Page 81 - Fisika Terapan for Engineers and Scientists

9.3 Circular Orbits 281

COMMUNICATIONS SATELLITES
PHYSICS IN PRACTICE
AND WEATHER SATELLITES

More than a hundred communications and weather
Concepts
in satellites have been placed in geostationary orbits.
Context
The communications satellites use radio signals to
relay telephone and TV signals from one point on
the Earth to another. The weather satellites capture pictures
of the cloud patterns and measure the heights of clouds, wind
speeds, atmospheric and ground temperatures, and moisture
in the atmosphere. These observations are especially useful
for monitoring weather conditions over the oceans, where
there are few observation stations at ground level. Data col-
lected by weather satellites permit early detection of danger-
ous tropical storms (hurricanes, typhoons) and forecasting of
the tracks and the strengths of these storms.
The launch vehicle for these satellites usually consists of
a two-stage rocket, which carries the satellite to a low-altitude
orbit. A small rocket motor attached to the satellite is then
used to lift the satellite from the low-altitude orbit to the
high-altitude geostationary orbit. Alternatively, the satellite
can be ferried to the low-altitude orbit by the Space Shuttle.
At the high altitude of the geostationary orbit there is no FIGURE 1 Astronauts handle an INTELSAT communications
atmospheric drag, and a satellite placed in such an orbit will satellite.
continue to orbit the Earth indefinitely. However, the orbital
motion of the satellite is disturbed by the Moon and the Sun, Figure 1 shows a recent model of the INTELSAT series
and it is also affected by the nonspherical shape of the Earth, of communications satellites. This satellite has a length of
which produces deviations from the ideal uniform centripetal 5.2 m, a diameter of 3.6 m, and a mass of 2240 kg. It is pow-
force. These disturbances cause the satellite to drift from its ered by solar panels that convert the energy of sunlight into
geostationary position.This requires an adjustment of the orbit electricity, delivering 2300 watts of power. It contains 50
every few weeks, which is done with small control nozzles on transponders and is capable of handling 40 000 telephone
the satellite. Typically, a satellite carries enough propellant to circuits simultaneously.
operate its control nozzles for 10 years, by which time other For intercontinental communications, three groups of
components in the satellite will also have worn out,or will have INTELSAT satellites are deployed at geostationary positions
been superseded by new technology, so it becomes desirable over the Atlantic, Pacific, and Indian Oceans. But communi-
to switch the satellite off, and replace it by a new model. cations satellites are also cost-effective for communications
Communications satellites contain a radio receiver and a over shorter ranges,when there is a shortage of telephone cables.
transmitter connected to dish antennas aimed at radio sta- Many countries have launched communications satellites to
tions on the ground.The signal received from one station on handle telephone traffic within their borders.Communications
the ground is amplified by the satellite, and then this ampli- satellites also relay TV transmissions. A small dish antenna
fied signal is retransmitted to the other station (the satellite connected to an amplifier permits home television sets to pick
acts as a transponder). up a multitude of TV channels from these satellites.

QUESTION 2: The period of the orbital motion of the Moon around the Earth is 27
days. If the orbit of the Moon were twice as large as it is, what would be the period of
its motion?
QUESTION 3: The mass of a planet can be determined by observing the period of a
moon in a circular orbit around the planet. For such a mass determination, which of