New microwave sensors for intrusion detection systems

Intrusion detection systems based on the utilisation of continuous wave (CW) radar and Doppler effect have several advantages over electrooptic sensors, namely, their ability to work in dark or smoky spaces, and under adverse weather conditions. However, they pose some practical problems such as: their inability to provide any range measure, nor to distinguish between different targets; high sensitivity to unexpected motion which leads to the need of precise delimitation of the area under surveillance, making its use difficult in certain applications; remarkable vulnerability to electronic countermeasures and high sensitivity to interference from other sensors. The advances in microwave and signal processing technologies permit the affordable use of spread-spectrum techniques, and particularly, the development of sensors based on frequency modulated continuous wave radar, which is the goal of this paper. These new sensors overcome the above mentioned limitations: since its operation no longer lies on the Doppler effect, they are insensitive to unexpected motion; as they are able to determine ranges and therefore separate different targets, they can illuminate a broader area than the one under vigilance, thus simplifying their installation. In addition, they provide the number of targets in the area of interest; due to the use of spread-spectrum signals, they offer higher robustness against interference, either deliberate or generated by other sensors. A prototype of this kind of sensors, operating at 22 GHz, is presented.