Communications Frequencies

Information on a satellite communications link is carried by electromagnetic (EM) waves; Figure 6.2 in Chapter 6 illustrated the various parts of the EM spectrum. As a consequence, the speed of communication is the speed of light, which is around 300,000 km per sec (186,000 miles per second), so that communication with spacecraft in LEO is effectively instantaneous. However, for a communication satellite in GEO, the altitude of the satellite is around 38,000 km, so that EM waves take just over a tenth of a second to travel from the ground to the spacecraft. This may not seem a lot, but bear in mind that for me (in the United Kingdom) to hold a telephone conversation with someone in the United States requires four such trips for the EM waves; my voice needs to travel up to the satellite, and then down to a ground station in the U.S. My friend's response then needs to make the same return trip, requiring about half a second of travel time. If I talk with someone in Australia, the communications route may involve more than one satellite, so the travel time can be large enough to produce awkward pauses that can disrupt the flow of a conversation. Of course, for interplanetary spacecraft the distances are such that a two-way conversation is not an option; for example, the travel time one-way for EM waves to a spacecraft at Saturn is at least an hour and a quarter.

The wavelength of EM radiation used for satellite communications is between 2 and 30 cm, which is referred to as the microwave part of the EM spectrum (see Figure 6.2). This is also the part of the spectrum used by microwave ovens to heat up dinner in the evening. This type of cooker heats food by bombarding it with EM radiation with a wavelength of typically about 12 cm. Fortunately, the microwave beam from a large satellite communications dish antenna at a ground station is well focused along the axis of the dish and is usually pointed skywards, so it is not a health hazard. Figure 9.10 reviews how the wavelength of EM radiation is defined, but it also shows some other important features. The intensity of the radiation is governed by the amplitude of the wave—or the wave height. In terms of the visible part of the spectrum, for example, a bright light has a larger amplitude than a dim one. The figure also shows how the phase of an EM wave is defined. This is measured in degrees, and indicates where you are on the wave along its wavelength; for example, the leading edge of the wave is defined to be at 0-degree phase, the first crest is at 90 degrees, the trough at 270 degrees, and so on. We will see later why this feature of the wave is important.



Figure 9.10: The wavelength, amplitude, and phase of an EM wave (see text).

Another aspect of communications is that it is common to talk about the part of the spectrum in terms offrequency, rather than wavelength. For every wavelength of EM radiation, there is a corresponding frequency. Generally speaking, short wavelength radiation has a high frequency and long wavelength radiation has a low frequency. This focus on frequency can be seen by looking at a domestic FM stereo radio. A radio station may be listed on the tuning dial as having a frequency of, say, 100 MHz, where the "M" is an abbreviation for "Mega," meaning a million, so we have a frequency of 100 million Hz. "Hz" is short for Hertz, which means cycles per second, named in honor of Heinrich Hertz, a German physicist who made important contributions to electromagnetism. Thus the frequency of the radio station is 100 million cycles per second but maybe this still doesn't mean much to you. Another way of thinking about this is that the wavelength of this signal is such that 100 million wavelengths pass the radio every second traveling at the speed of light. Given that the speed of EM waves is constant, for this to happen a simple calculation shows that the wavelength of a 100-MHz signal must be 3 m.

Satellite communications occur at a higher frequency, and so have a shorter wavelength. Typically the frequencies used are between 1 and 15 GHz, where the "G" stands for "Giga," meaning 1000 million. So the frequencies used are between 1000 million and 15,000 million cycles per second, which (as mentioned above) correspond to a wavelength range of30 cm to 2 cm, respectively. Why are these particular frequencies used? This choice is dictated by the physics of the atmosphere. For a ground station to talk to a spacecraft, the EM waves must necessarily pass through Earth's atmosphere. However, for frequencies less than about 1 GHz, the energy of the radiation is sapped by charged particles, such as electrons, in the ionosphere. The ionosphere is a region of Earth's atmosphere at heights greater than about 80 km where the atmosphere's molecules of oxygen, nitrogen, etc. are stripped of their electrons by the Sun's ultraviolet radiation. For frequencies higher than 15 GHz, the radiation is absorbed by molecules of water vapor and oxygen in the lower part of the atmosphere. Thus the 1- to 15-GHz frequency range provides a convenient "window" through which the ground station can talk to the spacecraft, and vice versa.

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