Which sensors can see through smoke

Sensor detects hidden sources of fire

Forest fires usually spread rapidly and uncontrollably. It is therefore important to discover the fires as early as possible. Researchers have now developed a new radiometric sensor that can identify the eruption points even when visibility is limited, for example through smoke or foliage.


The number and extent of forest fires has increased dramatically over the past few decades. Unforgotten are television pictures of the infernos of flames that took place in Russia in the summer,

Australia and California devastated land for miles. Many regions in Germany are also affected by climate change - Brandenburg, for example, is one of the most endangered areas in Europe. Often the fires can only be contained from the air.

Infrared cameras ineffective due to smoke

In order to be able to fight fire sources in a targeted manner, fire-fighting aircraft must be given precise instructions. A tried and tested aid for this are infrared cameras (IR), as fire is most intense in the infrared range. The IR cameras measure the thermal radiation and can thus localize the source of the fire. They also deliver high-resolution images. However, if there is a lot of smoke, these image recorders cannot find the breakout points, as infrared rays are too much attenuated by particles of dust and smoke.

The researchers at the Fraunhofer Institute for High Frequency Physics and Radar Technology FHR in Wachtberg have developed a solution to the problem: a radiometer that can monitor fires even when visibility is restricted: The radiometric sensor works in the microwave range between 8 and 40 GigaHertz. At these low frequencies, the scattering of the rays by dust particles is significantly less than at the high IR frequencies.

Radiometer makes smoke more or less transparent

“In our test measurements at 22 GHz, the attenuation was negligible. Particles from dust and smoke are more or less transparent in the microwave range, ”explains Nora von Wahl, a scientist at the FHR. “Nevertheless, the radiation output is high enough to detect fire pockets. From a height of 100 meters and with limited visibility, we were able to detect a fire with an area of ​​five by five meters. "

For the test flights, the researcher and her team mounted the microwave sensor on the underside of an unmanned airship from the FernUniversität Hagen. "In addition to the sensors, the radiometer also includes a calibration unit, a planar group antenna and software to record and visualize data," says the expert. The resolution of the system is determined by the opening angle of the antenna and thus depends on the antenna size, frequency and distance to the ground. With an antenna size of 20 centimeters edge length, a frequency of 22 GHz and at a height of 30 meters, the radiometer resolved square cells measuring 2.6 meters. “We cannot achieve the level of detail of infrared cameras with the radiometer. We enlarge the antenna and can thereby increase the resolution, ”says the researcher.

Burn nests even through foliage

With the radiometric sensor, the scientists are even able to determine fire pockets through foliage. And: “After a forest fire, new fires often ignite

the earth. To discover these, firefighters used to dug the ground by hand with hooks. Our radiometer can detect outbreaks under the top layer of the earth, ”explains Nora von Wahl. The system can mainly be used for fire protection with fire-fighting aircraft. It is also conceivable to monitor industrial systems with the radiometer, for example to locate smoldering fires in waste incineration plants at an early stage.

The radiometer, measuring 105 by 150 by 73 millimeters, is available as a prototype. The aim of the scientists is to make the device even smaller. The engineers also want to optimize the antenna. Future models should also be characterized by their chip-based construction.

(Fraunhofer-Gesellschaft, January 4th, 2011 - NPO)

January 4, 2011