AIRS IR Level 1B Calibrated Radiance Data

Page Contents:

• 1 Data Set Information
• 2 Data Source Information
• 3 Data Set Organization
• 4 Data Characteristics
• 5 Sample Read Programs
• 6 Data Access Information
• 7 References
• 8 Point of Contact

1. Data Set Information:

The AIRS Infrared (IR) level 1B data set contains EOS Aqua AIRS infrared calibrated and geolocated radiances in milliWatts/m**2/cm**-1/steradian. This data set is generated from AIRS level 1A digital numbers (DN) and contains 2378 infrared channels in the 3.74 to 15.4 µm region of the spectrum. A day's worth of AIRS data is divided into 240 scenes (granules), each of 6 minute duration. For the AIRS infrared measurements, an individual scene consists of 135 scanlines containing 90 cross-track footprints. Thus, there is a total of 135 x 90 = 12,150 footprints per AIRS IR scene.

AIRS employs a 49.5 degree cross-track scanning with a 1.1 degree instantaneous field of view (IFOV) to provide twice daily coverage of essentially the entire globe in a 1:30 PM sun synchronous orbit. Combined with the passive microwave measurements from the AMSU-A and HSB instruments aboard the same platform, the AIRS calibrated radiances are used primarily to retrieve vertical profiles of temperature and humidity.

2. Data Source Information:

AIR is a complex instrument, especially with regard to the downlink telemetry of science and engineering data. There are 2378 spectral channels, i e., one channel refers to a spectral resolution element, with two samples per channel. There are 4482 total detectors on the IR Focal Plane Assembly (FPA), with hybrid PV/PC:HgCdTe detector types. There are 2,104 IR spectral channels, with dual PV detectors, and 274 channels in the critical 13.6-15.4 µm region, with single PC detectors. The dual detectors are summed (or selected) on board prior to downlink. After digitizing and formatting, each AIRS science data packet contains 2666 detector samples that include both IR and VIS/NIR.

The heart of the AIRS system is the multi-aperture, echelle grating spectrometer and corresponding multiple detector arrays. The IR Spectrometer Assembly includes a pupil imaging (i.e. detectors are located at a pupil stop of the spectrometer optics as opposed to the detectors being at a field stop) telescope which views the Earth and calibrator assemblies through a rotating scan mirror in the scan head assembly. While in-flight calibration measurements are made once per scan line (every 2.667 seconds), data from ten or more scan lines are combined by the ground calibration software to update calibration coefficients.

Views of the flight blackbody and the cold space view provide a two-point radiometric calibration for gain measurement and for background signal an electrical offset correction. The radiometric accuracy of the AIRS instrument depends directly on the accuracy of its flight calibration source and the quality of the cold space view.

A VIS/NIR Sensor Assembly shares the scan mirror with the spectrometer so that each IR footprint is overlayed with a grid of visible detector pixels in four wavelength channels. The four channels provide spectral coverage from 0.4 µm to 1.0 µm. The VIS/NIR channels have nominally six times the spatial resolution of the IR Sensor Assembly. The VIS/NIR channels are essential to account for the effect of prevalent low level clouds. In these regions of the atmosphere, neither infrared nor microwave channels are capable of providing the required sensitivity. The photometric reference source is provided for calibration of the VIS/NIR channels.

Spatial coverage and calibration targets are included in the scan head assembly which contains a cross track rotary scan mirror and calibrators. The scan mirror has two speed regimes: During the first 2 seconds it rotates at 49.5 degree/second, generating a scan line with 90 ground footprints, each with a 1.1 degree diameter FOV. During the remaining 0.667 seconds the scan mirror finishes the remaining 261 degrees of a full revolution. Routine calibration related data are taken during this time. These consist of four independent Cold Space Views, one view into the Onboard Blackbody Calibrator (OBC), one view of the Onboard Spectral Reference Source (OBS), and one view into a photometric calibrator for the VIS/NIR Photometer.

AIRS Scan Geometry Diagram(Courtesy of AIRS Science Team, NASA/JPL)

3. Data Set Organization:

Format:

Data Granularity:

Each AIRS IR Level1B Radiance file contains a single 6-minute swath data and browse subset contains daily data for ascending or descending node.

Files are named using the following convention:

6-minute granule:

AIRS.{Year}.{Mon}.{Day}.{Gran}.{Level}.{FileType}.{VerID}.{PGenFac}{Cycle#}.hdf

where:

Year is 4 digit year of data; e.g., 2001.

Month is 2 digit month (1-based); e.g., 03

Day is 2 digit day of month (1-based); e.g., 31

Gran is 3 digit granule of day (001-240) for standard (45 scanset) granules.

Level is product level; e.g., L1B is for Level1B

FileType is a string defining the product file type; for example,AIRS_Rad.

VerID is the PGEVERSION.

PGenFac is the PRODUCTGENERATIONFACILITY char. "G" for GSFC DAAC

Cycle# is set during production using the "Times Processed" input field (Cycle# = Times Processed - 1) and is used by the data creator to assure uniqueness of the LOCALGRANULEID; Cycle# is 3 digits and 0-based; e.g., 002.

Examples: AIRS.2003.03.14.088.L1B.AIRS_Rad.v2.7.12.0.G03073231231.hdf

Volume average of data files:

AIRS Infrared Level 1B Radiances: 114.0 MB

Parameter Description:

Arrays shown below are those most likely of interest to the general user. Array not included are primarily those dealing with statistics of the scene and calibration source counts, radiance statistics, and channel gain/offset statistics, among others.

AIRIBRAD - AIRS IR Geolocated Radiances

Sample Data Record:

4. Data Characteristics:

Spatial Characteristics:

• Spatial Coverage:  Global
• Spatial Resolution:   Size of Field of View (FOV) along a scan line varies from 13 km in diameter at nadir to 41km X 22.4km.

Temporal Characteristics:

• Temporal Coverage:   (Future Entry)
• Temporal Resolution:   Twice daily (day and night)

5. Sample Read Programs:

Data Product Readers :  A selection of product readers - FORTRAN/C readers for Level 1B and Level 2 data (with accompanying Guide document);  IDL procedure to read L1B, L2 and L3 product files;  and the MATLAB module to read L1B, L2 and L3 product files.

HDF_READER : This is a command line program developed by the GES DAAC to allow a user to view the contents of an HDF file, as well as to subset the data therein. A list of options controls what is displayed. One may list any of the HDF objects within a file, and the data within them. They may be subsetted along any dimension, or the entire data may be dumped if no subset options are given. There is also a mode to print a heirarchical tree list of the objects in the file. Data can be sent to an ASCII text file, a set of flat binary files, or displayed on the screen (default).

AIRSMETA: This was developed by the GES DAAC to read various components of the data granule (file) and display it in ASCII format. The program was designed and tested on SGI UNIX workstations. The HDF-EOS calling interface library must be installed on your machine and linked to this program during compilation.

6. Data Access Information:

Contacts for Archive/Data Access Information:

Atmospheric Dynamic Data Support Team Goddard Distributed Active Archive Center

NASA/Goddard Space Flight Center

Code 610.2

Greenbelt, MD 20771

Phone: (301) 614-5323

Fax: (301) 614-5268

Email: atmdyn-dst@disc.gsfc.nasa.gov

You may access the AIRS data from:

Search and Order

• DAAC Data Pool
• EOS Data Gateway
• Mirador

7. References:

1. AIRS Algorithm Theoretical Basis Document, AIRS Team Unified Retrieval for Core Products (Level 2 ATBD) JPL D-17006, Version 2.1 15 December 1999 http://eospso.gsfc.nasa.gov/atbd/airstables.html

2. AIRS Calibration Plan, JPL D-16821, 14 November 1997. http://eospso.gsfc.nasa.gov/calibration/plans.

3. AIRS Team Science Validation Plan, Core Products, JPL D-16822, Version 2.1 15 December 1999 ftp://eospso.gsfc.nasa.gov/sterling/Validation/AIRSplan.pdf

4. AIRS/AMSU/HSB Data Processing and Data Products Quality Assessment Plan, JPL D-20748, Version 2.1 August 28, 2001

5. AIRS Version 2.7 Processing Files Description

8. Point of Contact:

The Goddard Earth Sciences Data and Information Services Center

Distributed Active Archive Center (GES DISC DAAC)
Phone: (301) 614-5224
Fax: (301) 614-5268
E-mail: help-disc@listserv.gsfc.nasa.gov