Ionosonde

Sometimes it happens while listening to a station in SSB on shortwave, to hear for a brief moment a tone with variable frequency, a sweep that lasts just an instant; in modern radios equipped with a spectrum analyzer (waterfall) it is possible to see an oblique line forming from left to right. Here, what we have heard and seen on the waterfall is the signal of an ionosonde, I usually see and listen to 2 in sequence, after the first sweep another one follows after a few seconds, the swept tone (with variable frequency) is heard from low to high frequency if you are in USB, vice versa from a high tone to a low one if you are listening in LSB.

The best period to receive this type of signal is the warm period: spring, summer and autumn, when propagation in the high ranges is better; receiving the signals of the ionosondes is synonymous with good/high propagation, their power is not high, in the order of a hundred Watts; reception is usually involuntary as they have an emission that ranges across the entire spectrum of short waves.

These ionosondes, not to be confused with meteorological radiosondes that operate in the UHF band, are used to measure the propagation of radio waves and various physical parameters of the ionosphere, the operating principle is simple: the ionosonding stations generally transmit a CW signal (sinusoidal, therefore free of harmonic components) of constant intensity but at variable frequency, generally 100kHz/second, from 1 to 30MHz, it is possible by receiving the direct signal or an echo from local or remote receivers, to measure:

• first of all the MUF (Maximum usable frequency) or the maximum usable frequency regardless of the power, it is easy to note that in HF the highest bands are those most sensitive to propagation and daytime time, at a certain time the band closes, in winter propagation is poor, etc., radio amateurs are therefore forced to use the lower frequency bands; here this measurement is based on this system, the ionosonde station transmits its carrier at a variable frequency from 3 to 30 MHz, and a remote station detects the maximum frequency at which it is possible to receive the signal.
• The minimum non-reflected frequency MNRF (Minimum non-reflected frequency), can be obtained by measuring the reflection of the transmitted signal towards the ionosphere, the MNRF is the minimum frequency from which there is no reflection by the ionosphere, therefore it continues towards space.
• Another physical parameter that is measured is the TEC (Total Electron Content), it would be the maximum density of free electrons in a specific area, this parameter is strongly influenced by solar activity, receiving the ionosonde signal from a remote station, it is possible to measure the propagation delay of the signal and define through complex formulas the quantity of electrons present in the space between the transmitter and the receiver and the height of the ionosphere between the two points.

Ionosounding stations generally transmit their signal from 1 to 30 MHz, the frequency variation speed is constant, in reality there are a handful of frequencies that are “skipped”, they are those attributed to the sample time and frequency stations and to the distress and safety frequencies of the coastal stations. This operating mode is called “Scanning ionogram“, in reality there is also a fixed frequency survey, called “Fixed scanning ionogram“, the “Doppler ionograms“, “Oblique ionograms” and “Drift ionograms” are high precision modes for particular studies (omissis).

In Italy we have an ionsounding station managed by the National Institute of Geophysics and Volcanology, the station is located at 41.8° N latitude and 12.5° E longitude, i.e. in the South Rome area, on the INGV website it is possible to consult the ionograms in real time, the signal is radiated by this station at minutes 02, 15, 30 and 45 of every hour. Another large ionsounding station is located in the north-east of Germany near Juliusruh, the antenna of the station is 70 meters high and radiates in the frequency range between 1 and 20 MHz; but there are many others scattered around the world, a radiosonde currently in use is the Lower Digisonde with loop antenna. Radio listening enthusiasts can experiment with the ChirpView program.

The ionosphere
The ionosphere is a portion of the upper atmosphere between 50 and 1000 km of altitude, in which the sun’s radiation and cosmic rays cause the ionization of gases; the density of ions and its height depend on the time and solar activity, it is possible to further divide it into three regions, D, E, and F.

The lowest region extends between 50 and 80 km of height from the ground and is responsible for the reflection of long waves and is produced mainly by the ionization of nitrogen oxide by solar radiation, it is called Region D.
The intermediate region is located between 80 and 150 km of altitude approximately, called Region E (or Kennelly-Heaviside) and is involved in the reflection of medium and medium-short waves and is formed mainly by the ionization of molecular oxygen by solar X-rays.
In particular sporadic conditions, currently being studied, a sub-region called ES appears for short periods of time (from a few minutes to a few hours) around 100km above sea level, characterized by highly ionized electronic clouds of lamellar shape and small thickness (about 2 km).
Radio amateurs who operate in the 6-meter band exploit the formation of the ES layer to be able to make connections on this band, which allows intercontinental connections in these particular moments.
The highest region is also the largest, it extends between 150 and 500km above sea level, called Region F (or Appleton) and is the most important for radio propagation, since it allows the reflection of short and VHF waves. During the day, it is possible to divide it into two layers: the first is located at about 200km above sea level and is called layer F1, while the second, layer F2 is located higher and is also more ionized.

Siti di riferimento e fonti

Vi lascio dei link che sono le fonti di questo articolo, e dai quali potrete approfondire l’argomento:

http://ionos.ingv.it/Roma/latest.html
http://www.mediasuk.org/iw0hk/ionosonda.htm
https://air-radiorama.blogspot.com/2015/09/labc-delle-ionosonde.html
https://it.wikipedia.org/wiki/Ionosonda
https://it.wikipedia.org/wiki/Ionosfera
https://www.ionosonde.iap-kborn.de/indexeng.htm
http://aintel.bi.ehu.es/chirps-data/chirps.html
https://www.hfunderground.com/wiki/index.php?title=Ionosonde&oldid=5940
https://www.earth-prints.org/bitstream/2122/15302/1/2021IonosondaSDR.pdf
https://www.rtl-sdr.com/tag/ionosonde/