Microwave scanner sees fires through smoke

A new scanner could help firefighting planes find the source of a forest fire even when smoke makes visibility limited.

Researchers from a German Fraunhofer Institute have developed a microwave sensor that can help find the hottest part of a fire on occasions when traditional infrared (IR) scanners would be restricted by smoky conditions.

Dust particles scatter infrared radiation and dim the higher-frequency signals produced by fires. But the new sensor detects microwaves at lower frequencies between 8 and 40 Ghz.

‘Measurements we took during testing showed that the dimming effect was negligible at 22 GHz,’ said Nora von Wahl of the Fraunhofer Institute for High Frequency Physics and Radar Techniques in Wachtberg

‘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 100 metres, we were able to locate fires measuring five metres by five metres in low visibility conditions.’

The sensor allowed the researchers to locate pockets of fire even behind a curtain of foliage. ‘After a forest fire, it is often the case that new fires start underground,’ said von Wahl.

‘To find them, firefighters have to go in and dig around by hand. Our radiometer can detect fires below the top layer of earth.

She added that the system is principally suited to fire protection with firefighting planes, but could also be used to monitor industrial sites. This would, for example, enable early detection of smoldering fires at waste incineration plants.

For the test flights, the researchers mounted the microwave sensors on the underside of an unmanned airship belonging to the FernUniversität (Distance University) of Hagen.

The system’s resolution depends on the size of the antenna, the frequency and the distance from the ground.

Using an antenna measuring 20cm at its outer rim, operated at a frequency of 22 GHz from a height of 30 metres, the radiometer can resolve details on the ground to a grid accuracy of 2.6 metres.

‘Along with the sensors themselves, the radiometer comprises a calibrating unit, a planar antenna array, and software for recording and visualizing the data,’ said von Wahl.

‘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.’

The radiometer, which measures 105 by 150 by 73 millimetres, is currently a prototype. The scientists’ aim is to make the device even smaller, and they also want to optimize the antenna. Future designs will be based on microchips.