JPL: NASA Mission Will Observe Earth's Salty Seas

[bystander]

NASA Mission Will Observe Earth's Salty Seas
NASA JPL-Caltech | Aquarius-SAC-D | 2011 May 17

[size=85]Artist's concept of the Aquarius/SAC-D spacecraft, a collaboration between NASA and Argentina's space agency, with participation from Brazil, Canada, France and Italy. Aquarius, the NASA-built primary instrument on the spacecraft, will take NASA's first space-based measurements of ocean surface salinity, a key missing variable in satellite observations of Earth that links ocean circulation, the global balance of freshwater and climate. The mission is scheduled to launch in June. [i](Credit: NASA)[/i][/size]

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Final preparations are under way for the June 9 launch of the international Aquarius/SAC-D observatory. The mission's primary instrument, Aquarius, will study interactions between ocean circulation, the water cycle and climate by measuring ocean surface salinity.

Engineers at Vandenberg Air Force Base in California are performing final tests before mating Aquarius/SAC-D to its Delta II rocket. The mission is a collaboration between NASA and Argentina's space agency, Comision Nacional de Actividades Espaciales (CONAE), with participation from Brazil, Canada, France and Italy. SAC stands for Satelite de Applicaciones Cientificas. Aquarius was built by NASA's Jet Propulsion Laboratory in Pasadena, Calif., and the agency's Goddard Space Flight Center in Greenbelt, Md.

In addition to Aquarius, the observatory carries seven other instruments that will collect environmental data for a wide range of applications, including studies of natural hazards, air quality, land processes and epidemiology.

The mission will make NASA's first space observations of the concentration of dissolved salt at the ocean surface. Aquarius' observations will reveal how salinity variations influence ocean circulation, trace the path of freshwater around our planet, and help drive Earth's climate. The ocean surface constantly exchanges water and heat with Earth's atmosphere. Approximately 80 percent of the global water cycle that moves freshwater from the ocean to the atmosphere to the land and back to the ocean happens over the ocean.

Salinity plays a key role in these exchanges. By tracking changes in ocean surface salinity, Aquarius will monitor variations in the water cycle caused by evaporation and precipitation over the ocean, river runoff, and the freezing and melting of sea ice.

Salinity also makes seawater denser, causing it to sink, where it becomes part of deep, interconnected ocean currents. This deep ocean "conveyor belt" moves water masses and heat from the tropics to the polar regions, helping to regulate Earth's climate.

"Salinity is the glue that bonds two major components of Earth's complex climate system: ocean circulation and the global water cycle," said Aquarius Principal Investigator Gary Lagerloef of Earth & Space Research in Seattle. "Aquarius will map global variations in salinity in unprecedented detail, leading to new discoveries that will improve our ability to predict future climate."

Aquarius will measure salinity by sensing microwave emissions from the water's surface with a radiometer instrument. These emissions can be used to indicate the saltiness of the surface water, after accounting for other environmental factors. Salinity levels in the open ocean vary by only about five parts per thousand, and small changes are important. Aquarius uses advanced technologies to detect changes in salinity as small as about two parts per 10,000, equivalent to a pinch (about one-eighth of a teaspoon) of salt in a gallon of water.

Aquarius will map the entire open ocean every seven days for at least three years from 408 miles (657 kilometers) above Earth. Its measurements will produce monthly estimates of ocean surface salinity with a spatial resolution of 93 miles (150 kilometers). The data will reveal how salinity changes over time and from one part of the ocean to another.

The Aquarius/SAC-D mission continues NASA and CONAE's 17-year partnership. NASA provided launch vehicles and operations for three SAC satellite missions and science instruments for two.

JPL will manage Aquarius through its commissioning phase and archive mission data. Goddard will manage Aquarius mission operations and process science data. NASA's Launch Services Program at the agency's Kennedy Space Center in Florida is managing the launch.

CONAE is providing the SAC-D spacecraft, an optical camera, a thermal camera in collaboration with Canada, a microwave radiometer,; sensors from various Argentine institutions and the mission operations center there. France and Italy are contributing instruments.

For more information about Aquarius/SAC-D, visit: http://www.nasa.gov/aquarius and http://www.conae.gov.ar/eng/principal.html.

[bystander]

For Aquarius, Sampling Seas No 'Grain of Salt' Task
NASA JPL-Caltech | Aquarius-SAC-D | 2011 May 25

[size=85]An overhead crane lifts the Aquarius/SAC-D spacecraft, secured inside its payload transportation canister, into the mobile service tower at NASA's Space Launch Complex-2 (SLC-2) at Vandenberg Air Force Base in California. There, the spacecraft will be integrated to a United Launch Alliance Delta II rocket in preparation for the targeted June liftoff. Aquarius, the NASA-built instrument on the SAC-D spacecraft, will provide new insights into how variations in ocean surface salinity relate to fundamental climate processes on its three-year mission. [i](Credit: NASA/VAFB)[/i][/size]

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The breakthrough moment for oceanographer Gary Lagerloef, the principal investigator for NASA's new Aquarius mission, came in 1991. That's when he knew it would be possible to make precise measurements of ocean salinity from space. It has taken nearly two decades to turn that possibility into a reality.

Lagerloef was looking at data collected by a NASA aircraft flying over the ocean off the coast of Maryland. It was testing a new radiometer, an instrument that can sense thermal signals emitted by land, clouds and the ocean surface. The instrument not only captured the unique signature of dissolved salt in the surface water below, it showed how the water's salt content varied from one side of the Gulf Stream to the other.

"That flight was a turning point," said Lagerloef, a senior scientist at Earth & Space Research, Seattle. "We could clearly see the range that we needed to study salinity from its lowest levels in the North Pacific to the highest salinity levels in the North Atlantic."

Salinity, or saltiness, plays a critical role in ocean circulation and is a key tracer for understanding the ocean's role in Earth's global water cycle. While satellites routinely provide information on sea surface temperature, sea level, ocean color and ocean winds, historically, no global view of ocean surface salinity had been available. Salinity measurements were limited to those by ships, buoys and floats until recently - and are still few and far between.

Measuring salinity from space is extremely challenging and has been one of the last frontiers for ocean remote sensing. The European Space Agency launched a mission to measure soil moisture and ocean salinity in 2009. And now the Aquarius/SAC-D mission developed by NASA and Argentina's space agency, the Comisión Nacional de Actividades Espaciales, is being readied for launch on June 9. The two missions are complementary, but differ in focus and technology. One important difference is that Aquarius uses both a passive radiometer to detect ocean salinity and an active scatterometer radar to correct the radiometer's salinity measurements for wind roughness (waves) at the sea surface. This is the first combination of this kind used in space for Earth observations, whereas the European Space Agency mission uses only a passive radiometer.

Aquarius is dedicated to making precise measurements of ocean salinity over months and years, providing important new information for climate studies. It will produce monthly maps of the surface salinity of the global ocean with a 93-mile (150-kilometer) resolution and an accuracy of 0.2 practical salinity units, which is equal to about one-eighth teaspoon of salt in a gallon of water. (Practical salinity is a scale used to describe the concentration of dissolved salts in seawater, nearly equivalent to parts per thousand.) The mission is to make these measurements continuously for at least three years.

"This is a level of accuracy and stability that has never been achieved in space before," said Aquarius Instrument Scientist Simon Yueh, of NASA's Jet Propulsion Laboratory, Pasadena, Calif., which is managing the mission for NASA through its commissioning phase.

"The first challenge is that the signal we are measuring is very small," said Aquarius Deputy Principal Investigator David Le Vine, of NASA's Goddard Space Flight Center, Greenbelt, Md. "It is a very tiny signal in a noisy environment. In addition, the dynamic range - the difference in the signal that comes from water with low salinity and water with high salinity - is also small."

The Aquarius instrument has three separate radiometers aimed at the ocean below. The radiometers are designed to detect and measure a particular wavelength of microwave energy being emitted by the ocean.

"Everything radiates energy," explained Le Vine. "When you see the glow of an electric stove, you're seeing thermal radiation. It is in a range that our eyes can see. Night-vision goggles let you see radiation in the infrared part of the spectrum. For Aquarius, we're measuring radiation at microwave frequencies."

The radiometers on Aquarius measure the microwave emissions from the sea surface at 1.4 gigahertz in the L-band portion of the electromagnetic spectrum. This energy, which is measured as an equivalent temperature called the "brightness temperature" in Kelvin, has a direct correlation to surface salinity.

"Lots of things interfere with the salinity signal Aquarius is measuring, such as land and atmospheric effects," said Le Vine. "Ocean waves are a particularly significant source of 'noise' that can confuse the signal from salinity. That's why we have an additional instrument, a scatterometer, on board to help correct for this." The scatterometer sends a radar pulse to the ocean surface that is reflected back to the spacecraft, providing information about the ocean surface.

Because of its importance, the 1.4 gigahertz band is protected for scientific use. Nevertheless, says Aquarius Science Team Member Frank Wentz, director of Remote Sensing Systems, Santa Rosa, Calif., stray signals from radar, telephone and radio occasionally cause problems. Aquarius' radiometers are designed to detect much of this interference and eliminate contaminated measurements.

Wentz is part of the team creating the complicated mathematical formula - called a retrieval algorithm - that Aquarius will use to translate brightness temperature into measurements of salinity. "It's basically a big subtraction process," he said. "We figure out all the things that interfere with the signal we want and eliminate their effect on our measurement. This would be challenging enough to do even if the ocean were perfectly flat like a mirror. Instead, because of waves, it's more like a funhouse mirror that distorts everything."

Aquarius' primary focus is to see how salinity varies from place to place and changes with time. This task would be easier if there were a greater difference in the signal between the regions with low salinity and those with high salinity.

Over the open ocean, salinity is relatively constant. It ranges from about 32 to 37 parts per thousand. The corresponding change in brightness temperature -- what Aquarius measures -- is very small. Aquarius has been designed to detect changes in salinity as small as about two parts per 10,000, which corresponds to about 0.1 Kelvin. "This will be about 10 times more accurate than previous spaceborne radiometer observations of other sea surface characteristics," said Yueh.

Aquarius has to be extremely stable as well as sensitive. "If we want to see small things over long periods of time, we have to make sure that the instrument itself doesn't change. We need to know that any variations we observe are real and not caused by the instrument itself," said Yueh. Making this happen required special engineering attention to temperature control and calibration.

Controlling the temperature, critical for maintaining precision and stability, within a large instrument is difficult. JPL engineers had to design special computer models to understand the system's thermal behavior and its electronics. "This had never been done before for an instrument of this size operating over months and years," said Yueh.

All microwave radiometers, including Aquarius, require a calibration reference point. Most operational spacecraft radiometers continuously turn to look at cold space in order to calibrate their instruments. Aquarius, however, uses internal calibration to help retain its stability. "We have a big antenna and a large instrument," said Yueh. "No one wants to spin this instrument around frequently to look at an external reference."

While Aquarius benefits from advanced technologies such as internal calibration and sophisticated radiometers, its ability to measure global ocean surface salinity with unprecedented accuracy is the result of years of research and planning.

The key to making this challenging measurement, say Aquarius science team members, is in the details. And from the tiny, detailed measurements of salinity that Aquarius makes, a new "big" picture of Earth's ocean will emerge.

For more information on Aquarius, visit: http://www.nasa.gov/aquarius.

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"Age Of Aquarius" Dawns With Launch From California
NASA Headquarters | 2011 Jun 10

[url=http://www.nasa.gov/multimedia/imagegallery/image_feature_1972.html]Aquarius Lifts Off![/url] [i](Credit: NASA/Bill Ingalls)[/i] NASA IOTD (2011 Jun 10)

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NASA's 'Age of Aquarius' dawned Friday with the launch of an international satellite carrying the agency-built Aquarius instrument that will measure the saltiness of Earth's oceans to advance our understanding of the global water cycle and improve climate forecasts.

The Aquarius/SAC-D observatory rocketed into space from Vandenberg Air Force Base in California atop a United Launch Alliance Delta II rocket at 7:20:13 a.m. PDT. Less than 57 minutes later, the observatory separated from the rocket's second stage and began activation procedures, establishing communications with ground controllers and unfurling its solar arrays.

Initial telemetry reports show the observatory is in excellent health. The SAC-D (Satélite de Aplicaciones Científicas) observatory is a collaboration between NASA and Argentina's space agency, Comisión Nacional de Actividades Espaciales (CONAE).

"Aquarius is a critical component of our Earth sciences work, and part of the next generation of space-based instruments that will take our knowledge of our home planet to new heights," said NASA Deputy Administrator Lori Garver. "The innovative scientists and engineers who contributed to this mission are part of the talented team that will help America win the future and make a positive impact across the globe."

Aquarius will make NASA's first space observations of the salinity or concentration of salt at the ocean surface, a key missing variable in satellite studies of Earth. Variations in salinity influence deep ocean circulation, trace the path of freshwater around our planet and help drive Earth's climate.

"Data from this mission will advance our understanding of the ocean and prediction of the global water cycle," said Michael Freilich, director of NASA's Earth Science Division in the Science Mission Directorate at agency headquarters in Washington. "This mission demonstrates the power of international collaboration and accurate spaceborne measurements for science and societal benefit. This would not be possible without the sustained cooperation of NASA, CONAE and our other partners."

In addition to Aquarius, the observatory carries seven instruments that will monitor natural hazards and collect a broad range of environmental data. Other mission partners include Brazil, Canada, France and Italy.

"This mission is the most outstanding project in the history of scientific and technological cooperation between Argentina and the United States," said CONAE Executive and Technical Director Conrado Varotto. "Information from the mission will have significant benefits for humankind."

Aquarius will map the global open ocean once every seven days for at least three years with a resolution of 93 miles (150 kilometers). The maps will show how ocean surface salinity changes each month, season and year. Scientists expect to release preliminary salinity maps later this year.

Aquarius will measure salinity by sensing thermal microwave emissions from the water's surface with three microwave instruments called radiometers. When other environmental factors are equal, these emissions indicate the saltiness of surface water. A microwave radar scatterometer instrument will measure ocean waves that affect the precision of the salinity measurement. Because salinity levels in the open ocean vary by only about five parts per thousand, Aquarius will be able to detect changes as small as approximately two parts per 10,000, equivalent to about one-eighth of a teaspoon of salt in a gallon of water.

During the next 25 days, the Aquarius/SAC-D service platform will be tested and maneuvered into its final operational, near-polar orbit 408 miles (657 kilometers) above Earth. Science operations will begin after the observatory's instruments are checked out. This commissioning phase may last up to 65 days.

Aquarius Rising
NASA Earth Observatory | 2011 Jun 10

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In mythology and astrology, Aquarius is known as the water-bearer. For NASA, Aquarius is the space agency's first new Earth-watching tool—actually, water-watching—since 2008.

At 7:20:13 a.m. Pacific Daylight Time on June 10, 2011, the Aquarius/SAC-D observatory was launched on a Delta II rocket from Vandenberg Air Force Base in California. About an hour later, the satellite separated from the rocket's second stage, established communication with ground controllers, and unfurled its solar arrays. Initial reports showed the observatory to be in excellent health, and operators will spend the next few weeks maneuvering it into a polar orbit and turning on the sensors.

The satellite is a joint project of Argentina's Comision Nacionale de Actividades Espaciales, NASA's Jet Propulsion Laboratory, and NASA's Goddard Space Flight Center. The full name of the spacecraft is the Satelite de Aplicaciones Cientificas-D, or SAC-D. Aquarius is a key instrument that will make NASA's first space observations of the salt content, or salinity, of the ocean.

Salinity has traditionally been measured from ships and robotic ocean sensors. Aquarius will map the open ocean once every seven days for at least three years, allowing scientists to make the first global maps of salinity in the surface layer. Observing changes in salinity helps scientists understand ocean circulation and currents, the global water cycle, and other fundamental features of the seas.

In addition to Aquarius, SAC-D carries seven instruments that will monitor natural hazards and collect a broad range of environmental data. Other mission partners include Brazil, Canada, France and Italy.

Photograph by Bill Ingalls, NASA Headquarters.
Caption by Michael Carlowicz and Alan Buis.