| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains Calibrated data taken by
the New Horizons Student Dust Counter instrument during
the KEM1 ENCOUNTER 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.
The Student Dust Counter (SDC), aboard the New Horizons spacecraft,
is the first dedicated and calibrated dust instrument to measure the
density and size distributions of interplanetary dust particles (IDPs)
between 18 and 35 AU, and during KEM will extended these measurements
to 50 AU near the edge of the KB. SDC provides a near continuous
mapping of the dust density distribution along the trajectory of New
Horizons with this data set spanning from roughly 36.8AU to 40.2AU.
These measurements will aid in the interpretation of the complementary
Voyager observations of the putative dust densities derived from the
Plasma Wave System (PWS) out to ~100 AU.
When considering this data set, note that the SDC channel thresholds
were increased to their 'high' settings starting on 9/21/2017
(or 39.5 AU) due to unexpected noise following instrument testing
for the MU-69 encounter. The instrument testing was neither a
stimulus test nor a noise calibration test, though a modified
version of the noise calibration commanding was used. The test was
scheduled during a period of high spacecraft activity similar to
activity levels expected during the MU-69 encounter, with the
intent of judging what threshold values for each channel would give
noise counts similar to those measured during low-activity periods.
Thresholds were reset to 'low' settings on 6/10/2018. Therefore,
during the interval from 9/21/2017 through 6/10/2018, detections
smaller than the cruise 'high' thresholds were not recorded.
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 1.0 of this data set.
This version includes data acquired by the spacecraft between 08/14/2018
and 12/31/2018. It only includes data downlinked before 01/01/2019.
Future datasets may include more data acquired by the spacecraft
after 08/13/2018 but downlinked after 12/31/2018.
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 below
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.
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
Also note that a dust hit may be recorded in more than one data
product. The product name reflects the MET when data was tranferred
from instrument memory. More than one transfer may have been requested
for any given interval from START_TIME to STOP_TIME. When processing
the data, check these intervals carefully. Obvious cases of duplicate
dust hit data spanning long time intervals have already been removed
from this dataset.
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
SDC Summary of Student Dust Counter (SDC) boresight direction
/DOCUMENT/SDCRAM.*
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.
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.
For SDC the TARGET_NAME is always DUST.
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. A list of these sequences has been provided in
file DOCUMENT/SEQ_SDC_*.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 occurred
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 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 SDC Principal Investigator:
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-A-SDC-2-KEM1-V1.0,
may not have corresponding data products in the calibrated data set,
NH-A-SDC-3-KEM1-V1.0.
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.
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
===============================================================
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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