# mGEODAR--A Mobile Radar System for Detection and Monitoring of Gravitational Mass-Movements
## 1 Introduction  
Gravitational mass-movements are a major hazard to infrastructure, inhabitants and tourists in mountainous regions. For snow avalanches in particular, authorities deal with the hazard by temporary or technical mitigation measures, for example timed closures of roads and ski resorts or the protection of infrastructure with long term mitigation measures like dams and galleries. However, real-time detection of avalanches is advantageous as it allows safety measures to be implemented only when needed, for example closing a road using traffic lights when mass movements like snow avalanches are detected on the mountain flank above. Currently, such real-time detection uses seismic or infrasound sensors in close proximity to the avalanche flow or radar technology that can observe the avalanche path from a stand-off distance. The first avalanche radars were of continuous wave type that measured speed utilizing the Doppler effect and a high spatial resolution was achieved by pointing a narrow beam to certainlocations of the slope [1]. Modern radars for avalanche detection also utilize the Doppler effect, furthermore they send pulses that estimate the range by time of flight analysis. Those pulse-Doppler radars can achieve a range resolution on the 10 m scale, and within these range gates a complete velocity spectrum up to 100 m/s is measured [2]. While such pulse-Doppler radars are effective for movement detection, they lack high spatial resolution, limiting their application for detailed scientific studies on the flow dynamics. To increase the range resolution, a second generation of gravitational mass-movement radars were developed based on the FMCW (frequency-modulated continuous wave) principle. The first FMCW radar for snow movement was developed in 2010 and initially tested on a small-scale snow chute [3], and later installed in the Swiss full-scale avalanche test-site at Vallee de la Sionne [4]. This radar is known as GEODAR (GEOphysical flow dynamics raDAR) and achieves a range resolution of 0.75 m at around 100 Hz pulse repetition. Although FMCW radars can resolve a Doppler velocity directly, avalanches act as a distributed target, making the velocity processing complicated or even impossible. Nevertheless, due to the high spatial and temporal resolution of GEODAR, features in the flow can be tracked in time and space and their velocity indirectly estimated [5]. GEODAR is the main sensor of several scientific studies that significantly improve the current understanding of avalanche dynamics, namely the identification and characterization of flow regimes within the avalanche [6]. GEODAR has been constantly modified and improved in the following years [7]. A major design change was introduced with the mark IV system when the laboratory waveform generator to produce the FMCW triangular wave was exchanged with a more compact and reliable direct digital synthesiser (DDS) [8]. Another capability of GEODAR is the phased-array receiver to localize the avalanche in cross-range, however, the large width of the avalanche made the array processing difficult [9]. In the mark V system, the receiver array was replaced with several directional antennas, each pointing at a slightly different cross-range angle. A disadvantage of the modular GEODAR system is its large size and measurements have only ever been taken from the slope of the Vallee de la Sionne test-site. In this paper, we present a portable FMCW radar which works in a similar fashion to GEODAR mark IV but consists of only one receiver channel. The main idea is to keep the radar compact and reconfigurable so that measurement campaigns can be performed quickly in response to weather and snow conditions at various locations. Therefore the radar must not only be portable but also requires a battery-based power supply. Furthermore, the radar needs the possibility to trigger a measurement on its own, since an external signal of the avalanche start is usually not provided.