Abstract: A radio telemetry system has a first transceiver and a large number of further transceivers of which some but not all are out of range of the first transceiver and none of which is out of range of all the others. The first transceiver transmits a radio message having a predetermined format, and each of the further transceivers receiving the message, whether directly or indirectly, re-transmits that message in synchronism or substantially in synchronism with all the other further transceivers which received the message at the same time. Thus, in general, a further transceiver out of range of the first transceiver will receive the message simultaneously from two or more sources. The message contains iteration data which determines the number of times the message is to be re-transmitted.