Satellite Communications and How They Work

Satellite communications is an innovation of discovery in the technology world. It has bridged the gap between so many countries. Without satellites or use of satellites as a means of communication, the world would be set back just a bit. Satellite communications is abbreviated as SATCOM, which I will be using for the duration of this hub.

A satellite used for telecommunications usually has 3 types of orbits around the earth. The first one, which the US primarily uses, is a geosynchronous orbit. A geosynchronous (geosync) orbit is a orbit in which the satellite is in synchronous orbit with the Earth and it remains in a constant position. For instance, you could connect to a satellite in New York and then connect to it in Arizona, all you have to do is orient your dish at the correct azimuth and elevation. In geosync orbit, the satellite is approximately 22,300 miles from the earth surface. The second type of orbit is Molniya orbit, which Molniya is Russian for lightning. A Molniya orbit is a "closest approach' orbit and primarily the Russians use this orbit on their satellites. The third orbit is an elliptical orbit. Not too many satellites use this type because it requires expensive gear on ground stations to track these types of satellites. Only government agencies of the highest clearance use these. Since I am an American and have used only the geosync satellites, these are going to be the ones that I will talk about.

When talking about SATCOM, one has to understand the theory behind SATCOM. The theory that it "should" work, as we used to joke about this in the Marine Corps. Hence, why it is still theory because even if you do everything right, it still could not work. In order to understand SATCOM theory, we would have to discuss radio wave propagation. I will explain further in detail about radio wave propagation later on in this HUB. But first, SATCOM theory. First, we shall cover some basic components of communication and briefly described their use. Each ground station has a parabolic dish, a transceiver (up/down converter), a modem, a diplexer, a spectrum analyzer, and a high powered amplifier (HPA). These are the basic components, there are numerous other components but these are the key components of communication. The transceiver contains the up converter and down converter, which when transmitting converts the 70 MHz IF frequency (base-band signal) to an output signal of 14-14.5 GHz Radio Frequency. The IF or Intermittent Frequency is not strong enough to travel the required distance to the geosync satellite. And it is reverse process for the down converter side of the transceiver. The modem (modulator/demodulator) modulates and demodulates the signal for transmission, also has other communication parameters stored in it required for communication. The diplexer couples the base-band IF Data and control data between the modem and the antenna control unit. The spectrum analyzer allows the user to visually see the receiving signal and their transmitting signal, provides to be quite helpful when troubleshooting a circuit. The high powered amplifier boosts the signal to travel the 22,300 miles to reach the geosync satellite.

Now time to get into the "nitty gritty", without trying to confuse anyone, I will keep it simple. So, you have a centralized ground station transmitting to a geosync satellite. Their signal, which is still in IF, travels through their diplexer and modem at the same time and is sent to the up converter. From their after it is converted from IF to RF, it goes the HPA. The HPA boosts the signal and sends it out to the ground terminal dish equipment and then out into space to the satellite. Once it hits the satellite, the signal is then cleaned up and re-transmitted back down to the receiving station. The signal hits the receiving station's dish and begins its travel. It goes to the down converter, gets converted back to IF. Then proceeds to the modem and diplexer. The signal gets demodulated for data (i.e. digital signal), the diplexer sends the signal to the spectrum analyzer. This allows the receiving station to monitor that signal if power of transmitting station needs to be raised or lowered. This basically the process of SATCOM, but all of this is at the speed of light because RF radiation is a wavelength of light. Again, like I had said before, everything could be setup perfectly and still not work, hence why it still only a theory. I will explain radio wave propagation and hopefully that will clarify why it is theory.

Propagation is the movement a of a waveform through a medium. With radio waves, it is normally the Earth's atmosphere, space, ground, and water (submarine communications). Since, radio waves are electromagnetic, they are subject to a number of phenomena whilst moving through the atmosphere. The ionosphere is an electrically charged region of the Earth's atmosphere that lies 50-500km above the surface of the earth. It contains four layers. "D" Layer is present when the sun is above the horizon not sufficient to refract a radio wave, and absorbs energy from a radio wave as it passes through it. "E" Layer is the lowest layer that is sufficiently ionized to refract radio waves, its behavior can be predicted to a degree based on time of day, season, and solar activity. "F" Layer consists of two layers (F1 and F2) during daylight hours and merge into one at night, behavior is irregular and difficult to predict.

Other factors affecting signal quality are absorption, reflection, diffraction, and refraction. Absorption is the dissipation of a wave's energy as it travels through a medium. Reflection is when a wave hits a denser medium and is reflected off at the same angle at which it struck. Diffraction is the bending of a radio wave around an obstacle in its path to reform the wave some distance behind the obstacle. Refraction occurs when a radio wave strike a region of different density and is bent off its original course. You also have solar effects as well such as sun spots, solar flares, etc. that will affect communication.

Hopefully, after explaining some of radio wave propagation, it brings the reader a clearer understanding of SATCOM. Now, both areas (SATCOM and radio wave propagation) go into a much more vast detail than what I have provided, this information was provided so that the reader would have the simplistic idea of how SATCOM works. Hopefully, this HUB has been informative and educational. Feel free to comment, however be warned, any distasteful comments will be deleted and that user will be reported to HUB.

References: Marine Corps training.