| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains raw calibration and test images acquired by the
Deep Impact Medium Resolution Instrument Visible CCD (MRI) during the
cruise phase of the mission. These observations were used for optical
and autonomous navigation (NAV) of the flyby spacecraft. These data
were collected from 14 January to 25 April 2005. Test images of comet
9P/Tempel 1 were acquired on 25 April.
Software on board the flyby spacecraft used the optical navigation
(OpNav) and autonomous navigation (AutoNav) images to compute the
brightness centroid of the target body for trajectory corrections.
Optical navigation was used for the cruise phase and most of the
encounter phase, until two hours before impact when the AutoNav
system took control of the flyby spacecraft in preparation for
impact imaging. For more information about the AutoNav system,
see Mastrodemos, et. al (2005) [MASTRODEMOSETAL2005].
A NAV observation consisted of one or more packets of data, each
containing an 80-byte header plus a rectangle of image data called a
''snip'', cropped from a full-frame image. Pixels outside the snips
were not returned. The Deep Impact Science Data Center (SDC) at
Cornell University, used the packets for one observation to
reconstruct a raw, full-frame image of 1008 by 1008 pixels. Pixels
outside the snips were set to a value of zero with one common header.
Locations of the snips within the frame were not captured by the
data pipeline. Further details about these images are included in
the processing section below.
Raw NAV images were grouped into daily directories by mid-observation
date. A list of the NAV images in the data set is provided here:
Exposure IDs
OBS Date DOY Minimum Maximum Observation Type
---------- --- ------- ------- ----------------------------------
2005-01-14 014 1000000 1000004 Quick alignment
2005-01-15 015 1000006 1000010 Quick alignment
2005-01-26 026 1200001 1200080 AutoNav test
2005-01-30 030 1500000 1500004 Image snip test
2005-02-04 035 1500000 1500004 Image snip test
2005-02-06 037 1200004 1200560 AutoNav test
1300008 1300015 AutoNav test
2005-03-03 062 2100000 2100047 Attitude Determination and Control
Subsystem (ADCS) alignment test
2005-04-01 091 2300000 2300184 OpNav Photometric calibration
2005-04-05 095 2010005 2010016 Focal length/Geometric distortion
2005-04-06 096 2030018 2030029 Focal length/Geometric distortion
2005-04-18 108 4200000 4200129 Nav engineering
2005-04-23 113 4200000 4200006 MRI-ADCS alignment
4200007 4200129 Stray light/Lookback attitude test
2005-04-25 115 4200000 4200029 MRI-ADCS alignment repeat;
images include a snip of Tempel 1
Essential Reading
-----------------
The following documents, located on the Deep Impact Documentation
volume, DIDOC_0001, are essential for the understanding and
interpretation of this data set:
NAV_IMAGES_REPORT.* : Description of how raw and reduced
NAV FITS images were created for and
processed by the Deep Impact science
data pipeline
NAV_MRI_RAW_CRUISE.* : Science-related image indices for
this data set
NAV_MRI_FILE_NAMES_RAW2CAL : Cross-reference of raw and reduced
file names
CALIBRATION_DOC.* : Instrument calibration by Klaasen,
et al. (2006) [KLAASENETAL2006]
INSTRUMENTS_HAMPTON.* : Instrument paper by Hampton, et al.
(2005) [HAMPTONETAL2005]
AUTO_NAVIGATION_MASTRODEMOS.* : Auto-navigation paper by
Mastrodemos, et. al (2005)
[MASTRODEMOSETAL2005]
MISSION_OVERVIEW_AHEARN.* : Mission overview by A'Hearn, et al.
(2005) [AHEARNETAL2005B]
SCLK_CORRELATION.* : Discussion of the discrepancy
between the spacecraft clocks and
UTC
AICD_FLIGHT_HRIV_MRI_ITS.* : Description of the data set and
definitions of label keywords
Related Data Sets
-----------------
The following PDS data sets are related to this one:
DIF-C-MRI-2-NAV-9P-ENCOUNTER-V1.0 : Raw MRI NAV encounter data
DIF-C-MRI-3-NAV-9P-ENCOUNTER-V1.0 : Calibrated MRI NAV enctr data
DI-C-SPICE-6-V1.0 : SPICE kernels
The related Deep Impact science data sets are:
DIF-CAL-MRI-2-9P-CRUISE-V1.0 : Raw MRI cruise data
DIF-C-MRI-2-9P-ENCOUNTER-V1.0 : Raw MRI encounter data
DIF-C-MRI-3/4-9P-ENCOUNTER-V2.0 : Calibrated MRI encounter data
Processing
==========
The FITS images in this data set were created by the Deep Impact
Science Data Center (SDC) at Cornell University and were assembled
from raw NAV snips received from the flyby spacecraft. The NAV images
report, listed above in the recommended reading section, discusses how
the NAV data were processed into raw images. A summary is provided
here.
NAV images were typically acquired during sequences designed to gather
science data. The major difference between a science and NAV image
was that a NAV observation was made of one or more packets of data,
each containing an 80-byte header plus a rectangle of image data
called a ''snip''. Because the snips were cropped from a full-frame
image, not all pixels were returned in many cases. However, some
images were returned in full as a single snip of 1008 by 1008 pixels.
Because one raw NAV observation was received as one or more snips,
the SDC reconstructed a full frame of 1008 by 1008 pixels from the
snips and stored the results as raw a FITS file. Since the raw NAV
snips excluded the serial- and parallel-overclock pixels around the
edges of the 1024 by 1024 array, these areas were not added to the
reconstructed, raw frames. Also, pixels not returned were set to
a value of zero in the raw FITS images.
Applied Coherent Technology Corporation in Herndon, VA, produced the
PDS data labels by extracting parameters from the FITS headers.
Data
====
File Naming Convention
----------------------
The naming convention for the data labels and FITS files was
DxMeeeeeee_yyyydddhhmmssuuu.LBL or FIT, where:
D : Deep Impact
x : Image usage (A for AutoNav or O for OpNav)
M : MRI instrument
eeeeeee : Exposure ID, same as for science data; image number
within an exposure ID was always 1 of 1
yyyy : Ground-received time (GRT) year
ddd : GRT day of year
hhmmss : GRT hours, minutes, and seconds
uuu : Suffix to provide uniqueness when two images were
acquired or processed within the same second
Some images were downloaded more than once and processed
by the SDC. For this case, each version of a raw image was
included in this data set. For multiple downloads of one
image, the exposure IDs in the file names are identical
but the GRTs are different.
FITS CCD Images
---------------
The two-dimensional, visible CCD images in this data set are in FITS
format. The primary data array contains the CCD image. Each FITS
file is accompanied by a detached PDS label. The document about NAV
images provides a more detailed description of the image format.
Although a lossless compression algorithm was available, it was
not used during flight. Therefore, all raw NAV FITS images in
this data set are uncompressed (i.e., never compressed on board
the spacecraft).
Target Name and Description
---------------------------
The TARGET_NAME keyword in the data labels was set to the intended
target, such as 'Calibration'. The TARGET_DESC keyword provides
the name of the specific calibration target, such as 'NGC 3114'.
True-Sky ''As Seen By Observer'' Display
----------------------------------------
A true-sky view is achieved by displaying the image using the
standard FITS convention: the fastest-varying axis (samples)
increasing to the right in the display window and the slowest-
varying axis increasing to the top. This convention is also
defined in the image labels:
SAMPLE_DISPLAY_DIRECTION = 'RIGHT'
LINE_DISPLAY_DIRECTION = 'UP'
This topic is discussed in detail in the 'Quadrant Nomenclature'
section of the 'Deep Impact Instrument Calibration' document.
The direction to Celestial North and Ecliptic North, measured
clockwise from the top of the displayed image, is provided in PDS
labels by CELESTIAL_NORTH_CLOCK_ANGLE and
SOLAR_NORTH_POLE_CLOCK_ANGLE, both of which assume the correct
display defined by SAMPLE_DISPLAY_DIRECTION and
LINE_DISPLAY_DIRECTION.
Parameters
==========
Data Units
----------
Raw image data are in units of raw data numbers.
Imaging Modes and Filters
------------------------
The unbinned, full-frame, instrument mode 1 was always used for NAV
images. Also, the only filters used were the two clear filters: 1
or 6, both centered at 650 nanometers. However, the exact filter
was not known when the raw FITS images were created. Therefore, the
filter-related keywords in the PDS labels are set to UNK (unknown).
The exact filter was determined later during the pre-processing of
raw NAV data for the calibration pipeline.
For descriptions of the imaging modes and filters, please see
the Deep Impact instrument paper by Hampton, et al. (2005)
[HAMPTONETAL2005].
Time-Related Keywords
---------------------
All time-related keywords in the data labels, except
EARTH_RECEIVED_TIME, are based on the clock on board the
spacecraft. EARTH_RECEIVED_TIME provides the UTC when an
Earth-based observer should be able to see an event recorded by
the instrument.
Although the time to impact was not calculated for the NAV data,
the estimated time of impact at the flyby spacecraft was UTC
05:44:34.265 on 4 Jul 2005. This is based on the conclusions
presented in the spacecraft clock correlation report provided
on the DI Documentation volume.
Geometry-Related Keywords
-------------------------
The SOLAR_NORTH_POLE_CLOCK_ANGLE in the data labels specified the
the direction of ecliptic north as projected onto the image plane.
It is measured from the 'upward' direction, clockwise to the
direction toward ecliptic north when the image is displayed as
defined by the SAMPLE_DISPLAY_DIRECTION and LINE_DISPLAY_DIRECTION
keywords.
The SDC pipeline was not able to automatically determine the
proper geometric information for the target of choice in many
cases. When these parameters could not be computed, the
corresponding keywords in the PDS data labels were set to a
value of unknown (UNK). Geometry-related keywords for most
calibration targets were set to UNK.
Geometric parameters provided in the data labels were computed at
the epoch specified by MID_IMAGE_TIME, except for the target-to-sun
and earth-observer-to-target parameters. Target-to-sun values were
calculated for the time when the light left the sun while
earth-observer-to-target were calculated for the time when the light
left the target.
Geometry-related parameters in the PDS data labels are uncertain at
a level of a few seconds because of a known 2-second discrepancy
between the clocks on board the flyby and impactor spacecraft and
between in-situ data and ground-based observations. After a
detailed analysis of the timing problem in early 2006, improved
self-consistent SPICE kernels were generated by the Deep Impact
project to correlate the spacecraft clocks; there is still a
1-2 second discrepancy between the in-situ data and the ground-
based observations. These improved kernels were included in the
DI SPICE data set and were used to calculate the geometric
parameters in the PDS data labels. For more information about
this discrepancy, please see the spacecraft clock correlation
report provided on the DI documentation volume, DIDOC_0001.
The SPICE kernels used to calculated the geometric parameters are
provided by the SPICE_FILE_NAME keyword in the PDS data labels. The
kernels were listed in the order they were loaded into memory for
processing.
Ancillary Data
==============
Geometric parameters included in the data labels were computed using
the DI SPICE data set archived in the PDS.
Coordinate System
=================
Earth Mean Equator and Vernal Equinox of J2000 (EME J2000) was the
inertial reference system used to specify observational geometry
parameters in the data labels.
|
.png)
.png)
