The SLI 259A detects the movement of mass through a protected area (i.e. an arm reaching into a car or a door being opened). The SLI 259A has dual-zone protection. It combines both perimeter pre-intrusion warning and instant interior trigger protection. It does this by transmitting and receiving a high-frequency RF signal. When an object moving towards the vehicle is detected, the high-frequency RF signal will change. This change will cause the microwave sensor to send a trigger signal to the alarm.