Data Product Descriptions
About AIRS Data
The AIRS instrument suite consisting of the hyperspectral (2378 infrared channels and 4 visible/near-infrared channels) AIRS instrument, the (15 microwave channel) AMSU-A instrument and the (4 microwave channel) HSB instrument was launched aboard NASA's Aqua Earth Observing System satellite on May 4, 2002. The advantage of the AIRS suite in orbit is the provision of rapid global coverage as radiosonde coverage of much of Earth's land mass and all but a few locations in the oceans is practically nonexistent. There are less than 1000 radiosonde launch sites worldwide, most based in Europe and North America and most launch two radiosondes per day. AIRS soundings are equivalent to launching 300,000 radiosondes on a 50 km grid over the globe each day.
The orbit of the Aqua satellite is polar sun-synchronous with a nominal altitude of 705 kilometers (438 miles) and an orbital period of 98.8 minutes, completing approximately 14.5 orbits per day. The repeat cycle period is 233 orbits (16 days) with a ground track repeatability of +/- 20 kilometers (12 miles). The satellite equatorial crossing local times are 1:30 a.m. in a descending orbit, and 1:30 p.m. in an ascending orbit.
The AIRS instrument suite was constructed to obtain atmospheric temperature profiles to an accuracy of 1 kelvin for every 1 kilometer layer in the troposphere and an accuracy of 1 kelvin for every 4 kilometer layer in the stratosphere up to an altitude of 40 kilometers. The temperature profile accuracy in the troposphere matches that achieved by radiosondes launched from ground stations. The advantage of the AIRS suite in orbit is the provision of rapid global coverage as radiosonde coverage of Earth's oceans is practically nonexistent.
In conjunction with temperature profiles, the AIRS instrument suite obtains humidity profiles to an accuracy of 15% in 2 kilometer layers in the lower troposphere and an accuracy of ~50% in the upper troposphere. It also provides integrated column burden for several trace gases. The infrared spectrum is rich in information on numerous gases in the atmosphere, and the primary data returned by AIRS is the infrared spectrum in 2378 individual frequencies. It forms a "fingerprint" of the state of the atmosphere for a given time and place that can be used as a climate data record for future generations.
The AIRS infrared radiance data product is stable to 10 milliKelvin per year and accurate to better than 200 milliKelvin. This product is the most accurate and stable set of hyperspectral infrared radiance spectra measurements made in space to date, and it meets the criteria identified by the National Research Council for climate data records.
AIRS processing software is updated every three to five years and the entire mission set reprocessed using the current software. The Level 1B radiance processing software updates occur less frequently than the Level 2 and Level 3 retrieval software updates.
Level 1B data product: Current processing state is Version 5 release (data are not processed to Version 6 because algorithms that create Level 1B are unchanged as of the Version 6 release)
Level 2 data product: Current processing state is Version 6 release
Level 3 data product: Current processing state is Version 6 release
Level 1B Data Products
Calibrated and geolocated radiances
The Level 1B data product contains measurements of the radiation in Earth's atmosphere measured at 2378 different frequencies. It contains geolocated and calibrated infrared, microwave and visible/near-infrared radiances at instrument spatial resolution along with Quality Assessment (QA) data. Each product granule contains 6 minutes of data resulting in 240 granules of each Level 1B product every day. Each Level 1B science file type contains data in HDF-EOS (based on HDF version 4) Swath format. The AIRS Version 6 retrieval code continues to use the Version 5 radiance data.
• AIRS infrared and visible/near-infrared geolocated, calibrated radiances
• AMSU-A and HSB microwave geolocated, calibrated brightness temperatures
• HSB geolocated, calibrated brightness temperatures
• AIRS infrared and visible/near-infrared quality assurance subsets
• AIRS and AMSU-A calibration subsets
"FLAVORS" OF AIRS LEVEL 2 AND LEVEL 3 DATA
There are three Version 6 processing streams of each Level 2 and Level 3 data type. They differ in the combination of instrument radiances used in the retrieval process. We refer to each of these processing streams as a different “flavor” of the Version 6 products.
AIRS+AMSU: The main Version 6 product (or flavor) is the retrieval based on the combined AIRS and AMSU observations.
AIRS+AMSU+HSB: This flavor is based on the combined AIRS, AMSU, and HSB observations. Since the HSB instrument stopped working very early in the mission (February 2003) the AIRS+AMSU+HSB retrieval is the shortest time series of the three flavors.
AIRS-only: A third flavor is the AIRS-only retrieval that does not use any microwave data. The AIRS-only algorithm is a backup algorithm that provides a continuous data product for the entire mission in the event the AMSU instrument fails.
Level 2 Data Products
Geophysical retrievals of atmospheric constituents and surface properties, and cloud-cleared radiances
The Level 2 data product includes geolocated cloud-cleared radiances and associated 2- and 3-dimensional retrieved geophysical quantities (e.g., temperature, moisture, ozone, carbon monoxide and methane profiles throughout the atmosphere and surface properties). Each product granule contains 6 minutes of data, resulting in 240 granules of each Level 2 product every day. Three types of AIRS Level 2 files are produced: the standard product, the support product, and cloud cleared radiance files. The standard retrieval product is designed for the general user, while the support product contains interim and experimental portions intended for use by the AIRS team and others willing to make a significant investment of time in understanding the product. Each Level 2 science file type contains data in HDF-EOS Swath format.
AIRS Standard Retrieval Product contains retrieved estimates of cloud and surface properties, plus profiles of retrieved temperature, water vapor, ozone, carbon monoxide and methane.
AIRS Support Product includes higher vertical resolution profiles of the quantities found in the Standard Product plus intermediate output (e.g., microwave-only retrieval), research products such as the abundance of trace gases, and detailed quality assessment information.
Level 3 Data Products
Gridded averages of geophysical retrievals
The AIRS Level 3 products contain gridded statistical summaries of the AIRS Level 2 geophysical quantities. Level 3 products differ in that they are Level 2 products that have been QC filtered and binned into 1°x1° grid cells and contain 1°x1° degree means, standard deviation and counts. The Level 3 daily product uses data from all 15 orbits (there are 15 orbits per day) and averages the data together. Level 3 standard files grid data from the Level 2 standard retrieval product in daily, eight-day, and monthly HDF-EOS Grid products. A more detailed set using 100-layer support profiles is included in Level 3 support products. A Level 3 research product is also available for internal project use with intermediary products and quality indicators.