Global Positioning System technology can provide position information with accuracy to a few millimetres in near real-time. Thanks to this level of precision, the movement of structures can be monitored. The falling tendency of GPS receiver pricing and their miniaturisation, along with their modest power requirements spurred a research and development project at SUPSI which resulted in a prototype system, deemed interesting for a number of applications. It is shown that using carrier phase differential GPS, a network of receiver modules can be installed in order to perform monitoring and surveillance operations for small movements. Typical applications for this type of sensor network include monitoring of structures such as buildings, dams, bridges, as well as measuring the movement of landslides and rock formations. The system consists of a number of small receivers installed on the object to be monitored. A radio-linked base station provides for data collection, post-processing, and monitoring for correct operation of the network. Ancillary sensors may be added to the single receiver units and their measurements synchronized to the positional measurements. The base station may be programmed to initiate a warning or alarm action when absolute position differences or the velocity of movement exceed a pre-set limit.