Abstract: In accordance with the method, the received signal is quadrature demodulated by mixing it with two signals with a relative phase difference of .pi./2 and having the same frequency as the emitted carrier, the absolute value of each of the demodulated signals of each pair of signals obtained is compared to a respective threshold to determine valid pairs if the absolute value of at least one of the two demodulated signals is higher than the threshold and invalid pairs otherwise and the number of successive rotations of the vector whose coordinates correspond to the values of the signals of the successive valid pairs is counted. For each invalid pair the value of the count is progressively reduced. The alarm is tripped if the value of the count reaches a given threshold.