We describe a technique for measuring time delays by using the cross spectrum of Pc 1 recorded at different stations. The method is based upon the computation of the coherency coefficient and of the phase of the cross spectrum. It is applied to data which were obtained during a campaign of observations at two pairs of conjugate stations (L=3.7 and L=4.5). We show that the coherency coefficient between signals recorded at conjugate points may be large (~1) for structured Pc 1, which implies that the optical path is constant, to within a fraction of a wavelength, over long periods of time (the integration time is of the order of 16 min). Measurements of time delay between the northern and southern hemispheres show that there is generally an asymmetry, the delay being longer for the north to south propagation. This asymmetry is tentatively interpreted in terms of differences between the ionospheric characteristics above the two stations but also in terms of a difference between the two magnetospheric paths. The detailed study of the cross-spectral power density phase versus frequency variation shows that on some occasions a reamplification process takes place at each bounce of the wave, but only on one of the two trajectories.