Radio-Wave Propagation Forecast

a prediction of conditions for shortwave radio communication. A distinction is made between long-range and short-range forecasts. Long-range forecasts for periods of a month or more are based on predicted monthly median normal ionospheric conditions. Short-range forecasts are updates of long-range forecasts and are based on the current data on ionospheric conditions, solar activity, and geomagnetic activity. The chief purpose of the forecasts is to determine in advance the frequencies to be used in given radio links. The choice depends on a number of factors—geographic location, length of the radio link, time of day, season, and level of solar activity. Since these are the same factors that determine ionospheric conditions, the reliability of the forecasts is determined by the level of our knowledge of the laws governing changes in the ionosphere.

Shortwave radio communication over long distances is made possible by the reflection of the waves from the layers of the ionosphere. For each case there exists a maximum usable frequency (MUF). Radio waves of a frequency higher than the MUF are not reflected but pass through the ionosphere and into outer space. The existing methods of forecasting are based on the calculations for world maps of the MUF of each layer of the ionosphere for different times of day, seasons, and levels of solar activity. Such maps take into account the results of observations made of the ionosphere over many years by a worldwide network of ground-based ionosondes and by rockets and satellites. The maps also take into account theoretical ideas on the aeronomic and ionization-recombination processes occurring in the ionosphere.

The greater the distance between the receiver and the transmitting station, the higher the frequency at which radio communication is possible. The reason for this is that as the angle of incidence of the radio waves becomes smaller, the MUF increases with the cosine law. For distances greater than 3,000–4,000 km, however, radio waves propagate by multiple reflections, or hops. In this case, the MUF is severely limited because it is determined by the smallest of the MUF’s at the points of reflection. This limitation is particularly important for long radio links that run along parallels of latitude, since the MUF’s at the points of reflection differ greatly owing to the differences in the local time. Forecasts are especially valuable in such cases.

Existing radiowave propagation forecasts have limited application. The maps of predicted MUF’s that are issued are borne out in only about 50 percent of all cases because the regular behavior of the normal ionosphere is often upset by solar flares and geomagnetic disturbances. When such flares and disturbances occur, radio communications become unstable, and the absorption of radio waves is increased. Forecasts cannot be made for the polar regions because the ionosphere changes in an irregular and unpredictable manner.