Abstract
Microwave Radar is an attractive solution for forest mapping and inventories because microwave signals penetrates into the forest canopy and the backscattering signal can provide information regarding the whole forest structure. Satellite-borne and airborne imaging radars have been used in forest resources mapping for many decades. However, their accuracy with respect to the main forest inventory attributes substantially varies depending on the wavelength and techniques used in the estimation. Systems providing canopy backscatter as a function of canopy height are, practically speaking, missing. Therefore, there is a need for a radar system that would enable the scientific community to better understand the radar backscatter response from the forest canopy. Consequently, we undertook a research study to develop an unmanned aerial vehicle (UAV)-borne profiling (i.e., waveform) radar that could be used to improve the understanding of the radar backscatter response for forestry mapping and inventories. A frequency modulation continuous waveform (FMCW) profiling radar, termed FGI-Tomoradar, was introduced, designed and tested. One goal is the total weight of the whole system is less than 7 kg, including the radar system and georeferencing system, with centimetre-level positioning accuracy. Achieving this weight goal would enable the FGI-Tomoradar system to be installed on the Mini-UAV platform. The prototype system had all four linear polarization measuring capabilities, with bistatic configuration in Ku-band. In system performance tests in this study, FGI-Tomoradar was mounted on a manned helicopter together with a Riegl VQ-480-U laser scanner and tested in several flight campaigns performed at the Evo site, Finland. Airborne laser scanning data was simultaneously collected to investigate the differences and similarities of the outputs for the same target area for better understanding the penetration of the microwave signal into the forest canopy. Preliminary analysis confirmed that the profiling radar measures a clear signal from the canopy structure and has substantial potential to improve our understanding of radar forest mapping using the UAV platform.
Highlights
Radar is an active sensor that transmits radio or microwave radiation and record echoes from the target area
In this research, (1) we investigated whether unmanned aerial vehicle (UAV)-borne frequency modulation continuous waveform (FMCW) profile radar is feasible with latest developed components and state-of-the-art devices; (2) we carried out the range calibration based on ground test to testify the system range linearity; (3) we confirmed the profile radar can penetrate boreal forest from tree canopy top to ground level through branches and trunks, and extracted the ground and vertical structure information; (4) we assured that, by combining topography information collected by laser scanner and density information unveiled by profile information archived by Tomoradar, a more composite forest model can be generated to assist the understanding of how radio frequency (RF) signal propagates in boreal forest
Usingaahelicopter helicopterfield field test, the developed system was analysed against Riegl VQ-480-U laser scanner
Summary
Radar is an active sensor that transmits radio or microwave radiation and record echoes from the target area. The SAR intensity is the strength of the backscattered signal comparing to the transmitted signal, and it depends on the system wavelength and the polarization and on the target parameters. The target parameters, such as forest biomass, canopy density and tree type, must be taken into account and calibrated in advance because from a physics viewpoint the propagation of a radio frequency (RF) signal is influenced by the objects or the materials through which it passes
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