Appleton and his colleagues were one of two teams to first to prove the existence of a reflecting layer at a height of about 100 km (now called the E layer). This was soon followed by the discovery of another layer at around 250 km (now called the F layer). This was done by broadcasting a continuous signal from one site and receiving the signal at a second site several miles away. By measuring the difference between the signal received along the ground and the signal reflected from the atmosphere it was possible to calculate the height of the atmospheric reflecting layer. The second team, Breit & Tuve in the U.S.A. were able to transmit a very short radio pulse and measure the height of the layer by measuring the time it took to be reflected. This quickly become the standard technique for measuring the height of the reflecting layers.
For a reflection from the E layer, the speed of the radio pulse was easily estimated, as there is little ionisation below the layer to slow the radio wave down. For the F-region echoes however, the underlying E-layer ionisation slows the radio pulse, making the delay from a F-layer echo much longer. If the radio pulse is estimated to be travelling at the speed of light, this added delay causes the F-layer appear to be much higher than it really is. By sounding the E layer at several radio frequencies, the underlying ionisation can be estimated and taken into consideration in the calculations of F-region layer height. As a result, the method of sounding soon changed from single frequency sounding to a 'frequency change' method, where the transmitter broadcast over a range of frequencies.