Shortwave Propagation Forecast
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.
Why is wave propagation prediction important?
The primary objective of forecasting shortwave propagation conditions is to choose the frequencies that will be used in specific radio links in advance. The decision is based on a number of variables, including geographic location, radio link length, time of day, season, and solar activity level.
How could we forecast wave propagation?
The accuracy of the forecasts depends on our understanding of the rules driving changes in the ionosphere because they are the same variables that produce ionospheric conditions.
- Propagation forecasts are divided into long-range and short-range categories.
- Long-term forecasts (a month or more ahead) are based on predicted monthly median normal ionospheric conditions.
- Short-range forecasts are updates of long-range forecasts and are based on the current ionospheric conditions, solar activity, and geomagnetic activity.
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 higher the frequency at which radio communication is possible, the greater the distance between the receiver and the transmitting station. The greater the frequency of occurrence, the more likely it is that the frequency of occurrence will increase. However, for distances greater than 3,000-4,000 km, radio waves propagate through multiple reflections, or hops. The MUF is severely limited in this case because it is determined by the smallest of the MUFs at the points of reflection. This limitation is especially important for long radio links that run along latitude parallels, because the MUFs at the points of reflection differ greatly due to differences in local time. Forecasts are especially useful in these situations.
What are the limits of wave propagation forecast?
Forecasts for radiowave propagation are currently limited. Because solar flares and geomagnetic disturbances frequently disrupt the ionosphere, the maps of predicted MUFs issued are only partially accurate. When such flares and disturbances occur, radio communications become unstable, and radio wave absorption increases. Forecasting for the polar regions is almost impossible due to the irregular and unpredictable changes in the ionosphere.
Read more about Radio-Wave Propagation Forecast. See also an index for HF Radio Propagarion.
