What are Electromagnetic Waves?
Mechanical waves and electromagnetic waves are two important ways that energy is transported in the world around us.
Electromagnetic waves differ from
mechanical waves in that they do not require a medium to propagate. This means
that they can travel not only through air and solid materials, but also through
the vacuum of space.
Electromagnetic waves have crests and troughs similar
to those of ocean waves. The distance between crests is the wavelength. The
shortest wavelengths are just fractions of the size of an atom, while the
longest wavelengths scientists currently study can be larger than the diameter
of our planet!
The number of crests that pass a
given point within one second is described as the frequency of the wave. One
wave—or cycle—per second is called a Hertz (Hz). A wave with two cycles that
pass a point in one second has a frequency of 2 Hz.
An electromagnetic wave can also be
described in terms of its energy—in units of measure called electron volts
(eV). An electron volt is the amount of kinetic energy needed to move an electron
through one volt potential. Moving along the spectrum from long to short
wavelengths, energy increases as the wavelength shortens.
Consider a jump rope with its ends
being pulled up and down. More energy is needed to make the rope have more
waves.
Depending on which range of
frequencies Electromagnetic Waves are moving, there are several types: Radio,
Microwaves, Infrared, Visible Light, Ultraviolet, X-ray and Gamma Ray.
What is a Communication Satellite?
A communication
satellite is a device used to receive, amplifies and transmit radio &
micro waves in space. The satellite has communications equipment including
receive and transmit antennas, power, and electronic components which enable to
receive a radio signal from a terminal, and then transmit that same radio signal
to another terminal.
The radio waves used for
telecommunications links travel by “line of sight” and so are obstructed by the
curve of the Earth. The purpose of satellites is to relay the signal around the
curve of the Earth allowing communication between widely separated geographical
points.
There are many functions and services which satellites are designed and used for: telephone communications, internet, video and TV distribution, etc
A vast array of satellites exist
with various Frequencies (C Band, Ku Band, L Band, etc), Altitudes (LEO, MEO,
GEO) and Orbital Planes (Equatorial, Circular, Inclined, Polar, etc)
Frequencies Used for Satellite Communications
Life of a Staellite
The design life of geostationary
satellites is approximatly 10-15 years.
Orbital Location and Footprint
The location of a satellite is
referred to its orbital position. All Geostationary Satellites are located in a
single ring above the equator. The requirement to space these satellites apart
means that there are a limited number “slots” available, thus only limited
number of satellites can be placed in geostationary orbit.
The location of a satellite is normally measured in
terms of longitudinal degrees East or West from the prime Meridian of 0
degrees.
The area of Earth’s surface for
coverage of transmit to or receive from is called the footprint which can be
tailored for different frequencies and power levels.
Uplink and Downlink
Signals transmitted from Earth to
Satellite are referred to as uplink signals, and signals received from the
Satellite are downlink signals.
The satellite, through the
transponder, converts the signal before it retransmits back to earth.
The signals going up to the satellite
are at one frequency range (band of frequencies) and the satellite changes them
to a different frequency range coming down so they won´t interfere with the
signals going up.
As an example: “C Band” uplinks at 6
GHz and downlinks at 4 GHz and “Ku Band” uplinks at 14 GHz and downlinks at 12
GHz.
Throughput rates (Mbps) say us how
much data actually is sent/received to/from a Satellite. Of course, throughput
rates depends on “Uplink/Downlink” signals and the frequencies assigned.
Satellites and Orbits
Geosynchronous Orbit (GEO) are located 35,786 km
above Earth. A single satellite can view approximately 1/3 of Earth´s surface.
They travel in the same direction and speed as Earths´s rotation so they appear
“stationary” and Earth station do not need to track the satellite.
Medium Earth Orbit (MEO) are located
8,000-20,000 Km above Earth. Typically, they have an elliptical (oval-shaped)
orbit, but some travel in near perfect circles. The orbital period is anywhere
from 2 to 12 hours. The most common use for satellites in this region is for
navigation, communication, and geodetic/space environment science. They are
used by GPS satellites. Communications satellites that cover the North and
South Pole use MEO satellites.
Low Earth Orbit (LEO) are located 500-2,000
Km above Earth. LEOs are much closer to earth and travel at high speed to avoid
being pulled out of orbit by Earth’s gravity. They orbit Earth about every 90
minutes. The international space station is a LEO.
What is Installed on the Ground?
All communications with a
geostationary satellite requires the use of Earth stations. They may be fixed
or mobile, from small to very large antennas.
The Earth station typically consists
of an antenna, RF (radio frequency) equipment to Transmit&Receive, indoor unit
and the final communications devices. Final communications devices could be
local or off site via terrestrial network.
A teleport or super hub is
essentially a large version of a typical Earth station. Teleports have similar
equipment to a remotes but the equipment will be hub centric since it is
looking at many remotes, rather than the remote just looking at the hub.
As well teleports will also have
extra reliability by means of backup power, redundancy of equipment, and
sometimes the ability to counteract the effects of fading (uplink power
control).
Types of Satellite Services
There are several defined types of
satellite service.
Fixed Satellites Services (FSS), so-called
because the terminals on the ground are in fixed locations
Mobile Satellite Services (MSS) , where the
terminals can be fixed, or in motion such as on a vehicle, a ship or even an
airplane.
The worldwide market for fixed
satellite services (FSS) is now over $10 billion annually and is significantly
larger than the worldwide market for mobile satellite services (MSS).
Historically, Fixed systems have
higher throughput and lower operating costs than Mobile systems as a rule. But
Fixed Satellite Services (FSS) hardware is more expensive and features larger
antennas. They are often susceptible to damage from sand, snow or rain.
Mobile systems have smaller antennas, lower hardware
costs and broader coverage. But the cost per minute of use is much higher than
Fixed systems, and the throughput rates are far lower than those for Fixed systems.
Demand for both FSS and MSS is
growing rapidly and the distinction between the two is becoming blurred as fixed
antennas get smaller and mobile terminals accommodate higher throughput speeds.
A third class is “broadcast satellite service”
(BSS). Signals are transmitted or retransmitted by space stations are intented
for direct reception by the general public. Subscribers receive signals
directly from geostationary satellites. Signals are broadcast in digital format
at microwave frequencies.
A subscriber needs an installation
of a dish antenna, a conventional TV set, a signal converter placed next to the
TV set, and a length of coaxial cable between the dish and the converter.
The dish intercepts microwave
signals directly from the satellite. The converter produces output that can be
viewed on the TV receiver.
What is a Backhaul?
A backhaul (or local loop) is the
intermediate link between a core network (teleport or hub) to smaller networks
or devices at the edge of the network. It is the physical link or circuit that
connects the customers premises to an Earth station.
A backhaul is usually more
cost-effective than a customer having their own hub or teleport.
Fading
Satellite Services are subject to
fading.
The higher the frequency the more
the signal may be affected. C Band is less affected than Ku. Ku is less
affected than Ka.
Fading can severely affect service
when heavy rain or snow is present.
Tolerances are built into the power
levels of the transmitted services to minimize the effect. These tolerances are
referred to as fade margins.
That means we transmit more power
than what is needed during clear sky conditions and the amount of this power is
determined by the link budget analysis.
System design will include a margin to
accommodate some signal reduction by precipitation. How much fade margin is
used will be determined by the customer’s service availability requirements.
Even with fade margin there still
will be some instances where the density of clouds and rain reduces the signal
enough that it affects data with errors, or a voice call gets noisy, or it
affects a TV channel.
