AIRS Vis/Near IR Level1B 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 Vis/Near Infrared (IR) level 1B data set contains AIRS visible and near-infrared calibrated and geolocated radiances in W/m**2/micron/steradian. This data set is generated from AIRS level 1A digital numbers (DN) and contains 4 channels in the 0.4 to 1.0 µm region of the spectrum. A day's worth of AIRS data is divided into 240 scenes, each of 6 minute duration. For the AIRS visible/near IR measurements, an individual scene contains 135 scanlines with a scanline containing 720 cross-track pixels by 9 along-track pixels. There is a total of 720 x 9 x 135 = 874,800 visible/near-IR pixels per scene.

AIRS employs a 49.5 degree crosstrack scanning with a 1.1 degree IR (Vis/NIR: 0.2 degree) instantaneous field of view (IFOV) to provide twice daily coverage of essentially the entire globe in a 1:30 PM sun synchronous orbit. The primary purpose of the visible/near IR channels is the detection and flagging of clouds and significant inhomogeneities in the AIRS infrared field-of-view, which may adversely impact the quality of the temperature and moisture soundings. The AIRS visible/near IR channels will also be used for comparative studies with the higher resolution MODIS channels contained within this spectral region.

2. Data Source Information:

AIRS has four Vis/NIR channels. Channel 1 (0.40 to 0.44 µm) is designed to be most sensitive to aerosols. Channels 2 (0.58 to 0.68 µm) and 3 (0.71 to 0.92 µm) approximate the response of AVHRR channels 1 and 2, respectively, and are particularly useful for surface studies. (AVHRR is an imaging instrument currently carried by NOAA polar orbiting satellites.) Channel 4 has a broadband response (0.49 to 0.94 µm) and can be used for energy balance studies. The VIS/NIR channels help to account for the effect of low-level clouds. The infrared and microwave channels sometimes have difficulty distinguishing low clouds from the surface. A photometric reference source is provided for calibration of the VIS/NIR channels.

The VIS/NIR channels have nominally six times the spatial resolution of the IR Sensor Assembly. Each IR footprint of the AIRS instrument is associated with a 9x8 array of Vis/NIR pixels. A single across-track scan of AIRS (90 IR footprints) corresponds to a Vis/NIR image of 720 pixels across-track, and 9 pixels along-track. The across-track pixels overlap with their neighbors by ~1/3 of a pixel on each side. In the along-track direction, successive scan-lines overlap by 1.57 pixels.

3. Data Set Organization:

Format:

Data Granularity:

Each AIRS Vis/Near-IR level1B Radiance file contains a single 6-minute swath 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,VIS_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.2003.03.12.140.L1B.VIS_Rad.v2.7.12.0.G03072225454.hdf

Volume average of data files:

AIRS Vis/Near-IR Level 1B Radiances: 16.0 MB

Parameter Description:

N.B. 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.

AIRVBRAD - AIRS Vis/Near-IR Geolocated Radiances

Sample Data Record:

An AIRS Level 1B data granule contains data fields, geolocation fields, dimension, and swath attributes for a single swath. A detailed description of each attribute can be found in AIRS processing Files Descriptions.

Data fields: Data fields are the main part of a swath from a science perspective and all the other parts of the swath exist to provide information about the data fields or to support particular types of access to them. In AIRS Level 1B data, Along-Track data fields appear once for scanline and Full Swath data fields appear for every footprint of every scanline in granule. Sample.

Geolocation fields: Geolocation fields allow scientific or engineering data to be accurately tied to particular points on the Earth's surface. These fields appear for every IR foot print and correspond to footprint center coordinates and "shutter" time.

To reduce the data volume, not every Vis/NIR pixel is geolocated. Instead, only the four "corner pixels" of the 9x8 grouping associated with each IR footprint are geolocated. (A bi-linear interpolation can be used to geolocate the remaining pixels.) In the data files, four-element arrays called "cornerlats" and "cornerlons" carry this information. The first array element is for the most along-track pixel closest to the start of the scanline (the upper-left pixel when viewing an image aligned with "up" being the along-track direction). The second element is the upper-right pixel (most along track, near the end of the scanline). The third and fourth elements refer to the lower-left and lower-right pixels, respectively.

Dimensions: Dimensions define the axes of the data and geolocation fields by giving them names and sizes. Sample .

Swath Attributes: Swath attribute includes DayNight flag, Quality Assessment (QA) flags and other swath information. Sample.

4. Data Characteristics:

Spatial Characteristics:

• Spatial Coverage:  Global
• Size of Field of View (FOV) is a square, 2.3km on side at nadir.

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