Vertical Sounders — the Ionosonde

Abstract

One of the first measurements of ionospheric layer height was reported by Appleton and Barnett in 1926, who used a continuous wave transmitter and a receiver located from 65 to 130 km distant from the transmitter. The ground wave and the skywave arrived at the receiver with a relative phase difference. When the frequency of the transmitter and the receiver were simultaneously and rapidly changed by a small amount, the investigators obtained maxima and minima as the ground and skywave signals reinforced and canceled each other, respectively. Ionospheric layer heights were deduced from these data (see Kelso 1964, for a complete description of the technique and of the results in terms of both phase path and group path changes). It is interesting that G.W. Pierce in his 1910 textbook suggested that waves reflected from the ionosphere might interfere with the direct groundwave. As a matter of history, perhaps the first observations of this kind, followed by a correct deduction as to the causative mechanism, were made in 1912 by de Forest and Fuller, using continuous wave arc transmitters and receivers of the Federal Telegraph Company in California (Villard 1976). The first vertical sounding technique employed to probe the terrestrial ionosphere was the “pulse-echo” equipment used by Breit and Tuve to measure ionospheric layer height in 1925 (reported in 1926). This was the progenitor of the ionosonde — the most ubiquitous of all ionospheric measuring devices.