PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = TEXT INTERCHANGE_FORMAT = ASCII PUBLICATION_DATE = 2001-09-01 NOTE = "N/A" END_OBJECT = TEXT END 1. Introduction ____________ This document provides an overview of the Near Earth Asteroid Rendezvous Gamma-Ray Spectrometer, On asteroid Level 2 data archive. This volume contains science data products from observations acquired by the Near Earth Asteroid Rendezvous (NEAR) Gamma-ray Spectrometer (GRS) instrument. 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 maitained 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. The following documentation describes the dataset delivered as part of the NEAR XGRS instrument team delivery to NASA's Planetary Data System. 2. Volume Set Information ______________________ 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 from the 3 longest integration times were taken and corrected on the ground. Representative spectra are availabe on the volume for both BGO and NAI instruments. Each of set of spectra were corrected for channel rollover which occured 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 resulting effect is 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. As a result three Level-2 spectra were generated for the BGO instrument and three for the NAI instrument. In addition, the NAI 1 and 2 escape spectra (only one for each mode) were collected as the last spectra integrated from the surface: The resulting 17 individual 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 x' shows availability in level-2 DATA product. NAI = Sodium Iodide BGO = Bismuth Germanate N1 = NAI 1 Escape N2 = NAI 2 Escape Spectra only. I-Time is integration time. 3. File Formats ____________ The GRS on asteroid Level 2 data are contained in ASCII TABLE`s files that contain corrected spectra and associated ancillary information specific to each integration as a seperate table. All TABLE files (*.TAB files) and detached labels (*.LBL files) on this volume are stream format 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 the MacOS, DOS, and UNIX operating systems. An example and description of the content and format of the two on asteroid Level-2 record files described below. Format : 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 (.LBL) files These products are located in in the DATA directory: 4. Volume Contents _______________ Files on this volume are organized into a series of subdirectories below the top-level directory. The following table shows the structure and content of these directories. In the table, directory names are enclosed in square brackets ([]). FILE CONTENTS Top-level directory | |- [DATA] | |- DATAINFO.TXT Descrip. files in DATA directory. (This) | |- ONSURxyz.* Data and Label files for the above files | |- x = NAI, ANTI, BGO, NAI 1, or NAI 2 | |- y = Ancillary or Spectra table | | z = denotes 1,2, 3, or 4 | | * = TAB or LBL | | | |- [DOCUMENT] GRS Documentation file. | |- DOCINFO.TXT Description of files DOCUMENT directory. |- |- GRS_REQS_7_2.DOC NEAR-GRS L-2 Requirement Specs (MS Word) |- |- GRS_REQS_7_2.LBL Label file for GRS L-2 Reqs Doc versions |- |- GRS_REQS_7_2.TXT Reduced form, NEAR GRS Reqs specs. (ASCII) |- |- GRS_REQS_7_2.JPG Reqs. saved as multi-page Jpeg file. |- |- GRS_REQS_7_2.TIF Reqs. saved as multi-page TIF file. |- |- GRS_REQS_7_2_* | |- Individual Pages NEAR GRS L-2 Reqs Spec. | |- Saved as 1 page per TIF (LBL) file, * = Page # | |- | |- [INSTRUMENT] Includes XGRS_DEFINITIONS file | |- [OBSERVATION_LOG] Has various logs and user guide | |- [GRS_RESULTS] Includes derived abundances (errors) and various | figures from MAPS paper on calibration |- [CATALOG] | |- CATINFO.TXT Catalog directory information | |- INSTHOST.TXT Instrument design information | |- MISSION.CAT NEAR Mission related information and refs | |- PERSON.CAT Personnel associated with this submission | |- REF.CAT References to other publications | |- GRSURFL3.CAT Description of this GRS dataset | |- XGRSINST.CAT Instrument related Information | 5. Cognizant Persons ----------------- NEAR/GRS data were provided by Jacob Trombka, NEAR XGRS Team Leader: 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