  This paper presents an introduction to Chapman's theory of a production layer
in the ionosphere relevant to the monitoring of VLF signals and Sudden
Ionospheric Distubances (SIDs). It shows how the diurnal pattern of VLF signal
strength can be derived, and how this pattern may be fitted to VLF signal
strength measurements. It also shows through an example some of the possible
problems for fitting this diurnal pattern (the fact that D-layer height cannot
always be derived directly from measurements) and some of the advantages when a
`fit' has been obtained (increased sensitivity of the instrument without any
changes to hardware).
