| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains Calibrated data taken by
the New Horizons Alice Ultraviolet Imaging Spectrograph
instrument during the KEM1 ENCOUNTER mission phase.
PERSI-Alice (P-ALICE; also ALICE) is a spectrograph on the New Horizons
spacecraft that is sensitive to extreme and far UltraViolet (UV) light
(520-1870 Angstroms). The ALICE instrument comprises a telescopic optics
section and a spectrograph section that includes a diffraction grating
and a photosensitive two-dimensional (2-D) detector. The optics and
diffraction grating physical arrangement configure one detector
dimension as a spatial dimension and the other as spectral. ALICE has
two separate entrance apertures that feed light to the telescope section
of the instrument: the AirGlow Channel (AGC) aperture; the Solar
Occultation Channel (SOCC) aperture. Both apertures pass light to the
detector through a lollipop-shaped slit comprising two contiguous
sections: a narrow, rectangular slit with a Field Of View (FOV) of 0.1
by 4.0 degrees; a fat, square slit with FOV 2.0 x 2.0 degrees. ALICE has
two data-taking modes: PixelList mode records each detector/photon
event location (pixel i.e. spectral and spatial), interleaved with time
sequence events (hacks), allowing sub-second resolution of the photon
events; histogram mode summarizes the per-pixel photon event counts into
a 2-D histogram over all detector pixels, collected over an extended
time which can range from a few seconds to several days. From both
modes, the common data product is the histogram (derived on the ground
in the PixelList case), which is functionally equivalent to a
spectral-by-spatial spectrogram (2-D image); other data products are
also provided and described in this data set.
Every observation provided in this data set was taken as a part of a
particular sequence. A list of these sequences has been provided in
file DOCUMENT/SEQ_ALICE_*.TAB.
N.B. Some sequences provided may have no corresponding observations.
For a list of observations, refer to the data set index table. This
is typically INDEX.TAB initially in the INDEX/ area of the data set.
There is also a file SLIMINDX.TAB in INDEX/ that summarizes key
information relevant to each observation, including which sequence
was in effect and what target was likely intended for the
observation.
Version
=======
This is VERSION 4.0 of this data set.
This version includes data acquired by the spacecraft between
08/14/2018 and 04/30/2020. It only includes data downlinked before
05/01/2020. Future datasets may include more data acquired by the
spacecraft after 08/13/2018 but downlinked after 04/30/2020.
This version includes high voltage (HV) dark tests, Airglow
observations of Arrokoth, Approach Solar Unocculted SOCC Histogram,
Departure Solar Unocculted SOCC Histogram, Stellar Occultation Airglow,
Stellar Occultation SOCC, a map of the Lyman Alpha Sky Emission,
IPM Observations, and Pointing and Flux Verifications.
Some data files may not contain housekeeping data because the
housekeeping packets were not downloaded by the time of archive
creation. In the level 3 (calibrated) dataset, some files may have a
countrate extension with 0 values caused by a high voltage and dark
test in which the housekeeping sample rate was changed.
V3.0
----
Version 3.0 of this data set included data acquired by the spacecraft
between 08/14/2018 and 07/31/2019. It only included data downlinked
before 08/01/2019.
This version included high voltage (HV) dark tests, Airglow
observations of Arrokoth, Approach Solar Unocculted SOCC Histogram,
Departure Solar Unocculted SOCC Histogram, Stellar Occultation Airglow,
Stellar Occultation SOCC, and a map of the Lyman Alpha Sky Emission.
V2.0
----
Version 2.0 of this data set included data acquired by the spacecraft
between 08/14/2018 and 01/31/2019. It only included data downlinked
before 02/01/2019.
This version included high voltage (HV) dark tests, Airglow
observations of Arrokoth, Departure Solar Unocculted SOCC Histogram,
Stellar Occultation Airglow, and Stellar Occultation SOCC.
An EXTENSION_HK_TABLE was added to several fits files as compared to
V1.0. An EXTENSION_CT_RATE_VEC_HK_ARRAY was added to several fits files
as compared to V1.0.
Two digits of precision have also been added to the EXPOSURE_DURATION
value in all data labels after V1.0.
General statement about data set versions after V1.0
----------------------------------------------------
The pipeline (see Processing below) was re-run on these data for each
version since the first (V1.0). A pipeline rerun usually changes the
FITS headers but not the FITS data of raw data sets. In some cases
calibrated FITS data may change because the calculated geometry of an
observation has changed. See data set version-specific sections above
for significant exceptions to this general statement, i.e. changes to
pipeline processing, calibration processing, and data delivered.
Note that even if this is not a calibrated data set, calibration
changes are listed as the data will have been re-run and there will be
updates to the calibration files, to the documentation (Science
Operations Center - Instrument Interface Control Document:
SOC_INST_ICD) and to the steps required to calibrate the data.
Processing
==========
The data in this data set were created by a software data
processing pipeline on the Science Operations Center (SOC) at
the Southwest Research Institute (SwRI), Department of Space Operations.
This SOC pipeline assembled data as FITS files from raw telemetry
packets sent down by the spacecraft and populated the data labels
with housekeeping and engineering values, and computed geometry
parameters using SPICE kernels. The pipeline did not resample
the data.
Data
====
The observations in this data set are stored in data files using
standard Flexible Image Transport System (FITS) format. Each FITS
file has a corresponding detached PDS label file, named according
to a common convention. The FITS files may have image and/or table
extensions. See the PDS label plus the DOCUMENT files for a
description of these extensions and their contents.
This Data section comprises the following sub-topics:
- Filename/Product IDs
- Instrument description
- Other sources of information useful in interpreting these Data
- Visit Description, Visit Number, and Target in the Data Labels
Filename/Product IDs
--------------------
The filenames and product IDs of observations adhere to a
common convention e.g.
ALI_0123456789_0X4B0_ENG.FIT
^^^ ^^^^^^^^^^ ^^^^^ ^^^\__/
| | | | ^^
| | | | |
| | | | +--File type (includes dot)
| | | | - .FIT for FITS file
| | | | - .LBL for PDS label
| | | | - not part of product ID
| | | |
| | | +--ENG for CODMAC Level 2 data
| | | SCI for CODMAC Level 3 data
| | |
| | +--Application ID (ApID) of the telemetry data
| | packet from which the data come
| | N.B. ApIDs are case-insensitive
| |
| +--MET (Mission Event Time) i.e. Spacecraft Clock
|
+--Instrument designator
Note that, depending on the observation, the MET in the data filename
and in the Product ID may be similar to the Mission Event Time (MET)
of the actual observation acquisition, but should not be used as an
analog for the acquisition time. The MET is the time that the data are
transferred from the instrument to spacecraft memory and is therefore
not a reliable indicator of the actual observation time. The PDS label
and the index tables are better sources to use for the actual timing of
any observation. The specific keywords and index table column names for
which to look are
* START_TIME
* STOP_TIME
* SPACECRAFT_CLOCK_START_COUNT
* SPACECRAFT_CLOCK_STOP_COUNT
Instrument Instrument designators ApIDs **
=========== ================================== =============
ALICE ALI 0X4B0 - 0X4B7 *
* Not all values in this range are in this data set
** ApIDs are case insensitive
There are other ApIDs that contain housekeeping values and
other values. See SOC Instrument ICD (/DOCUMENT/SOC_INST_ICD.*)
for more details.
Here is a summary of the types of files generated by each ApID
(N.B. ApIDs are case-insensitive) along with the instrument
designator that go with each ApID:
ApIDs Data product description/Prefix(es)
===== ===================================
0x4b0 - ALICE PixelList Lossless (CDH 1)/ALI
0x4b1 - ALICE PixelList Packetized (CDH 1)/ALI
0x4b4 - ALICE PixelList Lossless (CDH 2)/ALI
0x4b5 - ALICE PixelList Packetized (CDH 2)/ALI
0x4b2 - ALICE Histogram Lossless (CDH 1)/ALI
0x4b3 - ALICE Histogram Packetized (CDH 1)/ALI
0x4b6 - ALICE Histogram Lossless (CDH 2)/ALI
0x4b7 - ALICE Histogram Packetized (CDH 2)/ALI
Notes:
------
1) CDH 1 and CDH 2 refer to the spacecraft redundant Command and Data
Handling systems in general, and here specifically to their
respective Solid State Recorders (SSRs) 1 and 2, where ALICE data
be stored and prepared for downlink. ALICE can send data to SSR
1 or to SSR 2, or, for mission-critical data, to both redundantly.
ALICE shares its channel to the SSRs with the Long-Range
Reconnaissance Imager (LORRI), so both instruments cannot store
data simultaneously. ALICE has the capability to store histogram
data to instrument-internal storage, and to transfer it to the
SSR(s) later; such an operation is called a Held Histogram, and
it allows ALICE to take data at the same time that LORRI is taking
and writing data to the SSR(s).
2) Packetized and Lossless refer to the method used on-board to
convert raw, high-speed instrument data on the SSR to low-speed
data ready for downlink. The conversion process is generally
referred to as compression, even though Packetized conversion does
not reduce the data volume In practice, PixelList data always
use Packetized compression. Histogram data may use Packetized or
Lossless compression. Depending on the actual data contents,
Lossless compression reduces data volume by 60 to 90% or more;
for nominal science data a factor of 3 or more is normal.
Lossless compression is used whenever possible to reduce downlink
data volume. There is no difference, between Packetized and
Lossless compression, in the resultant FITS files after processing
by the Science Operations Center (SOC) data pipeline.
Instrument description
----------------------
Refer to the following files for a description of this instrument.
CATALOG
ALICE.CAT
DOCUMENTS
ALICE_SSR.*
SOC_INST_ICD.*
NH_ALICE_V###_TI.TXT (### is a version number)
Other sources of information useful in interpreting these Data
--------------------------------------------------------------
Refer to the following files for more information about these data
NH Trajectory tables:
/DOCUMENT/NH_MISSION_TRAJECTORY.* - Heliocentric
ALICE Field Of View definitions:
/DOCUMENT/NH_FOV.*
/DOCUMENT/NH_ALICE_V###_TI.TXT
Visit Description, Visit Number, and Target in the Data Labels
---------------------------------------------------------------
The observation sequences were defined in Science Activity Planning
(SAP) documents, and grouped by Visit Description and Visit Number.
The SAPs are spreadsheets with one Visit Description & Number per row.
A nominal target is also included on each row and included in the data
labels, but does not always match with the TARGET_NAME field's value in
the data labels. In some cases, the target was designated as RA,DEC
pointing values in the form ``RADEC=123.45,-12.34'' indicating Right
Ascension and Declination, in degrees, of the target from the
spacecraft in the Earth Equatorial J2000 inertial reference frame.
This indicates that either the target was a star, or the
target's ephemeris was not loaded into the spacecraft's attitude and
control system which in turn meant the spacecraft could not be pointed
at the target by a body identifier and an inertial pointing value had
to be specified as Right Ascension and Declination values. PDS-SBN
practices do not allow putting a value like RADEC=... in the PDS
TARGET_NAME keyword's value. In those cases the PDS TARGET_NAME value
is set to CALIBRATION. TARGET_NAME may be N/A (Not Available or Not
Applicable) for a few observations in this data set; typically that
means the observation is a functional test so N/A is an appropriate
entry for those targets, but the PDS user should also check the
NEWHORIZONS:OBSERVATION_DESC and NEWHORIZONS:SEQUENCE_ID keywords in
the PDS label, plus the provided sequence list (see Ancillary Data
below) to assess the possibility that there was an intended target.
These two keywords are especially useful for STAR targets as often
stars are used as part of instrument calibrations, and are
included as part of the sequencing description which is captured
in these keywords.
Specifically for ALICE observations, any observation that has an
observation description or sequence ID that includes the words dump
or held will usually have N/A as its target, but that indicates the
observation was actually taken as part of an earlier sequence and held
held locally in instrument memory (i.e. a Held Histogram; see the
Notes in the Data section below), and the Dump sequence represents the
commands that transferred the instrument data onto the spacecraft
Solid-State Recorders (SSRs). In the cases of Held Histograms, the
user should check the previous sequence in the sequence list. For
other cases note that if the characters _P_, _C_, or _PC_ are in the
sequence ID, then the intended target was likely Pluto, Charon, or
Pluto and Charon together, respectively.
Ancillary Data
==============
The geometry items included in the data labels were computed
using the SPICE kernels archived in the New Horizons SPICE
data set, NH-J/P/SS-SPICE-6-V1.0.
Every observation provided in this data set was taken as a part of a
particular sequence. The sequence identifier (REQID) and description
are included in the PDS label for every observation.
N.B. While every observation has an associated
sequence, every sequence may not have associated observations. Some
sequences may have failed to execute due to spacecraft events (e.g.
safing). No attempt has been made during the preparation of this data
set to identify such empty sequences, so it is up to the user to
compare the times of the sequences to the times of the available
observations from INDEX/INDEX.TAB to identify such sequences.
Time
====
There are several time systems, or units, in use in this dataset:
New Horizons spacecraft MET (Mission Event Time or Mission Elapsed
Time), UTC (Coordinated Universal Time), and TDB Barycentric
Dynamical Time.
This section will give a summary description of the relationship
between these time systems. For a complete explanation of these
time systems the reader is referred to the documentation
distributed with the Navigation and Ancillary Information
Facility (NAIF) SPICE toolkit from the PDS NAIF node, (see
http://naif.jpl.nasa.gov/).
The most common time unit associated with the data is the spacecraft
MET. MET is a 32-bit counter on the New Horizons spacecraft that
runs at a rate of about one increment per second starting from a
value of zero at
19.January, 2006 18:08:02 UTC
or
JD2453755.256337 TDB.
The leapsecond adjustment (DELTA_ET = ET - UTC) was 65.184s at
NH launch, and the first four additional leapseconds occurred
at the ends of 12/2009, 06/2012, 06/2015, and 12/2016.
Refer to the NH SPICE data set, NH-J/P/SS-SPICE-6-V1.0, and the
SPICE toolkit documentation, for more details about leapseconds.
The data labels for any given product in this dataset usually
contain at least one pair of common UTC and MET representations
of the time at the middle of the observation. Other portions
of the products, for example tables of data taken over periods
of up to a day or more, will only have the MET time associated
with a given row of the table.
For the data user's use in interpreting these times, a reasonable
approximation (+/- 1s) of the conversion between Julian Day (TDB)
and MET is as follows:
JD TDB = 2453755.256337 + ( MET / 86399.9998693 )
For more accurate calculations the reader is referred to the
NAIF/SPICE documentation as mentioned above.
Reference Frame
===============
Geometric Parameter Reference Frame
-----------------------------------
Earth Mean Equator and Vernal Equinox of J2000 (EMEJ2000) is the
inertial reference frame used to specify observational geometry items
provided in the data labels. Geometric parameters are based on best
available SPICE data at time of data creation.
Epoch of Geometric Parameters
-----------------------------
All geometric parameters provided in the data labels were computed at
the epoch midway between the START_TIME and STOP_TIME label fields.
Software
========
The observations in this data set are in standard FITS format
with PDS labels, and can be viewed by a number of PDS-provided
and commercial programs. For this reason no special software is
provided with this data set.
Contact Information
===================
For any questions regarding the data format of the archive,
contact
New Horizons ALICE Principal Investigator:
S. Alan Stern
Southwest Research Institute (SwRI)
Department of Space Studies
1050 Walnut Street, Suite 400
Boulder, CO 80302
USA
|
| CONFIDENCE_LEVEL_NOTE |
Confidence Level Overview
=========================
During the processing of the data in preparation for
delivery with this volume, the packet data associated with each
observation were used only if they passed a rigorous verification
process including standard checksums.
In addition, raw (Level 2) observation data for which adequate
contemporary housekeeping and other ancillary data are not available
may not be reduced to calibrated (Level 3) data. This issue is raised
here to explain why some data products in the raw data set,
NH-A-ALICE-2-KEM1-V4.0,
may not have corresponding data products in the calibrated data set,
NH-A-ALICE-3-KEM1-V4.0.
Data coverage and quality
=========================
Every observation provided in this data set was taken as a part of a
particular sequence.
Refer to the Confidence Level Overview section above for a summary
of steps taken to assure data quality.
The lollipop-shaped fuzz in images of some ALICE spectra, seen as high
signal levels at the box end of the slit around Hydrogen Lyman-alpha (H
Lya) wavelengths, is due to a characteristic of the detector and
aperture. To make the Micro Channel Plate (MCP) more sensitive to UV
light, it was coated with potassium bromide (KBr) photocathodes from
520 to 1180 Angstrom and with cesium iodide (CsI) photocathodes from
1250 to 1870 Angstrom. A vertical strip - a spectral band of 70
Angstrom centered at ~1216 Angstrom - of the MCP was masked and left
uncoated to reduce the sensitivity of the detector to H Lya
radiation. In the slit portion of the aperture (0.1deg wide x 4deg
high), the diffraction grating keeps the strong H Lya line within that
uncoated band. However, in the 2x2 degree box portion of the aperture
designed to capture the Sun during occultations, the H Lya spreads out
beyond the uncoated 70-Angstrom band over another ~55 Angstroms of more
sensitive photocathode-coated detector on either side. The quantum
efficiencies of the photocathode- coated surfaces are about an order of
magnitude more sensitive to H Lya wavelengths than the bare, uncoated
glass, which gives rise to high signal levels from the box area of the
slit i.e. the lollipop fuzz.
Observation descriptions in this data set catalog
=================================================
Some users will expect to find descriptions of the observations
in this data set here, in this Confidence Level Note. This data
set follows the more common convention of placing those
descriptions under the Data Set Description (above, if the user is
reading this in the DATASET.CAT file) of this data set catalog.
Caveat about TARGET_NAME in PDS labels and observational intent
===============================================================
The downlink team on New Horizons has
created an automated system to take various uplink products, decode
things like Chebyshev polynomials in command sequences representing
celestial body ephemerides for use on the spacecraft to control
pointing, and infer from those data what the most likely intended
target was at any time during the mission. This works well during
flyby encounters and less so during cruise phases and hibernation.
The user of these PDS data needs to
be cautious when using the TARGET_NAME and other target-related
parameters stored in this data set. This is less an issue for the
plasma and particle instruments, more so for pointing instruments.
To this end, the heliocentric ephemeris of the spacecraft, the
spacecraft-relative ephemeris of the inferred target, and the
inertial attitude of the instrument reference frame are provided
with all data, in the J2000 inertial reference frame, so the user
can check where that target is in the Field Of View (FOV) of the
instrument.
Finally, note that, within the FITS headers of the data products,
the sequence tables, and other NH Project-internal documents used
in this data set and/or inserted into the data set catalog,
informal names are often used for targets instead of the canonical
names required for the TARGET_NAME keyword. For example, during
the Pluto mission phase, instead of the TARGET_NAME '15810 ARAWN
(1994 JR1)' there might be found any of the following: 1994JR1;
1994 JR1; JR1. For all values where the PDS keyword TARGET_NAME
is used (e.g. in PDS labels and in index tables), the canonical,
PDS-approved names are used (if not, please bring this to the
attention of PDS so it can be rectified). However, within the
context of this data set, these project abbreviations are not
ambiguous (e.g. there is only one NH target with 'JR1' in its
name), so there has been, and will be, no attempt to expand such
abbreviations where they occur outside formal PDS keyword values.
Review
======
This dataset was peer reviewed and certified for scientific use by
the PDS.
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