# mGEODAR--A Mobile Radar System for Detection and Monitoring of Gravitational Mass-Movements
## 2 Materials and Methods
### Radar Hardware  
An overview of the mGEODAR hardware is shown in Figure 1. A key component in the signal generation chain is the DDS (AD9914, no. 1) on the transmitter board, which is clocked at \(f_{s}=3.48\) GHz with a variable oscillator (AD4351, no. 2). The DDS divides this clock signal by 24 and allows for integer arithmetic for all the frequency calculations. The DDS generates a linear frequency ramp, hereafter called chirp signal. The chirp signal contains the fundamental frequencies from 0 to \(f_{s}/2\), but also contains additional higher order harmonics called Nyquist-images [11]. A band-pass filter (Figure 2A) is used to select the image that gives the desired frequency ramp \(f_{ch}\) of 400 MHz bandwidth at centre frequencies of around 2.4 GHz in the first image. This chirp signal \(f_{ch}\) is up-converted with a local oscillator \(f_{lo}\) (ADF5355, no. 3) to yield the desired transmit frequency \(f_{tx}\) of 10.2 GHz to 10.6 GHz (Figure 1, annot. C). Figure 1: Hardware schematic with high-frequency components, data acquisition and controlling devices of radar mGEODAR. Three printed circuit boards (PCB) boards host the radar hardware: The transmitter board is shown on top-right, the receiver board is on the bottom-left and the interface board for control of HF components is in the middle. The data acquisition, storage and processing devices are shown on the far left. Signal generator and oscillators are enumerated (no. 1–3). Annotation with light gray boxes and letters (annot. A–C) indicate areas with same frequency. The frequency responses of all filters in these boxes are drawn in Figure 2.  
The power amplifier (AM31-10.2-10.7-37-37) with 37 dB gain is in between the transmitter board and the transmit antenna. To keep the radiation pattern focused towards the avalanche track, the antenna specifications are chosen with a narrow azimuth angle and wider elevation angle. Two sector antennas (SA15-90-104V-D1 from Cobham) with 15 dB gain are used for transmit and receive purpose. Their beamwidth is around 85\({}^{\circ}\) azimuth and 7\({}^{\circ}\) elevation, but we mount the antenna with 90\({}^{\circ}\) rotation to capture a complete vertical slice of the flow path. On the receiver board, the received signal is first conditioned with a series of low-noise amplifiers and a cascade of high- and low-pass filters. Then the signal is down-converted with the local oscillator frequency \(f_{lo}\) and the corresponding chirp signal \(f_{ch}\), both taken from signal splitters on the transmitter board. The transmit and receive chain contain digitally-programmable amplifiers and attenuators to match the signal levels prior to all mixing stages. All the programmable components are controlled with a micro controller (VM2 from Venom Control Systems, Cambridge) using I2C and SPI serial communication protocols. The resulting low-frequency deramp signal is passed through a custom built variable gain analogue baseband filter. The baseband filter characteristics are described in Section 2.4. Finally, the deramp signal is recorded using a 16-bit analog-to-digital converter (USB-6361 from National Instruments) with a maximum single-channel sampling rate of 2 MS/s. Since a binary marker signal indicating the beginning of a pulse is required as well, the sample rate reduces to 0.5 MS/s for a two channel acquisition to ensure multiplexing between the channels. The data is stored on a miniature computer with an Intel i7 CPU, 16 GB RAM and SSD storage. All digital signal processing (Section 2.5) is executed remotely on this computer, so that large data need not be transferred but rather the results are send as image files. For remote control and transfer of processed data an Teltonika RUT240 UMTS modern is installed that features an external antenna port. The radar hardware settings, acquisition and processing software can be remotely accessed and controlled with a Telegram messenger Bot interface implemented with Python. Figure 2: The insertion loss frequency responses of the HF-filter in mGEODAR.