Microwaves To Reveal Ocean Weather, Locate Land Mines

Joel T. Johnson, associate professor of electrical engineering at Ohio State, and his colleagues are developing sensors that detect the small amounts of microwave radiation naturally emitted by the ocean surface and by buried objects.

In two papers in the journal IEEE Transactions on Geoscience and Remote Sensing, Johnson and graduate student Min Zhang described computer models they constructed for passive microwave sensors -- one for weather satellites, and one for hand-held land mine detectors.

Johnson said passive microwave sensors may help scientists understand global climate change, by providing knowledge of the earth's weather patterns on a global scale.

"We can't predict global warming without global data," he said.The same technology could detect today's plastic mines,which are invisible to metal detectors. Since all objects on the earth emit some very low level of microwave radiation, a hand-held microwave sensor can detect radiation emitted by a mine buried beneath the soil.

Scientists have known since the early 1980s that microwave sensors could prove useful for these applications, but the necessary computer models have been difficult to construct.

Every object has its own microwave "signature," depending on the material it's made of, how warm the object is, and where it is located -- and models must take the different signatures into account, Johnson explained.

The situation becomes even more complicated when the object under study is the undulating surface of the ocean. Then the computer model requires knowledge of how wind creates waves, and how waves propagate.

Still, the payoff for a microwave-based weather satellite system is potentially huge, Johnson said.

He works with microwave sensors, called radiometers, that detect the roughness of the ocean surface -- the size of the waves. Roughness is an indicator of both wind direction and speed -- and, by extension -- weather conditions at the ocean surface.

The sensors are considered "passive" because they can only detect microwaves, not emit them. "Active" microwave sensors, more commonly known as microwave radar, can also detect the roughness of the ocean surface.

"Passive sensors are simpler devices, and have lower power consumption requirements," Johnson said. "Those are both important factors when designing a system to operate on a satellite."

Optical weather satellites can easily observe the ocean, but they can't see the surface of the water at night, and they can't see through clouds, either. In contrast, microwave sensors would allow for 24-hour satellite observation over all the earth's oceans.

Johnson and Zhang compared predictions of their model with microwave data taken by the WindRAD radiometer, a microwave sensor operated on a series of aircraft flights by the Jet Propulsion Laboratory (JPL) between 1994 and 2000. From the window of a NASA DC-8 aircraft, WindRAD took passive microwave measurements off the coast of northern California, while buoys deployed by the National Buoy Data Center (NDBC) measured the actual wind speed and wind direction below.

Johnson said that good models would be useful for the next generation of weather satellites currently under development for the National Polar-orbiting Operational Environmental Satellite System (NPOESS).

NPOESS is a tri-agency program shared by the National Oceanic and Atmospheric Administration, the Department of Defense, and NASA. Starting in 2008, the program plans to launch three new weather satellites, all featuring microwave sensors.

These satellites will be able to gauge wind speed and direction over patches of the ocean as small as 400 square kilometers, or about 150 square miles.

In their second IEEE Transactions paper, Johnson and Zhang describe how microwave sensors could be used to detect buried objects such as land mines. The two engineers created a computer model of the expected signal when a microwave sensor passes over a smooth object that is buried beneath a rough surface, such as soil. "Depending on the depth of the buried object and the material it's made of, we expect that we will obtain a unique microwave signal," Johnson said.

Soon he and Zhang will have a chance to test out their model. Through a grant from the Defense University Research Instrumentation Program (DURIP), Ohio State is purchasing a microwave radiometer. When the radiometer arrives on campus in August 2001, the engineers will begin experiments to detect buried objects such as land mines.

Both of Johnson's projects were sponsored by the Office of Naval Research. In addition, the land mine research was sponsored by the National Science Foundation, as well as a grant from the Multidisciplinary Research Program of the University Research Initiative (MURI) for humanitarian demining. The MURI initiative, in turn, is sponsored by the Office of the Secretary of Defense.