| DATA_SET_DESCRIPTION |
This description was written by B. Carcich with support from the
Stardust-NExT operations and science teams.
Data Set Overview
=================
This data set contains Level 2 (EDR) pre- and post-encounter and
encounter images taken by the Stardust Navigation Camera during
the encounter with comet 9P/Tempel 1 (1867 G1), plus calibration
images taken throughout the Stardust-NExT mission.
Every image provided in this data set was taken as a part of a
particular imaging sequence, each of which is described in the
Data Collection Periods section below.
Data Collection Periods
=======================
For the complete list of images and their parameters, refer to
the data set's index table, INDEX/INDEX.TAB. For additional
notes on individual images also consult with the document ``Log
of Stardust-NExT NAVCAM Flight Images', DOCUMENT/NEXTIMAGELOG.LBL,
provided with this data set.
N.B. The NAVCAM data collection periods listed here have gaps between
the stop time of one period and the start of the next; this is
intentional and consistent with the NAVCAM data set in that no
NAVCAM image data were taken between these periods.
N.B. The NAVCAM data collection periods listed here overlap, but are
defined differently than, the mission phases defined in the
mission catalog (NEXT.CAT).
N.B. Each data collection period listed here corresponds to one or
more two-letter codes in the NAVCAM image file names and
PRODUCT_IDs, corresponding to NAVCAM subphases (see
DATA/DATAINFO.TXT). The two-letter codes are listed here with
their corresponding NAVCAM subphase periods and the mission
catalog phases which they overlap:
Two-
Letter Overlapping Mission
Code NAVCAM Subphase Catalog Phase(s)
==== =============== ================
CO Checkout CRUISE
C5 Cruise 5 CRUISE
C6 Cruise 6 CRUISE
TE Tempel 1 Encounter APPROACH, ENCOUNTER, DEPARTURE
For the NAVCAM, operational mission subphases were defined as CO
(CHECKOUT), C5 and C6 (CRUISE 5 and 6), and TE (TEMPEL
ENCOUNTER), and those two-letter acronyms were used in the
NAVCAM PRODUCT_IDs and FILE_NAMEs. C6 was not intentional but
was added when the NAVCAM image ID was reset during C5,
temporarily causing duplicate FILE_NAMEs in the ground data
system for NAVCAM images taken at different times. An earlier
mission phase, C4 (CRUISE 4) was defined but no NAVCAM data were
taken during that phase.
The following sections list the NAVCAM data collection periods:
2007-01-25 to 2010-08-23 -- Checkout, Cruise 5, Cruise 6
--------------------------------------------------------
This section covers the entire CRUISE phase described in the
Mission Catalog.
The performance of the NAVCAM was monitored throughout the
Stardust-NExT extended mission using a standard calibration
sequence along with a few special calibrations. Calibrations
involved imaging of a variety of stars, several of which are
photometric standards, acquiring dark frames, and taking images
illuminated by the NAVCAM internal calibration lamp.
All of these images were taken with NAVCAM in CRUISE mode which
included the option to only store and downlink selected windows
of pixels from the full CCD array, and many of these images used
this windowing option.
The problem with recurring camera contamination (Hillier, et al.,
2011; Newburn, et al., 2003a & b; Tsou, et al., 2004; Li, et al.,
2009 [HILLIERETAL2011] [NEWBURNETAL2003] [NEWBURNETAL2003B]
[TSOUETAL2004] [LIETAL2009]) was successfully controlled by
periodic heating of the instrument using its internal electrical
heaters and by placing direct Sunlight on the camera radiator.
The cruise calibrations allowed characterization of camera imaging
performance in the areas of geometric fidelity, spatial resolution,
and radiometry (including zero-exposure signals, shutter times,
linearity, field flatness, noise, and radiometric response rate)
more accurately than had been possible during the primary mission.
Preliminary radiometric calibration results have been incorporated
into the image processing pipeline. Special observations allowed
determination of the NAVCAM periscope throughput as a function of
scan mirror angle, scattered light levels from the spacecraft
structure as functions of mirror angle and the Sun illumination
direction on the spacecraft, and charge bleeding and residual image
in the CCD detector.
2010-12-17 to 2011-02-25 -- 9P/Tempel 1 (1867 G1) Encounter
-----------------------------------------------------------
This section covers the entire APPROACH, ENCOUNTER and DEPARTURE
phases described in the Mission Catalog.
NAVCAM imaging of Tempel 1 was initiated 60 days before Encounter
(E-60d) and was repeated twice per week. Exposures of 10s and 20s
(the maximum commandable by the spacecraft) were used; however, the
comet was not bright enough to be detected in the initial images
even after summing all 8 images taken at each sampling time. Many
frames had several pixels of smear using these long exposure times.
At this time, the spacecraft was oriented with its dust shields
pointed away from the comet (to avoid having to image it through
the periscope) and with the high-gain communication antenna pointed
at Earth to allow data downlinking. As the spacecraft range to the
comet decreased, the mirror angle required to view the comet
progressively increased. When it exceeded 168deg, increasing
levels of scattered light began to raise the background signal and
decrease the signal-to-noise ratio (SNR). Starting at E-27d, the
mirror was moved back to 160deg, and the spacecraft was maneuvered
off Earth point to view the comet. This reduced the scattered
light and allowed the first detection of the comet in stacks of 8
summed images. Daily 8-frame image sets of 351x351-pixel subframes
were typically acquired after this time. These images were usable
for optical navigation, but the SNR was still too low for useful
science. At E-7d, the spacecraft was flipped around to put the
dust shields forward, and the scan mirror was set at 20deg for
comet imaging. The comet SNR in 8-frame stacks became
scientifically useful at about this time, and sets were taken every
2 hours from this point until E-2d when approach imaging was halted
to prepare the spacecraft for the encounter. Evidence of the
nucleus brightening the central pixel was first seen at about E-3d.
The close encounter image set was restricted by spacecraft memory
and software to 72 full-frame compressed images, as had also been
the case at Wild 2. These images were sequenced to occur within
E+/-4m (minutes). Images were taken on 8s centers outside
E+/-144s (seconds) and on 6s centers inside that period. Scan
mirror pointing was controlled by the onboard autonomous navigation
software, which worked flawlessly to keep the nucleus in the camera
FOV. The pixel scale in the encounter image set ranged from 158
m/pixel down to 11 m/pixel at closest approach. Four of the 72
images (the first, last, and those at E+/-72s) were intentionally
overexposed to allow better detection of any near-nucleus jets.
Other than those frames, nearly all images were well exposed.
Slight saturation of the bright limb occurred on two frames (E-33s
and E-15s) due to the actual arrival time being 15s earlier than
nominal. The first 24 images viewed the comet through the
periscope; 6 of those frames showed evidence of double images, as
expected for mirror angles between 8 and 15deg, where light reaches
the camera both through the periscope and from just outside it. No
evidence of any optical contamination was observed.
Departure imaging resumed at E+1d. With the dust shields forward,
the scan mirror angle began at about 174deg. Significant scattered
light was apparent, but the SNR was adequate for continued useful
science. Single 351x351-pixel subframes were acquired every 5
minutes to support high-time-resolution monitoring of coma
activity. Increased central pixel brightening due to the nucleus
was no longer seen after about E+5d. At E+7d, the sampling rate
was decreased to every 11 minutes due to decreasing Deep Space
Network ground receiving station coverage, and the subframe size
was reduced to 201x201 pixels one day later. The scattered light
level gradually decreased with time, as did the comet signal.
Useful science imaging was no longer achieved after E+10d, and
comet imaging was then terminated. All approach and departure
images were acquired uncompressed. No evidence of optical
contamination was observed except for the residual contamination
seen after the last heating procedure during cruise.
Calibration sequences similar to the standard cruise calibration
were executed at E-18d and at E+10d. The NAVCAM
performance remained essentially unchanged throughout the
Stardust-NExT mission. A publication with more complete NAVCAM
calibration results is available (Klaasen, et al., 2011
[KLAASENETAL2011B]).
All approach and departure imaging used CRUISE mode with the
windowing option. The close encounter images used ENCOUNTER mode
which does not have a windowing option, so all close encounter
images are full-frame images.
Instrument and data calibrations
================================
This section is duplicated in both calibrated and raw data sets, even
though it is not represented in the raw data, as calibration is an
integral part of understanding and using the data.
Calibration sources
-------------------
The calibration data for NAVCAM were derived from pre-launch and
in-flight testing.
The NAVCAM was specified as an engineering instrument for the prime
mission to Wild 2. Its main purpose was for navigation,
calibration was done on a best-efforts basis, and late hardware
deliveries severely hampered those efforts.
For Stardust-NExT, imaging was a key part of the science goals, and
review of existing data plus extensive in-flight calibration was
done to characterize NAVCAM performance [KLAASENETAL2011B].
This data set includes documents (see /DOCUMENT/DOCINFO.TXT),
references to published papers, and calibration files (see
/CALIB/CALINFO.TXT) detailing the calibration of the NAVCAM
instrument.
Data calibration process
------------------------
The data calibration pipeline comprised several steps: masking
pixels outside any windows; quality checks (saturation);
decompression of compressed data; bias estimation and subtraction;
dark-current estimation and subtraction; signal-to-noise ratio
calculation; flat-fielding to remove stable pixel-to-pixel
variations; calculation of DN rate; conversion to radiance.
The data calibration process does not remove coherent noise
(CNoise) or Fixed-Pattern Noise (FPN) from the images. See
below for a brief description of these effects.
N.B. Coherent Noise (CNoise)
----------------------------
Coherent Noise is usually only visible in underexposed,
uncompressed images when viewed using extreme contrast enhancement,
and appears as stripes of noisy dark and light pixels. The CNoise
variation is about +/-5DN in the raw images [KLAASENETAL2011B].
N.B. Fixed-Pattern Noise (FPN)
------------------------------
Fixed-Pattern Noise (FPN) is usually only noticeable in images
where the NAVCAM has been on for more than ten hours. The rise in
FPN is accompanied by an associated rise in CCD temperature. It
occurs independent of the contamination level of the camera. But
a peculiar aspect of the FPN is that even after long power-on times
with elevated CCD temperatures, the FPN does not show up in dark
frames, only in those that have had the shutter open to admit some
level of external photons (even if only a low-level scattered light
background).
Investigation of the FPN during NExT showed that the FPN level also
depends on the amount of background scattered light in an image.
The Wild 2 approach images had scattered light levels of <100 DN
and raw FPN amplitudes of 6-20 DN rms. But during the NExT
approach to Tempel 1, much higher levels of scattered light were
encountered, and the FPN amplitude increased to 25-45 DN rms even
when the camera had been powered on for only a short time and the
CCD temperature remained low.
The FPN can be largely eliminated by successive frame differencing
when identical frame pairs are acquired. No evidence of FPN is
found in the Tempel 1 close encounter images, which were acquired
using data compression, at short power-on time, and with minimal
scattered light. No attempts to correct for FPN are included in
the NAVCAM processing pipeline [KLAASENETAL2011B].
Data Product Type and Format Overview
=====================================
NAVCAM data files provided in this archive are divided by target,
9P/TEMPEL 1 (1867 G1) and other (CALIBRATION or N/A).
The images in this data set are in FITS format with detached PDS
labels.
The Primary Data Unit (PDU) of each image file in this data set is a
two-dimensional array of brightnesses as measured by the array of
pixels in the NAVCAM CCD, and as viewed through the NAVCAM optics.
The brightnesses in the PDU are the raw (unconverted) Data Number
(DN) values obtained from the NAVCAM Analog-to-Digital Converter
(ADC) as it read the voltages in the CCD pixels.
A single Extension Data Unit (EDU) contains BaseLine Stabilization
pixels which may be used to calculate the bias subtraction in the
data calibration.
Additional image-synoptic data such as CCD temperature, geometry
and windowing parameters are stored in the image labels.
In cases where only windows of the detector were stored and
downlinked, the program filled the pixels in the image corresponding
to the areas for which data had not been downlinked with raw zeroes.
In such images WINDOW OBJECTs define the areas containing non-null
data.
Parameters
==========
The primary parameters in this data set are brightness images,
two-dimensional arrays of brightnesses corresponding to the pixels in
the NAVCAM CCD, and as viewed through the NAVCAM optics.
The brightnesses are the raw (unconverted) Data Number (DN) values
from the NAVCAM Analog-to-Digital Converter (ADC).
Ancillary data include bias data and image-synoptic data such as CCD
temperature and observational geometry.
Data Processing
===============
The images in this data set were initially assembled at the Jet
Propulsion Laboratory (JPL) from raw telemetry packets sent down by
the spacecraft; attached preliminary PDS labels were populated with
housekeeping values and computed geometry parameters from SPICE
kernels.
The JPL image files were then multiplexed to several Science Data
Center (SDC) systems with identical processing pipelines operated by
Cornell University, where they were converted to FITS format and
where detached PDS labels for the FITS files were generated.
Ancillary Data
==============
The geometry items included in the image PDS labels were computed
using the SPICE kernels archived in the Stardust SPICE data set,
SDU-C-SPICE-6-V1.0 [SEMENOVETAL2004B]; refer to that data set for
details.
Lockheed Martin Astronautics (LMS) provided image command logs,
which were needed to calibrate the data; see /CALIB/CALINFO.TXT for
details.
Reference Frames
================
The geometry items provided in the files are relative to the J2000
reference frame. Refer to the description of the geometry table
columns in /INDEX/INDEX.TAB to see which parameters are defined in
which frame.
The J2000 reference frame is defined as follows:
- +Z axis is along Earth Mean Equator North at the J2000 epoch
(2000 JAN 01 12:00 ET);
- +X axis is along the vernal Equinox at the J2000 epoch;
- +Y completes the right hand frame.
The Stardust spacecraft reference frame is defined as follows:
- +X axis is along the longer side of the spacecraft bus and
points from the aerogel capsule side towards the dust shield
side;
- +Z is perpendicular to the deployed solar arrays surface and
points along the HGA pointing direction;
- +Y completes the right hand frame.
This diagram, which is not to scale, illustrates the spacecraft
reference frame:
^+Z Solar Array
.-. | Shield
Solar Array | | | .-.
===============o===============o======|========| |
.-------------------| | | `-'
| Periscope/| | | | -------->
(former | _ | | | | | +X Nominal
aerogel | NAVCAM / \ | | | | x-------> Comet-
capsule)| and| | |/ | | +Y relative
| Mirror \_/ | | (into Spacecraft
'-------------------| | page) velocity
| | during
| `-' Encounter
Direction to comet at | Whipple
closest approach | Shield
along spacecraft -Z |
|
V
As seen on the diagram, NAVCAM is located on the -Y side of the
spacecraft bus. The NAVCAM boresight before the mirror points along
the spacecraft -Y axis. The mirror redirects the boresight in the
spacecraft XZ plane, pointing near spacecraft +X on approach, along
spacecraft -Z at closest approach, and near spacecraft -Z on
departure.
Epoch of Geometric Parameters
-----------------------------
All geometric parameters provided in each image PDS label were
computed at the epoch specified in the start time for that label.
Software
========
The images in this data set conform to the FITS standard, and have
standard PDS image labels. They can be viewed by a number of
PDS-provided and/or open-source and/or commercial programs. For this
reason no data-set-specific software is provided with this data set.
Contact Information
===================
For any questions regarding the data format of the archive,
contact Stardust-NExT NAVCAM Science Lead:
Dr. Joseph F. Veverka [JVEVERKA]
or Science Data Center Manager
Brian Carcich [BCARCICH]
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
All telemetry data produced by the NAVCAM contained checksums that
were validated upon ground receipt.
During the processing of the data in preparation for delivery
with this volume, the structure of the image data was verified.
This verification included detection of both the sync word and
the length of each packet, which ensured that all packets were
complete and not damaged. The fundamental validity of the data has
been inferred by the NAVCAM Science Lead and Science team members
by display and subsequent review of the images.
Breaks in image sequences and missing data
==========================================
All NAVCAM image data for the Stardust-NExT mission, which were
successfully downlinked without corrupted or lost packets during the
data collection periods listed above, are included in this data set.
Images may appear to be missing because of breaks in the sequence of
image numbers. This section gives possible reasons for breaks in the
sequences, lists all breaks in the image number sequences, and states
whether any data are actually missing for each break.
Due to the operational constraints on telemetry playback, some of
the pre-encounter images taken by the camera could not have been
reconstructed due to an incomplete set of packets and, therefore,
are not present in this data set. Specifically an image could not
have been reconstructed if its first packet was missing or if
it was a windowed image and any packet was missing.
Some images were only intended for on-board auto-tracking (NAV) and
never intended for playback. However, their existence still
incremented the image number, so images taken from before and after
NAV image(s) will show a break in their sequence.
Some breaks in the image number sequence occurred because the image
number was intentionally reset.
A final break in the image number sequence occurred at the end of
science imaging, as the spacecraft receded from the comet, when it
was determined that the comet was too dim to be detected in any
remaining images, which were at that time stored on the spacecraft
and scheduled to be downlinked, nor would it be detected in future
images. At that point the operations and science teams agreed to
terminate science imaging operations, erase the remaining images from
spacecraft memory without downlinking them, and prepare for the final
NAVCAM calibration and end of mission activities.
The breaks in the sequence of image numbers are as follows:
Preceding Following
Image Image Reason for Absence
--------- --------- ------------------
N0005CO N0000C5 Reset sequence to zero; no missing data
N0133C5 N0000C6 Reset sequence to zero; no missing data
N0036C6 N10000TE Reset sequence to zero; no missing data
N10004TE N10006TE Missing packets; missing N10005TE
N10318TE N10320TE Missing packets; missing N10319TE
N10344TE N10346TE Missing packets; missing N10345TE
N10357TE N10359TE Missing packets; missing N10358TE
N10366TE N10368TE Missing packets; missing N10367TE
N10404TE N10409TE Missing packets; missing 4 from N10405TE
N10414TE N10416TE Missing packets; missing N10415TE
N10419TE N10421TE Missing packets; missing N10420TE
N10638TE N10641TE Missing packets; missing 2 from N10639TE
N10710TE N30004TE NAV, Reset sequence to 30000; no missing data
N30100TE N30102TE Missing packets; missing N30101TE
N30954TE N30956TE Missing packets; missing N30955TE
N30956TE N30958TE Missing packets; missing N30957TE
N30965TE N30969TE Missing packets; missing 3 from N30966TE
N31135TE N31137TE Missing packets; missing N31136TE
N31213TE N31215TE Missing packets; missing N31214TE
N31574TE N31648TE Science imaging terminated; no missing data
|
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