Data Calibrated Collection Description This document provides an overview of the Near Earth Asteroid Rendezvous Gamma-Ray level 2 and level 3 data archives. The data_calibrated collection contains science data products from observations acquired by the Near Earth Asteroid Rendezvous (NEAR) Gamma-ray Spectrometer (GRS) instrument. Within the document collection the subdirectory data_calibrated_documents contains all of the documentation for the the Near Earth Asteroid Rendezvous (NEAR) Gamma-ray Spectrometer (GRS) data calibrated data products. All products were derived in the Asteroid Body Fixed Coordinate System described in the near_coordinate_systems.txt document which can be found in the NEAR Mission Information Bundle document collection. Data in this archive have been reduced as part of mission data analysis activities of the NEAR XGRS Science Team. Data of questionable validity have been omitted from the solutions. These products were derived from a specialized ground processing system developed at the University of Arizona Lunar and Planetary Laboratory. This archival data set was reviewed by the NEAR XGRS Team prior to submission to the Planetary Data System (PDS). It was one of several data sets included in a comprehensive review by the PDS Small Bodies Nodes in August 2001, prior to PDS acceptance and ingestion. Table of Contents 1. Eros Orbit Level 2 Calibrated Data 1a: Introduction 1b: Observation Log 1c: Processing Information 1d: File Formats 2. Eros Surface Level 2 and 3 Calibrated Data 2a: Introduction 2b: Surface Observations 2c: Level 2 Eros Surface Data Products 2d: Level 2File Naming formats 2e: Level 3 Eros Surface Data Products 2f: Contents 2g: Cognizant Persons 1. Eros Orbit Level 2 Calibrated Data ===================================== 1a. Introduction _______________ The GRS Level 2 data products in this collection include daily time series of spectra collected by the instrument along with selected spacecraft engineering and instrument configuration and orbital ephemerides. Supporting documentation is included for the specifics of how these data were generated. Selected ephemerides and engineering parameters are averaged for the composite set of information. 1b. Observation Log __________________ The Near Earth Asteroid Rendezvous Gamma-Ray Level 2 Observation Log file can be found in the data_raw collection. The Observation Log documentation can be found in the data_raw_documents collection. These documents describe the systems that were used to command and control the spacecraft during times when XGRS was controlling attitude, as well as commands that were used to configure the instrumentation during the orbital encounter. 1c. Processing Information __________________________ Calibrated Level 2 Products The Level 2 data product comprises the time series of the GRS instrument's science data in physical units. At each integration time the instrument Data Processing Unit (DPU) samples instrument spectra accumulated during the integration period as well as instrument and engineering configuration information. Ground processing activities at the University of Arizona produce Derived Engineering and Spatial Ephemerides information. The Level-2 data are then combined into integral products within a relational database and exported as an integral Level-2 product. Each GRS record is comprised of the following logical data units: Spectra: 5, (1024 Channel) Spectra (NAI, BGO, ANTI, NAI1, NAI2, BGO1, BGO2) NAI = Sodium Iodide: Asteroid Pointing BGO = Bismuth Germanate: Asteroid Pointing ANTI= Anti-Coincidence: Asteroid Pointing NAI1= Sod. Iod 1 Escape: Asteroid Pointing NAI2= Sod. Iod 2 Escape: Asteroid Pointing 2, (21 Channel) Spectra BGO1= Bis. Germ. 1 Escape:Asteroid Pointing BGO2= Bis. Germ. 2 Escape:Asteroid Pointing Instrument HouseKeeping: Instrument Rates and activity Engineering Housekeeping: Engineering Configurations Derived Engineering: Ground derived data products Spatial Orbital Ephemerides 1d. File Formats _______________ The GRS Level 2 integral data are binary files that describe the integrated composition of spectral, orbital ephemerides, instrument configuration and derived products for each GRS integration period. An example format of each Level-2 binary record file is: Example: g2000011.dat where g = Gamma-ray Spectrometer 2000 = Year 011 = Day Number .dat = file extension For the complete description of the GRS Level-2 Products see: "The Near Earth Asteroid Rendezvous, Gamma-ray Spectrometer, Level-2 Requirements Specification": May 25, 2001 in the document directory 2. Eros Surface Level 2 and 3 Calibrated Data ============================================== 2a. Introduction ----------------- This data set contains reduced (integrated and corrected) science data products from observations acquired by the Near Earth Asteroid Rendezvous (NEAR) Gamma Ray Spectrometer (GRS) instrument during the Eros Surface phase. At the end of the orbital mission, the NEAR spacecraft was maneuvered to perform a final landing on the surface of the 433 Eros asteroid. Upon NEAR's successful landing, it was found that not only had the spacecraft remained intact, but it had also maintained an attitude capable of maintaining limited communications contact with NASA's Deep Space Network (DSN) and simultaneously having the Gamma-ray spectrometer pointed to the surface. At that time NEAR mission scientists proposed an extension to the NEAR mission which included turning on the Gamma-ray spectrometer to produce long duration, in-situ, integrations of the spectrometer to generate high resolution results. Data in this archive have been reduced as part of mission data analysis activities of the NEAR XGRS Science Team. Data of questionable validity have been omitted from the solutions. These products were derived from a specialized ground processing system developed at the University of Arizona Lunar and Planetary Laboratory. The data included in this archive were collected from 2001-02-19 to 2001-02-28 during the Eros Surface (i.e., On-asteroid) phase of the NEAR mission. 2b. Surface Observations ------------------------ It was found that the on-surface attitude of the spacecraft did not facilitate the use of the fixed high gain antenna and the low gain antenna was used for the duration of the on-surface mission. As a result available telemetry rates did not facilitate the downlink of the full complement of instrument, engineering and ancillary information with priority being given to selected spectra. The first two days of on-surface activity were used to calibrate the detectors to account for deviations of temperature between orbital and on surface operations. Adjustments were then made to the instrument high voltage to improve the energy gain in the escape peak spectra. In this configuration, several long term integrations were collected and spectra taken for the four longest integrations along with ancillary information were used to generate the Level 2 science products on the first volume of this data set, NXERSF_2001. For the Level 3 products on the second volume, NXERSF_2002, spectra from the two longest integration times (total duration of 586320 seconds) were summed for NAI, ANTI, NAI1, and NAI2 spectra. The higher counting rate BGO spectrum was taken for a single integration (total duration of 346480 seconds) and summed. Each of set of spectra were corrected for channel rollover which occurred in several instances due to long integration times causing accumulations of counts in some channels to exceed and rollover the maximum numerical range of the 2 byte channel. Higher counting rates in the lower energies of each spectrum incurred multiple rollovers and algorithms were developed to rectify this effect. A second correction made to the on-surface spectra is the removal of differential non-linearities (DNL) contained within the spectra. The effect being high frequency spikes in channels spaced at regular intervals within the spectrum. (DNL)`s were removed using a 3rd order polynomial fit and smoothing function for a sliding boxcar of 21 channels through each raw spectrum. 2c. Level 2 Eros Surface Data Products --------------------------------------- As a result three Level 2 spectra were generated for the BGO instrument and three for the NAI instrument and finally the NAI 1 and 2 escape spectra only were collected as the last spectra integrated from the surface. The resulting 17 individual Level 2 spectra are listed with ancillary information in the following table: ID MET (start) UTC (at Start) I-Time NAI ANT BGO N1 N2 ------------------------------------------------------------ 1 158012304 2001:050:17:00:48 23108 x x x x x 2 158035412 2001:050:23:25:56 237840 x x x x x 3 158273394 2001:053:17:32:18 346480 x x x x x 4 158619998 2001:057:17:49:02 169440 x x where 'x' = Available as a Level 2 DATA product. NAI = Sodium Iodide BGO = Bismuth Germanate N1 = NAI 1 Escape N2 = NAI 2 Escape Spectra only. I-Time = Integration time. The GRS On-asteroid Level 2 corrected spectra are provided as fixed-width ASCII table files. Ancillary information specific to each integration is provided as a separate ASCII table. All Level 2 tables (*.tab files) and detached labels (*.xml files) are ASCII files with a carriage return (ASCII 13) and a line feed character (ASCII 10) at the end of each line. This allows the files to be read by MacOS, DOS, and UNIX operating systems. 2d. File Naming Formats ----------------------- ONSURxAz.TAB - Asteroid Surface Position (Lat, Lon) West Longitude - Starting time tags (MET) for each summed integrations - End (MET) - Energy Calibration - Instrument Attitude - 'x' denotes (N = NAI, B = BGO, A = ANTI, 1 = NAI 1, 2 = NAI 2) - 'A' denotes ancillary - 'z' denotes integration number (1,2,3,4) - 11 columns in single record ONSURxSz*.TAB - Surface Spectrum - 'x' denotes (N = NAI, B = BGO, A = ANTI, 1 = NAI 1, 2 = NAI 2) - 'S' denotes Spectrum - 'z' denotes integration number (1,2,3,4) - 2 columns, 1024 channels - Plus label (.xml) files 2e. Level 3 Eros Surface Data Products -------------------------------------- The GRS On-asteroid Level 3 corrected and summed spectra are provided as fixed-width ASCII table files. Ancillary information specific to each integration is provided as a separate ASCII table. 2f. Contents ------------ A description of the contents of the on-asteroid files: File 1: ONSURNAS.TAB - Starting and ending time tags (MET) for each summed integration - Asteroid Surface Position (Lat, Lon) West Longitude - Energy Calibration - Instrument Attitude ONSURNSS.TAB - 4 Spectra (NAI, ANTI, NAI1, NAI2) - Summed from 2 separate integrations (total duration = 586320 sec) File 2: ONSURBAS.TAB - Starting and ending time tags (MET) for the single BGO integration - Energy Calibration - Surface Position (Lat, Lon) West Longitude - Instrument Attitude ONSURBSS.TAB - 1 Spectrum, BGO - Summed from one separate integration (total duration = 346480 sec) NEAR/GRS data were provided by Jacob Trombka, NEAR XGRS Team Leader: 2g. Cognizant Persons --------------------- Jacob Trombka Building 2, Rm# 165 Code 691, Astrochemistry Branch NASA/Goddard Space Flight Center Greenbelt, MD 20771. jack.trombka@gsfc.nasa.gov 301-286-5941 This volume was designed and produced by: Tim McClanahan Building 2, # 166 Code 691, Astrochemistry Branch NASA/ Goddard Space Flight Center Greenbelt, MD 20771 Migration Note: This document contains the information from the dataset catalog files, aareadme.txt files and the datainfo.txt files from the PDS3 NEAR datasets NEAR-A-GRS-3-EDR-EROS-ORBIT-V1.0 and NEAR-A-GRS-3-EDR-EROS-SURFACE-V1.0