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Ionospheric Forecast

To establish a long distance transmission using a shortwave radio signal propagation for a specific circuit condition it is necessary to know the range of frequencies that make the radio link possible. The range of usable frequencies will vary:

  • throughout the day
  • with the season
  • with the solar cycle
  • from place to place

The upper limit of frequencies depends mainly on the above mentioned factors while the lower limit depends on the circuit geometry and parameters and on ionospheric absorption (working parameters).

The working frequency must fall within the range of usable frequencies:

  • if it is higher than the Maximum Usable Frequency (MUF) it will pass straight through the ionosphere;
  • if it is lower than the Lowest Usable Frequency (LUF) it will not be sufficient to ensure the radio link.

To maintain a good communication link it is therefore necessary to have reliable predictions of the state of the ionosphere at a given time and location.

Fixed ionospheric predictions suitable for each time of the day and of the year have been produced for the Mediterranean area.

From 1981, a specific software package processing periodic ionospheric predictions, published every month, has been developed to provide detailed frequency guides in advance of two months. These predictions are used for radio links between two fixed points or between one fixed point and several moving points and they produce better results with respect to those obtained through fixed predictions.

The ionospheric F2 region (link alla pagina) plays the most important role in radio communication systems, therefore ionospheric predictions are mostly based on F2 layer models (link). The ionospheric prediction programme developed by the Servizio Ionosferico Nazionale of INGV uses a single station model based on the data collected at the Rome ionospheric observatory. This regional model does not take into account the longitudinal effect.

Ionospheric predictions issued by the Servizio Ionosferico Nazionale are presented in three different ways:

  • through MUF/LUF hourly diagrams, i.e. diagrams of the usable frequency range for radio links between fixed points;
  • through MUF/LUF charts recording the upper and lowest limits of the MUF/LUF field for different spatial positions of the receiver with respect to the transmitter;
  • through area maps showing the geographic area covered (via the ionosphere) by a specific transmitter, for some assigned circuit parameters.

Occasionally, using the monthly median values measured by vertical sounding at more than 100 ionospheric stations, maps of the predicted monthly values of the critical frequency foF2 and of the M(3000)f2 are produced. These maps are extended all over the globe through the expansion in spherical harmonics of the functions FoF2 and M(3000), which depend on the three variables latitude, longitude and time. Through these maps it is possible to draw diagrams of point to point forecasts valid all over the world although with a different reliability with respect to the three types of ionospheric forecasts mentioned above which apply only to the European area. 

 Example of ionospheric forecast over the Mediterranean area. The plotted curves represent the skip distance (the shortest reception distance via the ionosphere for a given frequency) for a transmitter located in Rome.


Example of ionospheric forecast: "MUF/LUF Charts": External curves have a 30% probability, internal curves have a 90% probability.

The ionospheric forecast method is based on regional, physic-empirical models. For long distance forecasts or for short-term forecasts, instead, methods based on global models or ad-hoc models are used.

For short-term ionospheric forecasts or "day-to-day forecasting" and for the prediction of particular disturbances such as, for example, the Sudden Ionospheric Disturbance (SID) over the Mediterranean area, the U.F. Fisica dell’Alta Atmosfera works jointly with CNET-France-Telecom (Lannion-France), providing daily observations of the ionospheric plasma frequencies.

Concerning the very short-term forecast or " nowcasting", the Rome Ionospheric Observatory provides the Rutherford Appleton Laboratory (RAL, UK) with real-time hourly ionograms and the associated measurable physical parameters. Using these measurements, RAL offers several products and services for the ionospheric nowcasting valid for the European region such as, for example, the MUF (Maximum Usable Frequency) maps for radio links, produced 24 hours in advance. The ionograms recorded at the Rome Observatory are also used by IPS (Ionospheric Prediction Service, Australia), for global and European medium- and short-term forecasts. Moreover, both electronic density vertical profiles and plasma frequencies of the E, F1 and F2 layers, available in real time through the INGV website, are used by IPS for ALERT and WARNING messages on radio propagation conditions at planetary and/or continental scale (Space Weather). These messages are necessary not only for managing air and naval traffic systems, but also for operating earth-satellite links and transmissions.

HF wave propagation paths.
Note that no signal can be received for distances shorter than path n. 3; the zone of silence between the point where the ground wave becomes too weak for reception and the point where the sky wave is first returned to Earth is called the skip zone. The size of the skip zone varies with the time of day, with the season and with the solar cycle and depends on the extent of the ground wave coverage and on the skip distance. The skip distance is the shortest distance that permits radio signals of a given frequency to travel from the transmitter to the receiver by reflection from the ionosphere. The ground wave may be, however, detected in the proximity of the transmitter; ground wave propagation does not depend on the state of the ionosphere but only on the ground conductivity and on the transmitter power.


A pictorial image of a radio link via the ionosphere.


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