Method of and a system for surveillance of an environment utilising electromagnetic waves

A system and method for surveillance of a physical environment. May utilise already installed equipment, such as WLAN for surveillance purposes. This makes the system cost effective to implement. Transmits a first and a second signal. The signals are received by receivers, and the received signals are compared in order to detect a difference in the signals, the difference being caused by a physical change present in the signal path between transmitter(s) and receiver(s). Does not require high spectral purity of the transmitted signals or heavy computations in order to establish that a change has occurred. Suitable for intruder alarm purposes.