InSAR often demands the installation of artificial reflectors, usually represented by Passive Corner Reflectors (PCR). This occurs when in the imaged areas the coherence is low, and the density of Persistent Scatterers (PS) is scarce or even non-existent. For missions based on C-band sensor as Sentinel-1, PCRs capable of providing a high phase accuracy are cumbersome, and heavy, and the installation is difficult and costly. For this reason, the use of Active Reflectors (AR), compact and smaller with respect to PCRs, can be a valid alternative especially in mountain vegetated or snow-covered areas, or glaciers. The use of these devices has not yet largely spread due to their high sensitivity over long interval to annual/seasonal temperature range of variation which reduces their reliability. This work focuses on the analysis of the performance of a C band low-cost AR tested in a real field campaign based on the interferometric processing of Sentinel-1 images aimed at monitoring a forested slope, located in Andorra, partially collapsed some years ago. Phase stability is investigated using data acquired during an experimental campaign carried out in where also several PCRs have been installed. The experimental tests confirm that the device is capable to provide a millimetric accuracy over a significant temporal interval and can be used as reference point for a network of PCR installed in instable areas.