AMSU-A Level 1B Brightness Temperature Data Set

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 AMSU-A level 1B data set contains AMSU-A calibrated and geolocated brightness temperatures in degrees Kelvin. This data set is generated from AMSU-A Level 1A digital numbers (DN) and contains 15 microwave channels in the 50 - 90 GHz and 23 - 32 GHz regions of the spectrum. A day's worth of data is divided into 240 scenes, each of 6 minute duration. An AMSU-A scene contains 30 cross-track footprints in each of 45 along-track scanlines, for a total of 45 x 30 = 1350 footprints per scene. The AMSU-A is co-aligned with the AIRS instrument onboard the Aqua platform so that successive blocks of 3 x 3 AIRS footprints are corresponding to AMSU-A footprint.

AMSU-A is primarily a temperature sounder that provides atmospheric information in the presence of clouds, which can be used to correct the AIRS infrared measurements for the effects of clouds. This is possible because non-precipitating clouds are for the most part transparent to microwave radiation, in contrast to visible and infrared radiation which are strongly scattered and absorbed by clouds. Combined with simultaneous measurements from the AIRS and HSB instruments, the calibrated AMSU-A brightness temperatures will also be used to initialize the surface temperature and atmospheric temperature profile required for the retrieval of the final AIRS geophysical products. AMSU level 1B daily summary browse product is also available to provide users with a global quick look capability when searching for data of interest.

2. Data Source Information:

The AMSU-A instrument consists of two independent modules (AMSU-A1 and AMSU-A2), with each module having separate spacecraft interfaces. Like AIRS, AMSU-A is a crosstrack scanner. AMSU-A1 has two antenna/receiver systems while AMSU-A2 has one. The three receiving antennas are parabolic focusing reflectors than rotate continuously, completing one revolution in 8 seconds. The 8-second scan cycle is divided into three segments. In the first segment the Earth is viewed at 30 different angles, symmetric around the nadir direction, in a step-and-stare sequence. Each of the 30 Earth views (scene stations) takes about 0.2 seconds, for a total of approximately 6 seconds. The second segment is a rapid scan covering a cold space view and an internal (warm) blackbody calibration target. Finally, each antenna returns to the starting position to start a new scan cycle. (There is also a stare mode, where the antenna is permanently pointed to the nearest-nadir direction, but that is only used for special purposes - such as for spatial calibration using coastline crossings.)

The capabilities of AMSU-A significantly exceed those of the NOAA Microwave Sounding Unit (MSU). While MSU has only 4 channels (in the 50-GHz oxygen band for temperature sounding) and samples eleven 7.5 ° scenes per 26.5-second crosstrack scan, AMSU-A has 12 temperature sounding channels as well as 3 moisture channels and samples thirty 3.3 ° scenes per 8-second crosstrack scan. The size of an AMSU-A "footprint" at nadir is therefore less than half the size of and MSU footprint. In addition, the AMSU-A is co-aligned with the AIRS instrument onboard the Aqua platform so that successive blocks of 3 x 3 AIRS pixels are contained within the AMSU-A footprint.

3. Data Set Organization:

Format:

Data Granularity:

Each AMSU-A 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,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.2003.03.14.105.L1B.AMSU_Rad.v2.7.12.0.G03074131756.hdf

Volume average of data files:

AIRS AMSU Level 1B Radiances: 0.4 MB

Parameter Description:

AMSU-A level 1B data contains calibrated and geolocated brightness temperatures in degrees Kelvin. Brightness temperature is the apparent temperature of the surface assuming a surface emissivity of 1.0. Setting the emissivity to one is equivalent to assuming the target is a blackbody, so the brightness temperature is defined as the temperature a blackbody would be in order to produce the radiance perceived by the sensor.

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.

AIRABRAD - AMSU-A Geolocated Radiances

Sample Data Record:

An AMSU-A 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 AMSU-A 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 foot print and correspond to footprint center coordinates and "shutter" time.

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

• Spatial Resolution:  Size of Field of View (FOV) is 40.5 km in diameter at nadir.

Temporal Characteristic:

• 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

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