Data Set Overview
 =================

   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 Level 1 dataset consists of files containing data processed from the
   Level 0 primary science, secondary science, and housekeeping raw data re-
   cords.  During processing, the raw data are converted with instrument-
   specific calibration and conversion factors to calibrated data records
   (CDR) containing science and engineering measurements and instrument oper-
   ating parameters.  The CDR are written to files in plain text, fixed re-
   cord format; each file contains CDR for a single UTC day. All times 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
   dataset 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 dataset all times are expressed in spacecraft clock units; space-
   craft location and instrument pointing data are not included.  Users
   seeking CRaTER data are instead encouraged to use the Level 2 derived data
   record (DDR) dataset.  The Level 2 data contain all Level 1 data supple-
   mented time values converted to UTC and computed spacecraft location and
   instrument pointing information.

   See the MISSION.CAT file for more information on the LRO mission.
   See the CRAT_INST.CAT file for more information on the CRaTER instrument.
   See SPENCEETAL2010 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 in-
   strument'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 measure-
   ment of the signals from all of the detectors.  The resulting detector
   signal amplitudes are compared to the values of the 'lower level discrim-
   inators' (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 mea-
   surements are not contaminated by system electronic noise.  Seperate LLD
   settings are required for the thick and thin detectors due to the dif-
   ference 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 register-
   ing 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.  Con-
   versely, 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 instru-
   ment'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 space-
   craft 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 con-
   verting the instrument binary output with conversion and calibration fac-
   tors to science and engineering data.

   The Level 1 dataset is composed of the three types of time-sequential
   calibrated data records (CDR): (1) primary science, (2) secondary
   science, and 3) housekeeping.  The three types of CDR are written to
   seperate data files in plain text, fixed record format.  Each file con-
   tains CDR for a single UTC day.

   The Level 1 primary science data consists of a sequence of interaction
   event CDR--one CDR for each measured event.  Each CDR 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).
   CDR 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 CDR 'INDEX'
   field.

   The Level 1 secondary science CDR contain the majority of instrument con-
   figuration settings, status flags, and event counters.  Reported con-
   figuration settings include the last command sent to CRaTER, detector
   LLD settings,  and coincidence mask values.  Status flags available in the
   secondary science CDR include detector bias status, selected pulse am-
   plitude 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 re-
   corded by CRaTER during the monitoring period.

   The Level 1 housekeeping CDR 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 CDR.

Confidence Level Overview
   -------------------------
   An assessment of the accuracy and precision of data in the
   LRO-L-CRAT-3-CDR-CALIBRATED-V1.0 dataset is limited to the measured de-
   posited 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 amplitud
   or 0.02% of each string's maximum output value. [The system electronic
   e 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.  A re-
   vision to this catalog file will be provided when the values become avail-
   able.

   The process of converting the detector signals into digital values re-
   quires discretizing the amplifier analog output signals into one of a pos-
   sible 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 sum-
   marized 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 L0 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 reprocess-
   ing 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-L-CRAT-3-CDR-CALIBRATED-
   V1.0 archival volume 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.
   There is only limited re-constructed ephemeris data currently available
   for the period between initial instrument power-up (2009-06-20) and LOI
   completion and transition to the nominal observing attitude.  CRaTER data
   obtained during Cruise Phase (instrument power-up - 2009-06-23), Lunar
   Orbit Acquisition (2009-06-23), and initial Commissioning Period
   (2009-06-23 - 2009-06-28) will be included when more complete ephemeris
   data from this early part of the mission becomes available.

   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 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; the log files are found in the DATA direc-
   tory with their corresponding data products.

   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 gen-
   eral 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 deter-
   mined.  No correlation has been found with spacecraft location, local
   space and spacecraft environment conditions, instrument boresite direc-
   tion, 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-L-CRAT-3-CDR-CALIBRATED-V1.0 data set includes all 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 CDR.

   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 CDR.

   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 CDR.  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 varia-
   bility 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.