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calib/ 29-Aug-2002 10:43 -
aareadme.txt 21-Jun-2002 12:05 20K
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PUBLICATION_DATE = 2001-09-01
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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
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.
The following documentation describes the dataset
and documentation delivered as part of the NEAR XGRS instrument
team delivery to NASA's Planetary Data System. for NEAR GRS
on asteroid activities.
2. Documentation
_____________
This document provides an overview of the Near Earth Asteroid
Rendezvous Gamma-Ray On-asteroid Level 2 document archive. These
documents describe the design, development, implementation,
calibration and operations of the NEAR GRS instrument. The
directory contains references for 2 documents that could not be
included in this archive due to copyright concerns and are cited
in the REF.CAT file in the CATALOG directory.
An additional publication specific to the analysis of the GRS
on-asteroid data has been submitted to Meteoritics and
Planetary Science for publication in Fall 2001.
Elemental Composition from Gamma-Ray Spectroscopy of the NEAR-
Shoemaker Landing Site on 433 Eros. Larry Evans et. al.
Additional documentation is found in the DOCUMENT directory,
including a document describing the GRS instrument calibrations.
3. 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 4 longest integration times
(NAI, ANTI,NAI1, and NAI2) and BGO spectra.
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-linearity's (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.
As a result three Level-2 spectra were
generated for the BGO instrument and 3 for the NAI instrument
and finally the NAI 1 and 2 escape spectra only were collected
as the last spectra integrated from the surface:
as a result 17 individual spectra are listed with ancillary
information as specified 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.
4. CD-ROM Format
_____________
The CD-ROM has been formatted so that a variety of computer systems
(e.g.,PC, Macintosh, and Sun) may access the data. Specifically, it
is formatted according to the ISO-9660 level 1 Interchange Standard.
For further information, refer to the ISO-9660 Standard Document:
RF# ISO 9660- 1988, April 15, 1988.
This CD-ROM does not contain any Extended Attribute Records (XAR).
Thus, VAX/VMS users may have some problems accessing files on this
volume.
5. 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-3 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:
6. 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
|
|- AAREADME.TXT The file you are reading.
|
|- [DATA]
| |- DATAINFO.TXT Description of files in the DATA directory.
| |- 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.
| |- Also included label file each TIF image
| |- Saved as 1 page per TIF file, * = Page #
| |- [INSTRUMENT] Includes the XGRS_DEFINITIONS file
| |- [OBSERVATION_LOG] Has various log files with user guide
| |- [GRS_RESULTS] Includes explanation file for abundances and
| | figures from the MAPS paper
|
|- [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
| |- GRSURFL2.CAT Description of this GRS dataset
| |- XGRSINST.CAT Instrument related Information
|
7. Whom To Contact For Information
_______________________________
For questions concerning this data set collection:
Planetary Data System (PDS)
Small Bodies Node (SBN)
Astronomy Department
University of Maryland
College Park, MD 20742
NEAR/XGRS Science Team
Code 691, Astrochemistry Branch
NASA / Goddard Space Flight Center
Greenbelt, MD 20771
WWW Site: http://leptpm.gsfc.nasa.gov/near.html
Electronic mail addresses: jack.trombka@gsfc.nasa.gov,
tim.mcclanahan@gsfc.nasa.gov
8. Cognizant Persons
_________________
NEAR/XGRS 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