Synchronizing radio communications

A radio device configured to:

• receive a succession of calibration signals transmitted by radio to the radio device, according to a predetermined schedule of nominal transmission times, by a second radio device;
• determine a plurality of timing offsets for the radio device and the second radio device, wherein each timing offset is determined by subtracting i) a nominal transmission time for a respective calibration signal of the succession of calibration signals from ii) a time of arrival at the radio device of the calibration signal, whereby the timing offset represents a time-of-flight for the respective calibration signal plus any transmission delays;
• identifying a largest timing offset in the plurality of timing offsets for the radio device and the second radio device;
• determine a start time for a transmission of an unmodulated-carrier ranging signal by the radio device by using the identified largest timing offset to adjust a nominal start time for the transmission of the unmodulated-carrier ranging signal to determine a start time for the transmission that is earlier than the nominal start time by an amount at least as large as the largest timing offset in the plurality of timing offsets;
• and
• transmit the unmodulated-carrier ranging signal at the determined start time.

A radio device configured to:

• transmit a plurality of calibration signals by radio to a second radio device;
• receive a plurality of calibration response signals from the second radio device;
• determine a plurality of time-of-flight values for the radio device and the second radio device, wherein each time-of-flight value is determined in dependence on a time of departure of a respective calibration signal and a time of arrival of a respective calibration response signal;
• identify a largest time-of-flight value of the plurality of time-of-flight values for the radio device and the second radio device;
• determine a start time for a transmission of a ranging signal by the radio device by using the identified largest time-of-flight value to adjust a nominal start time for the transmission of the ranging signal to determine a start time for the transmission that is earlier than the nominal start time by an amount at least as large as the largest time-of-flight value in the plurality of time-of-flight values;
• transmit a ranging signal to the second radio device at the determined start time;
• receive a ranging response signal from the second radio device;
• and
• process the ranging response signal to determine a range value representative of a distance between the radio device and the second radio device.