# UAV-Borne Profiling Radar for Forest Research
###### 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. A 2017 9 58 2017 9  
biomass). Therefore, microwave radar is an attractive system for forest mapping and forest resources estimating. Satellite-borne and airborne imaging radars, primarily synthetic aperture radars (SAR), have been studied and used in forest mapping for many decades [1; 2; 3; 4; 5]. Due to its all-weather imaging capability, satellite SAR enables wide-area mapping, which might not be possible to utilize other remote sensing techniques. The primary technique used in forest mapping is based on SAR intensity information. The SAR intensity is the strength of the backscattered signal comparing to the transmitted signal, and it depends not only on the system wavelength and the polarization but also 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. The characteristics of different radar wavelengths (i.e., microwave bands) with respect to the use of forest biomass from SAR data are discussed in [PERSON] et al. [6]. According to [PERSON] et al. [6], the SAR backscattering intensity of longer wavelengths (e.g., the L and P bands, which have wavelengths of about 20 and 60 cm, respectively) is more effective than that of shorter microwaves (e.g., the X and C bands, which have wavelengths of 2-6 cm) in forest biomass estimation. Using short microwaves, the scattering mainly occurs at the top parts of the forest canopy, whereas in long microwaves the signal is more dependent on the forest tree trunks. Thus, the tree trunk and the forest stem volume can be better mapped with a longer wavelength microwave signal.