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 2.0 Release Date = 2023-03-30 START_TIME = 2009-06-29T00:00:00.00 STOP_TIME = 2024-03-31T23:59:56.53 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.