A method for determining changes in internal delays of RF units, the RF units including a plurality of receivers and transmitters. The method includes providing initial direct signals' time of arrivals of the RF units initial internal delays of the RF units. Following this, each transmitter transmitting a direct signal, and the real-time direct signal's time of arrivals of the RF units, are measured. Then, changes in internal delays of the RF units are calculated based on the real-time direct signals' time of arrivals and initial direct signals' time of arrivals. And finally, real-time internal delays of the RF units are calculated based on the changes in internal delays and the initial internal delays of the RF units.