| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set includes the complete set of Stardust SPICE data
files (``kernel files'') for the primary (Stardust) and extended
(NEXT) missions. These kernels files can be accessed using SPICE
software. The SPICE data contains geometric and other ancillary
information needed to recover the full value of science
instrument data. In particular SPICE kernels provide spacecraft
and planetary ephemerides, instrument mounting alignments,
spacecraft orientation, spacecraft sequences of events, and data
needed for relevant time conversions.
Data Types (SPICE kernel types)
===============================
SPK kernels contain ephemerides for spacecraft, planets,
satellites, comets and asteroids as well as for moving or fixed
spacecraft and instrument structures. They provide position and
velocity, given in a Cartesian reference frame. SPK files are
located under the ``data/spk'' directory of this data set.
PCK kernels contain certain physical, dynamical and
cartographic constants for target bodies, such as size and shape
specifications, and orientation of the spin axis and prime
meridian. PCK files are located under the ``data/pck'' directory
of this data set.
IK kernels (Instrument description kernels) give descriptive and
operational data peculiar to a particular scientific instrument,
such as internal timing relative to the spacecraft clock and
field-of-view model parameters. IK files are located under the
``data/ik'' directory of this data set.
CK kernels describe pointing, containing a transformation
traditionally called the C-matrix which is used to determine
time-tagged pointing (orientation) angles for a spacecraft
structure upon which science instruments are mounted. CK files
are located under the ``data/ck'' directory of this data set.
EK (Events) kernels are derived from the integrated sequence of
events used to produce actual spacecraft commands. EK files are
located under the ``data/ek'' directory of this data set.
LSK (Leapseconds) kernels contain the leapseconds and the values
of other constants required to perform a transformation between
Universal Time Coordinated (UTC) and Ephemeris time (ET). LSK
files are located under the ``data/lsk'' directory of this data
set.
SCLK (Spacecraft Clock) kernels contain on-board clock calibration
data required to perform a mapping between Ephemeris time (ET)
and spacecraft on-board time (SCLK.) SCLK files are located under
the ``data/sclk'' directory of this data set.
FK (Frame Definitions) kernels contain information required to
define reference frames, sources of frame orientation data and
inter-connections between these frames and other frames supported
within the SPICE system. This includes mounting alignment
information for each instrument. FK files are located under
the ``data/fk'' directory of this data set.
Kernel File Details
===================
A brief overview of the different types of Stardust kernels
included in this data set is provided in the
CONFIDENCE_LEVEL_NOTE section of this file while details specific
to individual files are found in the ``*info.txt'' files in the
corresponding data directory. The most detailed description of
the data in each file is provided in metadata included inside the
file -- in the description area of text kernels or in the comment
area of binary kernels.
Software
========
The SPICE Toolkit contains software modules needed to read
SPICE kernel files. SPICELIB software is highly documented
via internal headers. Additional documentation is available
in separate ASCII text files called Required Reading files.
For example, the S- and P- Kernel (SPK) Required Reading File,
named SPK.REQ, describes use of the SPK kernel file readers
and contains sample programs.
The latest SPICE Toolkit for a variety of computer platforms such
as PC, Mac, SUN, etc. is available at the NAIF Node of PDS
electronically (via anonymous FTP and WWW servers). Refer to
information in ``software/softinfo.txt'' for details regarding
obtaining this software. Each version of the Toolkit is also
archived at the NASA National Space Science Data Center.
Loading Kernel Files into a SPICE-based Application
===================================================
The easiest way to make data from a collection of SPICE kernels
available to a SPICE-based application is to list these kernels
in a meta-kernel and load it into the program using the high
level SPICE data loader routine FURNSH. This data set provides
such meta-kernel(s) under the ``extras/mk'' directory. For more
information about the Stardust meta-kernel(s), see the file
``extras/mk/mkinfo.txt''.
|
| CONFIDENCE_LEVEL_NOTE |
This volume contains SPICE kernel files created during mission
operations and after mission data reconstruction and analysis.
Some general information about this collection follows here,
but the prospective user is also referred to extensive
descriptions about each file that are stored inside each kernel
file. These metadata provide detailed information regarding
the information source from which the data were derived, the
type of processing applied to the source data, applicability
of the data, etc. Metadata are located in the ``comment area''
for binary kernel types (SPK, CK, ESQ), accessible using either
the COMMNT or SPACIT utility program found in the NAIF Toolkit.
Metadata are located after ``\begintext'' markers within the
text kernel types (PCK, IK, FK, LSK, SCLK), accessible by
using any available text file display tool such as a word
processor, text editor, or the unix ``more'' or ``cat''
commands.
In some cases data accuracy information does not exist, or
may be estimated after mission completion.
Where there are questions about data accuracy or ``confidence''
not addressed herein the reader is invited to examine subsequent
SPICE archive releases associated with this data set (if such
exist), or to contact the NAIF node of the Planetary Data System
for possible further information.
SPK Files
=========
Majority of the SP-Kernel files included in this data set provide
the trajectory of the Stardust spacecraft. These files also
include ephemeris data for the primary mission targets -- Wild 2
and Annefrank -- and the NEXT mission target -- Tempel 1 -- along
with ephemeris data for Earth, Moon, and Sun. The spacecraft
trajectory stored in these SPKs was determined by the Stardust
Navigation (NAV) and Optical Navigation (OPNAV) teams, both from
JPL.
According to the NAV Team post-Annefrank flyby trajectory release
the converged B-plane solution -- B.R (b-plane radial), B.T
(b-plane transversal), and CLTOF (closest approach time, ET) --
were -1249.9km (-+ 4km), 2813.4km (-+ 8.5km), and 2-nov-2002
04:51:20.8 (-+ 0.8sec). The flyby distance was 3078.5km (-+9km).
For the Wild 2 encounter the most accurate spacecraft trajectory
relative to Wild 2 was computed by the OPNAV team by fitting the
NAVCAM image data. Based on this solution the closest approach
time and distance for the encounter are 2004-01-02T19:21:32 UTC
and 236.5 km. This solution is provided in a separate SPK file,
covering just the two hour period around the encounter. While it,
when used together with the adjusted spacecraft attitude CK file,
provides the best geometry for processing of the image data, it
does not provide an accurate representation of the heliocentric
location of the spacecraft. The boundary discontinuity between
this image-based solution and the primary radio navigation
solution, which is more accurate in an absolute sense, is about
440 km.
For the Tempel 1 encounter the most accurate spacecraft
trajectory relative to Tempel 1 was also computed by the OPNAV
team by fitting the NAVCAM image data. Based on this solution the
closest approach time and distance for the encounter are
2011-02-15T04:39:12.417 UTC and 182.0 km. This solution was
combined with the longer arc solution based on radio tracking
data and delivered in the SPK covering the first four months of
2011.
One additional SP-Kernel file included in this data set provides
locations of the spacecraft's center of gravity and antennas with
respect to the spacecraft reference frame origin. This file is
included in the data set because it may be useful for the future
radio science analysis.
Additional information about SPK files provided in this data set
is available in the file ``data/spk/spkinfo.txt''.
CK Files
========
C-kernel files included in this data set provide orientation of
the spacecraft bus, Navigation Camera instrument, and dust
collector grid. The former two kinds of CK files are produced
from engineering telemetry downlinked from the spacecraft. As is
often the case with spacecraft engineering telemetry-based
measurements, considerable uncertainty exists as to the accuracy
of such measurements, and, data quality (accuracy and
completeness) can vary significantly. The latter kind of CK files
is produced from commanded collector angle data.
Spacecraft Bus CK Files
-----------------------
These CK files containing the orientation of the spacecraft bus
(``the spacecraft'') are provided to support interpretation of
data obtained by all science experiments. The spacecraft
orientation CK file included in this data set were produced from
the merged stream of orientation quaternions computed on-board
and sent down in channelized telemetry, small forces packets, and
NAVCAM image headers.
The fact that the NAVCAM instrument could successfully point to
and track the desired targets (Annefrank, Wild 2, star fields,
etc.) and that the targets were found close to the expected
location in the images indicates that the accuracy of orientation
information sent down from the spacecraft was within the mission
requirements. At the same time it should be mentioned that at the
time of this archive release there are no known reports assessing
accuracy of the spacecraft orientation provided in these CK
files.
For the Wild 2 encounter the most accurate spacecraft orientation
was computed by the OPNAV team by fitting the NAVCAM image data.
This attitude solution is provided in a separate CK file,
covering just the 45 minute period around the encounter. When
used together with the OPNAV-determined spacecraft trajectory SPK
file, this solution provides the best geometry for processing of
the close encounter data, predicting the Wild 2 center position
in the NAVCAM images to within five to twenty pixels of the
actual, seen position (for the unadjusted, telemetry-based
spacecraft attitude CK this difference is tens of pixels.)
For the Tempel 1 encounter the most accurate spacecraft
orientation was computed by the OPNAV team by fitting the NAVCAM
image data. This attitude solution is provided in a separate CK
file, covering just 72 short intervals between to the exposure
start and stop times of the 72 encounter images. When used
together with the OPNAV-determined spacecraft trajectory SPK
file, this solution provides the best geometry for processing of
the close encounter data.
Navigation Camera CK Files
--------------------------
These CK files containing orientation of the NAVCAM instrument
relative to the spacecraft bus are provided to support the NAVCAM
image analysis. The orientation data in these files are comprised
of a number of individual transformations implementing the NAVCAM
pointing model defined and described in the Stardust Frames
Kernel (FK) file. Some of these transformations have been computed
from the NAVCAM mirror angle data contained in the image headers,
others -- using the OPNAV team pointing correction tables
provided in the NAVCAM IK file.
The original, type 1, NAVCAM CK files contain discrete data points
corresponding to the exact times when the images were taken and
cannot be used to compute the camera view direction for any other
times. To get these exact times users interested in computing
NAVCAM orientation using these CKs must start with SCLK values
provided in the SPACECRAFT_CLOCK_START_COUNT keywords in the
NAVCAM image labels and convert these SCLKs to other time
system(s) to access SPICE data. Starting with UTC times from the
NAVCAM labels will result in a small round off and the pointing
will not be found.
The type 2 NAVCAM CK files prepared for the archive contain the
same orientation as the original type 1 CKs but that orientation
covers small windows, from exposure start minus 1 SCLK tick to
exposure stop plus 1 SCLK tick, allowing computing NAVCAM
orientation at any time within the image exposures and making
these CKs more usable with high-level SPICE APIs. The mirror
orientation stored in these files is constant during each of the
exposure windows.
While OPNAV Team invested substantial effort in solving for
correction to be applied to the ideal NAVCAM pointing model in
order to estimate actual camera orientation, the correction that
had been derived prior to the Annefrank flyby and used to compute
orientation data for the NAVCAM CK files did not work well for
all image sequences. Even with correction applied, computed
pointing estimate had been off by as much as few tens of pixels
for some of the images.
Dust Collector CK File
----------------------
This file contains commanded orientation of the collector,
computed from commanded collector position angles, because no
direct indicators of the actual collector orientation were
available in the spacecraft engineering telemetry. It covers both
ISP collection period and the Wild 2 encounter.
Caution on Using CK Files
-------------------------
While not a data accuracy issue, the CK user is nevertheless
cautioned to understand and properly use the data found and
tolerance arguments that are inputs to the CK reader provided
in the SPICE Toolkit. Descriptions about how to read CK files
are found in the CK reader routine headers (top of the source
code files) and in the ``CK Required Reading'' document; these
are contained in each SPICE Toolkit package.
Additional information about CK files provided in this data set
is available in the file ``data/ck/ckinfo.txt''.
PCK Files
=========
PCK files provide size, shape and orientation data for ``target''
bodies such as planets, satellites, comets and asteroids. A
description of these data items and reference to their source is
provided inside the PCK file, which is a simple text file that
can be viewed using any word processor, text editor or text
display utility.
This data set contains generic PCK files based on the official
IAU/IAG/COSPAR values accepted in 1994 and 2006 and PCK files
for Stardust mission targets -- Wild 2 and Annefrank, -- and NEXT
mission target -- Tempel 1 -- that provide constants determined
after the encounters. While the PCK files for Wild 2 and Tempel 1
contains both rotation and shape data, the file for Annefrank
contains only the shape data because orientation of the asteroid
was not determined from the flyby data.
Additional information about PCK files provided in this data set
is available in the file ``data/pck/pckinfo.txt''.
FK Files
========
Frames kernel files provide specifications for how one reference
frame is defined relative to another frame. The Stardust FK
provided in this data set contains a complete set of frame
definitions for the spacecraft, its structures such as antennas,
and all of its science instruments. Meta information provided in
the comments included in this file consists of the frame
definitions, description of the frame relationships, source of
and accuracy of the mounting alignment information, etc.
Additional information about FK files provided in this data set
is available in the file ``data/fk/fkinfo.txt''.
IK Files
========
Instrument kernel files provide specifications for the optical
and physical instrument parameters and -- where applicable --
field-of-view size, shape and orientation for the named
instrument. Metadata describing these estimates are provided in
each IK file included in this archive.
Additional information about IK files provided in this data set
is available in the file ``data/ik/ikinfo.txt''.
SCLK Files
==========
Spacecraft Clock kernel files provide a tabulation of data
needed for converting time measurements between ephemeris time
(ET) and spacecraft clock time (SCLK). A SPICE SCLK file is
made from a similar file -- SCLK/SCET file, or SCLKvSCET
file -- produced by another mission entity. Each newly made SCLK
file fully replaces the previous SCLK file.
For Stardust the time correlation accuracy provided in SCLK/SCET
files is informally reported by the Lockheed Martin spacecraft
team to be better than 20 milliseconds. (NAIF has not attempted
to obtain or produce an independent corroboration of these LMA
estimates.) The SPICE SCLK file has essentially the same accuracy
as the SCLK/SCET file.
SPICE software does not prohibit an SCLK file from being used
for time conversions occurring for ``future times'' (or more
accurately, for epochs that occur later than the epoch of the
last telemetry data used in producing the last correlation
coefficients found in the SCLK file). Such ``predict'' time
conversions are likely to be inaccurate (changed somewhat)
once the epoch of interest has been passed.
Additional information about SCLK files provided in this data set
is available in the file ``data/sclk/sclkinfo.txt''.
LSK Files
=========
Leapseconds kernel files provide a tabulation of ``leapseconds''
and some other terms used in converting time measurements
between ephemeris time (ET) and Universal Time (UTC). ``Spacecraft
Event Time'' (SCET) is the commonly used name for UTC events
measured at the spacecraft.
Metadata describing how the LSK data are obtained or computed
is contained inside the LSK text file. The time conversion
provided by SPICE LSK files is accurate to approximately
0.000030 seconds.
Additional information about LSK files provided in this data set
is available in the file ``data/lsk/lskinfo.txt''.
EK Files
========
Two kinds of Events kernel (EK) files for the primary mission are
included in this archive: Experimenter's Notebook EKs and
Sequence EKs. No EK files were made during the NEXT mission.
Experimenter's Notebook EKs kernels are a digital analog to
the traditional paper laboratory notebook. Hence Experimenter's
Notebook entries represent truth only as seen by the author
of the notes--there is no attempt made to independently validate
any aspect of Experimenter's Notebook information.
Sequence EKs comprise a summary of the commands sent to the
spacecraft as well as instrument state (status) summaries as
modeled by the sequence generation tools. This data set includes
Sequence EKs created from integrated sequence Predicted Events
Files (PEFs) produced by the Stardust spacecraft team, LMA. As
seen from the source product name these files contain predicted
information -- background sequences, as should have executed by
the spacecraft. Because of that the EK records have times
different from the actual command execution times (although very
close in most cases.) These EKs are also missing any commands
that were not part of the background sequences (real-time
commands).
Additional information about EK files provided in this data set
is available in the file ``data/ek/ekinfo.txt''.
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