US Patent 7123545 Monitoring device and method using echo signals

An echo-signal monitoring device having a plurality of transceiver units for transmitting signals and receiving the echoes reflected by an external object and an analyzer unit for estimating the distance from the monitoring device to the external object on the basis of the received echoes is characterized in that the device has an operating mode for calculating the relative position of the transceiver units from one another on the basis of the reflected echoes.