A communication system comprising one or more transceiver units of a first type and one or more transceiver units of a second type capable of communicating with the transceiver units of the first type; each transceiver unit of the first type comprising: a frequency comparison unit for comparing the frequency of a signal received from a transceiver unit of the second type with a reference frequency; a feedback signal generator for generating a feedback signal dependent on the result of that comparison; and a transmitter for transmitting that signal to the transceiver unit of the second type; and each transceiver unit of the second type comprising: a local frequency reference unit on which the frequency of signals transmitted by it are dependent; and a frequency adjustment unit for receiving the feedback signal and adjusting the local frequency reference unit in dependence on the feedback signal; the communication system further comprising at least one transceiver unit of a third type with which the transceivers of the second type are capable of communicating; and a database accessible to the transceiver units of the first type and the transceiver units of the third type; and wherein: the transceiver units of the first type are arranged to, on comparing the frequency of a signal received from a transceiver unit of the second type with the reference frequency, store in the database an indication of the result of that comparison together with an identity of that transceiver unit of the second type; and each transceiver unit of the third type comprises: a local frequency reference unit on which the operating frequency of that transceiver unit is dependent; and a frequency adjustment unit for, on receiving a signal from a transceiver unit of the second type for which the database holds an indication of the result of a comparison, comparing the frequency of that signal with the frequency of the respective local frequency reference unit and adjusting the frequency reference unit in accordance with that comparison. a frequency comparison unit for comparing the frequency of a signal received from a transceiver unit of the second type with a reference frequency; a feedback signal generator for generating a feedback signal dependent on the result of that comparison; and a transmitter for transmitting that signal to the transceiver unit of the second type; and a local frequency reference unit on which the frequency of signals transmitted by it are dependent; and a frequency adjustment unit for receiving the feedback signal and adjusting the local frequency reference unit in dependence on the feedback signal; the communication system further comprising at least one transceiver unit of a third type with which the transceivers of the second type are capable of communicating; and a database accessible to the transceiver units of the first type and the transceiver units of the third type; the transceiver units of the first type are arranged to, on comparing the frequency of a signal received from a transceiver unit of the second type with the reference frequency, store in the database an indication of the result of that comparison together with an identity of that transceiver unit of the second type; and a local frequency reference unit on which the operating frequency of that transceiver unit is dependent; and a frequency adjustment unit for, on receiving a signal from a transceiver unit of the second type for which the database holds an indication of the result of a comparison, comparing the frequency of that signal with the frequency of the respective local frequency reference unit and adjusting the frequency reference unit in accordance with that comparison.