| CPC G08B 21/22 (2013.01) [G08B 21/0258 (2013.01); G08B 21/0266 (2013.01)] | 22 Claims |

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1. A distance-determining system comprising:
at least one first monitoring unit; and
at least one second monitoring unit,
wherein at least the first monitoring unit comprises a transmitter unit that is configured to emit a sound signal, and an electromagnetic signal,
wherein at least the second monitoring unit comprises a receiver unit that is configured to receive the sound signal and the electromagnetic signal such that a distance between the first monitoring unit and the second monitoring unit can be determined on the basis of a runtime difference between the sound signal received by the receiver unit and the electromagnetic signal received by the receiver unit and allocated to the received sound signal,
wherein the receiver unit comprises a sound receiver module for a reception of the sound signals and an EM receiver module for a reception of the electromagnetic signals, the receiver unit having one or several listening operation states and at least one measuring operation state that differs from the listening operation states,
wherein in the listening operation states only the sound receiver module or only the EM receiver module is active, and
wherein in the measuring operation state the sound receiver module and the EM receiver module are active, or the respective receiver module that is currently inactive is at least temporarily switched on in addition to a respective already active receiver module, or the respective receiver module that is active in the listening operation states is deactivated and the receiver module that is inactive in the respective listening operation states is activated.
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