| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains Raw data taken by New Horizons
Student Dust Counter
instrument during the PLUTO mission phase.
The mission of the SDC is to analyze the size and distribution of
Interplanetary Dust Particles (IDPs) along the New Horizons trajectory to the
Kuiper Belt. SDC comprises twelve thin, permanently polarized polyvinylidene
fluoride (PVDF) plastic film sensors, with a combined area of about 0.1 m**2,
mounted on the top surface of a support panel and normal to the spacecraft ram
direction (flight velocity). In addition, there are two reference sensors,
identical to the top surface sensors, mounted on the back side of the detector
support panel and protected from any dust impacts, used to monitor background
noise levels.
An impacting IDP causes a depolarization charge when it penetrates the PVDF
film on one of the sensors. That charge is then measured by that sensor's
electronics (channel); if the measurement is above a preset level, the
instrument records and stores the event for later downlink. The level preset
is adjusted based on in-flight Noise Floor Calibrations, and there are
extensive autonomy rules adjusting SDC behavior, even turning channels off for
up to thirty days at a time, to avoid overloading the storage system with
noise.
SDC was designed to detect events for particles down to about one picogram at
Pluto [BAGENALETAL2016]; that detection limit is lower than earlier in the
mission where the spacecraft velocity was higher. The SDC instrument has a
temperature- and velocity-dependent calibration, first converting the raw
measurement to charge, then converting charge to particle mass.
The common data product is a binary table of downlinked event data: time;
sensor channel; magnitude; threshold magnitude. Associated data products are
housekeeping data such as instrument temperatures for calibration and
near-in-time spacecraft thruster events, which may induce false positives i.e.
SDC events not caused by IDPs. The channels in the binary table for raw data
are numbered from 0 to 13; the channel in the binary table for calibrated data
are numbered from 1 to 14.
Some time between instrument delivery to the spacecraft and launch, the
detector on one channel began exhibiting symptoms of degraded electrical
contacts to the PVDF; data from that channel (channel number 10 in raw data;
channel number 11 in calibrated data) are still processed but should be
ignored.
During the Pluto Charon Encounter mission phase starting in January,
2015, there were several sub-phases: three Approach sub-phases, (AP1,
AP2 and AP3); a CORE sequence for the Pluto flyby on 14.July, 2015 (Day
Of Year 195), sometimes also referred to as NEP (Near-Encounter Phase);
three Departure sub-phases (DP1, DP2, DP3). For this second SDC
delivery for the Pluto mission phase, this data set includes all SDC
data through late-January, 2016, including all encounter data.
SDC was turned on throughout Approach. It was powered off then on
periodically when needed for spacecraft power reasons or for trajectory
correction maneuvers, and the channels were turned off and on around
the start of DSN tracks that contained tweakups, as per normal
operations. The thresholds were updated and set higher for Pluto -
making SDC less sensitive to dust hits - on January 1st, 2015 (DOY 001),
then set back to Cruise threshold settings on July 30th (DOY 211).
The threshold used for each dust hit is recorded in extension 1 of the
calibrated data products.
During the five days before and after the Pluto flyby, SDC detected
one probable dust hit event. The event happened on July 11, three days
prior to Pluto closest approach, at a distance of ~3000 Pluto radii.
Refer to figure 6 of Bagenal et al. (2016) [BAGENALETAL2016] for more
details.
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_SDC_PLUTO.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.
SDC updates for Pluto Encounter
Data Sets V2.0
==============
This P2 Pluto Encounter dataset release provides updates to the
Pluto dataset between P1 (data on the ground by 7/31/2015) and P2
(data on the ground by 1/31/2016). All liens from the initial
Pluto delivery have also now been resolved. For SDC, most of the
Pluto Encounter data was downlinked in the 15229 load in August
2015. Since then, SDC has stayed on and taken data continuously,
however due to power restrictions it has frequently been turned
off anytime there was a downlink of SSR Side 2 data. The channels
are also turned off and on around the start of certain DSN tracks,
occurring about every 3 days (these tracks are known as tweakups).
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.
SDC data calibration is a two-step process: raw data numbers from a
particle impact are converted to a charge, and the charge is
converted to a particle mass via the ground calibrations obtained at
a dust acceleration facility. Refer to the provided documentation
for more information. The latest calibration procedure is described
in James et al., (2010) [JAMESETAL2010].
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.
SDC_0123456789_0X700_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 **
=========== ================================== =============
SDC SDC 0X700
* 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)
===== ===================================
0x700 - SDC Science Data/SDC
Instrument description
----------------------
Refer to the following files for a description of this instrument.
CATALOG
SDC.CAT
DOCUMENTS
SDC_SSR.*
SOC_INST_ICD.*
NH_SDC_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
SDC Field Of View definitions:
/DOCUMENT/NH_FOV.*
/DOCUMENT/NH_SDC_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-PLUTO-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_SDC_PLUTO.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 SDC Principal Investigator:
Mihaly Horanyi, LASP, University of Colorado
Mihaly Horanyi
Laboratory for Atmospheric
and Space Physics
University of Colorado
Boulder, CO 80302-0392
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-P-SDC-2-PLUTO-V2.0,
may not have corresponding data products in the calibrated data set,
NH-P-SDC-3-PLUTO-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_SDC_PLUTO.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 SDC, the stimulus calibration activity is known to generate
false positive events in the science data. This data set includes
a PDS TABLE, DOCUMENT/SDC_STIM_Vnnnn.TAB, that lists time periods
when stimulus calibrations were active (several times during
Launch and Jupiter mission phases, and about half an hour per
year during Annual CheckOuts (ACO) in the Pluto Cruise mission
phase. Eventually, the Science Operations Center (SOC)
operational pipeline may be enhanced to filter individual events
that occur near stimulus events.
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
2017-01-05.
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