The phase of a received very-low-frequency signal, recorded as a function of time, has been utilized to investigate (a) the frequency off-set between two remotely located atomic frequency standards and (6) the variations of the diurnal phase shift— which is a characteristic of the phase comparison records.A comparison of the precision obtained in the measurement of the frequency off-set by using 20 and 60 kHz transmissions over a path length of 1800 km, reveals that the measurement is more precise on 20 kHz transmissions. The diurnal phase shift, studied from the standpoint of the two-mode model, shows that the second mode interference increases with increasing frequency in the range 16-20 kHz as can be seen from the curves theoretically computed for diurnal phase shift as a function of path distance. Experimental results are presented for comparison.Variations of the general phase pattern and the monthly variation of the diurnal phase shift are also presented. The diurnal phase shift is found to be a minimum during the period of solstice and a maximum during equinox. The general consistency of the phase shift is shown to depend both on path length and its orientation. The value of the change in effective height of ionospheric reflection calculated from the yearly mean diurnal phase shift for a long propagation path is found to be in good agreement with the value used in theoretical calculations.