Physics: Principles and Problems

Chapter 7: Gravitation

In the News

Communications satellites:

February 2005

Look at any satellite dish. You will see that it is aimed mainly toward the south. That is because all of our communications satellites orbit directly above the Equator. You have learned that the geosynchronous orbit has a period equal to our Earth day and has an altitude of about 22,300 miles. This makes the satellite appear to hover above a single spot on Earth and never change its position.

What exactly does a communication satellite do? (They're sure expensive.)

The satellite's job is to amplify and retransmit radio waves sent to it by antennas on Earth. Because it orbits high above the earth, it can handle signals that would otherwise be blocked by the curvature of the earth. The frequency of the radio waves is quite high, in the GHz region. This is because only a very small antenna is required for such high frequencies, and such a small antenna can be used with a parabolic dish reflector that is small enough to be manageable. The parabolic reflector makes the antenna very directional, which means that it is very sensitive to signals coming from only one direction. Note that the dish portion of a satellite antenna is only a reflector: the antenna itself is very small and located at the focus of the parabola.

Even though the satellite's transmitting antenna sends out a narrow beam of radio waves, the distance to earth is so great that the beam spreads over a third of our nation. That means that anyone living in that area can send and receive signals through that satellite.

How many communications satellites are there? (No, you can't just put up your own)

Though three satellites could provide service to the entire Earth, the capacity of only three satellites would be insufficient to handle all of the telephone calls, radio and television shows, and Internet traffic in the US. Moreover, there is a need for special geosynchronous satellites for defense purposes. Thus there are about thirty communications satellites in orbit over the continental United States at any time. The number cannot be greater than this because the satellites must stay in the geosynchronous orbit and also be located a minimum distance from each other so that their radio signals will not interfere with each other. Therefore a firm which wishes to launch a communications satellite must lease a location for it from the International Telecommunications Satellite Organization (INTELSAT), a consortium of companies which regulates communications satellites the world over.

Why might satellites interfere with each other? (One TV show at a time is quite enough)

The satellite receives and transmits signals at different frequencies. The channels used by each satellite are separated just like the stations on a radio dial. If the stations were to be set too close together, they would interfere with each other. Therefore, the number of channels that can be used on any satellite is limited, but still very high: a single satellite can handle thousands of simultaneous telephone and Internet circuits plus several hundred television transmissions.

Keeping the satellite aimed toward the earth (instead of, say, Mars)

It is critical that the satellite keep its antennas aimed accurately toward the earth. This is done in three ways. One is to make the satellite spin like a gyroscope. The antenna can be designed so that it will operate properly even though it is spinning on its axis. The solar batteries built into the body of the satellite are also designed to operate when the satellite spins. But better antennas can be used if they do not spin. Therefore later satellites spin the cylindrical section containing the solar batteries and the amplifiers, but use a motor to keep the antennas stationary. The most recent development keeps the satellite aimed with small rockets operated by compressed nitrogen. Enough nitrogen is stored so that the satellite can be kept aimed throughout its working life, which is generally limited by that of the storage batteries that keep the satellite operating at night.

Your batteries are dead. Adios.

When the satellite reaches the end of its life, special rocket motors are started. These push the satellite from its orbit and out into space to make room for its replacement.

Activity:

You are listening to the same radio broadcast on two different radios. One radio is tuned to a radio station located 100 miles away. The other radio is tuned to a local station which receives the same broadcast through a communications satellite in geosynchronous orbit. Because the local station's signal will have traveled up to the satellite and back, there will be a delay in the local station's signal compared with that of the station 100 miles away. Calculate this delay. Would you be able to hear it?

(answer: about ¼ second, and you can definitely hear the delay. Try beating seconds, and then divide the period into quarter seconds)

References:

http://www.hq.nasa.gov/office/pao/History/satcomhistory.html

http://www.lsi.usp.br/~rbianchi/clarke/ACC.ETRelays.html

http://www.pilotfriend.com/century-of-flight/Aviation%20history/space/Commercial%20Communications%20Satellites.htm

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