Amplitude and phase variations of the one-hop sky wave of pulsed 100 kHz Loran-C signals were observed at the distance of 1200 km from the transmitter. The period of variations usually ranges from a few minutes to an hour and the fluctuation of reflection height is the order of a kilometer. Average power spectrum of the phase variations for seven nights observations in summer has a minimum at 5 min, which corresponds to Brunt-Vaisala frequency at the reflection altitude of 85 to 90 km. These variations are suggested to be due to the effect of atmospheric gravity waves.
