# UAV-Borne Profiling Radar for Forest Research
## 2 FGI-Tomoradar Development
### System Design  
A simplified block diagram of Tomoradar is presented in Figure 1. A number of Tomoradar's initial design parameters are listed in Table 2. The triangular 1 GHz sweep frequency is generated by a direct digital synthesis (DDS) device. The sweep signal is up-converted to a Ku-band frequency-modulated (FM) RF signal (14.0 GHz) by a mixer with a local oscillator (LO) output. The RF signal is then band-pass filtered and amplified. Then, the amplified signal is divided between the transmit antenna and the LO port of the mixer in the receiver chain using a directional coupler. The ortho-mode transducers (OMTs) of the transmit and receive antennas have two ports, one for horizontal and one for vertical polarization. The signal from the directional coupler can be directed to either OMT port, selected with a PIN diode switch. The PIN diode switch is controlled by the digitizer, which also receives a synchronization pulse from the DDS at the beginning of each frequency sweep.  
The antennas are parabolic dishes with 6 degrees beamwidth. The transmitted RF signal that has been backscattered from the target is collected by a second parabolic antenna, which has an OMT also. Two antennas are used to achieve high sensitivity and to reduce noise due to the low voltage standing wave ratio (VSWR) of the separated antennas. Two identical receiver chains are required to be connected with the OMT ports to acquire all four transmit-receive polarization combinations (i.e., VV, VH, HV, HH). The received signals are amplified with low noise RF amplifiers and then mixed with the RF transmit signal, resulting in IF signals. In the IF signal, the distance to the target is directly proportional to the signal frequency, while the backscattering coefficient determines the intensity (i.e., amplitude) of the signal. The IF signals are amplified by a cascade amplifier with distance correction functionality to compensate for decreasing signals strength when the range increases. Finally, the amplified signals are recorded by the digitizer. To produce a lower VSWR of the different ports and to reduce the noise floor, RF isolators are required between system components (Figure 1).  
A georeferencing system that consists of a tactical-grade IMU and a GNSS receiver records the position and attitude of the aerial platform. A TTL pulse from the DDS to the GNSS is used to synchronize the radar and the GNSS IMU data. Since the Tomoradar is rigidly installed on the airborne platform, the coordinates of the targets can be calculated. Assuming that the aerial platform speed is 10 m/s, the Tomoradar modulation frequency is 163 Hz, which corresponds to an along-track distance of 6.1 cm. This distance is substantially smaller than the antenna footprint on the ground (i.e., 10.51 m from 100 m altitude).  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Specified Value** \\ \hline Frequency & 14.0 GHz (Ku) \\ \hline Modulation type & FMCW (triangular) \\ \hline Sweep Frequency & 1000 MHz \\ \hline \multirow{4}{*}{Polarization modes} & HH, VV, HV, VH \\  & (H: horizontal polarization) \\  & (V: vertical polarization) \\  & (The first term corresponds to the polarization of the \\  & emitted radiation, the second term to the received radiation) \\ \hline Polarization isolation & \textgreater{}30 dB \\ \hline Antenna Aperture & 330 mm \\ \hline Incidence angle & 0 degree nadir with current installation \\ \hline Intermediate Frequency range & \textgreater{}20 K \\ \hline Dynamic Range & \textgreater{}50 dB \\ \hline IMU data rate & 200 Hz (helicopter)/125 Hz (UAV) \\ \hline \multirow{4}{*}{Attitude Accuracy} & Roll: 0.005 degree (helicopter)/0.015 degree (UAV) \\  & Pitch: 0.005 degree (helicopter)/0.015 degree (UAV) \\  & Heading: 0.008 degree (helicopter)/0.08 degree (UAV) \\ \hline Gyro Rate Bias & \textless{}1 degree/h (helicopter)/0.5 degree/h (UAV) \\ \hline Accelerometer Bias & \textless{}1.0\,\text{mG}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Technical Characteristics of Tomoradar.