Sky Wave, Microwave Link Communication and Satellite Communication (SATCOM)

Overview

Sky Wave, Microwave Link Communication, and Satellite Communication  (SATCOM) are the focus of this article. Sky Waves are essentially AM waves that the ionosphere reflects. For long-distance communication on Earth, we employ standard microwave link transmission. However, we all know that the earth is not flat, but rather oval in shape. As a result, the signal can only reach a few kilometers on a straight line of sight path (LOS). The signal is then reflected by the earth's surface. But we know that with that microwave link, we can communicate hundreds of kilometers distance. We'll look at how this happens in this article. Terrestrial satellite communication has now replaced microwave relay link communication.

Figure: Ionosphere Reflection - suitable for AM band (Sky Wave)

1. Sky Wave

You can see how the ionosphere bounces the radio signal and enables the ground station to communicate with the transmitter hundreds of kilometers away. This method is ideal for communication at low or medium frequencies. Ionosphere reflects low (AM band) or medium (<30 MHz) range frequency.

Due to its high frequency, the microwave band cuts through the ionosphere, making it ideal for communicating with satellites. However, with the help of the ionosphere, we can efficiently receive a signal at a distance of roughly 650 kilometers from the transmitter.

1.1. Role of Ionosphere for Sky Wave

The ionosphere is one of the top levels of the earth's atmosphere, as we all know. Its altitude ranges from 30 to 600 miles above the earth's surface. We, as humans, reside in the troposphere. Atoms and molecules in the ionosphere are electrically charged. The ionosphere has the unique property of reflecting radio signals below 30 MHz. Signals with operating frequencies greater than 30 MHz penetrate the ionosphere layer. Also, do not return to Earth. In this situation, we always keep the frequency below 30 MHz. When we send a signal via a microwave link, the earth's ionosphere reflects it. In the diagram above, the transmitter emits a signal, which is then reflected by the ground and, in a similar way, by the ionosphere. According to our knowledge, a received signal reflected by the ionosphere is easier to receive by a receiver than a reflected signal by the ground.   

1.3. Frequency Range

535 to 1705 KHz

2. Microwave Link Communication

For terrestrial microwave communication, we depended largely on relay communication. For line-of-sight communication, each relay station is situated 150-200 kilometers apart. However, a key disadvantage of relay communication was that if one of the relay stations in the middle failed, the entire system would fail. As a result, we rely more on satellites. Microwave, on the other hand, is significantly more refracted by hills than low-frequency AM band. As a result, the considerable path loss is observed in the case of microwaves when employing a microwave relay link. In the case of microwave communication, the line of communication is also taken into account.  

 

 

Microwave link communication is used for long-distance microwave communication. In this situation, a powerful directional microwave beam is used to go further in the earth's troposphere. 

2.1. Frequency Range:

Although microwave frequencies can range from 300 MHz to 300 GHz, we often choose sub-6 GHz frequencies or frequencies between 1 and 6 GHz for microwave link communication.

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3. Satellite Communication

However, you should be aware that a line-of-sight connection between the transmitter and the receiver is not possible during the whole communication path. Before that signal is reflected by the ground, foliage, and other objects, it becomes scattered and weaker. On the other hand, if one intermediary relay fails, the entire communication system will stop working. As a result, we rely on the signal reflected by the satellites in this case. If there is no ionosphere or satellite, the signal can only travel 150 - 200 kilometers (approx.) as shown in the above figure. Satellite television is a real-world example of terrestrial satellite communication.

3.1. Frequency Range:

Frequencies range from 1 to 40 GHz.

Difference between advantages of tropospheric wave propagation and sky wave (waves reflected or refracted by ionosphere) propagation