NASA scientists are studying the possibility of placing microwave sensors aboard unmanned aircraft and microsatellites as a way to obtain soil-moisture measurements at a bargain price.

NASA's Marshall Space Flight Center in Huntsville, Ala., issued a notice May 16 seeking potential ideas and sources for outfitting unmanned aerial vehicles (UAVs) and microsatellites with advanced L-band antennas for gathering soil-moisture measurements.

"Antenna design must be scaled to the proper weight and size to be deployable from current UAVs with future follow-on work to embed the antenna into a microsatellite," NASA said in the solicitation.

Respondents should assume operating altitudes of 150-10,000 meters for UAVs and 700 kilometers for microsatellites, and scale their sensors accordingly to meet the resolution requirements and other technical specifications, the notice said.

Climate and environmental scientists have long had an interest in soil-moisture measurements, which can aid in long-term weather forecasts and which also have potential applications in agriculture and resource management, according to Charles Laymon, a research fellow at Marshall. "If we can increase the resolution and frequency of observation, there's a wide breadth of other disciplines that could make use of this information," he said.

Up until this year, scientists interested in soil-moisture measurements were looking forward to NASA's $175 million Hydros mission, which was funded as an alternate under the agency's Earth Systems Science Pathfinder series of small environmental-science satellites. But the agency axed Hydros at the beginning of the year due to a funding crunch affecting its science programs across the board.

UAVs and microsatellites are now seen as promising low-cost alternatives for gathering the measurements.

"With the recent termination of the Hydros mission, we started looking at exploring whether some of these platforms might serve as a technology bridge until the next opportunity to re-propose the mission came along," Laymon said in an interview.

The challenge, Laymon said, is to design L-band microwave sensors that are small enough to fit on UAVs. NASA is still determining what size UAV would be compatible with this kind of experiment, according to NASA spokesman Rick Smith. "There are some new technologies out there that are being developed we can quickly harvest and modify slightly to serve our needs," he said. Marshall has a history of using remote sensing techniques to obtain soil-moisture data, according to Steve Roy, a spokesman for the center. In recent years scientists have relied on direct measurements combined with data collected by sensors aboard NASA's Aqua satellite, launched in May 2002, he said. But the data collected by Aqua is not particularly effective for obtaining the information the scientists need, and they hope that data collected on the UAVs will help scientists make better long-term weather forecast predictions, Smith said.

UAVs and microsatellites could significantly reduce the cost of gathering these measurements, Laymon said.

"The traditional science platform that NASA builds for a science mission has a long life of three to 10 years, incorporates a large number of instruments, and costs hundreds of millions of dollars," Laymon said. "These technologies can do things more cheaply. ... A UAV is a stepping stone in getting to a microsatellite."

UAVs do not provide the global, broad-area coverage of satellites. But they can be deployed relatively quickly as dictated by weather conditions or other events and focus in on areas of particular interest, Laymon said.

Laymon expressed hope that the responses to Marshall's request for information will lead to funding for technology demonstrations in the next two years or so. "The [request] is a first step in identifying who wants to play, and who has something to offer in the arena," Laymon said.

NASA's Goddard Space Flight Center in Greenbelt, Md., working in conjunction with the agency's Wallops Flight Facility in Wallops Island, Va., already has had some success incorporating microwave instruments on UAVs, according to Larry Hilliard, an engineer at Goddard and principle investigator for the project.

Hilliard said the Goddard instrument is a simplified system, with the intention of adding more capabilities to the instrument as the project proceeds. Scientists encased the antenna in a type of foam structure that was of very low mass, allowing the UAV to handle the weight of the payload. The vehicle has not yet been flight tested, but could potentially be in the fall, Hilliard said.

Hilliard's team is a potential collaborator for Marshall for the project outlined in the request for information, Hilliard said.