Horizontal Motion in Ionospheric Regions-A Review

295-307A historical account of the discovery of a radio method of investigating horizontal drift in the ionosphere is presented. The discovery was made by the author around 1947, while he was working at the Cavendish Laboratory, Cambridge. The spaced-receiver technique, or A-1 method, involves three receivers spaced at the corners of a right-angled triangle spaced about one wavelength apart. The fading of a down-coming pulse transmitted from a nearby transmitter, as received by the three receivers, is simultaneously recorded on a 35-mm film and displacement between any two of the records gives the magnitude and direction of the velocity of the horizontal drift. The method has been followed throughout the world and specially during IGY and IGC. Subsequently, a few other methods for similar measurements have been developed. These have been compared for their relative merits. The spaced receiver technique has stood the test of time and is still the best method for investigating horizontal drift in the ionosphere at all times during day and night