| CONFIDENCE_LEVEL_NOTE |
This data set 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 because
could not be estimated even 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
=========
The SP-Kernel files included in this data set provide different
versions of the trajectory of the Deep Impact Flyby (DIF) and
Deep Impact Impactor (DII) spacecraft and ephemeris data for the
mission target comet Tempel 1. Some of these SPKs also include
ephemeris data for planets, the Sun, and a possible extended
mission target comet Boethin.
One version of the DIF and DII trajectories provided in this data
set (dif_preenc174_nav_v1, dii_preenc174_nav_v1) was generated by
merging regular orbit determination solutions based on radio
tracking and far approach optical navigation data produced by the
DI Navigation team, JPL during operations. This solution covers
the whole primary mission, from launch to the beginning of
August 2005, and extends four years past it, to January 2009.
While this solution does not provide accurate trajectory for
the close encounter because the encounter portion of it is based
on predicted data generated pre-encounter, it provides sufficient
coverage and accuracy for processing data taken at all other
times during the mission.
Another version of the DIF and DII trajectories provided in this
data set was generated as the result of post-encounter processing
of the close approach optical navigation data by Dan Kubitschek
and Nick Mastrodemos of the DI Navigation Team, JPL. This
solution covers only the two day period around the Tempel 1
encounter. While this solution provides much better accuracy
for processing of the close encounter data, it was found to be
inadequate for some applications because the spacecraft position
provided in it was referenced to the impact site rather than the
comet Tempel 1 center. Two SPKs files of this kind provide
early (di_finalenc_nav_v1.bsp) and final (di_finalenc_nav_v3.bsp)
versions of this solution with the earlier version being archived
only because it was used in processing image data in
November/December 2005.
The last version of the DIF and DII trajectories provided in this
data set (di_finalenc_nav_v3_to06048.bsp) was generated by
adjusting the final close approach optical navigation solution
mentioned above by Brian Carcich of the DI Science Data Center,
Cornell University. This solution also covers only the two day
period around the Tempel 1 encounter. The adjustment was done by
incorporating the offset between the impact site and the comet
center determined from the final optical navigation solution
SPK (v3) and the latest Tempel 1 shape model provided by
Peter Thomas. This solution was used to process the PDS archive
images in February 2006.
This data set also includes a separate, long-spanning SPK
(di_tempel1_ssd_v1.bsp) providing the latest ephemeris data for
comet Tempel 1 produced using all DI mission data by the Solar
System Dynamics Group, JPL. The ``original'' and ``adjusted''
close approach solutions mentioned are consistent with and
incorporate a portion of this file.
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 the orientation of
the DIF and DII spacecraft buses and the DIF high gain antenna
(HGA). Most of these CK files were 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.
DIF Spacecraft Bus CK files
---------------------------
Two kinds of the DIF spacecraft bus orientation CK files are
included in this data set. The first kind, based on the low rate
(1/8 Hz), provides coverage for the whole primary mission while
the second kind, based on the high rate telemetry (5 Hz) provides
coverage only for a short period in February 2005 and the for
about 50 minutes around closest approach to Tempel 1. Both types
of files need to be loaded to provide most complete coverage with
the high rate CKs, providing superior attitude, loaded last so
that data from them is accessed first. Also, while the low rate
CK covers the whole mission, its coverage contains numerous gaps
ranging from minutes to hours and, in some cases, even days. Due
to these gaps this CK does not provide attitude data for about
20% of the DIF images, particularly those taken during some cruise
calibration sequences and far approach. This lack of attitude can
be alleviated to some extent by modifying interpolation intervals
in the file by using the CKSPANIT program (as was done for
processing of the images for archiving in PDS in February 2006 -
see comments below on 'gap-filler CK files'.)
The first archived versions of both the low rate and the high
rate CKs had problems with internal time tags. The tags in the
low rate CKs were ``compressed'' to the start of each second due
to discrepancy in interpretation of the fractional SCLK part,
causing timing error of up to 50 milliseconds and the end of each
second (see additional information about this in the ``SCLK
Files'' section below.) The tags in the high rate CKs were
truncated by up to 2 milliseconds, introducing attitude errors of
almost 20 microradians during the fastest spacecraft turn rates
in the ~100 second interval before the closest approach to Tempel
1 (at other times attitude errors due to this truncation were
negligible). These problems were fixed in the version 2 of these
CKs.
The accuracy of the DIF spacecraft bus orientation determined
on-board varied significantly, from 50 micro-radians (~5 MRI
pixels) at the times when the attitude was determined based on
the star tracker measurements to a few hundreds of micro-radians
(tens of MRI pixels) for the periods when the attitude was
determined by propagating ACS gyro data without any star tracker
updates for extended periods of time. For this reason, the actual
nucleus center position in the images is different from the
center position computed using the CK files provided in this data
set by as many as 20-30 MRI pixels for a number of the early
pre-encounter and post-encounter look-back images. Some of this
discrepancy may also be attributed to fact that the linear
interpolation done by the Type 3 CK readers between the rather
sparse, 1/8th Hz attitude points stored in the low-rate CK
file does not adequately represent the actual orientation between
the points.
A number of times during the mission the ACS system was reset due
to a safe mode entry or an update to the flight software. At the
time of each reset the attitude quaternion computed by the ACS
software was re-initialized (in some cases to an identity
quaternion, in other cases to a random quaternion) and for a
period of time following the reset, until the ACS was able to
re-estimated the actual orientation from the star tracker data,
the attitude data sent down in the channelized telemetry was
bogus. This table lists the times and reasons of the ACS resets
during the mission:
2005 JAN 12 19:25:30.858 post-launch safe mode
2005 JAN 12 19:29:44.578 post-launch safe mode
2005 JAN 12 19:33:37.578 post-launch safe mode
2005 JAN 12 19:37:18.578 post-launch safe mode
2005 JAN 12 19:45:05.007 post-launch safe mode
2005 JAN 20 06:59:48.398 solar flare induced safe mode
2005 FEB 09 21:32:48.264 flight s/w 6.3.0 update
2005 MAR 16 08:28:18.000 SCU-A reset safe mode
2005 APR 12 23:27:45.377 flight s/w 7.0.0 update
2005 JUN 06 04:12:39.808 flight s/w 7.0.0 update
DII Spacecraft Bus CK Files
---------------------------
Two kinds of the DII spacecraft bus orientation CK files are
included in this data set. The first kind provides DII attitude
prior to its separation from DIF. This attitude -- constant with
respect to DIF -- was determined as the result of processing
instrument geometric calibration data. The second kind, based on
telemetry, provides attitude from separation through impact. The
coverage of this CK contains a number of gaps. Due to these gaps
this CK does not provide attitude data for some of the
post-separation DII images. This lack of coverage can be
alleviated by modifying the interpolation intervals in the file by
using the CKSPANIT program (as was done for processing of the
images for archiving in PDS in February 2006.) It is important to
note that during image archive preparation Science Data Center
team observed a bias (of up to 20 ITS pixels, decreasing to zero
at closer to the impact time) in image pointing solutions done
using this CK. No explanation was found for and no correction was
done to this bias at that time.
DIF HGA CK Files
----------------
Two kinds of the DIF HGA orientation CK files are included in
this data set. The first kind is based on the measured HGA gimbal
angles provided in telemetry (``V-channels'') while the second
kind is based on the commanded HGA gimbal angles provided in
telemetry (``B-channels''). These files should be loaded together
to provide the most complete coverage with the file based on
measured angles loaded last (to be accessed first.) Even when
loaded together, the combined coverage provided by the files is
full of gaps, which can be alleviated by modifying the
interpolation interval information by using the CKSPANIT program.
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 file based on the official
IAU/IAG/COSPAR values accepted in 2000 and PCK file for DI
mission target -- Tempel 1 -- providing constants determined
after the encounter. The PCK file for Tempel 1 contains both
rotation and shape/size data derived by the DI science team
during post encounter data analysis.
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 DI FK provided in
this data set contains a complete set of frame definitions for
the both spacecraft, their structures such as antennas, and all
of their science instruments. Comments in the FK file provide
the frame definitions, a description of the frame relationships,
the source of and accuracy of the mounting alignment information,
etc. The instrument frame definitions provided in the file
incorporate the latest mounting alignment data available at the
end of the mission.
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. While a majority of the of
parameters included in the IK files archived in this data set
are based on the nominal instrument data, the files incorporate
the latest version of optical distortion model derived by the DI
optical navigation team.
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
to convert time measurements between ephemeris time (ET) and
spacecraft clock time (SCLK). A SPICE SCLK file is made from a
similar file -- SCLK/SCET or SCLKvSCET file -- produced by
another mission entity. Each newly made SCLK file fully replaces
the previous SCLK file.
This data set includes SCLK kernels providing the correlation
between the DIF spacecraft primary (SCU-A) on-board clock, DIF
spacecraft secondary (SCU-B) on-board clock, and DII spacecraft
on-board clock. Two SCLK kernels -- the ``official'' version and
``science'' version -- are available for the DIF primary clock and
DII clock while only one SCLK kernel -- ``official'' version --
is available for the DIF secondary clock.
The main difference between the ``official'' versions of the SCLK
kernels and the ``science'' versions is that ``official'' versions
are based on the latest time correlations that were used by the
project during mission operations while the ``science'' versions
contain updated correlations that were used to process science
data for archiving in PDS.
The correlation updates for the DIF primary clock and for the DII
clock were done for two different reasons. The DIF correlation was
updated to eliminate ``artificial'' data points introduced by the
tool-set that created source SCLKvSCET file and to adjust clock
rates in order to connect actual data points in a continuous
fashion. The DII correlation, on the other hand, was updated to
simply enforce consistency between the DIF primary clock and the
DII clock based on the SCLK times of the impact determined from
the image data (DII SCLK = 1/173727875.105 +/-300ms per Dennis
Wellnitz and DIF SCLK = 1/173727702.218 +/-25ms per image
number 064 of ExpID 9000910) and DIF/DII trajectory solutions
based on close approach optical navigation data (time of impact
for DII trajectory at 2005-185T05:44:34.2 UTC).
The ``science'' versions of the SCLKs and trajectory solutions
based close approach optical navigation data were created in the
following sequence:
- correlation from ``official'' DIF primary clock SCLK kernel
was modified to get ``science'' DIF primary clock SCLK kernel
- ``science'' DIF primary clock SCLK kernel and ``official''
DII SCLK kernels were used to process close approach optical
navigation images to generate trajectory solution
- UTC of impact (2005-185T05:44:34.2) was determined by
converting DIF SCLK of impact determined from DIF images
(1/173727702.218) to UTC using ``science'' version of DIF
primary clock SCLK kernel
- correlation from ``official'' DII SCLK kernel
was modified so that UTC mentioned above matched DII SCLK
of impact determined from DII images (1/173727875.105)
- DII trajectory was shifted so that impact happened at the
UTC mentioned above.
Because of the tight coupling between the SPK and SCLK kernels,
the ``science'' SCLK must be used for encounter image processing.
Unfortunately, the absolute accuracy of both ``official'' and
``science'' correlations is no better than 1 second as was
determined by a DI Time Issues Working Group that included Ball
and JPL engineers and members of the DI science team. This group
was put together at the end 2005 with the goal to improve DIF and
DII time correlation knowledge. The group concluded that getting
improvement was not possible without substantial effort due a
number of factors among which were the lack of correlation data
during the last ten days before encounter, big changes in the
thermal conditions and strong temperature dependency of the clock
oscillators, and errors in the production of the time correlation
packets.
Note that the clock format defined by both ``official'' and
``science'' SCLK kernels (seconds with fractional seconds given
as a count of 1/256 second ticks) does not match the format of
on-board time tags in mission telemetry (seconds with fractional
seconds given as top 8 or 16 or full 20 bits of a count of
micro-seconds). For details on this discrepancy and the algorithm
to fix it see ``sclk140.txt'' provided in Deep Impact and EPOXI
documentation set (DI-C-HRII/HRIV/MRI/ITS-6-DOC-SET-V3.0).
While all science images sent to the ground contained SCLK time
in the headers (spacecraft primary, SCU-A, SCLK for DIF
and main SCLK for DII), optical- and auto-navigation image
headers contain a different time. This time was so a called
on-board Ephemeris Time (ET) that optical/auto navigation
on-board software used internally to perform autonomous
trajectory and attitude solutions. This on-board ET was computed
from on-board clock by adding a constant delta to it to make it
synchronous with the actual ET to the extent possible. Since the
on-board clock was drifting throughout the mission, the delta had
to be changed a number of times. The table below shows the delta,
in seconds, that must be subtracted from the on-board ET to get
corresponding SCLK (spacecraft primary, SCU-A, SCLK for
DIF and main SCLK for DII) and the time when a particular
delta had been uplinked and took effect in the on-board software
(shown as UTC and on-board ET):
DIF:
Delta Applicability Start UTC Apl. Start on-board ET
------- ----------------------- ----------------------
64.184 2005-014T02:50:20.1133 158943084.297
160.176 2005-083T11:14:00.1992 164934912.535
177.161 2005-103T02:14:28.7695 166630537.301
268.794 2005-157T04:17:00.0000 171303535.833
237.978 2005-175T03:58:33.1251 172857585.892
DII:
Delta Applicability Start UTC Apl. Start on-board ET
------- ----------------------- ----------------------
64.184 2005-014T02:50:20.1133 158943084.297
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 Events kernel (EK) files are included in this archive: DIF
Sequence EK and DIF Command Dictionary EK.
DIF Sequence EK contains a summary of the commands sent to the
spacecraft during the mission as well as instrument state
(status) summaries as modeled by the sequence generation tools.
This file was created from integrated sequence Predicted Events
File (PEF) provided by Steve Wissler, DI Mission Operations
Systems Engineer as the end of the primary mission. While
significant effort was made to collect all necessary information
for producing complete PEF file, command logs for some periods
have not been available and the commands for these periods are
missing from the EK. Also, because the PEF file was the result
of sequence modeling process the EK records have times different
from the actual command execution times (although very close in
most cases).
DIF Command Dictionary EK is based on the last version of the
command dictionary available at the end of the primary mission.
Additional information about EK files provided in this data set
is available in the file ``data/ek/ekinfo.txt''.
Kernel Files Not Included In This Data Set
===========================================
A number of kernel files used to process image data archived in
PDS and mentioned in the image labels were not included in this
data set because they either were superseded by the files
provided in this archive, or were known to contain incorrect or
out-of-date information, or could be easily re-created from the
data provided in this archive using utility program(s) available
from NAIF. Among such files are:
- predicted SPK files:
spk_bet_050112_101_impactor.bsp
spk_bet_050112_101_flyby.bsp
spk_od023.bsp
dif_ort11_dss_merged.bsp
spk_diipredict_2005177_175028.bsp
These SPKs were used for processing of the images for
archiving in PDS during November/December 2005. They
contained earlier versions of the spacecraft trajectory and
comet ephemerides and were superseded by the SPKs included
in this data set.
- predicted CK files:
dii_attached_v10.bc
launch_bet050112_101_20050102t205819_att.bc
dif_rec_ck_ort11_merged.bc
dii_predict_2005177_175028.bc
These CKs were used for processing of the images for
archiving in PDS during November/December 2005. They
contained predicted attitude data for DII and DIF spacecraft.
The main purpose of using these files was to provide ``some''
orientation data for under 20% of the images, for which
actual orientation data was not available in the
reconstructed CKs provided in this archive. While they served
this gap-filler purpose, the data provided by them was
incorrect in most/all cases due to differences between
planned and actual pointing profiles, timing issues, etc. For
this reason they were not included into this data set.
- nominal Tempel 1 PCK file:
di_tempel1_v00.tpc
This PCK was used for processing of the images for archiving
in PDS during November/December 2005. It contained nominal,
placeholder shape/size and rotation constants for comet Tempel
1. It was superseded by comet PCK ``di_tempel1_v01.tpc''
provided in this data set. For the record, the nominal
constants provided in that PCK were:
BODY1000093_POLE_RA = ( 0.0 0.0 0.0 )
BODY1000093_POLE_DEC = ( +90.0 0.0 0.0 )
BODY1000093_PM = ( 0.0 205.7142857142857 0.0 )
BODY1000093_RADII = ( 3.1 2.1 2.1 )
- star position SPK and PCK files:
naifstarnames_2005321_v01.tpc
stars_2005321_v01.bsp
These SPK and PCK were used in the Science Data Center image
processing pipeline to provide access via SPICE interfaces
to positions of a few stars used for instrument calibration
during the mission. While these kernels successfully served
their purpose, they were not fully compliant with official
NAIF practices. This reason they were not included into this
data set. On the other hand, recognizing the importance of
this data, it was decided to include the star positions
stored in these files into this data set's documentation for
the reference. The table below provides these positions.
Each line contains J2000 RA (hours, arc-minutes,
arc-seconds), J2000 DEC (degrees, arc-minutes, arc-seconds)
and name.
10,24,23.71,-74,1,53.8,PRETEND I CAR HD90589
0,25,45.07,-77,15,15.3,BET HYI HD2151
10,2,42,-60,7,0,NGC3114
6,23,57.11,-52,41,44.4,CANOPUS HD45348
1,37,42.85,-57,14,12.3,ACHERNAR HD10144
21,7,6,42,14,0,NGC7027
19,41,48.95,50,31,30.2,16 CYG A HD186408
7,33,27.32,-50,35,3.3,HD60753
18,36,56.34,38,47,1.3,VEGA HD172167
0,24,5.22,-72,4,57.9,47 TUC NGC104
6,45,08.92,-16,42,58.0,SIRIUS HD48915
9,11,16.72,-62,19,1.1,I CAR HD79447
11,5,33,-58,43,48,NGC3532
18,51,6,-6,16,0,M11
10,36,16.079,-58,16,38.20,HD92044
08,40,36,-53,02,0,OMI VEL CLUSTER IC2391
- ``gap-filler'' CK files
dii_predict_050703_050704.bc
dif_predict_050112_050809.bc
These CKs were used for processing of the images for
archiving in PDS during February to fulfill the same purpose
as the predicted CKs mentioned above -- to provide ``some''
orientation data for under 20% of the images, for which
actual orientation data was not available in the
reconstructed CKs. Unlike the other predicted CKs these two
kernels did not provide truly predicted orientation. Instead
they stored the same data as the reconstructed CKs but
allowed interpolation across gaps of any size between actual
attitude data points. These files were not included in this
data set because they were created -- and can be recreated at
any time -- from the reconstructed CKs provided in this data
set using CKSPANIT application with the tolerance of 44236800
ticks (= 2 days).
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