Part 3 of Section 1 (SATELLITE COMMUNICATIONS - A SHORT COURSE) of SATELLITE COMMUNICATIONS, prepared by Dr. Regis Leonard for NASA Lewis Research Center
Every communications satellite in its simplest form
(whether low earth or geosynchronous) involves the transmission of information from an
originating ground station to the satellite (the uplink), followed by a retransmission of
the information from the satellite back to the ground (the downlink). The downlink may
either be to a select number of ground stations or it may be broadcast to
everyone in a large area. Hence the satellite must have a receiver and a receive antenna,
a transmitter and a transmit antenna, some method for connecting the uplink to the
downlink for retransmission, and prime electrical power to run all of the electronics. The
exact nature of these components will differ, depending on the orbit and the system
architecture, but every communications satellite must have these basic components. This is
illustrated in the drawing below.
Basic Components of a Communications Satellite Link
The amount of power which a satellite transmitter needs to send out depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same. (Fortunately, of course, we change some other things so that we don't need 10,000 times as much power.)
OPTIONAL FOR THE MATHEMATICALLY INCLINED In looking at the relative power requirements of
satellites at different distances, it is useful to think of the total power (P0)
radiated as spreading out and striking the surface of a sphere which is centered on the
transmitter and has a radius equal to the distance between the transmitter and receiver. |
For either geosynchronous or low earth satellites, the power put out by the satellite transmitter is really puny compared to that of a terrestrial radio station. Your favorite rock station probably boasts of having many kilowatts of power. By contrast, a 200 watt transmitter would be very strong for a satellite.
One of the biggest differences between a low earth
satellite and a geosynchronous satellite is in their antennas. As mentioned earlier, the
geosynchronous satellite would require nearly 10,000 times more transmitter power, if all
other components were the same. One of the most straightforward ways to make up the
difference, however, is through antenna design. Virtually all antennas in use today
radiate energy preferentially in some direction. An antenna used by a commercial
terrestrial radio station, for example, is trying to reach people to the north, south,
east, and west. However, the commercial station will use an antenna that radiates very
little power straight up or straight down. Since they have very few listeners in those
directions (except maybe for coal miners and passing airplanes) power sent out in those
directions would be totally wasted.
The communications satellite carries this principle even further. All of its listeners are
located in an even smaller area, and a properly designed antenna will concentrate most of
the transmitter power within that area, wasting none in directions where there are no
listeners. The easiest way to do this is simply to make the antenna larger. Doubling the
diameter of a reflector antenna (a big "dish") will reduce the area of the beam
spot to one fourth of what it would be with a smaller reflector. We describe this in terms
of the gain of the antenna. Gain simply tells us how much more power will
fall on 1 square centimeter (or square meter or square mile) with this antenna than would
fall on that same square centimeter (or square meter or square mile) if the transmitter
power were spread uniformly (isotropically) over all directions. The larger antenna
described above would have four times the gain of the smaller one. This is one of the
primary ways that the geosynchronous satellite makes up for the apparently larger
transmitter power which it requires.
OPTIONAL FOR THE MATHEMATICALLY INCLINED Antenna gains, like many power specifications are
usually quoted in decibels (dB). The ratio of two power levels in decibels is defined as: |
One other big difference between the geosynchronous
antenna and the low earth antenna is the difficulty of meeting the requirement that the
satellite antennas always be "pointed" at the earth. For the geosynchronous
satellite, of course, it is relatively easy. As seen from the earth station, the satellite
never appears to move any significant distance. As seen from the satellite, the earth
station never appears to move. We only need to maintain the orientation of
the satellite. The low earth orbiting satellite, on the other hand, as seen from the
ground is continuously moving. It zooms across our field of view in 5 or 10 minutes.
Likewise, the earth station, as seen from the satellite is a moving target. As a result,
both the earth station and the satellite need some sort of tracking capability which will
allow its antennas to follow the target during the time that it is visible. The only
alternative is to make that antenna beam so wide that the intended receiver (or
transmitter) is always within it. Of course, making the beam spot larger decreases the
antenna gain as the available power is spread over a larger area , which in turn increases
the amount of power which the transmitter must provide.
You might wonder why we don't actually use
transmitters with thousands of watts of power, like your favorite radio station does. You
might also have figured out the answer already. There simply isn't that much power
available on the spacecraft. There is no line from the power company to the satellite. The
satellite must generate all of its own power. For a communications satellite, that power
usually is generated by large solar panels covered with solars cells - just like the ones
in your solar-powered calculator. These convert sunlight into electricity. Since there is
a practical limit to the how big a solar panel can be, there is also a practical limit to
the amount of power which can generated. In addition, unfortunately, transmitters are not
very good at converting input power to radiated power so that 1000 watts of power into the
transmitter will probably result in only 100 or 150 watts of power being radiated. We say
that transmitters are only 10 or 15% efficient. In practice the solar cells
on the most "powerful" satellites generate only a few thousand watts of
electrical power.
Satellites must also be prepared for those periods when the sun is not visible, usually
because the earth is passing between the satellite and the sun. This requires that the
satellite have batteries on board which can supply the required power for the necessary
time and then recharge by the time of the next period of eclipse.
Go to Part 4 (FUTURE COMMUNICATIONS SATELLITES) of Section 1
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