ENG. Mohamed Hamdi Omer
What is the satellite .
Orbital mechanics .
Orbital altitude and velocity.
Types of satellites.
Types of orbits.
Space junk .
A satellite is a moon, planet or machine that orbits a planet or star. For
example, Earth is a satellite because it orbits the sun. Likewise, the
moon is a satellite because it orbits Earth. Usually, the word "satellite"
refers to a machine that is launched into space and moves around
Earth or another body in space.
Earth and the moon are examples of natural satellites. Thousands of
artificial, or man-made, satellites orbit Earth. Some take pictures of
the planet that help meteorologists predict weather and track
hurricanes. Some take pictures of other planets, the sun, black
holes, dark matter or faraway galaxies. These pictures help scientists
better understand the solar system and universe.
Still other satellites are used mainly for communications, such as
beaming TV signals and phone calls around the world. A group of
more than 20 satellites make up the Global Positioning System, or
GPS. If you have a GPS receiver, these satellites can help figure out
your exact location.
The bird's-eye view that satellites have allows them to see large areas of
Earth at one time. This ability means satellites can collect more data, more
quickly, than instruments on the ground.
Satellites also can see into space better than telescopes at Earth's surface.
That's because satellites fly above the clouds, dust and molecules in the
atmosphere that can block the view from ground level.
Before satellites, TV signals didn't go very far. TV signals only travel in
straight lines. So they would quickly trail off into space instead of
following Earth's curve. Sometimes mountains or tall buildings would
block them. Phone calls to faraway places were also a problem. Setting up
telephone wires over long distances or underwater is difficult and costs a
With satellites, TV signals and phone calls are sent upward to a satellite.
Then, almost instantly, the satellite can send them back down to different
locations on Earth.
Satellites come in many shapes and sizes. But most
have at least two parts in common -- an antenna
and a power source. The antenna sends and
receives information, often to and from Earth. The
power source can be a solar panel or battery. Solar
panels make power by turning sunlight into
Many satellites carry cameras and scientific
sensors. Sometimes these instruments point
toward Earth to gather information about its land,
air and water. Other times they face toward space
to collect data from the solar system and universe.
Through a lifelong study of the motions of bodies in the solar
system, Johannes Kepler (1571-1630) was able to derive three basic
laws known as Kepler's laws of planetary motion. Using the data
compiled by his mentor Tycho Brahe (1546-1601), Kepler found
the following regularities after years of laborious calculations:
1. All planets move in elliptical orbits with the sun at one focus.
2. A line joining any planet to the sun sweeps out equal areas in
3. The square of the period of any planet about the sun is
proportional to the cube of the planet's mean distance from the
These laws can be deduced from Newton's laws of motion and
law of universal gravitation. Indeed, Newton used Kepler's work
as basic information in the formulation of his gravitational theory.
After completion, a newly manufactured satellite is transferred to the
launch site for final testing and fuelling, before being mated to the launch
vehicle. The launch is a complex operation conducted in several stages,
which differ according to the system used.
One common method of placing satellites into geostationary orbit
is based on the Hohmann transfer principle. Using this system the
satellite is placed into a low earth orbit with an altitude of around
300 kilometers. Once in the correct position in this orbit rockets
are fired to put the satellite into an elliptical orbit with the perigee
at the low earth orbit and the apogee at the geostationary orbit.
When the satellite reaches the final altitude the rocket or booster is
again fired to retain it in the geostationary orbit with the correct
Alternatively the satellite is launched directly into the elliptical
transfer orbit, typically between 200km and several thousand
kilometres from the Earth. Again when the satellite is at the
required altitude the rockets are fired to transfer it into the
required orbit with the correct velocity.
Mission control monitors the launch and once
the satellite has been released by the launch
vehicle into geosynchronous orbit, the satellite
operator takes over to acquire the telemetry
signal; monitor proper deployment of the solar
arrays; fire the necessary satellite motors to fly
the satellite to its proper location and
then, puts it through a series of in-orbit tests
before validating the satellite and ground
stations for commercial entry into service.
Most satellites are launched into space on rockets. A satellite
orbits Earth when its speed is balanced by the pull of Earth's
gravity. Without this balance, the satellite would fly in a
straight line off into space or fall back to Earth. Satellites
orbit Earth at different heights, different speeds and along
different paths. The two most common types of orbit are
"geostationary" (jee-oh-STAY-shun-air-ee) and "polar."
A geostationary satellite travels from west to east over the
equator. It moves in the same direction and at the same rate
Earth is spinning. From Earth, a geostationary satellite looks
like it is standing still since it is always above the same
Polar-orbiting satellites travel in a north-south direction
from pole to pole. As Earth spins underneath, these satellites
can scan the entire globe, one strip at a time.
SPUTNIK, OCT. 4, 1957
Because of Soviet government secrecy at the time, no
photographs were taken of this famous launch. Sputnik
was a 23-inch (58-centimeter), 184-pound (83-kilogram)
metal ball. Although it was a remarkable achievement,
Sputnik's contents seem meager by today's standards:
Radio transmitter - changed the tone of its beeps to
match temperature changes
Nitrogen gas - pressurized the interior of the satellite
On the outside of Sputnik, four whip
antennas transmitted on short-wave frequencies above
and below what is today's Citizens Band (27 MHz)
A satellite in a geostationary orbit appears to be
in a fixed position to an earth-based observer.
A geostationary satellite revolves around the
earth at a constant speed once per day over the
equator. The geostationary orbit is useful for
communications applications because ground
based antennas, which must be directed
toward the satellite, can operate effectively
without the need for expensive equipment to
track the satellite’s motion.
LEO is typically a circular orbit about 400 to 900 kilometres above
the earth’s surface and, correspondingly, has a much shorter
period (time to revolve around the earth) of about 90 minutes.
Because of their low altitude, these satellites are only visible from
within a small area (about 1000 km radius) beneath the satellite as
it passes overhead. In addition, satellites in low earth orbit change
their position relative to the ground position quickly. So even for
local applications, a large number of satellites are needed if the
mission requires uninterrupted connectivity. For this reason, LEO
satellites are often part of a group of satellites working in concert
otherwise known as a satellite constellation. Low earth orbiting
satellites are less expensive to launch into orbit than geostationary
satellites and, due to proximity to the ground, do not require as
high a signal strength.
Molniya orbits can be an appealing alternative. The Molniya orbit is
highly inclined, guaranteeing good elevation over selected positions
during the northern portion of the orbit. (Elevation is the extent of the
satellite’s position above the horizon. Thus, a satellite at the horizon has
zero elevation and a satellite directly overhead has elevation of
90 degrees).The Molniya orbit is designed so that the satellite spends the
great majority of its time over the far northern latitudes, during which its
ground footprint moves only slightly. Its period is one half day, so that
the satellite is available for operation over the targeted region for eight
hours every second revolution. In this way a constellation of three
Molniya satellites (plus in-orbit spares) can provide uninterrupted
coverage. Molniya satellites are typically used for telephony and TV
services over Russia. Another application is to use them for mobile radio
systems (even at lower latitudes) since cars travelling through urban
areas need access to satellites at high elevation in order to secure good
connectivity, e.g. in the presence of tall buildings
MEO is the region of space around the Earth above
low Earth orbit and below geostationary orbit . The
most common use for satellites in this region is for
navigation, such as the GPS (with an altitude of
20,200 kilometres), Glonass (with an altitude of
19,100 kilometres) and Galileo (with an altitude of
23,222 kilometres) constellations. Communications
satellites that cover the North and South Pole are
also put in MEO. The orbital periods of MEO
satellites range from about 2 to 24 hours.
Telstar, one of the first and most famous
experimental satellites, orbits in MEO.