A comparison of nighttime propagation anomalies observed at two very low frequencies

Recent nighttime phase and amplitude anomalies have been observed on the 10.2 and 13.6 kHz signals from Trinidad, West Indies, as received at Boulder, Colorado. The anomalies, in some cases, are comparable in size to the normal diurnal variations of these signals and occur predominantly at 13.6 kHz. These observations, when taken with independent confirming observations, suggest localized changes in the ionosphere or in VLF mode structure that may be significant to radio navigation.