Variations in phase path of man-made one-hop whistler mode signals at 18.6 kilocycles per second

The phase stability of one-hop whistler mode signals on 18.6 kc/s from the VLF transmitter NPG near Seattle, Washington, has been observed in New Zealand. These signals are characterized by a change in frequency, presumably introduced by variations in the phase path length. The observations indicate that the phase path length can alter at a rate of about 1 km/sec, and total phase path changes of the order of 200 km can occur in a period of 3 min. Possible methods of determining where and how the phase variations occur are suggested.