# Wireless Sensor Network-Based Rockfall and Landslide Monitoring Systems: A Review
## 3 Rockfall and Landslide Monitoring Based on WSNs
### WSN-Based Monitoring Systems
The nodes were equipped with an accelerometer for monitoring dynamic activity and variations in the inclination of the mounting barrier, and all the units communicated using a 433 MHz ISM radio module. The elements were battery-powered, and solar harvesting was used to achieve energetic sufficiency. A unit in the network could be configured as a bridge or repeater while retaining the sensing capabilities. A gateway towards the Internet was always listening for transmissions of accelerometric data and forwarded the received information using GSM. All the systems were described at the hardware and functionality level, and real scenario test results were reported. One of the main difficulties that the authors faced was the poor radio coverage of the zone where the system was developed. To this extent, a mixed mesh was used as network topology. A first approach to radio frequency identification (RFID)-based land movement monitoring was proposed in [108]. The authors applied the principle of phase unwrapping to observe the variation in the phase of a response signal to the interrogation of the tags. A base station was responsible for the interrogation, which produced an RFID response used to process the radial distance between the system origin and the single tag location. A dense network of tags, together with multiple frequency scanning were the key factors to enhance the accuracy of phase unwrapping methods [109]. The aforementioned study included the RFID data from four landslides that share the same measurement scheme. Several RFID tags were read by an acquisition system consisting of an interrogator and at least two reader antennas. The measurement rate depended on the available power and on the site. The autonomous stations relied on renewable energy and used a lower acquisition frequency with respect to power-grid supplied stations. The data reported a minimum of 100 phase readings per tag every day. Both the phase of arrival (PoA) and the received signal strength (RSSI) were measured. The scenario of a difficult tag reading due strong multipath interference or poor signal strength was one of the main difficulties of this work. To overcome this problem, the authors applied a data fusion approach, obtaining a data availability increase. Multi-technological monitoring approaches can be beneficial for risk assessment. In [110], the authors presented a mixed system located in the Montserrat Massif, Catalonia, Spain. A large rocky wall is located above an inhabited area, posing a threat to the population. Therefore, the researchers installed a WSN based on extensometers to monitor the movements of the rock masses. These devices enabled precision tracking, while larger area monitoring was carried out using radar scanners. This second method was employed both in the aforementioned location and in a second unpopulated section of the mountain chain. The information obtained with the local and radar sensing was integrated with temperature measurements to obtain the relationship between the displacement events and the environmental causes. One of the main difficulties that the authors encountered in the mentioned work was related to the atmospheric changes that resulted in a distortion of the interferometric phase measured using the SAR system. These effects are particularly strong in mountainous areas due to the rapid variability of weather conditions. In [111], the development of slope and river monitoring systems employing three types of LPWANs was reported. In particular, the installation included a total of 18 tilt sensors, 6 extensometers, an ultrasonic river water depth sensor, and a temperature and humidity sensor to carry out monitoring at selected observation sites in Japan (Nakayama, Aruse,Namikata, and Matsuyama). The network made use of combined technologies because two units used Sigfox, and LTE-M, while the rest communicated over LoRa technology. Batteries were used to power all the nodes, in conjunction with solar panels for the ultrasonic one to recharge the battery.