# mGEODAR--A Mobile Radar System for Detection and Monitoring of Gravitational Mass-Movements
## 3 Measurements and Results
### Signal Strength Optimization  
To find optimal settings for the chirp signal \(f_{ch}\) and the local oscillator \(f_{lo}\), we vary the settings of the DDS and the local oscillator according to the desired transmit frequencies (\(f_{tx}=f_{ch}+f_{lo}\)). As mentioned before, the frequency band for our research mode is 10.2 GHz to 10.6 GHz and for the detection mode is 10.41 GHz to 10.44 GHz. The measurement for signal strength optimizations were done with a 90 m long coaxial cable (S_04272_B from Huber&Suhner). This cable has a signal velocity reduction factor of 0.82, so the deramp signal peak is expected at around 55 m range in the radar data (Equation (1) for two-way travel). The cable has an attenuation of 1.073 dB/m at 10 GHz giving around 96 dB attenuation for the complete cable. We include the external 5 W power amplifier into the loop measurement to compensate for the high cable attenuation. The amplifier and attenuator on the transmitter board were set to not exceed the required input signal for the power amplifier of 10 dB, namely, the TX amplifier set to 27.9 dB and the TX attenuator to \(-\)4.5 dB. On the receiver board, the two variable amplifiers are set to 16 dB and the attenuator to 0 dB. All stages of the baseband filter were set to their high gain settings. The most limiting factor for the usable signal is the filtering of the chirp signal (Figure 2A) which suppresses frequency components beyond the first Nyquist image of the DDS fundamental frequencies. The width of this first image ranges conservatively from 1.9 GHz to 3.3 GHz. However, the output signal strength of the DDS follow a _sinc_-function, thus we expect decrease of signal with an increase of the chirp frequency \(f_{ch}\)[11]. We observe some spurious signals in the deramp signal. The instantaneous DDS frequency of these spurious is exactly a third of the set \(f_{lo}\) frequency, and they correspond to expected spurs from the mixer. Thus, for the mGEODAR frequency range, the spurs only exist in-band for \(f_{lo}\) between 7.68 GHz and 7.92 GHz, otherwise \(f_{lo}/3\) is outside the frequency of the deramp signal. We optimize the signal intensities of both measuring modes with the loop cable. For each \(f_{lo}\) we took a separate 1 s loop cable measurement with all settings \(f_{lo}\), \(f_{ch}\) and chirp rate \(\alpha=40\) MHz/s controlled with the Venom micro controller. All the test measurements are done for both measuring modes individually and indicated with subscripts 400 and 30 for the research and detection mode, respectively. If a spurious signal lies within the deramp signal, a custom cosine-tapered window is applied to the time-domain deramp signal to notch out the spike prior to the spectral analysis. We aim to maximize the magnitude of the Fourier transformed deramp signal by varying \(f_{lo}\) at the range of the cable length. For example, Figure 4B,D show the frequency spectrum of the research and detection mode, respectively. The peak at the cable length is clearly visible for these measurements at \(f_{lo}=8.12\) GHz, but its strength varies for other frequency settings. Figure 5A shows the intensity \(P\) at the apparent cable length (55 m radar range) and the error bars of \(P\) extend to each side for one standard deviation that is derived from approximately 100 pulses in the 1 s measurement. The standard deviation is derived with Equation (2) in [PERSON] et al. (2018) for conversion into decibel scale. Additionally, the intensity \(B\) of the background signal for both measuring modes is shown. There are some unwanted spectral components at for example 470 m and 580 m (see Figure 4B,D). We define the background signal \(B\) in a conservative way, to take these unwanted spectral components into account. We define the background signal \(B\) as the largest of those peaks. And again a standard deviation is used for error bars of \(B\), but this time refers to the variation of the background spectrum across range.