Introduction
   =============
     This document provides an overview of the Near Earth Asteroid
     Rendezvous Gamma-Ray Spectrometer On-asteroid Level 2 and 3 data
     archive.  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.
 
     The following sections describe this data set and its documentation
     delivered as part of the NEAR XGRS instrument team delivery to
     NASA's Planetary Data System.
 
 
   Data 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 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.
 
     Level 2 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 (*.LBL
       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.
 
       A description of the content and format of the On-asteroid
       Level-2 record files is provided next.
 
       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 Level 2 products are located in the DATA directory of the
       first volume of this data set, NXERSF_2001.
 
     Level 3 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.
 
       All Level 3 tables (*.TAB files) and detached labels (*.LBL
       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.
 
       A description of the content and format of the two On-asteroid
       Level-3 record files is provided next.
 
       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)
 
       These Level 3 products are located in the DATA directory of the
       second volume of this data set, NXERSF_2002.
 
 
   Coordinate System
   =================
     All products were derived in the Asteroid Body Fixed Coordinate
     System
 
 
   Software
   ========
     Requirement Specifications For Level-2 Parameters Required
     Stored or Derived by The Near Earth Asteroid Rendezvous
     (NEAR) Gamma-Ray Spectrometer (GRS) Ground System
 
 
   Media
   ======
     CD-ROM

Overview
    ========
      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.
 
 
    Review
    ======
      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 Node in August 2001, prior to
      PDS acceptance and ingestion.
 
 
    Data Coverage and Quality
    =========================
      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.
 
 
    Limitations
    ===========
    None