Viper 508D Invisible Field Disturbance Sensor with Easy Payments Features: Protect your vehicle with an invisible sensing field which detects moving high-density objects (including people) near your car using Radio Frequency (RF) field disturbance technology. Ideal for convertibles, or any application where zone-intrusion protection is desired. Violation of the outer field can provoke siren chirps (or a WarnAway™ message from an optional voice module) while intrusion into the inner zone will trigger the system’s siren.