The frequency change method, used by Appleton and Barnett in their historic experiments in 1924, was used to carry out regular soundings of the ionosphere. The transmitter was placed in Windsor great park, with a receiver at Ditton Park. Data from this experiment exists from 1930, but it was at noon on January 11 1931 that a 90 day sequence of ionospheric measurements began. These first measurements involved changing the transmitted frequency until the radio wave was no longer reflected by the layer, but instead passed through it. The frequency at which this happens is known as the critical frequency, and from this it is possible to determine the peak electron density of the layer.
The longest continuous sequence of ionospheric soundings was started on 20th September 1932 and is still continuing at RAL today. The pulse sounding technique, in which a series of individual pulses is broadcast sequentially over a range of frequencies, eventually replaced the frequency change method. The pulse sounding technique was first used by G. Breit and M. A. Tuve in the United States.
|Transmitter used for
first routine soundings