LRO CRaTER Data Bundle
LRO Derived (Level 2) Data Collections (Housekeeping, Primary and Secondary)
Original PDS3_DATA_SET_ID = LRO-L-CRAT-3/4-DDR-PROCESSED-V1.0
Original DATA_SET_RELEASE_DATE = 2019-12-13
Version 2.1 Release Date = 2025-12-15
START_TIME = 2009-06-29T00:00:00.00
STOP_TIME = 2025-09-30T23:59:59.01
PRODUCER_FULL_NAME = PROF. HARLAN SPENCE
Overview of Collections
=======================
The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) is a
stacked detector-absorber cosmic-ray telescope designed to answer key
questions to enable future human exploration of the Solar System.
CRaTER's primary measurement goal is to measure directly the average
lineal energy transfer (LET or 'y') spectra caused by space radiation
penetrating and interacting with shielding material. Such measured LET
spectra are frequently unavailable. In the absence of measurements,
numerical models are used to provide estimates of LET; the reliability of
the models require experimental measurements to provide a ground truth.
The derived (Level 2) collections consist of files containing data
processe from the calibrated (Level 1) primary science, secondary
science, and housekeeping calibrated data records. During processing,
derived data records are formed by combining Level 1 calibrated data
records with derived parameters such as average LET, detector event
flags, and instrument viewing geometry data. The derived data records are
written to files in plain text, fixed record format; each file contains
derived data records for a single UTC day. All times in Level 2 data
products are reported in both spacecraft clock units and UTC.
The Level 2 data products are intended as the primary CRaTER data source
for further data analyses or scientific research.
Science Objectives and Observation Strategy
-------------------------------------------
CRaTER is designed to achieve characterization of the global lunar
radiation environment and its biological impacts and potential mitigation
as well as investigation of shielding capabilities and validation of
other deep space radiation mitigation strategies involving materials.
CRaTER will fill knowledge gaps regarding radiation effects, provide
fundamental progress in knowledge of the Moon's radiation environment,
and provide specific path-finding benefits for future planned human
exploration.
Parameters
----------
LRO CRaTER flight instrument identification:
--instrument model = Flight Model 1 (FM1);
--instrument serial number (S/N) = 02;
--FPGA revision code = 3.
Data
----
CRaTER's principal measurement is the energy deposited in the 3 pairs of
silicon detectors by charged particles and photons passing through the
instrument's 'telescope' unit. Whenever the coulombic charge signal re-
sulting from the energy deposited in a detector exceeds a predefined and
fixed threshold, the instrument's electronics performs a detailed
measurement of the signals from all of the detectors. The resulting
detector signal amplitudes are compared to the values of the 'lower level
discriminators' (LLDs). LLDs establish minimum amplitudes for signals to
qualify as valid charged-particle or photon interactions. The LLD values
are generally set to insure that the desired charged-particle or photon
measurements are not contaminated by system electronic noise. Separate
LLD settings are required for the thick and thin detectors due to the
difference in their sensitivities; the thin and thick detector LLD values
are reported in the 'DiscThin' and 'DiscThick' parameters as part of the
secondary science packet.
In addition to the LLD settings, measurement filtering is achieved
through detector coincidence requirements--the combination of detectors
registering valid signals to qualify as a charged-particle or photon
measurement 'event'. To measure all charged particles arriving from the
instrument's zenith or nadir directions, for example, the coincidence
requirements would be valid signals in at least detectors 1, or 2, or 5,
or 6. Conversely, a coincidence consisting of valid signals in all six
detectors would ensure only zenith- or nadir-arriving charged particles
with high energies are reported. For CRaTER's six axially-coaligned
detectors there are 64 possible coincidence combinations. The desired
set of coincidence combinations are stored as a coincidence mask
parameter in the instrument's memory; the coincidence mask setting is
reported in the 'Mask' parameter as part of the secondary science packet.
To qualify as an 'event', therefore, a charged particle or photon passing
through CRaTER's telescope must interact and deposit sufficient energy to
generate signals with amplitudes in excess of the specified LLDs in a
specified combination of detectors; only data for valid 'events' are re-
ported in the instrument's telemetry.
The measured interaction event data is written as a series of primary
science packets to the instrument's output telemetry buffer for the
spacecraft to read. At ~1 second intervals CRaTER receives a timing pulse
from the spacecraft, at which time it flushes the primary science data
from the output buffer and writes a secondary science packet for the
spacecraft to read. Every 16 seconds a housekeeping packet is also
created and written to the output buffer.
The Level 2 data are by combining the Level 1 data with derived or
supplemental parameters including average LET in each detector, detector
event flags, instrument electrical power consumption, and instrument
viewing geometry information.
Level 2 collections are composed of the three types of time-sequential
derived data records: (1) primary science, (2) secondary science, and 3)
housekeeping. The three types of derived data records are written to
separate data files in plain text, fixed record format. Each file
contains derived data records for a single UTC day.
The Level 2 primary science data consists of a sequence of interaction
event derived data records--one derived data record for each measured
event. Each derived data record consists of the energy deposited in each
of the six detectors, the resulting average LET, and the spacecraft time
and UTC at the end of the measurement interval (receipt of spacecraft
timing pulse). Also included in the primary science derived data records
are two sets of flags related to the measured deposited energy in each
detector: one set flags deposited energies exceeding corresponding LLD
values; the second set flags deposited energies approaching the
saturation value for the associated amplifier-ADC strings (signals
exceeding 95% of the ADC's dynamic range). derived data records for
events recorded in the same measurement interval have the same time tags-
-the 'SECONDS','FRACT', and 'TIME' field values. Although numerous events
may have the same time value, the events are recorded in the order in
which they occurred; this relative order is captured in the derived data
records 'INDEX' field.
The Level 2 secondary science derived data records contain the majority
of instrument configuration settings, status flags, and event counters.
Reported configuration settings include the last command sent to CRaTER,
detector LLD settings, and coincidence mask values. The record's time tag
includes both spacecraft time and UTC. Status flags available in the
secondary science derived data records include detector bias status,
selected pulse amplitude range and rate for the internal calibration
pulser, and detector processing status. Counters report the number of
'singles' for each detector as well as the number of 'good', 'rejected',
and total events recorded by CRaTER during the monitoring period. Also
included in the secondary science derived data records is LRO's location
relative to the center of the Moon; the location is provided as three
orthogonal vectors (Px, Py, Pz) in the MOON_ME (Moon Mean Earth/Rotation
Axis) reference frame
The Level 2 housekeeping derived data records contain measured instrument
operating and environmental parameters used to assess the health and
performance of the instrument, such as power supply output voltages,
detector bias voltages and currents, pulse amplitudes from the internal
calibration pulser, and temperatures at five locations inside of the
instrument's housing. The analog output signal (voltage) from radiation
monitor is also included in the housekeeping calibrated data records. The
record's time tag includes both spacecraft time and UTC. Also included
in the housekeeping derived data records are two status flags related to
the relative orientation of the instrument's boresite axis: one flag
indicates when the boresite axis does not intercept the lunar surface
'OFFMOONFLAG'; the second indicates when LRO and CRaTER are in eclipse.
LRO's location and CRaTER's relative boresite orientation are derived
from definitive spacecraft ephemeris ('SPK') and orientation ('CK')
kernels, using transformation routines from the JPL NAIF (Navigation and
Ancillary Information Facility) toolkit.
Confidence Level Overview
-------------------------
An assessment of the accuracy and precision of data in the
LRO CRaTER Derived (Level 2) collections are limited to the measured
deposited energy in each detector and the resulting average lineal energy
transfer (LET or 'y') values (primary science derived data records). Gene-
ral instrument configuration and housekeeping parameters (e.g., tempera-
tures, voltages, currents, LLD voltages, pulser signal amplitudes, space-
craft clock value) are provided with no statement of uncertainty--the ac-
curacy of these parameters is assumed to be sufficient for general cor-
relation and trending analysis. The accuracy of the housekeeping temper-
ature parameters has an impact on the accuracy and precision of the con-
version from detector PHA channel numbers to deposited energy (and LET)
values; this impact, however, is very small in comparison to other
sources of systematic and stochastic error.
Potential sources of instrument systematic error include signal pulse
shaping output linearity, analog-to-digital conversion (ADC) linearity,
electronic calibration source stability and linearity, and the accuracy
of the gain and offset values determined for each detector-amplifier-ADC
string.
The linearity of the amplifier-ADC strings (i.e., pulse height
analyzer or PHA) was established with a precision external pulser. For a
given pulser output setting, the variability in output pulse amplitude is
determined to be 0.01%. Over the pulser's full range of output pulse
amplitude settings, the measured pulse amplitudes were found to be very
linear, with an RMS fit residual upper limit of 0.1%.
The external pulser was used to establish the linearity of the six CRaTER
PHA circuits. The precision external pulser served as a calibrated input
charge source by coupling it (via a precision capacitor) to the base of
each PHA circuit's preamplifier. Each PHA circuit's response was found
to be very linear, with RMS fit residuals significantly less than 0.1%.
Temporal stability of the PHA circuits was established through repeated
testing with the external pulser over an 15-month period. Between Sep
2007 and Jan 2009, each PHA circuit was tested five times at a fixed
pulser output setting. The output of each PHA circuit was determined to
be very stable, with ~0.06% variability in the value of the center of the
PHA peak.
Temperature dependence of the gain of each PHA circuit was measured over
the expected range of operating temperatures during the LRO mission. The
output of each PHA circuit to fixed amplitude pulses from the precision
external pulser was measured with the CRaTER instrument operating at -30
degrees C, -10 degrees C, +10 degrees C, and +35 degrees C (temperature
measured inside the instrument's case close to the analog and digital
circuit boards). The PHA circuit gains were found to be fairly stable
over this temperature range, with only a weak non-linear temperature de-
pendence. Detectors 2, 4, and 6 PHA circuits exhibited gain variations
of ~ +/- 0.1% over the temperature range; detectors 1, 3, and 5 PHA
circuits gains varied by ~ +/- 0.5%.
Potential sources of stochastic error include electronic noise, uncer-
tainty in the PHA-channel-to-deposited-energy conversion factors (i.e.,
'calibration values'), uncertainty in actual deposited energy values
due to digitization, and uncertainty in the derived LET values caused by
variability in particles' paths through the detectors.
From the standard deviations of the pulse amplitudes measured over the
full dynamic range of each amplifier-A-to-D-converter strings, the upper
limit on system electronic noise is approximately 0.15% of pulse
amplitude or 0.02% of each string's maximum output value. [The system
electronic noise measured with CRaTER operating at 10 degrees C].
PHA channel number is converted to deposited energy by
Ei [keV] = GiCi + Oi, where
Ei [keV] = deposited energy measured by detector/
PHA chain i,
Ci [ADU or channel #] = output from detector/PHA chain i,
Gi [keV/ADU] = gain of detector/PHA chain i, and
Oi [keV] = offset of detector/PHA chain i.
The calibration values Gi and Ci used to convert PHA output to deposited
energy were determined through a combination of alpha particle exposure
measurements and modeling of the instrument's response to moderate energy
protons. A more extensive description of the calibration process is
found in SPENCEETAL2010.
The LRO CRaTER instrument V1.0 calibration values are listed in
SPENCEETAL2010, table 6, and reproduced here.
Parameter Units D1 D2 D3 D4 D5 D6
-----------------------------------------------------------------
Gain, Gi keV/ADU 76.3 21.8 78.6 21.6 76.3 21.9
Offset, Oi keV 105.1 50.0 152.8 74.7 119.1 46.6
The uncertainty in the Gi and Ci values awaits further analysis.
The process of converting the detector signals into digital values re-
quires discretizing the amplifier analog output signals into one of a
possible 4096 linearly-spaced values. These 4096 'channel' or 'ADU'
values correspond to ranges of ~0-300 MeV and ~0-90 MeV for the thin and
thick detector PHA circuits, respectively. Each PHA channel corresponds
to a small but finite range of energies described by a probability
distribution rather than a discrete energy value. The calibration
process establishes an effective energy and energy width for each
channel. Assuming the actual deposited energy probability distribution
for a given PHA channel is approximately flat, the average energy and
uncertainty corresponding to the channel are the effective energy and
energy width established through calibrations. While the absolute
magnitude of the uncertainty resulting from discretization is a constant
value (one-half the gain), the relative uncertainty is a function of the
energy corresponding to the particular PHA channel--the lower the
channel's corresponding energy, the higher the realtive uncertainty.
The discretization uncertainty extremes are summarized in the
following table.
Detector/ Energy (keV) Energy (keV)
PHA Chain PHA = 0 ADU PHA = 4095
----------------------------------------------------------
D1 105.1 +/- 38.2 (36.3%) 312554 +/- 38.2 (0.012%)
D2 50.0 +/- 10.9 (21.8%) 89321 +/- 10.9 (0.012%)
D3 152.8 +/- 39.3 (25.7%) 322020 +/- 39.3 (0.012%)
D4 74.7 +/- 10.8 (14.5%) 88527 +/- 10.8 (0.012%)
D5 119.1 +/- 38.2 (32.0%) 312568 +/- 38.2 (0.012%)
D6 46.6 +/- 11.0 (23.5%) 89727 +/- 11.0 (0.012%)
For PHA values ❯ 48 ADU, the relative uncertainty in the deposited energy
due to discretization is ❮ 1% a for all detector/PHA chains.
The uncertainty in the deposited energy values contributes to the
final uncertainty in the LET values.
The LET uncertainty due to variability in particle paths through the de-
tectors arises from the limited collimation of the particles incident on
each detector. Particle-detector incidence angle is not necessarily per-
pendicular, but some value between perpendicular and the an angle deter-
mined by the detectors defining the event's coincidence. The result is
that the pathlength through a given detector can be significantly longer
than the detector's thickness, which in turn lowers the LET value for a
given deposited energy value. The particle pathlengths throught the de-
tectors--instead of a being known and fixed values--must instead be de-
scribed by probability distributions. The resulting LET value is there-
fore also described by a probability distribution. In practice, LET is
computed using the most likely pathlength value (as determined analyti-
cally or through Monte Carlo modeling), while the probabilitic nature of
the pathlength is included in the value's overall uncertainty. The path-
length uncertainty is greatest for particles which deposit energy only in
the outer-most pairs of detectors, and decreases as particles deposit
energy in the remaining pairs of detectors.
This overview has identified, described, and where possible enumerated
the various error/uncertainty components. The confidence levels for the
total cumulative uncertainty in the measured deposited energies and
derived LET values awaits further analysis. When the values become
available a revision will be provided to this catalog file.
Review
------
A minimal set of automated quality control steps are used by the data
processing system to verify the integrity of the data during the initial
creation of the raw data files. Each raw data packet's CCSDS header is
checked for format and content. Packets are discarded if their headers
are corrupted, incorrectly formatted, or containing invalid values. All
packets are sorted into time order and checked for temporal gaps. Dupli-
cate packets are also discarded. Metrics plus any detected anomalies are
written to process log files for review by scientists and engineers from
the instrument team. Anomalies noted during the processing are investi-
gated. Anomalies due to missing input files (e.g., instrument science
and housekeeping data files, spacecraft housekeeping data files,
spacecraft ephemeris kernels, and ancillary files such as leap second and
spacecraft clock kernels) are corrected by locating the missing input and
reprocessing the data.
All data is periodically analyzed using graphical and statistical methods
to check for out-of-range values as well as anomalous trends that may
indicate detector and/or amplifier-ADC string degradation.
Data Coverage and Quality
-------------------------
The start date for the initial version of the LRO CRaTER Derived
(Level 2) archival data is 2009-06-29T00:00:00.000. This date/time
is the beginning of the first full day following completion of LRO lunar
orbit insertion (LOI) and transition to the nominal nadir-pointing
observation attitude. It is also the first day for which complete
re-constructed ephemeris ('SPK') data was provided by the LRO Mission
Operations Center.
Data gaps are identified during initial data processing. The gap start
and stop times are recorded in gap files stored in the document directory
there are seperate gap files for the primary science, secondary science,
and house-keeping data sets. Each gap file contains a cumulative listing
of the missing data up to and including the days for the data current
volume. Description of overall data coverage and quality. This section
should include information about gaps in the data (both for times or re-
gions) and details regarding how missing or poor data are flagged or
filled, if applicable. The minimum duration between successive data
packets to qualify as a data gap is specified during data processing. The
default durations are 2 seconds for both primary and secondary science
data packets, and 20 seconds for housekeeping data packets. These values
may be over ridden at the time of data processing, however. The actual
durations used while processing a specific set of data are recorded in
the corresponding process log file.
Aperiodic episodes of sporadic, significant elevation in the thick detec-
tor (D2, D4, and D6) singles rates have been observed during all phases
of mission phases. The elevated singles rates most commonly occur in
detector D2, but have also been observed in detector D6; a detector's
singles rate may increase by a factor of 20 or more. During these
periods increases may occur in both the 'reject' and 'good' event rates.
Episodes tend to last for three to five weeks, followed by extended
periods with nominal singles rates. During an episode singles rates
vary sporadically between nominal and extremely elevated levels, although
there seems to be a general gradual build-up and decline in the peak
magnitude of the singles rates over the course of an episode. Despite
intensive analysis, the cause for the periods of elevated singles rates
has not yet been determined. No correlation has been found with
spacecraft location, local space and spacecraft environment conditions,
instrument boresite direction, or spacecraft and instrument operations.
Users are urged to first plot the detector singles rates and 'good' and
'reject' event rates as a function of time to identify periods with
elevated singles rates which may impact their particular use of the data.
Limitations
-----------
The LRO CRaTER Derived (Level 2) data set includes all derived data
obtained by the CRaTER instrument, including data from periods when
the instrument was placed into special configurations. Special configur-
ations include the instrument start-up tests that occur whenever the in-
strument is power cycled to (e.g., initial instrument start-up, recovery
following spacecraft transition to sun-safe mode) as well routine cali-
brations (90-degree off-nadir GCR background measuerments, internal pul-
ser sweeps, LLD zero crossing measurements, and LLD sweeps). These per-
iods can be detected by monitoring the values in the 'CalLow', 'CalHigh',
'DiscThin', and 'DiscThick' fields in the secondary science derived data
records.
Timing resolution for the set of events recorded between two successive
timing pulses (buffer readouts)is limited to the corresponding spacecraft
times. If, for example, 560 particle 'events' are measured between two
successive timing pulses, the exact time of each event's occurrence is
unknown--all that is known is that event was measured between the times
of the two timing pulses. The sequence in which the events were measured,
however, is preserved-for a given time interval, the first reported event
was measured before the second reported event, etc.
The maximum rate at which detector measurements can be reported in the
primary science data is ~1200 events per second; the true number of
events in each time interval is reported in the secondary science derived
data records.
Users should be aware of the impact of the LLD settings on the primary
and secondary science data. The LLD settings establish the minimum
amplitudes of the amplifier output pulse heights (i.e., minimum deposited
energies) to qualify as a valid signal and trigger the ADC process. In
addition to determining the lower limit of the PHA and LET spectra, the
choice of LLD values directly affects the number of 'good' and 'reject'
events reported in the secondary science data derived data records. For a
given set of incident charged-particle energy spectra, as the LLD values
increase, the 'good' and 'reject' event rates will decrease. Users
analyzing the temporal variability of 'good' and 'reject' event rates
should ensure the LLD settings do not change over the analysis period.
The nominal instrument operating mode maintains constant LDD settings.
Modes using varying LLD settings, however, occur during instrument
power-up tests and routine calibration procedures. In addition, as the
mission progresses changes in noise levels due to instrument component
aging may require adjustments to the baseline LLD settings.
Reference
=========
[Spence et al. 2010] Spence, H.E., A.W. Case, M.J. Golightly,
T. Heine, B.A. Larsen, J.B. Blake, P. Caranza, W.R. Crain, J. George,
M. Lalic, A. Lin, M.D. Looper, J.E. Mazur, D. Salvaggio, J.C. Kasper,
T.J. Stubbs, M. Doucette, P.Ford, R. Foster, R. Goeke, D. Gordon,
B. Klatt, J. O'Conner, M. Smith, T. Onsager, C. Zeitlin, L.W. Townsend,
Y. Charara (2010), CRaTER: The Cosmic Ray Telescope for the Effects of
Radiation Experiment on the Lunar Reconnaissance Orbiter Mission, Space
Sci. Rev., 150, 243-284, DOI: 10.1007/s11214-009-9584-8.
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