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Basics Satellite TV
Introduction
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A satellite is an object that orbits around
another object in space. The satellites that concern us transmit
television directly to the consumer. These satellites require a special
orbit, so a little information about orbits is a good place to start in
understanding satellite television basics.
The time it takes for a satellite to complete an orbit depends upon its
distance from the object that it orbits. The moon is a natural satellite
that is 238,328 miles from Earth. It takes 27.32 days for the moon to
complete one orbit of the Earth. The space shuttle operates in a low
Earth orbit. The shuttle orbits less than 200 miles to about 350 miles
above the Earth. The average time for the space shuttle to complete an
orbit around the Earth is about 90 minutes. The moon takes longer to
complete an orbit of the Earth because it is much further from the Earth
than is the space shuttle. The type of orbit that is required of the
satellites that transmit television signals to the Earth lies between
these two extremes.
In order for a satellite to be used for television transmission, it must
"hang" over one spot above the Earth. The types of orbits
described so far would require a receiving dish that is constantly
moving, in order to keep up with the transmitting satellite. Satellite
television satellites are in a geo synchronous orbit that exactly
matches the speed that the Earth spins. When a satellite is in a geo
synchronous orbit, the satellite appears to be stationary when viewed
from the ground. In order to accomplish a geo synchronous orbit, a
satellite must be directly over the equator and about 22,300 miles from
the Earth. This area around the Earth is often called the Clarke Belt.
Satellites maintain proper positioning in the Clarke Belt with onboard
fuel. Ground stations constantly monitor these satellites to make any
adjustments that are necessary to keep a satellite in it's orbital slot.
The television signals transmitted by a satellite are quite different
from the television or radio signals that are broadcast over the air.
The particulars of a satellite TV signal are beyond the scope of this
small article, but there are some basics that you should know. Satellite
TV is transmitted by microwaves. Microwaves don't behave like the lower
frequency radio waves of off-air television or radio, which can bounce
off obstructions, clouds, and the ground. Microwaves are strictly line
of sight. In order for a satellite dish to receive a signal, there can
be no obstruction between the transmitting satellite and the receiving
satellite dish.
Satellite television in the USA is divided into two major types. The
first major type is TVRO. TVRO satellite systems have a large dish which
is movable. The movable dish enables a TVRO system to view programs on
the many satellites that are positioned in the Clarke Belt. TVRO
satellite systems are also called BUD, Big Dish, C-Band, and Full View
satellite TV. Just remember that if the dish is large (usually 6 - 12
feet across) and it moves, it is a TVRO satellite system. To find out
more about TVRO satellite systems go to TVRO Basics.
The second major type of satellite TV is DBS. DBS systems have a small
dish (18 inches to 3 feet across) that does not move. In the US there
are currently 2 major types of DBS satellite systems. Each DBS system
requires it's own special receiving equipment and has it's own
programming line up. The 2 major types of DBS systems are DIRECTV and
DISH Network.
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