The ADEP package used to scale the Chilton data has been designed as a tool for predicting ionospheric parameters as well as measuring them. As a result, where no foE is measured, the software requires a nominal value of 0.6 Mhz to be supplied before it will allow the automatic scaling of other parameters (this is a requirement of the true-height profile calculation).
In contrast, scaling foE from the Slough data is done by visual inspection only. If no foE can be measured, none is attributed. The fact that the analysis of the Slough data is valid is demonstrated by the fact that on not one of the occasions where ADEP 'predicts' a value 0.6 for foE, does the Slough sounder measure foE. In other words, a value of 0.6 Mhz at Chilton is always equivalent to no E layer. Of the common points (approximately thirty percent, but this figure is distorted by the above), there is good agreement, with no obvious outliers when the data are plotted against each other. A least squares fit to the spring equinox data gives a gradient of 1.