A time reference point can be determined in a radio system by sending a synchronization sequence s(i), i=0, . . . N−1, from transmitter X, 40 to receiver Y, 48, and by detecting the peak value at the output of a matched-filter, 44, h(i)=s(N−1-i) on Y. In practical systems the accuracy of this peak detection is limited by interference and noise on the radio channel. To increase the peak detection accuracy, we propose to repeat the transmission of the same synchronization sequence K times. The interval L between repetitions is constant and the amplitude of the synchronization sequence in each repetition varies according to a given variation pattern a(jL), j=0, . . . , K−1. The receiver Y, 48 knows L and a(jL), j=0, . . . , K−1. After comparison to a threshold, the matched-filter 44 at the receiver 48 may deliver peaks resulted from repeatedly received synchronization sequences or peaks caused by interference and/or noise.