Microwave sensors help fight wildfires from the air

In the last decade the number and intensity of forest fires seems to have been on the increase around the globe, with massive and devastating wildfires in California, Greece, Russia and Australia. The best tool for fighting these intense wildfires is accurate and timely information. Traditional airborne infrared cameras have long been a vital device for mapping fire intensity though their limited ability to find the heart of a fire through thick smoke poses a major drawback. A new radiometric sensor that works in the microwave range can now pinpoint the heart of the wildfire, even when visibility is poor.

When wildfires break out in remote and thickly vegetated areas often they can only be contained from the air. In order to target the most intense areas of the fire, firefighting planes need to be given precise information. Infrared (IR) cameras have long been used to provide this information, since fire glows most intensely in the infrared range. But despite the high-resolution images these cameras produce, they cannot be used to find the source of a fire in very smoky conditions because the dust and smoke scatters the infrared rays.

Researchers from the Fraunhofer Institute for High Frequency Physics and Radar Techniques (Fraunhofer FHR) in Wachtberg, Germany have solved this problem by developing a radiometer that overcomes this limited visibility. The radiometric sensor works in the microwave range between 8 and 40 GHz. At these low frequencies, radiation is scattered far less by smoke and dust particles meaning that a much clearer map of the fire's intensity can be produced. The radiometric sensor also allows pockets of fire hidden behind thick foliage to be detected.

“Measurements we took during testing showed that the dimming effect was negligible at 22 GHz,” Nora von Wahl of Fraunhofer FHR said. “Particles of dust and smoke are practically transparent in the microwave range, but the radiation is still strong enough for the source of a fire to be detected. From a height of 109 yards (100m), we were able to locate fires measuring five meters by five meters in low visibility conditions. Along with the sensors themselves, the radiometer comprises a calibrating unit, a planar antenna array, and software for recording and visualizing the data.”

For the test flights, the scientist and her team mounted the microwave sensors on the underside of an unmanned airship belonging to the FernUniversität Hagen. The system’s resolution is determined by the antenna’s size, the frequency measured and the distance from the ground. Using an antenna measuring 7.9 inches (20 cm) at its outer rim, operated at a frequency of 22 GHz and from a height of 32.8 yards (30 m), the radiometer can resolve details on the ground to a grid accuracy of 2.8 yards (2.6 m).

“The radiometer doesn’t give us as much detail as an infrared camera, but if we increase the size of the antenna we can achieve higher resolution,” von Wahl said. “After a forest fire, it is often the case that new fires start underground. To find them, firefighters have to go in and dig around by hand. Our radiometer can detect fires below the top layer of earth.”

The radiometer, which measures 4.1 inches (105mm) by 5.9 inches (150mm) by 2.9 inches (73mm), is currently a prototype. Next the researchers hope to make the device even smaller, optimizing the antenna using microchips.

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