Ionospheric models and measurements required by HF communication system designers and operators

Communications, radar and navigation systems which rely on the propagation of electromagnetic (EM) waves through or via the ionosphere are affected by the environment through which those waves must propagate. The ionosphere, which extends from ~50 to ~1000 km, significantly affects the propagation of all radio systems up to and including the top end of the ultrahigh frequency (UHF) band at 3 GHz. Above this frequency there are second order effects which may be important in certain specialised applications. The ionospheric effects are varied but include refraction, retardation, attenuation and scintillation. HF communications and radar systems exploit some of these effects to advantage but other systems are degraded. Positional errors and loss of phase lock in GPS (Global Positioning System) are examples of such deleterious effects. Quantitative estimates of some of these effects are provided. If the ionospheric environment was isotropic and stable in time, it would be relatively easy to determine its effects on the propagation of EM waves. Unfortunately this is not the case. The spatial scales of ionospheric variations range from thousands of kilometres to turbulence with scale sizes of a less than a metre. Likewise the temporal scales vary over many orders of magnitude from many years (solar cycle effects on ionospheric propagation) to hours or even minutes (the scale of weather phenomena). The paper identifies that ionospheric information required by the civilian HF system designer and system operator