Methods for controlling deployment of an occupant restraint device in a vehicle and determining whether an object is a child seat in which one ultrasonic transducer is arranged on a ceiling of the vehicle and another ultrasonic transducer is arranged at a different location in the vehicle, ultrasonic waves are transmitted from each transducer into the passenger compartment and received thereby, and the distance from each transducer to the object is calculated based on the time difference between the transmitted waves when transmitted from the transducer and the reflected waves when received at the transducer. Deployment of the occupant restraint device is controlled based on the calculated distances and/or a determination is made whether the object is a child seat based on the calculated distances.