LRO CRaTER Data Bundle
LRO Calibrated (Level 1) Data Collections (Housekeeping, Primary and Secondary)
Original PDS3_DATA_SET_ID = LRO-L-CRAT-3-CDR-CALIBRATED-V1.0
Original PDS3 DATA_SET_RELEASE_DATE = 2019-12-13
Version 2.4 Release Date = 2026-09-15
START_TIME = 2009-06-29T00:00:00.00
STOP_TIME = 2026-06-30T23:59:59.01
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 calibrated (Level 1) data consists of files containing data processed
from the raw primary science, secondary science, and housekeeping data re-
cords. During processing, the raw data are converted with instrument-
specific calibration and conversion factors to calibrated data records
containing science and engineering measurements and instrument oper-
ating parameters. The calibrated data records are written to files in
plain text, fixed record format; each file contains calibrated data records
for a single UTC day. All time values in Level 1 data products are in
spacecraft clock units.
The Level 1 data are an intermediate data product meant to be used for
data processing diagnostics and troubleshooting. Although the Level 1
collections can be used for some data analyses, it is not intended as the
primary source for further data analyses or scientific research. In the
Level 1 collections all times are expressed in spacecraft clock units;
spacecraft location and instrument pointing data are not included. Users
seeking CRaTER data are instead encouraged to use the Level 2 derived data
record collections. The Level 2 data contain all Level 1 data supplemented
time values converted to UTC and computed spacecraft location and
instrument pointing information.
See [Spence et al. 2010] for detailed description of LRO flight version
of the instrument, its operations, and data processing.
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 1 data are created from the corresponding Level 0 data by
converting the instrument binary output with conversion and calibration
factors to science and engineering data.
The Level 1 collections are composed of the three types of time-sequential
calibrated data records: (1) primary science, (2) secondary
science, and 3) housekeeping. The three types of calibrated data records
are written to separate data files in plain text, fixed record format.
Each file contains calibrated data records for a single UTC day.
The Level 1 primary science data consists of a sequence of interaction
event calibrated data records--one CDR for each measured event. Each
calibrated data record consists of the energy deposited in each of the six
detectors and the spacecraft time at the end of the measurement interval
(receipt of spacecraft timing pulse). calibrated data records for events
recorded in the same measurement interval have the same time tags--the
'SECONDS' and 'FRACT' 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 calibrated data records
'INDEX' field.
The Level 1 secondary science calibrated 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. Status flags available
in the secondary science calibrated 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.
The Level 1 housekeeping calibrated 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 the housekeeping calibrated data records.
Confidence Level Overview
-------------------------
An assessment of the accuracy and precision of data in the LRO CRaTER
calibrated (Level 1) collections is limited to the measured deposited
energy in each detector. General instrument housekeeping param-
eters (e.g., temperatures, voltages, currents, LLD voltages, pulser
signal amplitudes, spacecraft clock value) are provided with no statement
of uncertainty--the accuracy of these parameters is assumed to be suf-
ficient for general correlation and trending analysis. The accuracy of
the housekeeping temperature parameters has an impact on the accuracy and
precision of the conversion from detector PHA channel numbers to de-
posited energy 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'), and uncertainty in actual deposited energy values
due to digitization.
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 [Spence et al. 2010].
The LRO CRaTER instrument V1.0 calibration values are listed in
[Spence et al. 2010], 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.
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 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 Calibrated (Level
1) 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 separate gap files for the primary science, secondary science,
and housekeeping data sets. Each gap file contains a cumulative listing
of the missing data up to and including the days for the data current vol-
ume. 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 Calibrated (Level 1) data include all Level 1 data obtained
by the CRaTER instrument, including data from periods when the instrument
was placed into special configurations. Special configurations include
the instrument start-up tests that occur whenever the instrument is power
cycled to (e.g., initial instrument start-up, recovery following space-
craft transition to sun-safe mode) as well routine calibrations (90-degree
off-nadir GCR background measuerments, internal pulser sweeps, LLD zero
crossing measurements, and LLD sweeps). These periods can be detected by
monitoring the 'CalLow' and 'CalHigh' flags and 'DiscThin' and 'DiscThick'
LLD values in the secondary science calibrated 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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