| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains Level 3 (RDR; calibrated) approach and
encounter images taken by the Stardust Navigation Camera (NAVCAM)
targeting comet 81P/Wild 2 (1978 A2).
The impetus for this data set delivery is to apply the updated NAVCAM
calibration procedure, developed during the later Stardust-NExT
extended mission, to the approach and encounter images of the target
body. Therefore, the images in this data set are only a subset of all
images taken during approach and encounter; calibration images, and
early approach images in which the target body cannot be detected,
have been intentionally excluded.
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 documents ``Log
of Stardust NAVCAM Flight Images'', DOCUMENT/PIIMGLOG.LBL, and
the detailed data set description, from earlier data sets containing
these data, in CATALOG/OLD_DATASET_CAT.TXT.
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 catalogs for this mission and for the extended mission
(NExT).
The following sections list the NAVCAM data collection periods.
Approach -- 2003-11-13 to 2003-12-30 -- Images 528-1014
------------------------------------------------------
Approach imaging was mostly windowed, and comprised attempts to test
and/or characterize various items: background stars; Wild 2
presence; auto-NAV pattern matching both through and off the
periscope; different mirror angles and windowing sizes; optical
navigation; exposure time response; a failed photometric calibration;
assess camera decontamination. Exposure durations ranged from 1s to
20s, with most at 5s, 10s or 15s.
Final approach -- 2003-12-30 to 2004-01-02 -- Images 1015-1122
--------------------------------------------------------------
Final approach imaging during the last few days comprised windowed
imaging at 5s and 10s exposure durations.
Wild 2 Encounter -- 2004 JAN 02 -- Images 2005-2115
---------------------------------------------------
Images 2005 through 2115 are the 81P/Wild 2 encounter set.
There are 72 complete images in this set, the missing numbers
having been used only to establish the auto-tracking on the
nucleus. There was sufficient memory only for the 72 images.
Commanding constraints limited in the number of changes that
could be made in the exposure time, so most were taken at
settings of either 100ms or 10ms. Many of the longer exposures
were saturated, but serve to bring out the many jets of gas and
dust ejected by the comet. Scan mirror angles range from 1 to 176
The images with mirror angles over 170 degrees all exhibit a great
deal of scattered light, probably from the sample return capsule.
There are lesser amounts of scattered light in images back to
about 160 degrees. There is a problem with images taken near 0
degrees as well, from light scattered from the launch vehicle
adapter ring which actually occludes a bit of the periscope.
Instrument and data calibrations
================================
Calibration sources
-------------------
The calibration data for NAVCAM were derived from pre-launch and
in-flight testing; the calibration analysis and pipeline development
were done during the Stardust-NExT extended mission. Re-calibration
of the 5535 Annefrank and 81P Wild 2 subsets of prime mission data
was performed in 2013/2014, which resulted in this data set.
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 activity
(approach; encounter).
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 Data Numbers (DNs) from
the NAVCAM Analog-to-Digital Converter (ADC) as it read the voltages in
the CCD pixels. In some cases these DN data have been compressed via
lookup table; see CALIB/NC_COMPR.LBL.
Note that, to save on limited downlink at the time some image
observations were made, only rectangular window subsets of the the
full-frame of pixels were telemetered to Earth. This can be
identified by WINDOW objects in PDS labels, and by the appearance of
the image as gray/white rectangle against a rectangles against a
black background in the BROWSE/ images. Note that BLS pixels are not
avalable in these windowed images, which affects their calibration.
Refer to the documentation and the calibration paper
[KLAASENETAL2011B] for more detail about the effect windowing has on
data calibration.
The data calibration converts the raw Data Numbers
(DNs) to radiance, typically, or to bias- and dark-subtracted DN for
images with zero exposure duration.
Extension Data Units (EDUs) of calibrated data contain maps of
parameters associated with each PDU image pixel: a quality map; an
uncertainty map; a signal-to-noise ratio map.
The value of a quality map pixel indicates whether the corresponding
pixel in the PDU could be calibrated or not, and if not, then why.
The value of an uncertainty map pixel gives the calculated
uncertainty for the corresponding pixel in the PDU.
The value of a signal-to-noise ratio (SNR) map pixel gives the
calculated SNR for the corresponding pixel in the PDU.
Refer to the data labels and the calibration documentation for more
details about the EDUs.
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 calibrated Data Number (DN) values from the
NAVCAM Analog-to-Digital Converter (ADC), converted to engineering
units: radiance or bias- and dark-subtracted DN.
Ancillary data include quality, uncertainty signal-to-noise ratio
maps, 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 converted to FITS format, the data were
calibrated and also stored in FITS format, and finally detached PDS
labels for the FITS files were generated.
The data calibration portion of the pipeline comprised several steps,
summarized in Data Calibration above.
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 this archive, contact the Planetary Data
System Small Bodies Node.
|
| 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 mission, which were
successfully downlinked without corrupted or lost packets during the
data collection periods listed above, are included in various data sets.
This data set contains only those images where the target body,
comet 81P/Wild 2 (1978 A2),
is calculated from the image geometry to be in or near the image field
of view. As a result, there may appear to be breaks in the image
sequencing as interpreted from the image frame numbers. Other possible
reasons for breaks in the sequencing follow.
A list of files, which were included in data sets delivered before
October, 2015 and excluded in the data sets delivered in October, 2015,
may be found in document EXCESSOLD.* in the DOCUMENT/ sub-directory
of this data set.
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.
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