# UAV-Borne Profiling Radar for Forest Research
###### Abstract
Recently, SAR satellite systems with short wavelengths (i.e., the X band) have also been found powerful in forest resources mapping when using forest canopy height measurements rather than SAR intensity information. When X-band SAR elevation measurements obtained by using SAR stereo-radargrammetry [7; 8] or SAR interferometry [9; 10] are subtracted from the terrain elevation values known from other source, forest tree height can be estimated, and consequently, forest stem volume and biomass can be estimated. In X-band SAR satellite data, the backscattering signal per resolution cell is coherently averaged, and the phase centre of the echo is relatively close to the top of the forest canopy. Therefore, the elevation measurement is highly correlated with the tree height. A drawback is that the terrain elevation cannot be easily observed from the measurements. Therefore, a digital terrain model obtained from another source, typically airborne laser scanning (ALS), is required to assist in obtaining tree height estimates. However, it has also been shown that a short microwave radar signal penetrates deeply into the forest canopy, even to the ground level. Stand profiles that present tree height and density in forest using helicopter-borne C-band (5.4 GHz) microwave radar were first reported by [PERSON] et al. [11]. Subsequent studies on forest inventory using microwave radar data have been conducted in the past decades [12; 13; 14; 15; 16]. A helicopter-borne profiling radar system known as HUTSCAT was designed and demonstrated by [PERSON] et al. [14]. Subsequently, intensive studies were performed based on HUTSCAT measurement. Based on these results, the nadir-looking profiling radar response from boreal forest clearly not only contains a signal from the ground surface but also echoes above-ground tree structures, which implies that gathering tree height estimation together with canopy density information can be achieved using a single instrument. In practice, these systems are similar to waveform LiDARs, which have been used in laser scanning since the early 2000s, and obtain similar data. In the early 2000s, the primary challenge was to accurately determine the location of the radar antenna and its orientation. Therefore, the problem of accurately locating the target remained unidentified. When airborne LiDARs emerged, the development of profiling radar did not continue. However, a number of studies have focused on unmanned air vehicle (UAV)-based radar systems, such as [PERSON] et al. [17] and [PERSON] et al. [18], in which the investigated radar technology was SAR rather than FMCW profile radar. A 3D imaging version of radar, as proposed, e.g., by [PERSON] [19], was not realized in operative airborne remote sensing since airborne laser scanning turned out to be a more cost-effective technique for forest inventory. However, profiling radar was compared with other remote sensing techniques for forestry with respect to information content by [PERSON] et al. [20]. Laser scanning was previously found to be superior to forest inventory by [PERSON] et al. [21]. Since the 1990s, direct positioning and orientation technology has substantially improved. However, in the scientific community, a need remains to better understand the radar response from forest structures. Therefore, there is a demand for a profiling radar system equipped with state-of-the-art positioning technology, which would assist scientists to more comprehensively understand the radar and SAR response from forest and possibly develop more accurate methods of satellite SAR-based forest resource mapping. The goals of this study were to design, construct, and test a light-weight short wavelength profiling radar system capable of acquiring highly accurate vertical intensity profiles from forest using both a helicopter and a UAV platform, as well as to compare these vertical intensity profiles with other remote sensing data from the same test sites. In general, we tested the prototype profile radar at the EVO test site in this research [22].