# UAV-Borne Profiling Radar for Forest Research
## 3 Field Test
### Ground and Canopy Information Extraction  
Since the Tomoradar was installed at a 0 degree incidence angle and the existence of forest gap, besides the canopy structure, the backscatter from underneath layers such as trunk, bush and ground can also be detected with Tomoradar. This procedure is critical for future measurements in areas in which a DEM is unavailable or the DEM resolution is inadequate. Even in presence of dense canopy, the ground reflection signal is strong and can be easily recognized. In this section, two test cases are investigated: the first case is that the helicopter turned when Tomoradar surveyed a mature forest with co-polarization mode in Figure 9; the second case is that Tomoradar data was used to process co-polarization and cross-polarization signals, Figure 10.  
It can be observed from the Normal Flight segment in Figure 9. The blue solid line is the ground level reference extracted from DEM coincides with the ground reflection. However, when the helicopter turns, the ground reflection becomes blurred together with the forest canopy in Figure 9. When the helicopter was turning, the incidence angle changed from 0 degree to tens of degrees. Therefore, Tomoradar sees the increased attenuation of the forest and the number of multiple reflections are increased since there do not exist any canopy gaps as in nadir looking measurements. The bright signal in nadir measurements is due to canopy gaps, and they disappear between 10-30 degrees based on earlier FGI measurements. This topic is further studied in future studies. Figure 8: Range-calibrated (**upper**) and non-calibrated (**lower**) stand profiles created during the test; the red lines represent the ground level acquired from DEM. Figure 9: Ground level and stand profile from co-polarization (HH) while the helicopter is banking and during level flight. Figure 10 presents a case in which the co-polarization (HH) measurement and cross-polarization (HV) measurement of a stripe were simultaneously collected during the Tomoradar EVO field test. The following phenomena can be observed. * The general reflection of co-polarization is stronger than cross-polarization. * The Tomoradar can collect the profile information from tree canopy top to ground level through the branch and trunk both for co-polarization channel and cross-polarization channel. * Strong cross-polarization could be observed in the top level tree canopy and ground level but not in branch and trunk section beneath the tree canopy since cross-polarization is caused by needles and small ground level features. * Strong co-polarization could be observed also from tree canopy and ground level. However, the penetration depth for the co-polarization is deeper than cross-polarization mode since also branches are giving backscattering but not that much of cross-polarization. * The ground backscatter is determined by various factors, such as: \(\circ\) Canopy gap: the backscatter is the strongest with larger canopy gaps, such as in sites B and E in both co-polarization and cross-polarization channels; \(\circ\) Canopy density: dense canopy may totally block the radiation from ground floor, such as in sites A, C and D. High canopy density results in high attenuation of ground signal. Dense forests having large number of needles seem to be more noticeable from the cross-polarized signal. From the aforementioned observations, we can generally conclude that the microwave radiation propagation model in boreal forest is more complex than a homogeneous media. It is a function of canopy gap and canopy density resulting in attenuation. Penetration capability of Ku band profile radar is considerable stronger than its SAR counterpart, it can penetrate the boreal forest throughout from canopy to ground level, and collect the profile information both in cross-polarization and co-polarization modes. The structure under the tree canopy are more readily detected by co-polarization channel while the dense canopy structure are more noticeable in cross-polarization.