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Ionospheric Sounding

Ionospheric sounding is performed using a HF radar system known as ionosonde. The transmitter (TX) sends short pulses of radio energy into the ionosphere, which reflects them, and the receiver (RX) records the time delay between transmission and reception of pulses.
By varying the carrier frequency of pulses the time delay at different frequencies is recorded. This record is usually presented in the form of a graph and it is called an ionogram.
From the analysis of the ionograms several important characteristics can be found that play a significant role in the studies concerning ionospheric physics and for radio communication purposes.

Vertical Sounding

To carry out a vertical sounding, TX and RX are located in the same place (Figure 1).

Figure 1Ionospheric vertical sounding Ionospheric vertical sounding

By varying the carrier frequency of pulses typically from 1 to 20 MHz, the time delay at different frequencies is recorded. This record is referred to as a vertical ionogram (Figure 2) and is usually presented in the form of a graph, where normally in place of time delay the virtual height is plotted according to the following equation:

(1) h′ = (cΔt)/2 ,

where Δt is the time delay, h′ is the virtual height and c is the free space speed of electromagnetic waves.

Figure 2Vertical ionogram Vertical ionogram

From the analysis of a vertical ionogram an operator can retrieve several important characteristics. Figure 3 shows the virtual heights and the critical frequencies of the main ionospheric layers for a diurnal ionogram.

Figure 3Parameters from vertical ionogram Parameters from vertical ionogram

The importance of real time ionospheric data to be used for space weather purposes has greatly increased over the past years. For this reason since the 1980’s much work has been performed by the ionospheric community to develop computer programs able to automatically scale vertical ionograms giving as output the standard ionospheric characteristics. Recently the Istituto Nazionale di Geofisica e Vulcanologia designed and developed an ionosonde called Advanced Ionospheric Sounder (AIS-INGV) along with a program, called Autoscala, to automatically scale the main ionospheric characteristics from an ionogram. Figure 4 shows the ionogram as in Figure 3 elaborated by Autoscala.

Figure 4Ionogram as elaborated by Autoscala software Ionogram as elaborated by Autoscala software

The INGV performs vertical soundings at the ionospheric stations of Rome (Italy, 41.8° N, 12.5° E), Gibilmanna (Italy, 37.9° N, 14.0° E), Mario Zucchelli Station (Baia Terra Nova, Antarctica, 74.7° S, 164.1° E), and Tucumán (Argentina, 26.9° S, 294.6 E).
The corresponding data is stored in a relational database and published on the Web within the eSWua project.
The data recorded at Rome is also used within the GIFINT, DIAS, and MIRTO projects.

Oblique Sounding

The technique for producing oblique soundings is similar to that for vertical ones except that TX and RX are separated (Figure 5).

Figure 5Oblique sounding Oblique sounding

By varying the carrier frequency of pulses, typically from 2 to 30 MHz, the time delay at different frequencies is recorded. This record is referred to as an oblique ionogram and is usually presented in the form of a graph, where the time delay vs frequency is plotted (Figure 6).


Figure 6Oblique ionogram Oblique ionogram

From an oblique ionogram important characteristics can be scaled. Among these the MUF (Maximum Usable Frequency), is the most important, this being the highest frequency at which a radio wave can propagate between given terminals by ionospheric refraction alone exploiting the one-hop F layer reflection. In an oblique ionogram the MUF corresponds with the frequency at which the high and low-angle rays join, also known as the Junction Frequency (JF), as shown in Figure 6.
In April 2005 the INGV installed a RCS-5B sweeping HF receiver for oblique sounding in Chania (Crete, Greece, 35.7° N, 24.0° E) to perform a radio link with Inskip (UK, 53.5° N, 2.5° W).

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