In a method of estimating the relative frequency uncertainty between two parts of a mobile radio system able to communicate via two mobile radio networks at first and second particular frequencies, each part of the mobile radio system has a clock from which the particular frequency is derived and the uncertainty is estimated by measuring the number of pulses of each of the clocks during a time window of duration T starting at a time t. The method includes the following steps, with the time window T sliding: a) measuring the number of pulses of each of the clocks at n intermediate times t+(T/n), t+(2T/n), . . . , t+(nT/n), where n is a positive integer, b) storing the numbers of clock pulses thereby obtained in memory, c) calculating the relative frequency uncertainty at time t+T from the numbers of clock pulses stored in memory.