Remote Sensing Hands-On Lesson, using CASSINI (Python)¶
November 20, 2017
Overview¶
In this lesson you will develop a series of simple programs that demonstrate the usage of SpiceyPy 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¶
This section lists SPICE documents referred to in this lesson.
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 the 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
The following SPICE tutorials serve as references for the discussions in this lesson:
Name Lesson steps/functions 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
DSK Detailed Target Shape (Topography) Data
These tutorials are available from the NAIF ftp server at JPL:
http://naif.jpl.nasa.gov/naif/tutorials.html
Required Readings
The Required Reading documents are provided with the Toolkit and are located under the “cspice/doc” directory in the CSPICE Toolkit installation tree.
Name Lesson steps/functions that it describes
--------------- -----------------------------------------
ck.req Obtaining spacecraft orientation data
dsk.req Obtaining detailed body shape data
frames.req Using reference frames
naif_ids.req Determining body ID codes
pck.req Obtaining planetary constants data
sclk.req SCLK time conversion
spk.req Obtaining ephemeris Data
time.req Time conversion
The Permuted Index
Another useful document distributed with the Toolkit is the permuted index. This is located under the “cspice/doc” directory in the C installation tree.
This text document provides a simple mechanism by which users can discover which SpiceyPy functions perform functions of interest, as well as the names of the source files that contain these functions.
SpiceyPy API Documentation
A SpiceyPy function’s parameters specification is available using the built-in Python help system.
For example, the Python help function
>>> import spiceypy
>>> help(spiceypy.str2et)
describes of the str2et function’s parameters, while the document
https://naif.jpl.nasa.gov/pub/naif/toolkit_docs/C/cspice/str2et_c.html
describes extensively the str2et functionality.
Kernels Used¶
The following kernels are used in examples provided in this lesson:
# FILE NAME TYPE DESCRIPTION
-- ------------------------- ---- -----------------------------------
1 naif0008.tls LSK Generic LSK
2 cas00084.tsc SCLK Cassini SCLK
3 981005_PLTEPH-DE405S.bsp SPK Solar System Ephemeris
4 020514_SE_SAT105.bsp SPK Saturnian Satellite Ephemeris
5 030201AP_SK_SM546_T45.bsp SPK Cassini Spacecraft SPK
6 cas_v37.tf FK Cassini FK
7 04135_04171pc_psiv2.bc CK Cassini Spacecraft CK
8 cpck05Mar2004.tpc PCK Cassini Project PCK
9 phoebe_64q.bds DSK Phoebe DSK
10 cas_iss_v09.ti IK ISS Instrument Kernel
These 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/
In addition to these kernels, the extra credit exercises require the following kernels:
# FILE NAME TYPE DESCRIPTION
-- --------------- ---- ---------------------------------------------
11 jup310_2004.bsp SPK Generic Jovian Satellite Ephemeris
These SPICE kernels are available from the NAIF server at JPL:
https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/satellites/
SpiceyPy Modules Used¶
This section provides a complete list of the functions and kernels that are suggested for usage in each of the exercises in this lesson. (You may wish to not look at this list unless/until you “get stuck” while working on your own.)
CHAPTER EXERCISE FUNCTIONS NON-VOID KERNELS
------- --------- --------------- --------------- ----------
1 convtm spiceypy.furnsh spiceypy.str2et 1,2
spiceypy.unload spiceypy.etcal
spiceypy.timout
spiceypy.sce2s
extra (*) spiceypy.unitim 1,2
spiceypy.sct2e
spiceypy.et2utc
spiceypy.scs2e
2 getsta spiceypy.furnsh spiceypy.str2et 1,3-5
spiceypy.unload spiceypy.spkezr
spiceypy.spkpos
spiceypy.vnorm
spiceypy.convrt
extra (*) spiceypy.kclear 1,3-5,11
3 xform spiceypy.furnsh spiceypy.str2et 1-8
spiceypy.unload spiceypy.spkezr
spiceypy.sxform
spiceypy.mxvg
spiceypy.spkpos
spiceypy.pxform
spiceypy.mxv
spiceypy.convrt
spiceypy.vsep
extra (*) spiceypy.kclear 1-8
4 subpts spiceypy.furnsh spiceypy.str2et 1,3-5,8,9
spiceypy.unload spiceypy.subpnt
spiceypy.vnorm
spiceypy.subslr
extra (*) spiceypy.kclear spiceypy.reclat 1,3-5,8
spiceypy.dpr
spiceypy.bodvrd
spiceypy.recpgr
5 fovint spiceypy.furnsh spiceypy.str2et 1-10
spiceypy.unload spiceypy.bodn2c
spiceypy.getfov
spiceypy.sincpt
spiceypy.reclat
spiceypy.dpr
spiceypy.illumf
spiceypy.et2lst
(*) Additional APIs and kernels used in Extra Credit tasks.
Use the Python built-in help system on the various functions listed above for the API parameters’ description, and refer to the headers of their corresponding CSPICE versions for detailed interface specifications.
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” (time.req) and” SCLK Required Reading” (sclk.req) documents.
When completing the “calendar format” step above, consider using one of two possible methods: spiceypy.etcal or spiceypy.timout.
Solution¶
Solution Meta-Kernel
The meta-kernel we created for the solution to this exercise is named ‘convtm.tm’. 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.
The names and contents of the kernels referenced by this
meta-kernel are as follows:
File name Contents
-------------------------- -----------------------------
naif0008.tls Generic LSK
cas00084.tsc Cassini SCLK
\begindata
KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
'kernels/sclk/cas00084.tsc' )
\begintext
Solution Source Code
A sample solution to the problem follows:
#
# Solution convtm
#
from __future__ import print_function
from builtins import input
import spiceypy
def convtm():
#
# Local Parameters
#
METAKR = 'convtm.tm'
SCLKID = -82
spiceypy.furnsh( METAKR )
#
# Prompt the user for the input time string.
#
utctim = input( 'Input UTC Time: ' )
print( 'Converting UTC Time: {:s}'.format( utctim ) )
#
# Convert utctim to ET.
#
et = spiceypy.str2et( utctim )
print( ' ET Seconds Past J2000: {:16.3f}'.format( et ) )
#
# Now convert ET to a calendar time string.
# This can be accomplished in two ways.
#
calet = spiceypy.etcal( et )
print( ' Calendar ET (etcal): {:s}'.format( calet ) )
#
# Or use timout for finer control over the
# output format. The picture below was built
# by examining the header of timout.
#
calet = spiceypy.timout( et, 'YYYY-MON-DDTHR:MN:SC ::TDB' )
print( ' Calendar ET (timout): {:s}'.format( calet ) )
#
# Convert ET to spacecraft clock time.
#
sclkst = spiceypy.sce2s( SCLKID, et )
print( ' Spacecraft Clock Time: {:s}'.format( sclkst ) )
spiceypy.unload( METAKR )
if __name__ == '__main__':
convtm()
Solution Sample Output
Execute the program:
Input UTC Time: 2004 jun 11 19:32:00
Converting UTC Time: 2004 jun 11 19:32:00
ET Seconds Past J2000: 140254384.185
Calendar ET (etcal): 2004 JUN 11 19:33:04.184
Calendar ET (timout): 2004-JUN-11T19:33:04
Spacecraft Clock Time: 1/1465674964.105
Extra Credit¶
In this “extra credit” section you will be presented with more complex tasks, aimed at improving your understanding of time conversions, the Toolkit routines that deal with them, and some common errors that may happen during the execution of these conversions.
These “extra credit” tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks.
Task statements and questions
1. Extend your program to convert the input UTC time string to TDB
Julian Date. Convert "2004 jun 11 19:32:00" UTC.
2. Remove the LSK from the original meta-kernel and run your
program again, using the same inputs as before. Has anything
changed? Why?
3. Remove the SCLK from the original meta-kernel and run your
program again, using the same inputs as before. Has anything
changed? Why?
4. Modify your program to perform conversion of UTC or ephemeris
time, to a spacecraft clock string using the NAIF ID for the
CASSINI ISS NAC camera. Convert "2004 jun 11 19:32:00" UTC.
5. Find the earliest UTC time that can be converted to CASSINI
spacecraft clock.
6. Extend your program to convert the spacecraft clock time
obtained in the regular task back to UTC Time and present it in
ISO calendar date format, with a resolution of milliseconds.
7. Examine the contents of the generic LSK and the CASSINI SCLK
kernels. Can you understand and explain what you see?
Solutions and answers
1. Two methods exist in order to convert ephemeris time to Julian
Date: spiceypy.unitim and spiceypy.timout. The difference
between them is the type of output produced by each method.
spiceypy.unitim returns the double precision value of an input
epoch, while spiceypy.timout returns the string representation
of the ephemeris time in Julian Date format (when picture input
is set to 'JULIAND.######### ::TDB'). Refer to the function
header for further details. The solution for the requested
input UTC string is:
Julian Date TDB: 2453168.3146318
2. When running the original program without the LSK kernel, an
error is produced:
Traceback (most recent call last):
File "convtm.py", line 67, in <module>
convtm()
File "convtm.py", line 30, in convtm
et = spiceypy.str2et( utctim )
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 76, in with_errcheck
checkForSpiceError(f)
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 59, in checkForSpiceError
raise stypes.SpiceyError(msg)
spiceypy.utils.support_types.SpiceyError:
=====================================================================
===========
Toolkit version: N0066
SPICE(NOLEAPSECONDS) --
The variable that points to the leapseconds (DELTET/DELTA_AT) could n
ot be located in the kernel pool. It is likely that the leapseconds
kernel has not been loaded via the routine FURNSH.
str2et_c --> STR2ET --> TTRANS
=====================================================================
===========
This error is triggered by spiceypy.str2et because the variable
that points to the leapseconds is not present in the kernel
pool and therefore the program lacks data required to perform
the requested UTC to ephemeris time conversion.
By default, SPICE will report, as a minimum, a short
descriptive message and a expanded form of this short message
where more details about the error are provided. If this error
message is not sufficient for you to understand what has
happened, you could go to the "Exceptions" section in the
SPICELIB or CSPICE headers of the function that has triggered
the error and find out more information about the possible
causes.
3. When running the original program without the SCLK kernel, an
error is produced:
Traceback (most recent call last):
File "convtm.py", line 67, in <module>
convtm()
File "convtm.py", line 58, in convtm
sclkst = spiceypy.sce2s( SCLKID, et )
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 76, in with_errcheck
checkForSpiceError(f)
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 59, in checkForSpiceError
raise stypes.SpiceyError(msg)
spiceypy.utils.support_types.SpiceyError:
=====================================================================
===========
Toolkit version: N0066
SPICE(KERNELVARNOTFOUND) --
The Variable Was not Found in the Kernel Pool.
SCLK_DATA_TYPE_82 not found. Did you load the SCLK kernel?
sce2s_c --> SCE2S --> SCE2T --> SCTYPE --> SCLI01
=====================================================================
===========
This error is triggered by spiceypy.sce2s. In this case the
error message may not give you enough information to understand
what has actually happened. Nevertheless, the expanded form of
this short message clearly indicates that the SCLK kernel for
the spacecraft ID -82 has not been loaded.
The UTC string to ephemeris time conversion and the conversion
of ephemeris time into a calendar format worked normally as
these conversions only require the LSK kernel to be loaded.
4. The first thing you need to do is to find out what the NAIF ID
is for the CASSINI ISS NAC camera. In order to do so, examine
the ISS instrument kernel listed above and look for the "NAIF
ID Code to Name Mapping" and there, for the NAIF ID given to
CASSINI_ISS_NAC (which is -82360). Then replace in your code
the SCLK ID -82 with -82360. After executing the program using
the original meta-kernel, you will be getting the same error as
in the previous task. Despite the error being exactly the same,
this case is different. Generally, spacecraft clocks are
associated with the spacecraft ID and not with its payload,
sensors or structures IDs. Therefore, in order to do
conversions from/to spacecraft clock for payload, sensors or
spacecraft structures, the spacecraft ID must be used.
Note that this does not need to be true for all missions or
payloads, as SPICE does not restrict the SCLKs to spacecraft
IDs only. Please refer to your mission's SCLK kernels for
particulars.
5. Use spiceypy.sct2e with the encoding of the Cassini spacecraft
clock time set to 0.0 ticks and convert the resulting ephemeris
time to UTC using either spiceypy.timout or spiceypy.et2utc.
The solution for the requested SCLK string is:
Earliest UTC convertible to SCLK: 1980-01-01T00:00:00.000
6. Use spiceypy.scs2e with the SCLK string obtained in the
computations performed in the regular tasks and convert the
resulting ephemeris time to UTC using either spiceypy.et2utc,
with 'ISOC' format and 3 digits precision, or using
spiceypy.timout using the time picture 'YYYY-MM-DDTHR:MN:SC.###
::RND'. The solution of the requested conversion is:
Spacecraft Clock Time: 1/1465674964.105
UTC time from spacecraft clock: 2004-06-11T19:31:59.999
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 spiceypy.spkezr call. Discover the difference between spiceypy.spkezr and spiceypy.spkpos. Familiarity with the Toolkit utility “brief”. Exposure to unit conversion with SpiceyPy.
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” (spk.req) document.
When deciding which SPK files to load, the Toolkit utility “brief” may be of some use.
“brief” is located in the” cspice/exe”directory for C toolkits. Consult its user’s guide available in “cspice/doc/brief.ug” for details.
Solution¶
Solution Meta-Kernel
The meta-kernel we created for the solution to this exercise is named ‘getsta.tm’. 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.
The names and contents of the kernels referenced by this
meta-kernel are as follows:
File name Contents
-------------------------- -----------------------------
naif0008.tls Generic LSK
981005_PLTEPH-DE405S.bsp Solar System Ephemeris
020514_SE_SAT105.bsp Saturnian Satellite Ephemeris
030201AP_SK_SM546_T45.bsp Cassini Spacecraft SPK
\begindata
KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
'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:
#
# Solution getsta.py
#
from __future__ import print_function
from builtins import input
import spiceypy
def getsta():
#
# Local parameters
#
METAKR = 'getsta.tm'
#
# 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.
#
spiceypy.furnsh( METAKR )
#
#Prompt the user for the input time string.
#
utctim = input( 'Input UTC Time: ' )
print( 'Converting UTC Time: {:s}'.format(utctim) )
#
#Convert utctim to ET.
#
et = spiceypy.str2et( utctim )
print( ' ET seconds past J2000: {:16.3f}'.format(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.
#
[state, ltime] = spiceypy.spkezr( 'PHOEBE', et, 'J2000',
'LT+S', 'CASSINI' )
print( ' Apparent state of Phoebe as seen '
'from CASSINI in the J2000\n'
' frame (km, km/s):' )
print( ' X = {:16.3f}'.format(state[0]) )
print( ' Y = {:16.3f}'.format(state[1]) )
print( ' Z = {:16.3f}'.format(state[2]) )
print( ' VX = {:16.3f}'.format(state[3]) )
print( ' VY = {:16.3f}'.format(state[4]) )
print( ' VZ = {:16.3f}'.format(state[5]) )
#
# Compute the apparent position of Earth as seen from
# CASSINI in the J2000 frame. Note: We could have
# continued using spkezr and simply ignored the
# velocity components.
#
[pos, ltime] = spiceypy.spkpos( 'EARTH', et, 'J2000',
'LT+S', 'CASSINI', )
print( ' Apparent position of Earth as '
'seen from CASSINI in the J2000\n'
' frame (km):' )
print( ' X = {:16.3f}'.format(pos[0]) )
print( ' Y = {:16.3f}'.format(pos[1]) )
print( ' Z = {:16.3f}'.format(pos[2]) )
#
# We need only display LTIME, as it is precisely the
# light time in which we are interested.
#
print( ' One way light time between CASSINI and '
'the apparent position\n'
' of Earth (seconds):'
' {:16.3f}'.format(ltime) )
#
# Compute the apparent position of the Sun as seen from
# PHOEBE in the J2000 frame.
#
[pos, ltime] = spiceypy.spkpos( 'SUN', et, 'J2000',
'LT+S', 'PHOEBE', )
print( ' Apparent position of Sun as '
'seen from Phoebe in the\n'
' J2000 frame (km):' )
print( ' X = {:16.3f}'.format(pos[0]) )
print( ' Y = {:16.3f}'.format(pos[1]) )
print( ' Z = {:16.3f}'.format(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.
#
[pos, ltime] = spiceypy.spkpos( 'SUN', et, 'J2000',
'NONE', 'PHOEBE' )
#
# Compute the distance between the body centers in
# kilometers.
#
dist = spiceypy.vnorm( pos )
#
# Convert this value to AU using convrt.
#
dist = spiceypy.convrt( dist, 'KM', 'AU' )
print( ' Actual distance between Sun and '
'Phoebe body centers:\n'
' (AU): {:16.3f}'.format(dist) )
spiceypy.unload( METAKR )
if __name__ == '__main__':
getsta()
Solution Sample Output
Execute the program:
Input UTC Time: 2004 jun 11 19:32:00
Converting UTC Time: 2004 jun 11 19:32:00
ET seconds past J2000: 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 (km):
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 (km):
X = 376551465.272
Y = -1190495630.303
Z = -508438699.110
Actual distance between Sun and Phoebe body centers:
(AU): 9.012
Extra Credit¶
In this “extra credit” section you will be presented with more complex tasks, aimed at improving your understanding of state computations, particularly the application of the different light time and stellar aberration corrections available in the spiceypy.spkezr function, and some common errors that may happen when computing these states.
These “extra credit” tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks.
Task statements and questions
1. Remove the Solar System ephemerides SPK from the original
meta-kernel and run your program again, using the same inputs
as before. Has anything changed? Why?
2. Extend your program to compute the geometric position of
Jupiter as seen from Saturn in the J2000 frame (J2000), in
kilometers.
3. Extend, or modify, your program to compute the position of the
Sun as seen from Saturn in the J2000 frame (J2000), in
kilometers, using the following light time and aberration
corrections: NONE, LT and LT+S. Explain the differences.
4. Examine the CASSINI frames definition kernel and the ISS
instrument kernel to find the SPICE ID/name definitions.
Solutions and answers
1. When running the original program without the Solar System
ephemerides SPK, an error is produced by spiceypy.spkezr:
Traceback (most recent call last):
File "getsta.py", line 128, in <module>
getsta()
File "getsta.py", line 47, in getsta
'LT+S', 'CASSINI' )
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 76, in with_errcheck
checkForSpiceError(f)
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 59, in checkForSpiceError
raise stypes.SpiceyError(msg)
spiceypy.utils.support_types.SpiceyError:
=====================================================================
===========
Toolkit version: N0066
SPICE(SPKINSUFFDATA) --
Insufficient ephemeris data has been loaded to compute the state of -
82 (CASSINI) relative to 0 (SOLAR SYSTEM BARYCENTER) at the ephemeris
epoch 2004 JUN 11 19:33:04.184.
spkezr_c --> SPKEZR --> SPKEZ --> SPKACS --> SPKGEO
=====================================================================
===========
This error is generated when trying to compute the apparent
state of Phoebe as seen from CASSINI in the J2000 frame because
despite both Phoebe and CASSINI ephemeris data being relative
to the Saturn Barycenter, the state of the spacecraft with
respect to the solar system barycenter is required to compute
the light time and stellar aberrations. The loaded SPK data are
enough to compute geometric states of CASSINI with respect to
the Saturn Barycenter, and geometric states of Phoebe with
respect to the Saturn Barycenter, but insufficient to compute
the state of the spacecraft relative to the Solar System
Barycenter because the SPK data needed to compute geometric
states of Saturn Barycenter relative to the Solar System
barycenter are no longer loaded. Run "brief" on the SPKs used
in the original task to find out which ephemeris objects are
available from those kernels. If you want to find out what is
the 'center of motion' for the ephemeris object(s) included in
an SPK, use the -c option when running "brief":
BRIEF -- Version 4.0.0, September 8, 2010 -- Toolkit Version N0066
Summary for: kernels/spk/981005_PLTEPH-DE405S.bsp
Bodies: MERCURY BARYCENTER (1) w.r.t. SOLAR SYSTEM BARYCENTER (0)
VENUS BARYCENTER (2) w.r.t. SOLAR SYSTEM BARYCENTER (0)
EARTH BARYCENTER (3) w.r.t. SOLAR SYSTEM BARYCENTER (0)
MARS BARYCENTER (4) w.r.t. SOLAR SYSTEM BARYCENTER (0)
JUPITER BARYCENTER (5) w.r.t. SOLAR SYSTEM BARYCENTER (0)
SATURN BARYCENTER (6) w.r.t. SOLAR SYSTEM BARYCENTER (0)
URANUS BARYCENTER (7) w.r.t. SOLAR SYSTEM BARYCENTER (0)
NEPTUNE BARYCENTER (8) w.r.t. SOLAR SYSTEM BARYCENTER (0)
PLUTO BARYCENTER (9) w.r.t. SOLAR SYSTEM BARYCENTER (0)
SUN (10) w.r.t. SOLAR SYSTEM BARYCENTER (0)
MERCURY (199) w.r.t. MERCURY BARYCENTER (1)
VENUS (299) w.r.t. VENUS BARYCENTER (2)
MOON (301) w.r.t. EARTH BARYCENTER (3)
EARTH (399) w.r.t. EARTH BARYCENTER (3)
MARS (499) w.r.t. MARS BARYCENTER (4)
Start of Interval (UTC) End of Interval (UTC)
----------------------------- -------------------------
----
2004-JUN-11 05:00:00.000 2004-JUN-12 12:00:00.000
Summary for: kernels/spk/020514_SE_SAT105.bsp
Bodies: MIMAS (601) w.r.t. SATURN BARYCENTER (6)
ENCELADUS (602) w.r.t. SATURN BARYCENTER (6)
TETHYS (603) w.r.t. SATURN BARYCENTER (6)
DIONE (604) w.r.t. SATURN BARYCENTER (6)
RHEA (605) w.r.t. SATURN BARYCENTER (6)
TITAN (606) w.r.t. SATURN BARYCENTER (6)
HYPERION (607) w.r.t. SATURN BARYCENTER (6)
IAPETUS (608) w.r.t. SATURN BARYCENTER (6)
PHOEBE (609) w.r.t. SATURN BARYCENTER (6)
SATURN (699) w.r.t. SATURN BARYCENTER (6)
Start of Interval (UTC) End of Interval (UTC)
----------------------------- -------------------------
----
2004-JUN-11 05:00:00.000 2004-JUN-12 12:00:00.000
Summary for: kernels/spk/030201AP_SK_SM546_T45.bsp
Body: CASSINI (-82) w.r.t. SATURN BARYCENTER (6)
Start of Interval (UTC) End of Interval (UTC)
----------------------------- ---------------------------
--
2004-JUN-11 05:00:00.000 2004-JUN-12 12:00:00.000
2. If you run your extended program with the original meta-kernel,
the SPICE(SPKINSUFFDATA) error should be produced by the
spiceypy.spkpos function because you have not loaded enough
ephemeris data to compute the position of Jupiter with respect
to Saturn. The loaded SPKs contain data for Saturn relative to
the Solar System Barycenter, and for the Jupiter System
Barycenter relative to the Solar System Barycenter, but the
data for Jupiter relative to the Jupiter System Barycenter are
missing:
Additional kernels required for this task:
File name Contents
----------------------- ----------------------------------
jup310_2004.bsp Generic Jovian Satellite Ephemeris
available in the NAIF server at:
https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/satellites/
Download the relevant SPK, add it to the meta-kernel and run
again your extended program. The solution for the input UTC
time "2004 jun 11 19:32:00" when using the downloaded Jovian
Satellite Ephemeris SPK:
Actual position of Jupiter as seen from Saturn in the
J2000 frame (km):
X = -436016583.291
Y = -1094176737.323
Z = -446585337.431
3. When using 'NONE' aberration corrections, spiceypy.spkpos
returns the geometric position of the target body relative to
the observer. If 'LT' is used, the returned vector corresponds
to the position of the target at the moment it emitted photons
arriving at the observer at `et'. If 'LT+S' is used instead,
the returned vector takes into account the observer's velocity
relative to the solar system barycenter. The solution for the
input UTC time "2004 jun 11 19:32:00" is:
Actual (geometric) position of Sun as seen from Saturn in the
J2000 frame (km):
X = 367770592.367
Y = -1197330367.359
Z = -510369088.677
Light-time corrected position of Sun as seen from Saturn in the
J2000 frame (km):
X = 367770572.921
Y = -1197330417.733
Z = -510369109.509
Apparent position of Sun as seen from Saturn in the
J2000 frame (km):
X = 367726456.168
Y = -1197342627.879
Z = -510372252.747
Spacecraft Orientation and Reference Frames (xform)¶
Task Statement¶
Write a program that prompts the user for an input time string, computes and displays 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 (HGA).
The HGA boresight direction is provided by the kernel variable
TKFRAME_-82101_BORESIGHT, which is defined in the Cassini frame
kernel cited above in the section "Kernels Used." In this
kernel, the HGA boresight vector is expressed relative to the
CASSINI_HGA reference frame.
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 spiceypy.pxform and spiceypy.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 step using SpiceyPy functions with which you are already familiar. The “long way” presented above is intended to facilitate the introduction of the functions spiceypy.pxform and spiceypy.sxform.
You may find it useful to consult the permuted index, the headers of various source modules, and the following toolkit documentation:
1. Frames Required Reading (frames.req)
2. PCK Required Reading (pck.req)
3. SPK Required Reading (spk.req)
4. CK Required Reading (ck.req)
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.tm’. 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.
The names and contents of the kernels referenced by this
meta-kernel are as follows:
File name Contents
-------------------------- -----------------------------
naif0008.tls Generic LSK
cas00084.tsc Cassini SCLK
981005_PLTEPH-DE405S.bsp Solar System Ephemeris
020514_SE_SAT105.bsp Saturnian Satellite Ephemeris
030201AP_SK_SM546_T45.bsp Cassini Spacecraft SPK
cas_v37.tf Cassini FK
04135_04171pc_psiv2.bc Cassini Spacecraft CK
cpck05Mar2004.tpc Cassini Project PCK
\begindata
KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
'kernels/sclk/cas00084.tsc',
'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:
#
# Solution xform.py
#
from __future__ import print_function
from builtins import input
import spiceypy
def xform():
#
# Local parameters
#
METAKR = 'xform.tm'
#
# 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)
#
spiceypy.furnsh( METAKR )
#
# Prompt the user for the input time string.
#
utctim = input( 'Input UTC Time: ' )
print( 'Converting UTC Time: {:s}'.format(utctim) )
#
#Convert utctim to ET.
#
et = spiceypy.str2et( utctim )
print( ' ET seconds past J2000: {:16.3f}'.format(et) )
#
# Compute the apparent state of Phoebe as seen from
# CASSINI in the J2000 frame.
#
[state, ltime] = spiceypy.spkezr( 'PHOEBE', et, 'J2000',
'LT+S', 'CASSINI' )
#
# 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.
#
sform = spiceypy.sxform( 'J2000', 'IAU_PHOEBE', et-ltime )
#
# Now rotate the apparent J2000 state into IAU_PHOEBE
# with the following matrix multiplication:
#
bfixst = spiceypy.mxvg ( sform, state, 6, 6 )
#
# Display the results.
#
print( ' Apparent state of Phoebe as seen '
'from CASSINI in the IAU_PHOEBE\n'
' body-fixed frame (km, km/s):' )
print( ' X = {:19.6f}'.format(bfixst[0]) )
print( ' Y = {:19.6f}'.format(bfixst[1]) )
print( ' Z = {:19.6f}'.format(bfixst[2]) )
print( ' VX = {:19.6f}'.format(bfixst[3]) )
print( ' VY = {:19.6f}'.format(bfixst[4]) )
print( ' VZ = {:19.6f}'.format(bfixst[5]) )
#
# It is worth pointing out, all of the above could
# have been done with a single use of spkezr:
#
[state, ltime] = spiceypy.spkezr(
'PHOEBE', et, 'IAU_PHOEBE',
'LT+S', 'CASSINI' )
#
# Display the results.
#
print( ' Apparent state of Phoebe as seen '
'from CASSINI in the IAU_PHOEBE\n'
' body-fixed frame (km, km/s) '
'obtained using spkezr directly:' )
print( ' X = {:19.6f}'.format(state[0]) )
print( ' Y = {:19.6f}'.format(state[1]) )
print( ' Z = {:19.6f}'.format(state[2]) )
print( ' VX = {:19.6f}'.format(state[3]) )
print( ' VY = {:19.6f}'.format(state[4]) )
print( ' VZ = {:19.6f}'.format(state[5]) )
#
# Note that the velocity found by using spkezr
# to compute the state in the IAU_PHOEBE frame differs
# at the few mm/second level from that found previously
# by calling spkezr and then sxform. Computing
# velocity via a single call to spkezr as we've
# done immediately above is slightly more accurate because
# it accounts for the effect of the rate of change of
# light time on the apparent angular velocity of the
# target's body-fixed reference frame.
#
# 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.
#
[pos, ltime] = spiceypy.spkpos( 'EARTH', et, 'J2000',
'LT+S', 'CASSINI' )
#
# 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 = [ 0.0, 0.0, 1.0]
#
# Now compute the rotation matrix from CASSINI_HGA into
# J2000.
#
pform = spiceypy.pxform( 'CASSINI_HGA', 'J2000', et )
#
# And multiply the result to obtain the nominal
# antenna boresight in the J2000 reference frame.
#
bsight = spiceypy.mxv( pform, bsight )
#
# Lastly compute the angular separation.
#
sep = spiceypy.convrt( spiceypy.vsep(bsight, pos),
'RADIANS', 'DEGREES' )
print( ' Angular separation between the '
'apparent position of\n'
' Earth and the CASSINI high '
'gain antenna boresight (degrees):\n'
' {:16.3f}'.format(sep) )
#
# Or alternatively we can work in the antenna
# frame directly.
#
[pos, ltime] = spiceypy.spkpos(
'EARTH', et, 'CASSINI_HGA',
'LT+S', 'CASSINI' )
#
# The antenna boresight is the Z-axis in the
# CASSINI_HGA frame.
#
bsight = [ 0.0, 0.0, 1.0 ]
#
# Lastly compute the angular separation.
#
sep = spiceypy.convrt( spiceypy.vsep(bsight, pos),
'RADIANS', 'DEGREES' )
print( ' Angular separation between the '
'apparent position of\n'
' Earth and the CASSINI high '
'gain antenna boresight computed\n'
' using vectors in the CASSINI_HGA '
'frame (degrees):\n'
' {:16.3f}'.format(sep) )
spiceypy.unload( METAKR )
if __name__ == '__main__':
xform()
Solution Sample Output
Execute the program:
Input UTC Time: 2004 jun 11 19:32:00
Converting UTC Time: 2004 jun 11 19:32:00
ET seconds past J2000: 140254384.185
Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
body-fixed frame (km, km/s):
X = -1982.639762
Y = -934.530471
Z = -166.562595
VX = 3.970833
VY = -3.812498
VZ = -2.371663
Apparent state of Phoebe as seen from CASSINI in the IAU_PHOEBE
body-fixed frame (km, km/s) obtained using spkezr directly:
X = -1982.639762
Y = -934.530471
Z = -166.562595
VX = 3.970832
VY = -3.812496
VZ = -2.371663
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
Extra Credit¶
In this “extra credit” section you will be presented with more complex tasks, aimed at improving your understanding of frame transformations, and some common errors that may happen when computing them.
These “extra credit” tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks.
Task statements and questions
1. Run the original program using the input UTC time "2004 jun 11
18:25:00". Explain what happens.
2. Compute the angular separation between the apparent position of
the Sun as seen from CASSINI and the nominal boresight of the
CASSINI high gain antenna (HGA). Is the HGA illuminated?
Solutions and answers
1. When running the original software using as input the UTC time
string "2004 jun 11 18:25:00":
Traceback (most recent call last):
File "xform.py", line 183, in <module>
xform()
File "xform.py", line 130, in xform
pform = spiceypy.pxform( 'CASSINI_HGA', 'J2000', et )
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 76, in with_errcheck
checkForSpiceError(f)
File "/home/bsemenov/local/lib/python3.5/site-packages/spiceypy/spi
ceypy.py", line 59, in checkForSpiceError
raise stypes.SpiceyError(msg)
spiceypy.utils.support_types.SpiceyError:
=====================================================================
===========
Toolkit version: N0066
SPICE(NOFRAMECONNECT) --
At epoch 1.4025036418463E+08 TDB (2004 JUN 11 18:26:04.184 TDB), ther
e is insufficient information available to transform from reference f
rame -82101 (CASSINI_HGA) to reference frame 1 (J2000). Frame CASSINI
_HGA could be transformed to frame -82000 (CASSINI_SC_COORD). The lat
ter is a CK frame; a CK file containing data
pxform_c --> PXFORM --> REFCHG
=====================================================================
===========
spiceypy.pxform returns the SPICE(NOFRAMECONNECT) error, which
indicates that there are not sufficient data to perform the
transformation from the CASSINI_HGA frame to J2000 at the
requested epoch. If you summarize the CASSINI spacecraft CK
using the "ckbrief" utility program with the -dump option
(display interpolation intervals boundaries) you will find that
the CK contains gaps within its segment:
CKBRIEF -- Version 6.1.0, June 27, 2014 -- Toolkit Version N0066
Summary for: kernels/ck/04135_04171pc_psiv2.bc
Segment No.: 1
Object: -82000
Interval Begin UTC Interval End UTC AV
------------------------ ------------------------ ---
2004-JUN-11 05:00:00.000 2004-JUN-11 09:25:02.019 Y
2004-JUN-11 09:26:14.019 2004-JUN-11 18:24:37.152 Y
2004-JUN-11 18:26:13.152 2004-JUN-12 05:53:26.012 Y
2004-JUN-12 05:54:56.012 2004-JUN-12 10:32:08.016 Y
2004-JUN-12 10:33:26.016 2004-JUN-12 11:59:59.998 Y
whereas if you had used ckbrief without -dump you would have
gotten the following information (only CK segment begin/end
times):
CKBRIEF -- Version 6.1.0, June 27, 2014 -- Toolkit Version N0066
Summary for: kernels/ck/04135_04171pc_psiv2.bc
Object: -82000
Interval Begin UTC Interval End UTC AV
------------------------ ------------------------ ---
2004-JUN-11 05:00:00.000 2004-JUN-12 11:59:59.998 Y
which has insufficient detail to reveal the problem.
2. By computing the apparent position of the Sun as seen from
CASSINI in the CASSINI_HGA frame, and the angular separation
between this vector and the nominal boresight of the CASSINI
high gain antenna (+Z-axis of the CASSINI_HGA frame), you will
find whether the HGA is illuminated. The solution for the input
UTC time "2004 jun 11 19:32:00" is:
Angular separation between the apparent position of the Sun and the
nominal boresight of the CASSINI high gain antenna (degrees):
73.130
HGA illumination:
CASSINI high gain antenna IS illuminated.
since the angular separation is smaller than 90 degrees.
Computing Sub-s/c and Sub-solar Points on an Ellipsoid and a DSK (subpts)¶
Task Statement¶
Write a program that prompts the user for an input UTC time string and 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.
The program computes each point twice: once using an ellipsoidal shape model and the
near point/ellipsoid
definition, and once using a DSK shape model and the
nadir/dsk/unprioritized
definition.
The program 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 SpiceyPy 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: how would the results change if the sub-solar and sub-observer points were computed using the
intercept/ellipsoid
and
intercept/dsk/unprioritized
definitions? Which definition is appropriate?
Solution¶
Solution Meta-Kernel
The meta-kernel we created for the solution to this exercise is named ‘subpts.tm’. 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.
The names and contents of the kernels referenced by this
meta-kernel are as follows:
File name Contents
-------------------------- -----------------------------
naif0008.tls Generic LSK
981005_PLTEPH-DE405S.bsp Solar System Ephemeris
020514_SE_SAT105.bsp Saturnian Satellite Ephemeris
030201AP_SK_SM546_T45.bsp Cassini Spacecraft SPK
cpck05Mar2004.tpc Cassini Project PCK
phoebe_64q.bds Phoebe DSK
\begindata
KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
'kernels/spk/981005_PLTEPH-DE405S.bsp',
'kernels/spk/020514_SE_SAT105.bsp',
'kernels/spk/030201AP_SK_SM546_T45.bsp',
'kernels/pck/cpck05Mar2004.tpc'
'kernels/dsk/phoebe_64q.bds' )
\begintext
Solution Source Code
A sample solution to the problem follows:
#
# Solution subpts.py
#
from __future__ import print_function
from builtins import input
#
# SpiceyPy package:
#
import spiceypy
def subpts():
#
# Local parameters
#
METAKR = 'subpts.tm'
#
# Load the kernels that this program requires. We
# will need:
#
# A leapseconds kernel
# The necessary ephemerides
# A planetary constants file (PCK)
# A DSK file containing Phoebe shape data
#
spiceypy.furnsh( METAKR )
#
#Prompt the user for the input time string.
#
utctim = input( 'Input UTC Time: ' )
print( ' Converting UTC Time: {:s}'.format(utctim) )
#
#Convert utctim to ET.
#
et = spiceypy.str2et( utctim )
print( ' ET seconds past J2000: {:16.3f}'.format(et) )
for i in range(2):
if i == 0:
#
# Use the "near point" sub-point definition
# and an ellipsoidal model.
#
method = 'NEAR POINT/Ellipsoid'
else:
#
# Use the "nadir" sub-point definition
# and a DSK model.
#
method = 'NADIR/DSK/Unprioritized'
print( '\n Sub-point/target shape model: {:s}\n'.format(
method ) )
#
# Compute the apparent sub-observer point of CASSINI
# on Phoebe.
#
[spoint, trgepc, srfvec] = spiceypy.subpnt(
method, 'PHOEBE', et,
'IAU_PHOEBE', 'LT+S', 'CASSINI' )
print( ' Apparent sub-observer point of CASSINI '
'on Phoebe in the\n'
' IAU_PHOEBE frame (km):' )
print( ' X = {:16.3f}'.format(spoint[0]) )
print( ' Y = {:16.3f}'.format(spoint[1]) )
print( ' Z = {:16.3f}'.format(spoint[2]) )
print( ' ALT = {:16.3f}'.format(spiceypy.vnorm(srfvec)) )
#
# Compute the apparent sub-solar point on Phoebe
# as seen from CASSINI.
#
[spoint, trgepc, srfvec] = spiceypy.subslr(
method, 'PHOEBE', et,
'IAU_PHOEBE', 'LT+S', 'CASSINI' )
print( ' Apparent sub-solar point on Phoebe '
'as seen from CASSINI in\n'
' the IAU_PHOEBE frame (km):' )
print( ' X = {:16.3f}'.format(spoint[0]) )
print( ' Y = {:16.3f}'.format(spoint[1]) )
print( ' Z = {:16.3f}'.format(spoint[2]) )
#
# End of computation block for "method"
#
print( " )
spiceypy.unload( METAKR )
if __name__ == '__main__':
subpts()
Solution Sample Output
Execute the program:
Input UTC Time: 2004 jun 11 19:32:00
Converting UTC Time: 2004 jun 11 19:32:00
ET seconds past J2000: 140254384.185
Sub-point/target shape model: NEAR POINT/Ellipsoid
Apparent sub-observer point of CASSINI on Phoebe in the
IAU_PHOEBE frame (km):
X = 104.498
Y = 45.269
Z = 7.383
ALT = 2084.116
Apparent sub-solar point on Phoebe as seen from CASSINI in
the IAU_PHOEBE frame (km):
X = 78.681
Y = 76.879
Z = -21.885
Sub-point/target shape model: NADIR/DSK/Unprioritized
Apparent sub-observer point of CASSINI on Phoebe in the
IAU_PHOEBE frame (km):
X = 95.373
Y = 40.948
Z = 6.610
ALT = 2094.242
Apparent sub-solar point on Phoebe as seen from CASSINI in
the IAU_PHOEBE frame (km):
X = 79.111
Y = 77.338
Z = -22.028
Extra Credit¶
In this “extra credit” section you will be presented with more complex tasks, aimed at improving your understanding of spiceypy.subpnt and spiceypy.subslr functions.
These “extra credit” tasks are provided as task statements, and unlike the regular tasks, no approach or solution source code is provided. In the next section, you will find the numeric solutions (when applicable) and answers to the questions asked in these tasks.
Task statements and questions
1. Recompute the apparent sub-solar point on Phoebe as seen from
CASSINI in the body fixed frame IAU_PHOEBE in kilometers using
the 'Intercept/ellipsoid' method at "2004 jun 11 19:32:00".
Explain the differences.
2. Compute the geometric sub-spacecraft point of CASSINI on Phoebe
in the body fixed frame IAU_PHOEBE in kilometers using the
'Near point/ellipsoid' method at "2004 jun 11 19:32:00".
3. Transform the sub-spacecraft Cartesian coordinates obtained in
the previous task to planetocentric and planetographic
coordinates. When computing planetographic coordinates,
retrieve Phoebe's radii by calling spiceypy.bodvrd and use the
first element of the returned radii values as Phoebe's
equatorial radius. Explain why planetocentric and
planetographic latitudes and longitudes are different. Explain
why the planetographic altitude for a point on the surface of
Phoebe is not zero and whether this is correct or not.
Solutions and answers
1. The differences observed are due to the computation method. The
"Intercept/ellipsoid" method defines the sub-solar point as
the target surface intercept of the line containing the Sun and
the target's center, while the "Near point/ellipsoid" method
defines the sub-solar point as the the nearest point on the
target relative to the Sun. Since Phoebe is not spherical,
these two points are not the same:
Apparent sub-solar point on Phoebe as seen from CASSINI in
the IAU_PHOEBE frame using the 'Near Point: ellipsoid' method
(km):
X = 78.681
Y = 76.879
Z = -21.885
Apparent sub-solar point on Phoebe as seen from CASSINI in
the IAU_PHOEBE frame using the 'Intercept: ellipsoid' method
(km):
X = 74.542
Y = 79.607
Z = -24.871
2. The geometric sub-spacecraft point of CASSINI on Phoebe in the
body fixed frame IAU_PHOEBE in kilometers at "2004 jun 11
19:32:00" UTC epoch is:
Geometric sub-spacecraft point of CASSINI on Phoebe in
the IAU_PHOEBE frame using the 'Near Point: ellipsoid' method
(km):
X = 104.497
Y = 45.270
Z = 7.384
3. The sub-spacecraft point of CASSINI on Phoebe in planetocentric
and planetographic coordinates at "2004 jun 11 19:32:00" UTC
epoch is:
Planetocentric coordinates of the CASSINI
sub-spacecraft point on Phoebe (degrees, km):
LAT = 3.710
LON = 23.423
R = 114.121
Planetographic coordinates of the CASSINI
sub-spacecraft point on Phoebe (degrees, km):
LAT = 4.454
LON = 336.577
ALT = -0.831
The planetocentric and planetographic longitudes are different
("graphic" = 360 - "centric") because planetographic
longitudes on Phoebe are measured positive west as defined by
Phoebe's rotation direction.
The planetocentric and planetographic latitudes are different
because the planetocentric latitude was computed as the angle
between the direction from the center of the body to the point
and the equatorial plane, while the planetographic latitude was
computed as the angle between the surface normal at the point
and the equatorial plane.
The planetographic altitude is non zero because it was computed
using a different and incorrect Phoebe surface model: a
spheroid with equal equatorial radii. The surface point
returned by spiceypy.subpnt was computed by treating Phoebe as
a triaxial ellipsoid with different equatorial radii. The
planetographic latitude is also incorrect because it is based
on the normal to the surface of the spheroid rather than the
ellipsoid, In general planetographic coordinates cannot be used
for bodies with shapes modeled as triaxial ellipsoids.
Intersecting Vectors with an Ellipsoid and a DSK (fovint)¶
Task Statement¶
Write a program that prompts the user for an input UTC time string and, for that time, computes the intersection of the CASSINI ISS NAC camera boresight and field of view (FOV) boundary vectors with the surface of Phoebe. Compute each intercept twice: once with Phoebe’s shape modeled as an ellipsoid, and once with Phoebe’s shape modeled by DSK data. The program presents each point of intersection as
1. A Cartesian vector in the IAU_PHOEBE frame
2. Planetocentric (latitudinal) coordinates in the IAU_PHOEBE
frame.
For each of the camera FOV boundary and boresight vectors, if an intersection is found, the program displays the results of the above computations, otherwise it indicates no intersection exists.
At each point of intersection compute the following:
3. Phase angle
4. Solar incidence angle
5. Emission angle
These angles should be computed using both ellipsoidal and DSK shape models.
Additionally compute the local solar time at the intercept of the camera boresight with the surface of Phoebe, using both ellipsoidal and DSK shape models.
Use this program to compute values at the epoch:
"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 target bodies whose shapes are modeled as ellipsoids or provided by DSKs. Discover another high level geometry function and another time conversion function in SpiceyPy.
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 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 modeled as an
ellipsoid or using DSK data
-- If this intercept is found, convert the position vector of the
intercept into planetocentric coordinates.
Then compute the phase, solar incidence, and emission angles at
the intercept. Otherwise indicate to the user no intercept was
found for this vector.
-- Compute the planetocentric longitude of the boresight
intercept.
Finally
-- Compute the local solar time at the boresight intercept
longitude on a 24-hour clock. The input time for this
computation should be the TDB observation epoch minus one-way
light time from the boresight intercept to the spacecraft.
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.tm’. 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.
The names and contents of the kernels referenced by this
meta-kernel are as follows:
File name Contents
-------------------------- -----------------------------
naif0008.tls Generic LSK
cas00084.tsc Cassini SCLK
981005_PLTEPH-DE405S.bsp Solar System Ephemeris
020514_SE_SAT105.bsp Saturnian Satellite Ephemeris
030201AP_SK_SM546_T45.bsp Cassini Spacecraft SPK
cas_v37.tf Cassini FK
04135_04171pc_psiv2.bc Cassini Spacecraft CK
cpck05Mar2004.tpc Cassini Project PCK
cas_iss_v09.ti ISS Instrument Kernel
phoebe_64q.bds Phoebe DSK
\begindata
KERNELS_TO_LOAD = ( 'kernels/lsk/naif0008.tls',
'kernels/sclk/cas00084.tsc',
'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'
'kernels/dsk/phoebe_64q.bds' )
\begintext
Solution Source Code
A sample solution to the problem follows:
#
# Solution fovint.py
#
from __future__ import print_function
from builtins import input
#
# SpiceyPy package:
#
import spiceypy
from spiceypy.utils.support_types import SpiceyError
def fovint():
#
# Local parameters
#
METAKR = 'fovint.tm'
ROOM = 4
#
# 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.
# A DSK file containing Phoebe shape data.
#
spiceypy.furnsh( METAKR )
#
#Prompt the user for the input time string.
#
utctim = input( 'Input UTC Time: ' )
print( 'Converting UTC Time: {:s}'.format(utctim) )
#
#Convert utctim to ET.
#
et = spiceypy.str2et( utctim )
print( ' ET seconds past J2000: {:16.3f}\n'.format(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.
#
try:
nacid = spiceypy.bodn2c( 'CASSINI_ISS_NAC' )
except SpiceyError:
#
# Stop the program if the code was not found.
#
print( 'Unable to locate the ID code for '
'CASSINI_ISS_NAC' )
raise
#
# Now retrieve the field of view parameters.
#
[ shape, insfrm,
bsight, n, bounds ] = spiceypy.getfov( nacid, ROOM )
#
# `bounds' is a numpy array. We'll convert it to a list.
#
# Rather than treat BSIGHT as a separate vector,
# copy it into the last slot of BOUNDS.
#
bounds = bounds.tolist()
bounds.append( bsight )
#
# Set vector names to be used for output.
#
vecnam = [ 'Boundary Corner 1',
'Boundary Corner 2',
'Boundary Corner 3',
'Boundary Corner 4',
'Cassini NAC Boresight' ]
#
# Set values of "method" string that specify use of
# ellipsoidal and DSK (topographic) shape models.
#
# In this case, we can use the same methods for calls to both
# spiceypy.sincpt and spiceypy.ilumin. Note that some SPICE
# routines require different "method" inputs from those
# shown here. See the API documentation of each routine
# for details.
#
method = [ 'Ellipsoid', 'DSK/Unprioritized']
#
# Get ID code of Phoebe. We'll use this ID code later, when we
# compute local solar time.
#
try:
phoeid = spiceypy.bodn2c( 'PHOEBE' )
except:
#
# The ID code for PHOEBE is built-in to the library.
# However, it is good programming practice to get
# in the habit of handling exceptions that may
# be thrown when a quantity is not found.
#
print( 'Unable to locate the body ID code '
'for Phoebe.' )
raise
#
# Now perform the same set of calculations for each
# vector listed in the BOUNDS array. Use both
# ellipsoidal and detailed (DSK) shape models.
#
for i in range(5):
#
# Call sincpt to determine coordinates of the
# intersection of this vector with the surface
# of Phoebe.
#
print( 'Vector: {:s}\n'.format( vecnam[i] ) )
for j in range(2):
print ( ' Target shape model: {:s}\n'.format(
method[j] ) )
try:
[point, trgepc, srfvec ] = spiceypy.sincpt(
method[j], 'PHOEBE', et,
'IAU_PHOEBE', 'LT+S', 'CASSINI',
insfrm, bounds[i] )
#
# 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 surface intercept '
'in the IAU_PHOEBE frame (km):' )
print( ' X = {:16.3f}'.format( point[0] ) )
print( ' Y = {:16.3f}'.format( point[1] ) )
print( ' Z = {:16.3f}'.format( point[2] ) )
#
# Display the planetocentric latitude and longitude
# of the intercept.
#
[radius, lon, lat] = spiceypy.reclat( point )
print( ' Planetocentric coordinates of '
'the intercept (degrees):' )
print( ' LAT = {:16.3f}'.format(
lat * spiceypy.dpr() ) )
print( ' LON = {:16.3f}'.format(
lon * spiceypy.dpr() ) )
#
# Compute the illumination angles at this
# point.
#
[ trgepc, srfvec, phase, solar, \
emissn, visibl, lit ] = \
spiceypy.illumf(
method[j], 'PHOEBE', 'SUN', et,
'IAU_PHOEBE', 'LT+S', 'CASSINI', point )
print( ' Phase angle (degrees): '
'{:16.3f}'.format( phase*spiceypy.dpr() ) )
print( ' Solar incidence angle (degrees): '
'{:16.3f}'.format( solar*spiceypy.dpr() ) )
print( ' Emission angle (degrees): '
'{:16.3f}'.format( emissn*spiceypy.dpr()) )
print( ' Observer visible: {:s}'.format(
str(visibl) ) )
print( ' Sun visible: {:s}'.format(
str(lit) ) )
if i == 4:
#
# Compute local solar time corresponding
# to the light time corrected TDB epoch
# at the boresight intercept.
#
[hr, mn, sc, time, ampm] = spiceypy.et2lst(
trgepc,
phoeid,
lon,
'PLANETOCENTRIC' )
print( '\n Local Solar Time at boresight '
'intercept (24 Hour Clock):\n'
' {:s}'.format( time ) )
#
# End of LST computation block.
#
except SpiceyError as exc:
#
# Display a message if an exception was thrown.
# For simplicity, we treat this as an indication
# that the point of intersection was not found,
# although it could be due to other errors.
# Otherwise, continue with the calculations.
#
print( 'Exception message is: {:s}'.format(
exc.value ))
#
# End of SpiceyError try-catch block.
#
print( " )
#
# End of target shape model loop.
#
#
# End of vector loop.
#
spiceypy.unload( METAKR )
if __name__ == '__main__':
fovint()
Solution Sample Output
Execute the program:
Input UTC Time: 2004 jun 11 19:32:00
Converting UTC Time: 2004 jun 11 19:32:00
ET seconds past J2000: 140254384.185
Vector: Boundary Corner 1
Target shape model: Ellipsoid
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 91.026
Y = 67.190
Z = 2.030
Planetocentric coordinates of the intercept (degrees):
LAT = 1.028
LON = 36.432
Phase angle (degrees): 28.110
Solar incidence angle (degrees): 16.121
Emission angle (degrees): 14.627
Observer visible: true
Sun visible: true
Target shape model: DSK/Unprioritized
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 78.770
Y = 61.570
Z = 0.964
Planetocentric coordinates of the intercept (degrees):
LAT = 0.552
LON = 38.013
Phase angle (degrees): 28.110
Solar incidence angle (degrees): 31.132
Emission angle (degrees): 16.539
Observer visible: true
Sun visible: true
Vector: Boundary Corner 2
Target shape model: Ellipsoid
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 89.991
Y = 66.726
Z = 14.733
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
Observer visible: true
Sun visible: true
Target shape model: DSK/Unprioritized
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 76.586
Y = 60.579
Z = 13.657
Planetocentric coordinates of the intercept (degrees):
LAT = 7.962
LON = 38.344
Phase angle (degrees): 27.894
Solar incidence angle (degrees): 32.013
Emission angle (degrees): 11.845
Observer visible: true
Sun visible: true
Vector: Boundary Corner 3
Target shape model: Ellipsoid
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 80.963
Y = 76.643
Z = 14.427
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
Observer visible: true
Sun visible: true
Target shape model: DSK/Unprioritized
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 68.677
Y = 71.100
Z = 13.444
Planetocentric coordinates of the intercept (degrees):
LAT = 7.745
LON = 45.993
Phase angle (degrees): 28.171
Solar incidence angle (degrees): 36.039
Emission angle (degrees): 14.474
Observer visible: true
Sun visible: true
Vector: Boundary Corner 4
Target shape model: Ellipsoid
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 81.997
Y = 77.106
Z = 1.698
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
Observer visible: true
Sun visible: true
Target shape model: DSK/Unprioritized
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 73.186
Y = 73.131
Z = 0.934
Planetocentric coordinates of the intercept (degrees):
LAT = 0.517
LON = 44.978
Phase angle (degrees): 28.385
Solar incidence angle (degrees): 41.268
Emission angle (degrees): 17.493
Observer visible: true
Sun visible: true
Vector: Cassini NAC Boresight
Target shape model: Ellipsoid
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 86.390
Y = 72.089
Z = 8.255
Planetocentric coordinates of the intercept (degrees):
LAT = 4.196
LON = 39.844
Phase angle (degrees): 28.139
Solar incidence angle (degrees): 18.247
Emission angle (degrees): 17.858
Observer visible: true
Sun visible: true
Local Solar Time at boresight intercept (24 Hour Clock):
11:31:50
Target shape model: DSK/Unprioritized
Position vector of surface intercept in the IAU_PHOEBE frame (km):
X = 74.326
Y = 66.602
Z = 7.247
Planetocentric coordinates of the intercept (degrees):
LAT = 4.153
LON = 41.863
Phase angle (degrees): 28.139
Solar incidence angle (degrees): 33.200
Emission angle (degrees): 9.230
Observer visible: true
Sun visible: true
Local Solar Time at boresight intercept (24 Hour Clock):
11:39:55
Extra Credit¶
There are no “extra credit” tasks for this step of the lesson.