It has already been explained that one has to correct for the effect of the ionosphere upon the doppler shift in order to track the Transit satellites with sufficient accuracy. At the Radio Research Station, Slough, the converse problem is being investigated; we are looking at the effect of the ionosphere upon the Transit satellite signals in order to find out about the ionosphere itself. In fact the doppler correction term is directly related to one of the quantities of major interest, the electron content of the ionosphere. This is the number of electrons in a unit column up to the satellite height. We are interested in the daily, seasonal and solar variation of this quantity.