The paper discusses radio-wave propagation in the 70–300-kc/s band for long-distance navigational applications. Signals may reach the receiver by the ground path and by reflection from the E-layer of the ionosphere, the resultant signal at the receiver terminals being displaced in phase or time to a degree dependent upon the relative magnitude and phase of the component signals. Time-error curves are shown for typical propagational conditions, both by day and by night, at various frequencies. The errors which occur are classified as follows:(a) Systematic errors for which corrections corresponding to the propagational conditions present at the time can be applied.(b) Random errors which depend upon the difference between the ground- and ionospheric-wave paths.(c) Random errors arising from the imperfections of the ionosphere as a reflecting layer.The characteristics of these propagational errors are discussed, and the magnitude of navigational errors arising therefrom are deduced, using suitable examples. It is shown that, at intermediate distances, the random position-line errors are mainly dependent on the difference in the overground path distance, and that at large distances, they are dependent on the imperfections of the ionosphere as a reflecting layer.From phase and amplitude observations made with receivers in aircraft, during both day and night, the relative values of the ground-and ionosphere-reflected wave components were obtained as a function of distance from the transmitters; from these observations the virtual height of the reflecting layer and the reflection coefficient at oblique incidence were also deduced. The mean height of the reflecting layer is about 70 km in day-time and 90 km at night, with reflection coefficients at oblique incidence of 0.05 and 0.25 respectively.Using the Consol, Decca and Post Office Position Indicator transmitting systems, data collected from fixed receiving points at various distances from the transmitters show the variations of signal amplitude and phase which result from imperfections of the E-layer of the ionosphere when used as a reflecting layer. By collating these data, it is shown that navigational accuracy is improved by making the base-line distances as large as possible.