In communications systems such as telemetry systems in which a receiver (34) is periodically energised to receive transmissions from a central station, the receiver includes a power control means (12) for controlling the energisation of the receiver, a circuit (48) for detecting the presence of a carrier, which circuit (48) causes the power control means to de-energise the receiver (34) in the event of carrier not being detected, and a signal quality measuring stage (50) for determining if the demodulated signal is decodable, the stage (50) causing the power control means to de-energise the receiver (34) in the event of the signal quality not being acceptable.