A water-sensing satellite orbits high above Earth. Airplanes packed with research instruments, including one from NASA's Jet Propulsion Laboratory, Pasadena, Calif., circle 25,000 feet above three U.S. states and Brazil. Scientists, college students and other volunteers troop into the countryside, armed with sensors and notepads. It's all about "getting the dirt." In this case, collecting detailed information about the soil.

The objectives are two-fold - validating soil moisture data gleaned from satellites and working to find the optimum instrument for conducting soil moisture remote sensing. By learning how to better gauge the amount of moisture in the soil, scientists are pursuing the long-range goal of eventually helping to improve the accuracy of weather forecasts and better estimate crop yields through remote-sensing methods.

Led by Dr. Thomas Jackson of the U.S. Department of Agriculture, Soil Moisture Experiments in 2003 is a collaboration between NASA, the U.S. Department of Agriculture's Agricultural Research Service, the Brazilian Agricultural Research Corporation, several U.S. universities and the Center for Hydrology, Soil Climatology, and Remote Sensing of Alabama A&M University in Huntsville. The campaign began June 22 in Huntsville and gathered data in Alabama and Georgia through July 2. It is continuing in Oklahoma through July 19 and concludes in Brazil Sept. 16-26.

"By gathering comprehensive soil moisture data from space, air and land, we hope to better understand how these measurements correlate and how the data can help farmers, weather forecasters and others who depend on Mother Nature for their livelihood," said Dr. Charles Laymon, a hydrologist and remote sensing scientist with Universities Space Research Association at the Global Hydrology and Climate Center in Huntsville.

For example, an improved understanding of soil moisture could aid irrigation, allowing farmers to irrigate when and precisely where necessary. This is important, Laymon said, because simple ground observations don't always tell the whole story. That's why scientists leading the campaign will look skyward for much of their data.

Aqua, a NASA satellite launched in May 2002, will fill in part of the puzzle. Orbiting 692 kilometers (430 miles) above Earth, its sensors collect information about Earth's water cycle -- including water vapor in the atmosphere, clouds, precipitation, and snow and ice cover. The Advanced Microwave Scanning Radiometer for Earth Observing System, a National Space Development Agency of Japan instrument, is the Aqua instrument scientists hope can provide information about soil moisture.

A challenge will be taking the "big picture" offered by that Aqua radiometer instrument and filling in the gaps. "Aqua's Advanced Microwave Scanning Radiometer for Earth Observing System was designed primarily to monitor oceans and polar ice," Laymon said. "So the sensor provides a very broad view of terrestrial soil moisture. To get a more detailed look at soil moisture, we will use information from this campaign to fine-tune the radiometer's results, and more importantly, correlate the satellite data to measurements gleaned by airborne instruments in the sky and by people on the ground."

The research aircraft are NASA's P-3B turboprop and DC-8 jet. Equipped with a suite of remote sensing instruments developed for airborne observations in support of satellite validation, including JPL's Airborne Synthetic Aperture Radar instrument, or Airsar, they will document patterns of surface moisture by measuring microwave energy in units of brightness temperature and power reflected off the surface.

On the ground, teams of scientists, college students and volunteers -- rain or shine -- will disperse into the countryside daily, taking measurements that include soil moisture and temperature, ground cover type and plant height.

One proposed soil moisture mission that the campaign will assist in the development of is the JPL-led Hydros remote global soil moisture and freeze-thaw state observing system. Hydros would provide soil moisture observations every three days or less over most of Earth's unfrozen, non-forested regions (dense vegetation limits the ability to sense the underlying soil moisture). The data would be used to better understand how water, energy and carbon are exchanged between Earth's land and atmosphere.

Dr. Eni Njoku, a JPL scientist and co-organizer of the Aqua radiometer validation and Hydros development campaign components, said Airsar data will be combined with ground data on soil and vegetation conditions to develop the problem-solving procedures Hydros will use for generating global soil moisture maps. "We hope to be able to answer key questions, such as how well Hydros will be able to collect soil moisture data in vegetated areas," he said. "We also expect to gain insight into how to best combine radar and radiometer data to get the most accurate soil moisture maps possible."

Participating NASA centers include the Marshall Space Flight Center in Huntsville, Goddard Space Flight Center, Greenbelt, Md., and JPL. Campaign aircraft are based at NASA's Wallops Flight Facility, Wallops Island, Va., and Dryden Flight Research Center, Edwards, Calif. JPL is managed for NASA by the California Institute of Technology, Pasadena.