A novel C-Band UAV-Radar for 3D characterisation of forest canopy backscatter profiles - Preliminary results

This work reports on a novel C-band monostatic UAV-radar system deployed over two forested wetlands in arctic Sweden, near to the Abisko research station. A Videodrone X4S drone acted as the carrying body, allowing programmable and repeatable flight paths. The radar system is multi-polarized (VV, VH, HV, HH), using one transmitter and optionally one or two receivers. The radar operates in a sawtooth FMCW mode, monotonically stepping in frequency across 5.2 GHz to 5.6 GHz. The choice of sweep time (1 to 8 ms) and number of data points (128 to 2048) are programmable and selected before a flight. The radar is triggered in flight manually from the ground using a Wi-Fi link, and which then repeatedly loops over a pre-set number of 10 s scans. Here, the choice of a drone speed of 5 m s-1 meant that each scan covered a 50 m flight line. There is a re-setting time of 0.3 s between scans. The wetlands are covered by a sparse forest, primarily of birch typically 3 to 7 m tall. We used the tomographic profiling (TP) scheme to collect high-resolution maps of the vertical scattering through the forest canopy. Such information is not available from the coarser satellite imagery, which provides no information on the vertical distribution of the backscatter, not even on the relative strengths of the ground and canopy returns. As the TP scheme has the antennas forward facing, only a narrow image transect beneath the flight path is collected. A synthetic aperture technique is used both to sharpen the real beam in the along track direction, and additionally steer it in angle. Thus, post-measurement, a single flight can be processed to capture the incidence angle response of the whole scene at a single incidence angle, selectable over a ~40 degree range. The results show how the forest backscatter response changes from one dominated by a ground return close to nadir viewing, to one dominated by the canopy above 20 degrees incidence angle. Comparisons and comment will also be provided of the differing responses with polarisation.