# 2D Phase-Based RFID Localization for On-Site Landslide Monitoring
## 1 Introduction  
Ground deformation monitoring with high resolution both in space and time remains a challenge due to the high cost of existing solutions, and to environmental limitations, such as meteorological phenomena, rough terrain or dense vegetation. Amongst several remote sensing methods [1; 2], surface monitoring of large landslides can be typically performed through interferometric synthetic aperture radar (InSAR), either by space-borne measurements [3; 4] or using ground-based stations [5; 6; 7; 8; 9]. Despite the high space resolution of these methods, the station cost remains high and the time resolution can be multiple days in the case of satellite remote sensing. More localized techniques, such as GPS [10; 11; 12; 13] and radiofrequency-transponders [14; 15], show higher time resolution, but also require on-board power sources which greatly increase initial cost and maintenance.  
In this context, radio-frequency identification (RFID) has shown increasing potential for earth science applications [16; 17]. Amongst other applications, it is foreseen as a promising alternative for landslide and civil engineering structure deformation monitoring [18] due to its low cost relative to other solutions, and because it works under rain, snow and vegetation cover conditions [19; 20]. It can thus be used as a tool for landslide early-warning [21], forecasting or long-term monitoring [22]. A wide range of solutions exist for tag localization using RFID [23; 24], with various possibilities both in measured quantity and in terms of the measuring scheme.  
The quantities most used for localization are the received signal strength and the back-scattered phase of arrival. Signal-strength-based methods have been widely used for tag localization [25; 26; 27; 28; 29]. However, phase-based methods have shown better precision and reliability in recent years [30; 31; 32], primarily because they are less sensitive to environmentalvariations and because the phase of the signal varies more rapidly with distance than the received signal strength.  
Phase-based localization is divided into multiple schemes, which are extensively presented elsewhere [33; 34; 35; 36]. These schemes generally rely on either multistatic stationary antennas and different carrier frequencies [30; 37; 38], or on a moving antenna with a known trajectory (e.g., the synthetic aperture radar technique) [39; 40; 41; 42; 43]. This paper focuses on a monostatic multi-antenna time-domain phase difference (TD-PD)-inspired scheme, as TD-PD has shown the best results for measuring relative displacements outdoors [18], with a precision of about 1 cm over long time periods for 1D displacement tracking. To date, RFID systems deployed to monitor moving ground only provide one-dimensional distance information and are subject to phase unwrapping issues that could be solved by using multiple antennas. In this article, we test the stationary configuration for 2D RFID tag localization using a set of four antennas in a TD-PD relative localization approach, and also discuss 3D localization perspectives. To the best of our knowledge, this is the first attempt at 2D-localization of RFID tags in an outdoor scenario, using a monostatic, monofrequency multi-antenna setup.  
In the following section, we present the instrumentation of the experimental site and the methodology for data acquisition and processing. Section 4 provides theoretical background and experimental validation of the RFID measurement error in order to decide on ideal antenna positioning by optimizing the localization accuracy and phase ambiguity. Section 5 reports on an example of 12 months of surface deformation monitoring on the slow-moving Harmaliere landslide.