspice_vhat routine (``icy/doc/html/icy/cspice_vhat.html''). Putting it
   all together, we get:
 
      target = 'SUN'
      frame  = 'CASSINI_INMS'
      corrtn = 'LT+S'
      observ = 'CASSINI'
 
      cspice_spkpos, target, et, frame, corrtn, observ, sundir, ltime
      cspice_vhat, sundir, sundir
 
   The updated program with added calls, required declarations and a simple
   print statements produces the following output (the output below was
   generated by this script compiled on a OS X; your output may differ
   slightly in its format and numeric precision):
 
      IDL> spice_example
      utc  = 2004-06-11T19:32:00
      et   =     140254384.184625
      sclk = 1465674964.105
      et   =     140254384.183426
      state =   -3.7659906e+08   1.2944878e+09      -7064853.1
            -5.1642262   0.80171891     0.040603057
      sundir =      -0.29020402      0.88163119      0.37216673
      IDL >
 
 
``Sun Direction'' Code
--------------------------------------------------------
 
   Program ``spice_example.pro'':
 
      PRO spice_example
 
         mkfile  =  'spice_example.mk'
         cspice_furnsh, mkfile
 
         utc     =  '2004-06-11T19:32:00'
         cspice_str2et, utc, et
 
         print,                      "utc  = ", utc
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         scid   = -82
         sclk   = '1465674964.105'
         cspice_scs2e, scid, sclk, et
 
         print,                      "sclk = ", sclk
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         target = 'CASSINI'
         frame  = 'ECLIPJ2000'
         corrtn = 'NONE'
         observ = 'SUN'
 
         cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
 
         print, "state = ", state
 
         target = 'SUN'
         frame  = 'CASSINI_INMS'
         corrtn = 'LT+S'
         observ = 'CASSINI'
 
         cspice_spkpos, target, et, frame, corrtn, observ, sundir, ltime
         cspice_vhat, sundir, sundir
 
         print, "sundir = ", sundir
 
         cspice_unload, mkfile
 
      END
 
   Meta-kernel file ``spice_example.mk'':
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           'kernels/ck/04135_04171pc_psiv2.bc'
                           'kernels/fk/cas_v37.tf'
                           )
      \begintext
 
 
Step-5: ``Sub-Spacecraft Point''
===========================================================================
 
 
``Sub-Spacecraft Point'' Task Statement
--------------------------------------------------------
 
   Extend the program from Step-4 to compute planetocentric longitude and
   and latitude of the sub-spacecraft point and the direction from the
   spacecraft to that point in the INMS frame.
 
 
``Sub-Spacecraft Point'' Hints
--------------------------------------------------------
 
   Find the Icy routine that computes sub-observer point coordinates. Use
   ``Permuted Index'' or ``subpt'' cookbook program for that.
 
   Refer to the routine's header to determine which additional kernels
   should be loaded to support this direction computation. Get these
   kernels from the NAIF's FTP site. Modify the meta-kernel to load
   this(these) kernels.
 
   Determine which input arguments should used in the call to this routine.
   Use the routine's header for that.
 
   Convert Cartesian vector returned by this routine to latitudinal
   coordinates. Use ``Permuted Index'' to find the routine that does this
   conversion. Refer to the routine's header for input/output argument
   specifications.
 
   Given that the direction from the spacecraft to the sub-spacecraft point
   is the difference between the sub-point position vector (which you
   already have) and the spacecraft position vector (which you don't have),
   compute the spacecraft position vector in the same frame (Phoebe
   body-fixed frame) using cspice_spkpos and subtract the two vectors. Use
   ``frames.req'' and/or ``Frames'' tutorial to find the name of the frame
   to be used in cspice_spkpos call.
 
   Since the difference vector is computed in the Phoebe body-fixed frame,
   it should be transformed into the instrument frame get the direction we
   are looking for. Refer to ``frames.req'' and/or ``Frames'' tutorial to
   determine the name of the routine computing transformations and use it
   to compute transformation from Phoebe body-fixed to the INMS frame.
 
   Using ``Permuted Index'' find the routine that multiplies 3x3 matrix by
   3d vector and use it to rotate the difference vector to the instrument
   frame.
 
   Add calls to the routine(s), necessary variable declarations and output
   print statements to the program. Re-compile and re-link it against Icy.
 
 
``Sub-Spacecraft Point'' Solution Steps
--------------------------------------------------------
 
   The cspice_subpt routine (``icy/doc/html/icy/cspice_subpt.html'') can be
   used to compute sub-observer point with a single call. To determine this
   point as the closest point on the Phoebe ellipsoid, the method argument
   has to be set to 'Near point'. For our case the target is 'PHOEBE' and
   the observer is 'CASSINI'.
 
   Since the s/c is close to Phoebe, light time does not need to be taken
   into account and, therefore, the abcorr argument can be set to 'NONE'.
 
   In order for cspice_subpt to compute the nearest point location, a PCK
   file containing Phoebe radii has to be loaded into the program (see
   ``Files'' section of the routine's header.) All other files required for
   this computation are already being loaded by the program. With PCK file
   name added to it, the updated meta-kernel will look like this:
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           'kernels/ck/04135_04171pc_psiv2.bc'
                           'kernels/fk/cas_v37.tf'
                           'kernels/pck/cpck05Mar2004.tpc'
                           )
      \begintext
 
   The sub-spacecraft point Cartesian vector can be converted to
   planetocentric radius, longitude and latitude using the cspice_reclat
   routine (``icy/doc/html/icy/cspice_reclat.html'').
 
   The other vector needed to compute direction from the spacecraft to the
   sub-spacecraft point -- the spacecraft position with respect to Phoebe
   -- can be computed using a call to cspice_spkpos. Note that both
   positions -- one of the sub-spacecraft point and the other of the
   spacecraft -- must be in the same frame in order to find the difference
   between them. Therefore, the frame in the cspice_spkpos call should be
   'IAU_PHOEBE' which is Icy's built-in name for Phoebe body-fixed frame.
   The other arguments will be 'CASSINI' (TARG), 'PHOEBE' (OBS), and 'NONE'
   (ABCORR), as the correction is also not essential in this case.
 
   The cspice_vsub routine (``icy/doc/html/icy/cspice_vsub.html'') can be
   used to subtract the spacecraft position from the sub-spacecraft point
   position.
 
   The computed difference vector has to be rotated from the body-fixed
   frame to the instrument frame. The name of the routine that computes 3x3
   matrices rotating vectors from one frame to another is cspice_pxform
   (``icy/doc/html/icy/cspice_pxform.html''). In our case the from argument
   should be set to 'IAU_PHOEBE' and the to argument should be set to
   'CASSINI_INMS' The difference vector should be then multiplied by this
   matrix using cspice_mxv routine (``icy/doc/html/icy/cspice_mxv.html'')
   to rotate it to the instrument frame. Then, if desired, it can be
   unitized using the cspice_vhat routine.
 
   For output the longitude and latitude angles returned by cspice_reclat
   in radians can be converted to degrees by multiplying by cspice_dpr
   function (``icy/doc/html/icy/cspice_dpr.html''). Putting it all
   together, we get:
 
      method = 'NEAR POINT'
      target = 'PHOEBE'
      corrtn = 'NONE'
      observ = 'CASSINI'
 
      cspice_subpt, method, target, et, corrtn, observ, spoint, alt
 
      cspice_reclat, spoint, srad, slon, slat
 
      target = 'CASSINI'
      frame  = 'IAU_PHOEBE'
      corrtn = 'NONE'
      observ = 'PHOEBE'
 
      cspice_spkpos, target, et, frame, corrtn, observ, scpos, ltime
 
      cspice_vsub, spoint, scpos, sbpdir
 
      fromfr = 'IAU_PHOEBE'
      tofr   = 'CASSINI_INMS'
 
      cspice_pxform, fromfr, tofr, et, m2imat
 
      cspice_mxv,  m2imat, sbpdir, sbpdir
      cspice_vhat, sbpdir, sbpdir
 
      print, "alt    =  ", alt
      print, "lon    =  ", slon * cspice_dpr()
      print, "lat    =  ", slat * cspice_dpr()
      print, "sbpdir =  ", sbpdir
 
   The updated program with added calls, required declarations and a simple
   print statements produces the following output (the output below was
   generated by this script compiled on a OS X; your output may differ
   slightly in its format and numeric precision):
 
      IDL> spice_example
      utc  = 2004-06-11T19:32:00
      et   =     140254384.184625
      sclk = 1465674964.105
      et   =     140254384.183426
      state =   -3.7659906e+08   1.2944878e+09      -7064853.1
            -5.1642262   0.80171891     0.040603057
      sundir =      -0.29020402      0.88163119      0.37216673
      alt    =         2084.1129
      lon    =         23.423158
      lat    =         3.7097972
      sbpdir =    -0.00077620706     -0.99987320    -0.015905456
 
 
``Sub-Spacecraft Point'' Code
--------------------------------------------------------
 
   Program ``spice_example.pro'':
 
      PRO spice_example
 
         mkfile  =  'spice_example.mk'
         cspice_furnsh, mkfile
 
         utc     =  '2004-06-11T19:32:00'
         cspice_str2et, utc, et
 
         print,                      "utc  = ", utc
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         scid   = -82
         sclk   = '1465674964.105'
         cspice_scs2e, scid, sclk, et
 
         print,                      "sclk = ", sclk
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         target = 'CASSINI'
         frame  = 'ECLIPJ2000'
         corrtn = 'NONE'
         observ = 'SUN'
 
         cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
 
         print, "state = ", state
 
         target = 'SUN'
         frame  = 'CASSINI_INMS'
         corrtn = 'LT+S'
         observ = 'CASSINI'
 
         cspice_spkpos, target, et, frame, corrtn, observ, sundir, ltime
         cspice_vhat, sundir, sundir
 
         print, "sundir = ", sundir
 
         method = 'NEAR POINT'
         target = 'PHOEBE'
         corrtn = 'NONE'
         observ = 'CASSINI'
 
         cspice_subpt, method, target, et, corrtn, observ, spoint, alt
 
         cspice_reclat, spoint, srad, slon, slat
 
         target = 'CASSINI';
         frame  = 'IAU_PHOEBE';
         corrtn = 'NONE';
         observ = 'PHOEBE';
 
         cspice_spkpos, target, et, frame, corrtn, observ, scpos, ltime
 
         cspice_vsub, spoint, scpos, sbpdir
 
         fromfr = 'IAU_PHOEBE'
         tofr   = 'CASSINI_INMS'
 
         cspice_pxform, fromfr, tofr, et, m2imat
 
         cspice_mxv,  m2imat, sbpdir, sbpdir
         cspice_vhat, sbpdir, sbpdir
 
         print, "alt    =  ", alt
         print, "lon    =  ", slon * cspice_dpr()
         print, "lat    =  ", slat * cspice_dpr()
         print, "sbpdir =  ", sbpdir
 
         cspice_unload, mkfile
 
      END
 
   Meta-kernel file ``spice_example.mk'':
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           'kernels/ck/04135_04171pc_psiv2.bc'
                           'kernels/fk/cas_v37.tf'
                           'kernels/pck/cpck05Mar2004.tpc'
                           )
      \begintext
 
 
Step-6: ``Spacecraft Velocity''
===========================================================================
 
 
``Spacecraft Velocity'' Task Statement
--------------------------------------------------------
 
   Extend the program from Step-5 to compute the spacecraft velocity with
   respect to Phoebe in the INMS frame.
 
 
``Spacecraft Velocity'' Hints
--------------------------------------------------------
 
   Compute velocity of the spacecraft with respect to Phoebe in some
   inertial frame, for example J2000. Recall that velocity is the last
   three components of the state vector returned by cspice_spkezr.
 
   Since the velocity vector is computed in the inertial frame, it should
   be rotated to the instrument frame. Look at the previous step the
   routine that compute necessary rotation and rotate vectors.
 
   Add calls to the routine(s), necessary variable declarations and output
   print statements to the program. Re-compile and re-link it against Icy.
 
 
``Spacecraft Velocity'' Solution Steps
--------------------------------------------------------
 
   All kernels required for computations in this step are already being
   loaded by the program, therefore, the meta-kernel does not need to be
   changed.
 
   The spacecraft velocity vector is the last three components of the state
   returned by cspice_spkezr. To compute velocity of CASSINI with respect
   to Phoebe in the J2000 inertial frame the cspice_spkezr arguments should
   be set to 'CASSINI' (TARG), 'PHOEBE' (OBS), 'J2000' (REF) and 'NONE'
   (ABCORR).
 
   The computed velocity vector has to be rotated from the J2000 frame to
   the instrument frame. The cspice_pxform routine used in the previous
   step can be used to compute the rotation matrix needed for that. In this
   case the frame name arguments should be set to 'J2000' (FROM) and
   'CASSINI_INMS' (TO).
 
   As in the previous step the difference vector should be then multiplied
   by this matrix using the cspice_mxv routine. Then, if desired, it can be
   unitized using the cspice_vhat routine.
 
   Putting it all together, we get:
 
         target = 'CASSINI'
         frame  = 'J2000'
         corrtn = 'NONE'
         observ = 'PHOEBE'
 
         cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
         scvdir = state[3:5]
 
         fromfr = 'J2000'
         tofr   = 'CASSINI_INMS'
         cspice_pxform, fromfr, tofr, et, j2imat
 
         cspice_mxv, j2imat, scvdir, scvdir
         cspice_vhat, scvdir, scvdir
 
         print, "scvdir = ", scvdir[0:2]
 
   The updated program with added calls, required declarations and a simple
   print statements produces the following output (the output below was
   generated by this script compiled on a OS X; your output may differ
   slightly in its format and numeric precision):
 
      IDL> spice_example
      utc  = 2004-06-11T19:32:00
      et   =     140254384.184625
      sclk = 1465674964.105
      et   =     140254384.183426
      state =   -3.7659906e+08   1.2944878e+09      -7064853.1
            -5.1642262   0.80171891     0.040603057
      sundir =      -0.29020402      0.88163119      0.37216673
      alt    =         2084.1129
      lon    =         23.423158
      lat    =         3.7097972
      sbpdir =    -0.00077620706     -0.99987320    -0.015905456
      scvdir =       0.39578487     -0.29280767      0.87041255
      IDL>
 
   Note that computing the spacecraft velocity in the instrument frame by a
   single call to cspice_spkezr by specifying 'CASSINI_INMS' in the ref
   argument will produce an incorrect result. Such computation will take
   into account the spacecraft angular velocity from the CK files, which
   should not be considered in this case.
 
 
``Spacecraft Velocity'' Code Program ``spice_example.pro'':
--------------------------------------------------------
 
      PRO spice_example
 
         mkfile  =  'spice_example.mk'
         cspice_furnsh, mkfile
 
         utc     =  '2004-06-11T19:32:00'
         cspice_str2et, utc, et
 
         print,                      "utc  = ", utc
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         scid   = -82
         sclk   = '1465674964.105'
         cspice_scs2e, scid, sclk, et
 
         print,                      "sclk = ", sclk
         print, FORMAT='(A7,F20.6)', "et   = ", et
 
         target = 'CASSINI'
         frame  = 'ECLIPJ2000'
         corrtn = 'NONE'
         observ = 'SUN'
 
         cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
 
         print, "state = ", state
 
         target = 'SUN'
         frame  = 'CASSINI_INMS'
         corrtn = 'LT+S'
         observ = 'CASSINI'
 
         cspice_spkpos, target, et, frame, corrtn, observ, sundir, ltime
         cspice_vhat, sundir, sundir
 
         print, "sundir = ", sundir
 
         method = 'NEAR POINT'
         target = 'PHOEBE'
         corrtn = 'NONE'
         observ = 'CASSINI'
 
         cspice_subpt, method, target, et, corrtn, observ, spoint, alt
 
         cspice_reclat, spoint, srad, slon, slat
 
         target = 'CASSINI';
         frame  = 'IAU_PHOEBE';
         corrtn = 'NONE';
         observ = 'PHOEBE';
 
         cspice_spkpos, target, et, frame, corrtn, observ, scpos, ltime
 
         cspice_vsub, spoint, scpos, sbpdir
 
         fromfr = 'IAU_PHOEBE'
         tofr   = 'CASSINI_INMS'
 
         cspice_pxform, fromfr, tofr, et, m2imat
 
         cspice_mxv,  m2imat, sbpdir, sbpdir
         cspice_vhat, sbpdir, sbpdir
 
         print, "alt    =  ", alt
         print, "lon    =  ", slon * cspice_dpr()
         print, "lat    =  ", slat * cspice_dpr()
         print, "sbpdir =  ", sbpdir
 
         target = 'CASSINI'
         frame  = 'J2000'
         corrtn = 'NONE'
         observ = 'PHOEBE'
 
         cspice_spkezr, target, et, frame, corrtn, observ, state, ltime
         scvdir = state[3:5]
 
         fromfr = 'J2000'
         tofr   = 'CASSINI_INMS'
         cspice_pxform, fromfr, tofr, et, j2imat
 
         cspice_mxv, j2imat, scvdir, scvdir
         cspice_vhat, scvdir, scvdir
 
         print, "scvdir = ", scvdir[0:2]
 
      END
 
   Meta-kernel file ``spice_example.mk'':
 
      \begindata
         KERNELS_TO_LOAD = (
                           'kernels/lsk/naif0008.tls'
                           'kernels/sclk/cas00084.tsc'
                           'kernels/spk/020514_SE_SAT105.bsp'
                           'kernels/spk/030201AP_SK_SM546_T45.bsp'
                           'kernels/spk/981005_PLTEPH-DE405S.bsp'
                           'kernels/spk/sat128.bsp'
                           'kernels/ck/04135_04171pc_psiv2.bc'
                           'kernels/fk/cas_v37.tf'
                           'kernels/pck/cpck05Mar2004.tpc'
                           )
      \begintext
 
