JUNE 20, 2002

AQUA Earth Observing System Satellite

Aqua is the second satellite in NASA's Earth Observing System series. The first is Terra, launched in 1999 and in operation since February 2000. [ See archived article "TERRA Earth Observation System Satellite", datelined June 29, 2000. ]

Aqua [Latin for water] is so named because its instruments focus on the study of water – on land, in the oceans, and in the atmosphere in the three physical states. "By extension, (Aqua) will gather data on systems dependent on water -- vegetation growth on land and phytoplankton in oceans." This information will help in understanding the Earth's water cycle, changes in global ecosystems, and the global environment.

The satellite was launched on May 4, 2002. Data collection with the entire suite of scientific instruments is scheduled to begin at the end of June. Orbit characteristics are: 705 km altitude; polar, ascending node; 1:30 p.m. daily equatorial crossing. [ Terra has a similar orbit except it crosses the equator daily between 10:30 and 10:45 a.m. in a descending node. ] Aqua's afternoon and Terra's morning observations together will "provide important insights into the daily cycling (diurnal variability) of key scientific parameters such as precipitation and ocean circulation."

Aqua is a joint project between the United States, Japan, and Brazil. The U.S. provided the spacecraft and four of the six scientific instruments, while Japan and Brazil provided one each. (See below.)

( Note: Below is a brief summary of information gleaned from online and published sources. Coverage is not entirely consistent or complete. Please e-mail updates or corrections to oars@oars.on.ca. )

Sensors onboard are:

1. Atmospheric Infrared Sounder (AIRS) *

What is it:	Multispectral infrared scanner.
What it does:	Provides highly accurate temperature profiles within the atmosphere.
Characteristics:	Senses in 2,378 IR bands (3.74 to 15.4 microns) plus 4 visible and near-IR bands (0.4 to 1.0 microns). Scanning mode: cross-track. Spatial resolution:13.5 km at horizontal at nadir, 1 km vertical. Swath: 1,650 km. Accuracy: "1 K temperature retrieval accuracy per 1 km layer in the troposphere. 0.05 emissivity accuracy." The 1-degree accuracy is achievable presently only with balloon soundings.

2. Advanced Microwave Scanning Radiometer-EOS (AMSR-E)

What is it:	Multi-frequency, multi-channel passive microwave radiometer provided by Japan.
What it does:	"Measures precipitation rate, cloud water, water vapor, sea surface winds, sea surface temperature, ice, snow, and soil moisture."
Characteristics:	Measures brightness temperature at 6 frequencies -- 6.925, 10.65, 18.7, 23.8, 36.5, and 89.0 GHz. 12 channels. Dual polarization: vertically and horizontally polarized measurements taken at all channels. Scanning mode: conical. Spatial resolution: from 6 x 4 km at 89.0 GHz to 75 x 43 km at 6.9 GHz. Swath: 1,445 km. Accuracy: 1 K or better

3. Advanced Microwave Sounding Unit (AMSU-A) *

What is it:	Multi-frequency, multi-channel passive microwave radiometer.
What it does:	Measures atmospheric temperature from surface to 40 km to "obtain temperature profiles in the upper atmosphere (especially the stratosphere) and to provide a cloud-filtering capability for tropospheric temperature observations."
Characteristics:	Frequency range: 15 - 90 Ghz. 15 channels. Scanning mode: cross-track Spatial resolution: 40 km horizontal at nadir Field of view: ± 49.5°

4. Humidity Sounder for Brazil (HSB) *

What is it:	Dual frequency, multi-channel passive microwave radiometer provided by Brazil.
What it does:	Measures water vapor in the atmosphere to provide humidity profiles even under conditions of heavy cloudiness and haze.
Characteristics:	Senses at 150 Ghz and 183 Ghz. 4 channels. Spatial resolution: 13.5 km horizontal at nadir. Field of view: ± 49.5° cross track from nadir. ( * The Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU-A), and the Humidity Sounder for Brazil (HSB) are closely integrated to provide accurate temperature and humidity measurements even under heavy clouds and haze.)

5. Moderate Resolution Imaging Spectroradiometer (MODIS)

What is it:	Multiband medium resolution scanning spectroradiometer. Same instrument as the one onboard the Terra satellite.
What it does:	Measures visible and IR radiation "to derive products ranging from vegetation, land surface cover, and ocean chlorophyll fluorescence to cloud and aerosol properties, fire occurrence, snow cover on land, and sea ice cover... "
Characteristics:	Reported to be excellent for monitoring sea ice. 36 bands: 21 within 0.4 to 3.0 microns and 15 within 3.0 to 14.5 microns. Daylight reflection and day/night emission spectral imaging. Scanning mode: cross-track. Swath: 2,300 km from 705 km altitude. Continuous global coverage every 1 to 2 days.

6. Clouds and the Earth's Radiant Energy System (CERES)

What is it:	Two identical broadband scanning radiometers. Same as the two onboard the Terra satellite.
What it does:	Measures reflected solar radiation, emitted terrestrial radiation, and total radiation "to measure the Earth's total thermal radiation budget and, in combination with MODIS data, detailed information about clouds."
Characteristics:	Individual radiometer: 3 bands: Shortwave: 0.3 - 5 microns. Measures reflected sunlight. Long wave: 8 - 12 microns. Measures Earth-emitted radiation. Total radiance: 0.3 - 100 microns. Measures all radiations. Scanning mode: One cross-track, the other biaxially (rotating azimuth plane). Spatial resolution: 20 km at nadir Field of view: ± 78° cross-track, 360° rotating azimuth plane.

Contrasting Aqua's emphasis on water, the third satellite in the Earth Observing series will focus on the atmosphere, particularly the ozone layer. It will be called Aura. Planned launch is in 2004.

For more information, go to http://aqua.nasa.gov/ or http://eos-pm.gsfc.nasa.gov/.