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FEATURE
The Aqua Satellite - Vital Statistics

04.22.02

Aqua will make measurements of the Earth at the same time, all the time. This is because the satellite will orbit the planet on a nearly polar route, passing over different points on the ground at approximately 1:30 pm and 1:30 am. By maintaining a consistent time for taking readings, the integrated suite of sensors on Aqua will facilitate sophisticated measurements of planetary processes that until now have been challenging to collect and calibrate. Working in concert or independently, Aqua's scientific hardware will be used to study climate change, vegetation, water vapor in the atmosphere, clouds, precipitation, soil moisture, sea ice, land ice, snow cover and more.

It's a broadly defined mission overall, specifically composed of both narrowly and widely tailored objectives. The processes targeted for study directly affect life on Earth; they describe the tangible world: rainfall that waters crops, snow and ice that reflect heat back into space, and changes in vegetation that describe how our planet's biosphere may be shifting due to human induced climate pressures. FLIGHT HARDWARE: AQUA'S RESEARCH PAYLOAD Modis First, a review: color is a description of light. Light is electromagnetic radiation. Therefore, color, at least in the visible sense, is a description of electromagnetic radiation. Even beyond the realm of what's visible to the human eye, the specific characteristics of a particular color can tell a great deal about the material that's reflecting or emitting it. From this simple axiom comes the development of a sensor that's designed specifically to capture the subtleties of color. It's called MODIS, the Moderate-resolution Imaging Spectroradiometer, and by many estimations, this is the future of color. Aqua's MODIS will measure visible and infrared light in 36 distinct bands-it will view the world in 36 colors, in other words. MODIS will not be helpless at night, either. The dramatic reduction in visible light when the planet turns away from the Sun does not curtail the instrument's ability to collect data of other wavelengths. These measurements will be used to generate scientific products about land surface cover, vegetation, phytoplankton in the oceans, fires on land, sea ice cover, snow cover on land, and properties of clouds and aerosols in the atmosphere. CERES CERES stands for Clouds and the Earth's Radiant Energy System. As the name suggests, the instrument looks at clouds and the Earth's radiation balance. More specifically, CERES studies how much heat is emitted and reflected by the Earth on a broad scale. CERES scientists will also deliver detailed information about clouds by analyzing the data in concert with the data from the MODIS instrument. By studying how clouds function in relation to the larger planet, scientists can develop new predictive models about weather systems and how the Earth maintains its delicate temperature balance. The first CERES instrument went into space aboard the TRMM satellite, launched in 1997. Designed to study rainfall in the tropics, TRMM successfully showed how a device of this kind could provide valuable data for weather and climate related research. The second and third CERES instruments launched onboard the Terra platform in 1999; Aqua will enable the fourth and fifth of these to fly above the Earth. AMSR-E The water cycle propels the Earth's climate. Not only is water vapor the most important greenhouse gas, but its circulation through the Earth's many parts also distributes energy. The water cycle is an extremely complex process, and a better understanding of its subtleties can provide valuable information to scientists. One of the key instruments on the Aqua satellite to explore these questions and more is AMSR-E, the Advanced Microwave Scanning Radiometer for the Earth Observing System. By collecting emitted radiation in the microwave range of the spectrum, AMSR-E will study precipitation, near-surface wind speeds, sea surface temperature, soil moisture, snow cover, and more. Compared to the other instruments onboard Aqua, AMSR-E is also distinctive for its appearance. Rising like a golden sail above the bow of the vehicle, the instrument spins 40 times a minute. It will gather data from an 867-mile (1445 km) swath of the planet as it orbits. HSB The Humidity Sounder for Brazil (HSB) is Brazil's latest foray into space. Charged with the task of deriving water vapor readings in the lower atmosphere, this instrument will be used largely to provide humidity measurements beneath overcast conditions and help make readings by its collaborating instrument AIRS more accurate. HSB will be able to make humidity measurements even if clouds or thick haze heavily covers the sky beneath it. HSB measurements will also be used to measure the intensity of rain over given areas, as well as provide supplemental information about clouds, both valuable on an orbiting observatory designed to study the Earth's climate. AMSU The two completely separate sensor units that make up AMSU, the Advanced Microwave Sounding Unit, collect data in 15 spectral bands. That means its sensors read information by measuring emitted light from observed surface features in 15 distinct "colors", although these microwave "colors" are not visible to the human eye. Scientists will treat the data collected by the two AMSU units as if they came from a single piece of hardware. Onboard Aqua the twin sensors are designed to operate in synchrony; back on the ground the data processing system will combine the measurements. AMSU will make measurements of atmospheric temperature, with particular attention paid to the stratosphere (the upper atmosphere), and a secondary goal of providing temperature observations in the troposphere (the lower atmosphere). When it looks at lower altitude parts of the sky, AMSU will be able to filter out data signatures from low lying clouds. The instrument sees the Earth in a swath of 1014 miles (1690 km). AIRS: A NEW SATELLITE, A NEW INSTRUMENT While the other five instruments on Aqua are all highly sophisticated pieces of technology, the Atmospheric Infrared Sounder, or AIRS, breaks new ground. It's designed to collect data about the Earth using 2378 distinct spectral channels in the infrared range, plus four more channels in the visible and near-infrared part of the spectrum. Where AIRS makes its most compelling statement of purpose is in its synergy with two other instruments onboard Aqua. By using microwave data from AMSU and HSB to take into account the effects of clouds, AIRS will be able to make unprecedented measurements of temperature through vertical columns of the atmosphere, as well as making temperature measurement for the planet's surface. By fusing data about water vapor, temperature, and clouds, powerful new tools become available for research and analysis into the Earth's climate at a systemic, process-oriented level. THE AIRS TRIO: MEASURING MORE THAN AIR TEMPERATURE AIRS may be the headliner, but in conjunction with AMSU and HSB, this is one trio worthy of lofty high notes. This is the most advanced atmospheric sounding system ever put into space. Scientists are eager to put it to the test. The idea is that each instrument will be able to make measurements that complement data gathered by the other two. The AIRS instrument itself has the major role of looking at infrared light, taking measurements that the scientist's data processing system can use to derive highly sensitive temperature information. In this visualization we get a better sense of what the trio will be trying to see. We're looking at a simulated three-dimensional section of the atmosphere, using data from NASA's advanced NSIPP (NASA's Seasonal to Interannual Prediction Project) climate model to illustrate the point. NSIPP simulates the Earth's climate inside a powerful supercomputer, enabling experts to conduct virtual "experiments" on the natural world. By looking through the total volume of atmosphere, AIRS, AMSU, and HSB are able to quantify water vapor, cloud density, and temperature simultaneously. This co-incident data acquisition is vital if experts are to better understand their interrelationship. The visualization also emphasizes the three-dimensionality of the measurements themselves. Just as the atmosphere is not a flat component of the total planet, neither are its features. They vary over distance: as you climb a mountain, the air not only gets colder but also drier. With the AIRS trio, experts can compile accurate profiles of vast regions of the atmosphere. Listed specifically, some of the products AIRS will provide include temperature and humidity profiles, a quantified value for how much a particular region is covered by clouds, cloud-top height, and cloud-top temperature. It will also be able to precisely measure the differences in day-night surface temperatures and provide a reliable measurement of total atmospheric water in a given region. This kind of data will likely have a major influence in our ability to forecast weather and help us better understand the nature of Earth's climate system.

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