Sensor could map global weather and land mines

Microwave technology under development at Ohio State University may improve global weather mapping and even detect buried 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 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.

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 computer 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.

Johnson works with microwave sensors (radiometers) that detect the roughness of the ocean surface — the size of the waves. The sensors are considered ‘passive’ because they can only detect microwaves but not emit them. ‘Active’ microwave sensors, more commonly known as microwave radar, can also detect the roughness of the ocean surface.

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. 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 US National Buoy Data Centre (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.

According to a statement, these satellites will be able to gauge wind speed and direction over patches of the ocean as small as 400 square kilometres.

In their second paper, Johnson and Zhang describe how microwave sensors could be used to detect buried objects such as land mines.

The 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,’ concluded Johnson.