# mGEODAR--A Mobile Radar System for Detection and Monitoring of Gravitational Mass-Movements
## 4 Discussion  
Wide bandwidth radars for high-resolution gravitational mass-movement monitoring like the mGEODAR, and also its predecessor EEEDAR, are challenging to construct as matching of wideband components and in-band spurious signals limit their performance. For example, the optimal frequency settings for the mGEODAR radar are slightly different from what to expect from the filter responses in Figure 2A. And some unwanted components in the frequency spectrum of the deramp signal appear  
Figure 6: A car departs from the radar location at range 0 m. The colouring is the intensity of the moving target identification for (**A**) the high resolution research mode and (**B**) the low resolution detection mode. The insets show a zoom of approximate similar range and time extent, thus the difference in resolution of both measuring modes can be seen. Note, the panels show two different data sets as both modes cannot be measured simultaneously. as ghost targets (Figure 4B). Thus, the performance of the complete system and the interplay of the components need to be verified and optimized. Usually optimizations of high-frequency systems require costly laboratory test equipment. However, due to the integrated design of the radar boards, they cannot be used in intermediate stages between most of the components. Thus, we directly used the capabilities of the radar itself to find frequency settings that optimize the received signal strength. This approach has the advantage of including the whole signal chain in the optimization process. A similar optimization should be performed for each installation locality as for example the antenna cross-talk depends on the installation site and its geometry. Instead of a loop cable, such measurements could be done with active targets at several distances in the radar field of view. By knowing the geometric arrangement of the radar installation, potential cable lengths and the location of the active target with GPS measurements, the signal strength optimization procedure can done before any experiment. An active target may simply consist of two antennas with an amplifying component in between [19]. These targets prove also useful for monitoring the radar performance throughout longer time spans like the complete winter season. For the current mGEODAR system we find an optimal LO frequency setting for the 400 MHz research mode at \(f_{lo}=8.12\) GHz. Then the chirp signal \(f_{ch}\) that is generated in the first Nyquist image of the DDS covers frequencies between 2.08 GHz and 2.48 GHz. The signal strength of a super-Nyquist output generally follows a _sinc_-function although gain flatness is desired [13]. We postulate using a slightly higher frequency for the chirp signal and a slightly lower frequency for the LO would be better and could avoid more low frequency attenuation of the HFCN-2275 (Figure 2A). However, this rejection filter actually equalizes the gain and thus compensates the decrease in signal strength towards the higher frequencies of the chirp signal. For the 30 MHz bandwidth detection mode the gain flatness is of lower importance, however, single spurs may have stronger effects on the signal degradation. We find an optimal LO frequency setting at \(f_{lo}=8.12\) GHz also for the detection mode. Another possibility to increase the gain flatness would be to change the clock frequency \(f_{s}\) of the DDS. A slight increase of \(f_{s}\) towards 3.9 GHz may already shift the chirp signal \(f_{ch}\) into a flatter region. Even though over-clocking of the DDS chip has been successfully tested [20], the chip may overheat more quickly in continuous monitoring usage and may cause erratic behaviour. The current radar system can still improve on the pulse coherency in the research mode as the standard deviation between 100 pulses is relative high (Figure 4A). As a result, the background noise level of MTI image in the range-time-diagram (Figure 6A) sometimes shows strong speckles.