# Drone SAR Imaging for Monitoring an Active Landslide Adjacent to the M25 at Flint Hall Farm
## 3 Materials and Methods
### Hardware Development
#### 3.2.2 Software Defined Radio/Radar (SDR)  
Software Defined Radio is a Radio Frequency (RF) communication system that implements Digital Signal Processing (DSP) components such as modulators, mixers, filters, and amplifiers through software rather than hardware. Hardware implementation is the traditional approach to radio communications; however, these are typically highly complex systems to design, implement and adapt if necessary. Software implementation through embedded systems and dedicated computer software takes advantage of recent, rapid improvements in computational power. An embedded system is a computer system with a dedicated function, comprising a computer processor, memory and input/output (I/O) devices. The ability to adapt and change the software components on-the-fly makes SDR operations very flexible and thus ideal for these research and development purposes. Importantly, SDR hardware may be used to implement radar functionality, since this falls within the RF spectrum. The use of the aforementioned SDR to implement radar is more specifically referred to as Software Defined Radar (SDR). For clarification purposes, the acronym SDR herein refers to the latter, Software Defined Radar, since all operations of Software Defined Radio in this research are for radar implementation.  
The SDR utilised in the drone radar payload is the Ettus Universal Software Radio Peripheral (USRP) E312 [32]. Ettus Research is owned by National Instruments (NI) and located in Mountain View, CA, USA. The E312 features an AD9361 transceiver from Analog Devices, which provides 56 MHz of instantaneous bandwidth, a 10 MS/s sample rate, and a wide frequency range of operation from 70 MHz to 6 GHz; the latter being compatible with the custom antenna centre frequency of 5.4 GHz, described later in Section 3.2.3. Regarding the system architecture, the baseband processor utilises the Xilinx Zynq 7020 System-on-a-Chip (SoC) for accelerated Field Programmable Gate Array (FPGA) computations, with 512 MB Double Data Rate 3 (DDR3) Random-Access Memory (RAM) dedicated to the FPGA logic. Importantly, stand-alone operation is enabled by an ARM Cortex A9 866 MHz dual-core Central Processing Unit (CPU) with 1 GB DDR3 RAM; this A-class processor provides application cores for performance intensive systems. Transmission (Tx) and reception (Rx) filter banks suppress harmonics and reduce out-of-band signal interference, respectively.  
Regarding SwaP-C, the E312 hardware is designed for mobile field deployment. The form factor is compact at 133 \(\times\) 68 \(\times\) 32 mm and is lightweight at 0.45 kg. Power is providedby an internal and rechargeable 3200 mAh Lithium-ion (Li-ion) battery for mobile operations up to two hours, depending on the centre frequency, sample rate and maximum gain settings. The SDR is not required to be completely weatherproof since it is mounted to the underside of the drone, thus shielding it from precipitation and wind effects. Furthermore, the DJI Matrice 600 Pro does not have an Ingress Protection (IP) rating and has a maximum wind resistance of 8 m/s (17.90 mph) [33]; therefore, the drone is not flown in precipitation or heavy wind. Nevertheless, the material properties of the E312 brushed-metal exterior make it durable, rugged and not easily damaged in challenging fieldwork conditions.