Early identification of bushfires using radar to detect burning plant ionisation
Creators: name: Masoumi, Sarah
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320601
subject: Gene and molecular therapy

description: Forest fires are one of the most serious and common types of disasters. They endanger human life and property, agricultural and forest productivity, animals, biodiversity, air and water quality, cultural heritage, and infrastructure. Early detection, correct location and rapid notification to the firefighting units are essential to enable a quick response to control and minimise the impact. This work aimed to investigate the unexplored technique of detecting fire utilising radio wave incoherent scattering from the ionised electron density of combusting plant material. The conception of this project was initiated in response to the 2009 Black Saturday fires. The project's driving force was to discover alternative technologies that could enhance existing optical and infrared detection systems which suffer significantly in varying environmental conditions such as wind-tossed trees, cloud shadows, reflections, and human activity.

Overheated forest material in the combustion zone of a forest fire induces a certain degree of plant salt ionisation, resulting in a weakly ionised gaseous medium with a natural plasma frequency in the radio frequency (RF) range. The formation of plasmas and ionised gases causes electromagnetic (EM) wave scattering at frequencies lower than the plasma frequency. As a result, a radar system operating at the fire's plasma frequency will experience higher signal reflection than the surrounding environment, allowing the fire to be detected. A radar system working within this frequency range possess the capability to detecting spot fires igniting well ahead of the fire front. All current fire detection methods such as IR, visual or weather radar are unable to provide this type of fire detection and analysis.

To prove the concept, a series of actual fire experiments were conducted in both laboratory and open-space environments with two different purposes. The open space experiment was carried out to investigate the formation of plasma states in the fire's combustion zone, whereas the laboratory setup was used to measure the reflection from fire. The open space experiment was an extension of the free-space method for characterising eucalyptus litter fire at frequencies ranging from 5 to 40 GHz, using the measured transmission phase shift of a propagating signal. A 72 cm × 47 cm rectangular brick area was prepared at ground level, with weighed eucalyptus ground litter biomass used as fuel for the fire. The measured phase shift varied from 12.2 to ‒ 166.7° at 5.35 and 39.74 GHz, resulting in a real part of the fire's relative permittivity ranged between 0.877 and 1, and an imaginary part ranging between 0.02 and 0.09.  The relative permittivity of less than one confirms the formation of a plasma in the combustion zone of the fire and is in agreement with previous studies.
 
The laboratory setup was then designed to investigate this plasma using a calibrated transmission line to identify the electromagnetic plasma frequency of forest fire. A plasma frequency of approximately 500 MHz has been identified by burning 4 grams and 6 grams of dry eucalyptus leaves, and the reflection from the burning leaves was approximately 20 dB higher than the reflection of the background environment below the plasma frequency. The plasma frequency was found to be stable as long as the biomass density was constant. The obtained results strongly suggest that such a model can be used to detect forest fires effectively.

The measurements confirm that there is a plasma frequency and reflection of the electromagnetic radiation by the plasma. This is a valuable finding as it enables the assignment of a specific (or range) plasma frequency to individual forest areas versus temperature. As a result, radar systems can be designed below the plasma frequency for reliable detection of forest fires in real-time from far distances. The measurements in this work were carried out in a controlled lab environment. In real fire (uncontrolled), the plasma frequency can vary when the temperature and vegetation type change. Therefore, a range of plasma frequencies should be considered to account for the change in temperature and vegetation type.