| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains Calibrated data taken by New Horizons
Pluto Energetic Particle Spectrometer Science Investigation
instrument during the PLUTOCRUISE mission phase.
PEPSSI (Pluto Energetic Particles Spectrometer Science Investigation)
is a particle telescope and a time-of-flight (TOF) spectrometer that
measures ions and electrons over a broad range of energies and
angles. Particle composition and energy spectra are measured for H to
Fe from ~ 30 keV to ~1 MeV (but not all species are uniquely separated)
and for electrons from ~30 keV to 700 keV. PEPSSI comprises a
time-of-flight (TOF) section and a solid-state detector (SSD) array
that measures particle energy. The combination of measured energy and
TOF provides unique particle identification by mass and particle energy
depending on the range: for protons from ~30 keV to ~1 MeV; for heavy
(CNO) ions from ~80 keV to ~1 MeV. Lower-energy (>3 keV) ion fluxes are
measured by TOF only, but without the SSD signal, providing velocity
spectra at these energies as well. Due to storage and bandwidth
limitations, all event data cannot be stored or telemetered to the
ground. Instead, a round-robin algorithm is used to save Energy, TOF,
and timing data for select events. The common data products contain
these event and summary measurements, accumulated over fixed periods of
86,400 seconds, with each period in a single file comprising multiple
binary tables. The documentation provided with this data set describes
the data format.
PEPSSI Data Summary
===================
During the Annual Check-out (ACO) periods 1-4 in 2007 though 2010,
PEPSSI performed a combination of calibration operations, table and
macro loads, and statistics gathering, which may be suitable for
science.
From 2011 through 2014, PEPSSI collected mostly science data during
hibernation, with breaks for checkouts during ACO periods 5-8.
PEPSSI Data Details
===================
The calibration operations ranged from functional checks, which run
the instrument for a short period to see if operations are nominal,
to operations in which the New Horizons spacecraft attitude is set
to aim the PEPSSI apertures in specified directions to permit
angular calibration akin to flat fielding for remote sensing
instruments. Table loads were performed to make improvements, e.g.
to PEPSSI's capability to identify particle species, and macro
loads were done to simplify operations so that a single spacecraft
command can execute a large number of PEPSSI activities within the
instrument, such as a power up sequence that we wish to be
identical whenever the instrument is turned on. The statistics
gathering activities were done primarily to populate the
time-of-flight vs. energy histograms. Those histograms are needed
to understand the PEPSSI mass identification response at Pluto, but
these periods also offer valuable interplanetary science data.
Almost all PEPSSI data in this data set are part of calibration,
functional tests and instrument checkout operations. However,
during periods when there were no interfering operations, such as
calibration operations or table loads, the term 'potential' is used
here to alert the user to periods where major sources of conflict
are lacking and the data taken may be scientifically useful. This
does not obviate the need to carefully examine all the spacecraft
and instrument states coincident with the data period under study,
for example the spacecraft attitude and background particles in a
given energy/species channel. The following is an overview of the
data taken during the four ACOs in this delivery.
In ACO-1, which ran from 2007/257-303 for PEPSSI, the major
operations included table loads, a diagnostic mode test, statistics
gathering, a full-sky scan activity, anisotropy test #1 for angular
calibration, and solid state detector (SSD) tests. The potential
periods for science data include the following periods, given at
daily granularity: 2007/257-258, 259-260, 278-282, and 300-303.
In ACO-2, which ran from 2008/276-341 for PEPSSI, the major
operations included table loads, a diagnostic mode test, statistics
gathering, start and stop discriminator threshold scans, anisotropy
test #2, a scan of the Sun through the PEPSSI field of view, a
macro load, and a memory refresh. The potential periods for
science data include the following periods, given at daily
granularity: 2008/276,278-280,282-283,284-287, and 303-341.
In ACO-3, the instrument function was tested by running PEPSSI for
24 hours on 2009/223. All data taken during this period are
potential science data.
Nearly all data taken during ACO-4 (2010/162-207) are potential
science data. Activities included a scan of the Sun across the
PEPSSI field of view, a memory refresh and statistics gathering.
The non-science activities are the sun scan, which took place from
2010/176 23:11:00 to 177 00:23:23, including two sun scans and set
up slews, and the 2010/184 20:05 EEPROM memory refresh.
From 2011 through 2014, analysis of spacecraft power margins
indicated that PEPSSI could operate during hibernation; prior to
that time PEPSSI was normally off during hibernation. So after
2010, PEPSSI collected science data for most of the rest of the
Pluto Cruise mission phase, with breaks for ACO periods 5-8 for
instrument monitoring, testing and characterization.
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_PEPSSI_PLUTOCRUISE.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 2.0 of this data set.
The pipeline (see Processing below) was re-run on these data for each
version since the first (V1.0). As a result, ancillary information,
such as observational geometry and time (SPICE), may be updated.
This will affect, for example, the calibration of the data if parameters
such as the velocity or orientation of the target relative to the
instrument, or the recorded target itself, have changed.
See the following sections for details of what has changed over each
version since the first (V1.0). Note that even if this is not a
calibrated data set, the 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.
PEPSSI updates for Pluto Cruise
Data Sets V2.0
==============
The previous Pluto Cruise data sets' deliveries (V1.0) went
through peer review in December, 2014. When subsequent versions
of those data sets were being delivered, some with additional data
(from August, 2014 through January, 2015) before all of those
previous liens were resolved, the previous data sets were left as
is, with those liens folded into the newer data sets.
New observations added with this version (V2.0) include ongoing
cruise observations from August, 2014 through January, 2015.
Other changes for this version were re-running of the ancillary
data in the data product, updated geometry from newer SPICE
kernels, minor editing of the documentation, catalogs, etc., and
resolution of liens from the December, 2014 review, plus those
from the May, 2016 review of the Pluto Encounter data sets.
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.
PEP_0123456789_0X691_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 **
=========== ================================== =============
PEPSSI PEP 0X691 - 0X698 *
* 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)
===== ===================================
0x691 - PEPSSI High Priority Science (long integration)
0x692 - PEPSSI Medium Priority Science (short integration)
0x693 - PEPSSI Low Priority Science (Up to 500 PHA events)
0x694 - PEPSSI Low Priority Science (Up to 500 PHA events)
0x695 - PEPSSI High Priority Science Diagnostic Mode data
0x696 - PEPSSI Medium Priority Science Diagnostic Mode data
0x697 - PEPSSI Diagnostic Mode Event data
0x698 - PEPSSI Diagnostic Mode Event data
For historical reasons, PEPSSI products always use an ApID of '0x691'
in the filename. Each product actually contains all the data types
(ApIDs) available for that day.
ApIDs '0x693' and '0x694' are combined during ground processing.
So are ApIDs '0x697' and '0x698'.
Instrument description
----------------------
Refer to the following files for a description of this instrument.
CATALOG
PEPSSI.CAT
DOCUMENTS
PEPSSI_SSR.*
SOC_INST_ICD.*
NH_PEPSSI_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
PEPSSI Field Of View definitions:
/DOCUMENT/NH_FOV.*
/DOCUMENT/NH_PEPSSI_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 either that the target was either a star, or that 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.
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-X-SPICE-6-PLUTOCRUISE-V1.0.
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_PEPSSI_PLUTOCRUISE.TAB. In addition, the
sequence identifier (ID) 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 three additional leapseconds occured
in at the ends of December, 2009, June, 2012 and June, 2015.
Refer to the NH SPICE data set, NH-J/P/SS-SPICE-6-V1.0, and the
SPICE toolkit docmentation, 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 PEPSSI Principal Investigator:
Ralph McNutt, Jr., Johns Hopkins Univ., Applied Physics Lab
Ralph McNutt, Jr.
Johns Hopkins University
Applied Physics Laboratory
Space Department
11100 Johns Hopkins Road
Room MP3-E116
Laurel, MD 20723
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-X-PEPSSI-2-PLUTOCRUISE-V2.0,
may not have corresponding data products in the calibrated data set,
NH-X-PEPSSI-3-PLUTOCRUISE-V2.0.
Data coverage and quality
=========================
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_PEPSSI_PLUTOCRUISE.TAB. N.B. Some sequences
provided may have zero corresponding observations.
Refer to the Confidence Level Overview section above for a summary
of steps taken to assure data quality.
For PEPSSI, electron detector channels are dominated by cosmic rays
in post-Jupiter mission phases.
The PEPSSI data are considered suspect for the first forty
minutes after an instrument power-on event, called a Bad Time
Interval (BTI); this file lists those time windows. The entire
Post-launch commissioning mission phase is also considered a
BTI.
See the Science Operations Center - Instrument Interface Control
Document (ICD - found at DOCUMENT/SOC_INST_ICD*.*) and the BTI TABLE
file for more detail.
The PEPSSI Time Of Flight only (TOF-only) Pulse Height Analysis (PHA)
event data may show differences in the 'N2 data' and 'N3 data' taken
simultaneously but using different collection algorithms. Refer to
the instrument description in the PEPSSI instrument catalog
(PEPSSI.CAT) under 'Data sampling and priority for TOF-only data'
in the 'Operational modes' section.
Some subset of the PHA event data is noise or other instrumental
artifacts. PHA events with parameters outside the stated instrument
sensitivity limits (see the SPECIFICATIONS section in the PEPSSI
instrument catalog file) should be ignored, or, at the very least,
used with extreme caution.
It should be noted that the Primary HDU and the first 5 extension
HDUs (the Image HDUs containing spectrograms) of the level 3 data
are 'quick-look' or 'browse' products only. They are constructed
with one minute averages for the whole mission, so that, if the data
collection period (the DT in the table in the FLUX extension) is not
an even multiple or factor of 1 minute, the spectrogram image will
exhibit aliasing artifacts. Further, the second extension, the
Helium spectrogram includes the Alpha source channels (see above).
In short, the Image HDUs in the level 3 data are not for scientific
use but for browsing or quick-look purposes and for researchers to
determine if they are able to correctly read the data in the table
extensions.
Data at higher resolution, up to 1 second for the Pluto Encounter
time period, can be found in the Flux and PHA Data HDUs. Please see
the ICD, section 11, and specifically sections 11.2 and 11.4, for
details regarding interpreting rates and the timing of events.
Please see the 'Data Validity' section of PEPSSI.CAT for details
regarding information on channels which should be excluded from
analysis.
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
===============================================================
A fundamental truth of managing data from some spacecraft missions
is that the intent of any observation is not suitable for insertion
into the command stream sent to the spacecraft to execute that
observation. As a result, re-attaching that intent to the data
that are later downlinked is problematic at best. For New Horizons
that task is made even more difficult as the only meta-data that
come down with the observation is the unpredictable time of the
observation. The task is made yet even more difficult because
uplink personnel, who generate the command sequences and initially
know the intent of each observation, are perpetually under
deadlines imposed by orbital mechanics and can rarely be spared for
the time-intensive task of resolving this issue.
To make a long story short, 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 point to be made is that 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. Furthermore, for pointing instruments with one or more
spatial components to their detectors, a table has been provided
in the DOCUMENT/ area with XY (two-dimensional) positions of each
inferred target in the primary data products. If those values are
several thousand pixels off of a detector array, it is a strong
indication that the actual target of that observation is something
other than the inferred target, or no target at all e.g. dark sky.
Review
======
This dataset was peer reviewed and certified for scientific use on
2017-03-29.
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