Radio telemetry systems with channel selection

A radio telemetry system comprising:

• a master station;
• and a plurality of transponders, each transponder having a stand-by state in which it listens on each of a number of channels in turn, the master station listening on each of said number of channels in turn to establish a relatively quiet channel and then transmitting a wake-up signal on said relatively quiet channel, each transponder listening on said relatively quiet channel at the time the master station transmits said wake-up signal responding to receipt of said wake-up signal by transmitting an acknowledgement signal on said relatively quiet channel, the master station responding to receipt of one of the acknowledgement signals transmitted to receive data from the transponder which transmitted that one acknowledgement signal, thereafter that transponder ignoring further wake-up signals for a predetermined time, and the master station transmits a further wake-up signal after receipt of data from the transponder which transmitted said one acknowledgement signal.