| In-situ Sensing Hands-On Lesson (C) |
Table of ContentsIn-situ Sensing Hands-On Lesson (C) Overview Note About HTML Links References Tutorials Required Reading Documents Source Code Header Comments Kernels Used CSPICE Routines Used Step-1: ``UTC to ET'' ``UTC to ET'' Task Statement ``UTC to ET'' Hints ``UTC to ET'' Solution Steps ``UTC to ET'' Code Step-2: ``SCLK to ET'' ``SCLK to ET'' Task Statement ``SCLK to ET'' Hints ``SCLK to ET'' Solution Steps ``SCLK to ET'' Code Step-3: ``Spacecraft State'' ``Spacecraft State'' Task Statement ``Spacecraft State'' Hints ``Spacecraft State'' Solution Steps ``Spacecraft State'' Code Step-4: ``Sun Direction'' ``Sun Direction'' Task Statement ``Sun Direction'' Hints ``Sun Direction'' Solution Steps ``Sun Direction'' Code Step-5: ``Sub-Spacecraft Point'' ``Sub-Spacecraft Point'' Task Statement ``Sub-Spacecraft Point'' Hints ``Sub-Spacecraft Point'' Solution Steps ``Sub-Spacecraft Point'' Code Step-6: ``Spacecraft Velocity'' ``Spacecraft Velocity'' Task Statement ``Spacecraft Velocity'' Hints ``Spacecraft Velocity'' Solution Steps ``Spacecraft Velocity'' Code Program ``spice_example.c'': In-situ Sensing Hands-On Lesson (C)
Overview
Note About HTML Links
In order for the links to be resolved, create a subdirectory called ``lessons'' under the ``doc/html'' directory of the Toolkit tree and copy this document to that subdirectory before loading it into a Web browser. References
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 ``cspice/exe'' of Toolkit installation tree. Tutorials
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 framesThese 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
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 velocitiesAnother very useful document, also distributed with the Toolkit, is ``Permuted Index'', called ``spicelib.idx'' for FORTRAN or ``cspice.idx'' for C, IDL, and MATLAB also found in the ``doc'' directory. 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.) Source Code Header Comments
For example the source code of the STR2ET/str2et_c routine is
toolkit/src/spicelib/str2et.forin the FORTRAN Toolkit and in
cspice/src/cspice/str2et_c.cin the C Toolkit. Since some of the FORTRAN routines are entry points they are usually part of a source file that has different name. The ``Permuted Index'' document mentioned above can be used to locate the name of their source file. Kernels Used
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 FKThese SPICE kernels are included in the lesson package available from the NAIF server at JPL:
ftp://naif.jpl.nasa.gov/pub/naif/toolkit_docs/Lessons/ CSPICE Routines Used
Name Function that it performs ---------- --------------------------------------------------- furnsh_c Loads kernels, individually or listed in meta-kernel str2et_c Converts UTC to ET scs2e_c Converts SCLK to ET spkezr_c Computes states (position & velocity) spkpos_c Computes positions vhat_c Find unit vector along a 3d vector subpnt_c Computes body-fixed coordinates of sub-observer point reclat_c Converts rectangular coordinated to latitudinal vsub_c Subtracts 3d vectors pxform_c Computes 3x3 matrix rotating vectors between frames mxv_c Multiplies 3d vector by 3x3 matrix vpack_c Packs 3 number into a 3d vectorThe most detailed documentation source for these routines are their headers. Step-1: ``UTC to ET''``UTC to ET'' Task Statement
``UTC to ET'' Hints
Find necessary kernel(s) on the NAIF's FTP site. Find out what routine should be called to load necessary kernel(s). Reference 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 CSPICE. ``UTC to ET'' Solution Steps
As any other SPICE kernel this file can be loaded by the furnsh_c routine. For that, the name of the file can put be provided as a sole argument of this routine:
#include "SpiceUsr.h" ... SpiceChar * lskfle = "naif0008.tls"; furnsh_c ( 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.tm'', can be then
provided as a sole argument of the furnsh_c routine:
#include "SpiceUsr.h" ... SpiceChar * mkfile = "spice_example.tm"; furnsh_c ( 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 CSPICE time routine converting UTC to ET is str2et_c (``cspice/src/cspice/str2et_c.c''). It has two arguments -- input time string representing UTC in a variety of formats (see str2et_c header's section ``Particulars'' for the complete description of input time formats) and output DP number of ET seconds past J2000. A call to str2et_c converting a given UTC to ET could look like this:
SpiceChar * utc = "2004-06-11T19:32:00"; SpiceDouble et; ... str2et_c ( utc, &et );By combining furnsh_c and str2et_c 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 CSPICE. Assuming that the program was saved in a file called "spice_example.c", this can be done with the following command on a Sun workstation: Sun C:
cc -c -Xc -o spice_example spice_example.c cspice.a -lmgcc:
gcc -c -ansi -Wall -o spice_example spice_example.c cspice.a -lmThe command assumes that the library file(s) ``cspice.a'' and the CSPICE include files *.h are located in current directory, which may not be the case. When you run the program's executable, ``spice_example'', it produces the following output (the output below was generated by this program compiled with gcc on a PC running Linux; your output may differ slightly in its format and numeric precision):
> ./spice_example utc = 2004-06-11T19:32:00 et = 140254384.184625 > ``UTC to ET'' Code
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
SpiceChar * mkfile;
SpiceChar * utc;
SpiceDouble et;
mkfile = "spice_example.tm";
furnsh_c ( mkfile );
utc = "2004-06-11T19:32:00";
str2et_c ( utc, &et );
printf ( "utc = %s \n", utc );
printf ( "et = %20.6f \n", et );
return ( 0 );
}
Meta-kernel file ``spice_example.tm'':
\begindata
KERNELS_TO_LOAD = (
'kernels/lsk/naif0008.tls'
)
\begintext
Step-2: ``SCLK to ET''``SCLK to ET'' Task Statement
``SCLK to ET'' Hints
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) needed 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 CSPICE. ``SCLK to ET'' Solution Steps
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 CSPICE routine converting SCLK to ET is scs2e_c
(``cspice/src/cspice/scs2e_c.c'').
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 str2et_c converting given SCLK to ET could look like this:
SpiceChar * sclk = "1465674964.105"; SpiceInt scid = -82; ... scs2e_c ( scid, sclk, &et );By adding the scs2e_c 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 CSPICE. It can be done using the same compile command as in Step-1:
Sun C: cc -c -Xc -o spice_example spice_example.c cspice.a -lm gcc: gcc -c -ansi -Wall -o spice_example spice_example.c cspice.a -lmWhen you run the program's executable, ``spice_example'', it produces the following output (the output below was generated by this program compiled with gcc on a PC running Linux; your output may differ slightly in its format and numeric precision):
> ./spice_example utc = 2004-06-11T19:32:00 et = 140254384.184625 sclk = 1465674964.105 et = 140254384.183426 > ``SCLK to ET'' Code
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
SpiceChar * mkfile;
SpiceChar * utc;
SpiceChar * sclk;
SpiceDouble et;
SpiceInt scid;
mkfile = "spice_example.tm";
furnsh_c ( mkfile );
utc = "2004-06-11T19:32:00";
str2et_c ( utc, &et );
printf ( "utc = %s \n", utc );
printf ( "et = %20.6f \n", et );
scid = -82;
sclk = "1465674964.105";
scs2e_c ( scid, sclk, &et );
printf ( "sclk = %s \n", sclk );
printf ( "et = %20.6f \n", et );
return ( 0 );
}
Meta-kernel file ``spice_example.tm'':
\begindata
KERNELS_TO_LOAD = (
'kernels/lsk/naif0008.tls'
'kernels/sclk/cas00084.tsc'
)
\begintext
Step-3: ``Spacecraft State''``Spacecraft State'' Task Statement
``Spacecraft State'' Hints
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. Determine the routine(s) needed 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. Reference 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 CSPICE. ``Spacecraft State'' Solution Steps
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 CSPICE routine computing states is spkezr_c
(``cspice/src/cspice/spkezr_c.c'').
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 we need only the geometric position, the `abcorr' argument of the routine should be set to "NONE" (see aberration correction discussion in the (``cspice/src/cspice/spkezr_c.c''). header). Putting it all together, we get:
SpiceChar * target; SpiceChar * frame; SpiceChar * corrtn; SpiceChar * observ; SpiceDouble state [6]; SpiceDouble ltime; ... target = "CASSINI"; frame = "ECLIPJ2000"; corrtn = "NONE"; observ = "SUN"; spkezr_c ( target, et, frame, corrtn, observ, state, <ime );The updated program with added calls, required declarations and simple print statements produces the following output (the output below was generated by this program compiled with gcc on a PC running Linux; your output may differ slightly in its format and numeric precision):
> ./spice_example utc = 2004-06-11T19:32:00 et = 140254384.184625 sclk = 1465674964.105 et = 140254384.183426 state = -3.765991e+08 1.294488e+09 -7.064853e+06 -5.164226e+00 8.017189e-01 4.060306e-02 > ``Spacecraft State'' Code
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
SpiceChar * mkfile;
SpiceChar * utc;
SpiceChar * sclk;
SpiceChar * target;
SpiceChar * frame;
SpiceChar * corrtn;
SpiceChar * observ;
SpiceDouble et;
SpiceDouble state [6];
SpiceDouble ltime;
SpiceInt scid;
mkfile = "spice_example.tm";
furnsh_c ( mkfile );
utc = "2004-06-11T19:32:00";
str2et_c ( utc, &et );
printf ( "utc = %s \n", utc );
printf ( "et = %20.6f \n", et );
scid = -82;
sclk = "1465674964.105";
scs2e_c ( scid, sclk, &et );
printf ( "sclk = %s \n", sclk );
printf ( "et = %20.6f \n", et );
target = "CASSINI";
frame = "ECLIPJ2000";
corrtn = "NONE";
observ = "SUN";
spkezr_c ( target, et, frame, corrtn, observ, state, <ime );
printf ( "state = %e %e %e %e %e %e\n",
state[0], state[1], state[2],
state[3], state[4], state[5] );
return ( 0 );
}
Meta-kernel file ``spice_example.tm'':
\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
``Sun Direction'' Hints
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 the proper input arguments for the spkpos_c call to calculate the 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 CSPICE. ``Sun Direction'' Solution Steps
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 CSPICE routine computing positions, spkpos_c, 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 (``cspice/src/cspice/spkpos_c.c'') header). If desired, the position can then be turned into a unit vector using vhat_c function (``cspice/src/cspice/vhat_c.c''). Putting it all together, we get:
SpiceDouble sundir [3]; ... target = "SUN"; frame = "CASSINI_INMS"; corrtn = "LT+S"; observ = "CASSINI"; spkpos_c ( target, et, frame, corrtn, observ, sundir, <ime ); vhat_c ( sundir, sundir );The updated program with added calls, required declarations and simple print statements produces the following output (the output below was generated by this program compiled with gcc on a PC running Linux; your output may differ slightly in its format and numeric precision):
> ./spice_example utc = 2004-06-11T19:32:00 et = 140254384.184625 sclk = 1465674964.105 et = 140254384.183426 state = -3.765991e+08 1.294488e+09 -7.064853e+06 -5.164226e+00 8.017189e-01 4.060306e-02 sundir = -2.902040e-01 8.816312e-01 3.721667e-01 > ``Sun Direction'' Code
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
SpiceChar * mkfile;
SpiceChar * utc;
SpiceChar * sclk;
SpiceChar * target;
SpiceChar * frame;
SpiceChar * corrtn;
SpiceChar * observ;
SpiceDouble et;
SpiceDouble state [6];
SpiceDouble sundir [3];
SpiceDouble ltime;
SpiceInt scid;
mkfile = "spice_example.tm";
furnsh_c ( mkfile );
utc = "2004-06-11T19:32:00";
str2et_c ( utc, &et );
printf ( "utc = %s \n", utc );
printf ( "et = %20.6f \n", et );
scid = -82;
sclk = "1465674964.105";
scs2e_c ( scid, sclk, &et );
printf ( "sclk = %s \n", sclk );
printf ( "et = %20.6f \n", et );
target = "CASSINI";
frame = "ECLIPJ2000";
corrtn = "NONE";
observ = "SUN";
spkezr_c ( target, et, frame, corrtn, observ, state, <ime );
printf ( "state = %e %e %e %e %e %e\n",
state[0], state[1], state[2],
state[3], state[4], state[5] );
target = "SUN";
frame = "CASSINI_INMS";
corrtn = "LT+S";
observ = "CASSINI";
spkpos_c ( target, et, frame, corrtn, observ, sundir, <ime );
vhat_c ( sundir, sundir );
printf ( "sundir = %e %e %e\n",
sundir[0], sundir[1], sundir[2] );
return ( 0 );
}
Meta-kernel file ``spice_example.tm'':
\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
``Sub-Spacecraft Point'' Hints
Refer to the routine's header to determine the additional kernels needed for this direction computation. Get these kernels from the NAIF's FTP site. Modify the meta-kernel to load this(these) kernels. Determine the proper input arguments for the routine. Refer to the routine's header for that information. Convert the surface point 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. Since the Cartesian vector from the spacecraft to the sub-spacecraft point 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 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 CSPICE. ``Sub-Spacecraft Point'' Solution Steps
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 subpnt_c 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 reclat_c routine
(``cspice/src/cspice/reclat_c.c'').
The vector from the spacecraft to the sub-spacecraft point returned by subpnt_c 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 pxform_c (``cspice/src/cspice/pxform_c.c''). In our case the `from' argument should be set to "IAU_PHOEBE" and the `to' argument should be set to "CASSINI_INMS" The vector should be then multiplied by this matrix to rotate it to the instrument frame. The mxv_c routine performs that function (``cspice/src/cspice/mxv_c.c'') After applying the rotation, normalize the resultant vector using the vhat_c function. For output the longitude and latitude angles returned by reclat_c in radians can be converted to degrees by multiplying by dpr_c function (``cspice/src/cspice/dpr_c.c''). Putting it all together, we get:
SpiceChar * method;
SpiceChar * fromfr;
SpiceChar * tofr;
SpiceDouble spoint [3];
SpiceDouble trgepc;
SpiceDouble srfvec [3];
SpiceDouble srad;
SpiceDouble slon;
SpiceDouble slat;
SpiceDouble sbpdir [3];
SpiceDouble m2imat [3][3];
...
method = "NEAR POINT: ELLIPSOID";
target = "PHOEBE";
frame = "IAU_PHOEBE";
corrtn = "NONE";
observ = "CASSINI";
subpnt_c ( method, target, et, frame, corrtn, observ,
spoint, &trgepc, srfvec );
reclat_c ( spoint, &srad, &slon, &slat );
fromfr = "IAU_PHOEBE";
tofr = "CASSINI_INMS";
pxform_c ( fromfr, tofr, et, m2imat );
mxv_c ( m2imat, srfvec, sbpdir );
vhat_c ( sbpdir, sbpdir );
printf ( "lon = %e \n", slon * dpr_c() );
printf ( "lat = %e \n", slat * dpr_c() );
printf ( "sbpdir = %e %e %e \n",
sbpdir[0], sbpdir[1], sbpdir[2] );
The updated program with added calls, required declarations and simple
print statements produces the following output (the output below was
generated by this program compiled with gcc on a PC running Linux; your
output may differ slightly in its format and numeric precision):
> ./spice_example utc = 2004-06-11T19:32:00 et = 140254384.184625 sclk = 1465674964.105 et = 140254384.183426 state = -3.765991e+08 1.294488e+09 -7.064853e+06 -5.164226e+00 8.017189e-01 4.060306e-02 sundir = -2.902040e-01 8.816312e-01 3.721667e-01 lon = 2.342316e+01 lat = 3.709797e+00 sbpdir = -7.762071e-04 -9.998732e-01 -1.590546e-02 > ``Sub-Spacecraft Point'' Code
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
SpiceChar * mkfile;
SpiceChar * utc;
SpiceChar * sclk;
SpiceChar * target;
SpiceChar * frame;
SpiceChar * corrtn;
SpiceChar * observ;
SpiceChar * method;
SpiceChar * fromfr;
SpiceChar * tofr;
SpiceDouble et;
SpiceDouble state [6];
SpiceDouble sundir [3];
SpiceDouble ltime;
SpiceDouble spoint [3];
SpiceDouble trgepc;
SpiceDouble srfvec [3];
SpiceDouble srad;
SpiceDouble slon;
SpiceDouble slat;
SpiceDouble sbpdir [3];
SpiceDouble m2imat [3][3];
SpiceInt scid;
mkfile = "spice_example.tm";
furnsh_c ( mkfile );
utc = "2004-06-11T19:32:00";
str2et_c ( utc, &et );
printf ( "utc = %s \n", utc );
printf ( "et = %20.6f \n", et );
scid = -82;
sclk = "1465674964.105";
scs2e_c ( scid, sclk, &et );
printf ( "sclk = %s \n", sclk );
printf ( "et = %20.6f \n", et );
target = "CASSINI";
frame = "ECLIPJ2000";
corrtn = "NONE";
observ = "SUN";
spkezr_c ( target, et, frame, corrtn, observ, state, <ime );
printf ( "state = %e %e %e %e %e %e\n",
state[0], state[1], state[2],
state[3], state[4], state[5] );
target = "SUN";
frame = "CASSINI_INMS";
corrtn = "LT+S";
observ = "CASSINI";
spkpos_c ( target, et, frame, corrtn, observ, sundir, <ime );
vhat_c ( sundir, sundir );
printf ( "sundir = %e %e %e\n",
sundir[0], sundir[1], sundir[2] );
method = "NEAR POINT: ELLIPSOID";
target = "PHOEBE";
frame = "IAU_PHOEBE";
corrtn = "NONE";
observ = "CASSINI";
subpnt_c ( method, target, et, frame, corrtn, observ,
spoint, &trgepc, srfvec );
reclat_c ( spoint, &srad, &slon, &slat );
fromfr = "IAU_PHOEBE";
tofr = "CASSINI_INMS";
pxform_c ( fromfr, tofr, et, m2imat );
mxv_c ( m2imat, srfvec, sbpdir );
vhat_c ( sbpdir, sbpdir );
printf ( "lon = %e \n", slon * dpr_c() );
printf ( "lat = %e \n", slat * dpr_c() );
printf ( "sbpdir = %e %e %e \n",
sbpdir[0], sbpdir[1], sbpdir[2] );
return ( 0 );
}
Meta-kernel file ``spice_example.tm'':
\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
``Spacecraft Velocity'' Hints
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 CSPICE. ``Spacecraft Velocity'' Solution Steps
The spacecraft velocity vector is the last three components of the state returned by spkezr_c. To compute velocity of CASSINI with respect to Phoebe in the J2000 inertial frame the spkezr_c arguments should be set to "CASSINI" (TARG), "PHOEBE" (OBS), "J2000" (REF) and "NONE" (ABCORR). For convenience the velocity can be copied from the output state in to a 3d vector using the vpack_c routine (``cspice/src/cspice/vpack_c.c''). The computed velocity vector has to be rotated from the J2000 frame to the instrument frame. The pxform_c 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 rotation matrix using the mxv_c routine. After applying the rotation, normalize the resultant vector using the vhat_c routine. Putting it all together, we get:
SpiceDouble scvdir [3];
SpiceDouble j2imat [3][3];
...
target = "CASSINI";
frame = "J2000";
corrtn = "NONE";
observ = "PHOEBE";
spkezr_c ( target, et, frame, corrtn, observ, state, <ime );
vpack_c ( state[3], state[4], state[5], scvdir );
fromfr = "J2000";
tofr = "CASSINI_INMS";
pxform_c ( fromfr, tofr, et, j2imat );
mxv_c ( j2imat, scvdir, scvdir );
vhat_c ( scvdir, scvdir );
printf ( "scvdir = %e %e %e \n",
scvdir[0], scvdir[1], scvdir[2] );
The updated program with added calls, required declarations and simple
print statements produces the following output (the output below was
generated by this program compiled with gcc on a PC running Linux; your
output may differ slightly in its format and numeric precision):
> ./spice_example utc = 2004-06-11T19:32:00 et = 140254384.184625 sclk = 1465674964.105 et = 140254384.183426 state = -3.765991e+08 1.294488e+09 -7.064853e+06 -5.164226e+00 8.017189e-01 4.060306e-02 sundir = -2.902040e-01 8.816312e-01 3.721667e-01 lon = 2.342316e+01 lat = 3.709797e+00 sbpdir = -7.762071e-04 -9.998732e-01 -1.590546e-02 scvdir = 3.957849e-01 -2.928077e-01 8.704125e-01 >Note that computing the spacecraft velocity in the instrument frame by a single call to spkezr_c by specifying "CASSINI_INMS" in the `ref' argument returns 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.c'':
#include <stdio.h>
#include "SpiceUsr.h"
int main()
{
SpiceChar * mkfile;
SpiceChar * utc;
SpiceChar * sclk;
SpiceChar * target;
SpiceChar * frame;
SpiceChar * corrtn;
SpiceChar * observ;
SpiceChar * method;
SpiceChar * fromfr;
SpiceChar * tofr;
SpiceDouble et;
SpiceDouble state [6];
SpiceDouble sundir [3];
SpiceDouble ltime;
SpiceDouble spoint [3];
SpiceDouble trgepc;
SpiceDouble srfvec [3];
SpiceDouble srad;
SpiceDouble slon;
SpiceDouble slat;
SpiceDouble sbpdir [3];
SpiceDouble m2imat [3][3];
SpiceDouble scvdir [3];
SpiceDouble j2imat [3][3];
SpiceInt scid;
mkfile = "spice_example.tm";
furnsh_c ( mkfile );
utc = "2004-06-11T19:32:00";
str2et_c ( utc, &et );
printf ( "utc = %s \n", utc );
printf ( "et = %20.6f \n", et );
scid = -82;
sclk = "1465674964.105";
scs2e_c ( scid, sclk, &et );
printf ( "sclk = %s \n", sclk );
printf ( "et = %20.6f \n", et );
target = "CASSINI";
frame = "ECLIPJ2000";
corrtn = "NONE";
observ = "SUN";
spkezr_c ( target, et, frame, corrtn, observ, state, <ime );
printf ( "state = %e %e %e %e %e %e\n",
state[0], state[1], state[2],
state[3], state[4], state[5] );
target = "SUN";
frame = "CASSINI_INMS";
corrtn = "LT+S";
observ = "CASSINI";
spkpos_c ( target, et, frame, corrtn, observ, sundir, <ime );
vhat_c ( sundir, sundir );
printf ( "sundir = %e %e %e\n",
sundir[0], sundir[1], sundir[2] );
method = "NEAR POINT: ELLIPSOID";
target = "PHOEBE";
frame = "IAU_PHOEBE";
corrtn = "NONE";
observ = "CASSINI";
subpnt_c ( method, target, et, frame, corrtn, observ,
spoint, &trgepc, srfvec );
reclat_c ( spoint, &srad, &slon, &slat );
fromfr = "IAU_PHOEBE";
tofr = "CASSINI_INMS";
pxform_c ( fromfr, tofr, et, m2imat );
mxv_c ( m2imat, srfvec, sbpdir );
vhat_c ( sbpdir, sbpdir );
printf ( "lon = %e \n", slon * dpr_c() );
printf ( "lat = %e \n", slat * dpr_c() );
printf ( "sbpdir = %e %e %e \n",
sbpdir[0], sbpdir[1], sbpdir[2] );
target = "CASSINI";
frame = "J2000";
corrtn = "NONE";
observ = "PHOEBE";
spkezr_c ( target, et, frame, corrtn, observ, state, <ime );
vpack_c ( state[3], state[4], state[5], scvdir );
fromfr = "J2000";
tofr = "CASSINI_INMS";
pxform_c ( fromfr, tofr, et, j2imat );
mxv_c ( j2imat, scvdir, scvdir );
vhat_c ( scvdir, scvdir );
printf ( "scvdir = %e %e %e \n",
scvdir[0], scvdir[1], scvdir[2] );
return ( 0 );
}
Meta-kernel file ``spice_example.tm'':
\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
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