FOR SATELLITE COMMUNICATIONS
|Catch The Wave
Whenever we listen to radio broadcasts, watch TV, use a cellular
phone or talk on a CB radio, invisible waves of electro-magnetic
energy are bringing us messages from distant locations. These invisible
waves continually bombard us as we walk down the street, play sports
or putter about the garden. We only become aware of the electro-magnetic
soup that surrounds us if we have the right antenna and receiver
for tuning in to these signals.
Back at the turn of the Century, Marconi
discovered that it was possible to combine messages with invisible
waves of electro-magnetic energy that radiate through space at the
speed of light. For the first time ever, mankind was able to communicate
over vast distances almost instantaneously. By the late-1920s, millions
of people world-wide were tuning in to AM radio stations: the world's
first electronically generated virtual realities. Back
then, these transmissions were simply called radio waves. As time
went on, however, it became apparent that waves of electro-magnetic
energy could be used to transmit all sorts of information, including
many respects, electro-magnetic waves are similar to waves on the
ocean. In one complete cycle of a wave, water rises
from sea level until it swells to reach the crest of the wave, then
plummets downward into the wave trough before rising again to sea
level. A communication signal is the electro-magnetic equivalent
of a message in a bottle; it rides a never ending succession of
waves before arriving at its final destination. The frequency
of any communications signal is the number of cycles per second
at which the radio wave vibrates or cycles. Electro-magnetic
waves cycle at phenomenal rates: one thousand cycles per second
is called a kilohertz (kHz), one million cycles per second a megahertz
(MHz), and one billion cycles per second a gigahertz (GHz). Today
we refer to the continuum of frequencies used to propagate communications
signals, 100 kHz to 100 GHz and beyond, as the electro-magnetic
The distance that each wave travels
during a single cycle is called its wave length. There
is an inverse relationship between frequency and wave length: the
higher the frequency, the shorter the wave length.
Each sub-set or band of
frequencies within the electromagnetic spectrum has unique properties
that are the result of changes in wave length. For example, Medium
Wave signals (500 kHz to about 3 MHz) radiate along the Earth's
surface over hundreds of miles, perfect for relaying AM radio stations
throughout a region.
International radio stations use the
Short Wave bands (3 to 30 MHz) to span distances of
thousands of miles; the ionosphere, upper layers of the Earth's
atmosphere that are electrically charged by the sun, reflects these
short waves back down to Earth, much as a mirror or any other shiny
metal object can reflect beams of light.
TV and FM radio broadcasters use the
Very High Frequency (VHF) and Ultra High Frequency (UHF) bands located
from 30 to 300 MHz and 300 to 900 MHz because these signals only
cover short distances; they cannot travel very far along the Earth's
surface or skip off the ionosphere. The advantage to using these
frequency bands for local communications is that dozens of TV and
FM radio stations can use identical frequencies within any one country
or region without causing interference.
|Dawn of the Satellite Age
The International Telecommunication
Union, an agency of the United Nations, has set aside space in the
super high frequency (SHF) bands located between 2.5 and 22 GHz
for satellite transmissions. At these frequencies, the wave length
of each cycle is so short that the signals are called microwaves.
These microwaves have many characteristics of visible light: they
travel directly along the line of sight from any satellite to its
primary coverage area and are not impeded by the Earth's ionosphere.
The scientists who developed the first
microwave radar systems during World War II assigned a letter designation
to each microwave frequency band. For example, the 800 MHz to 2
GHz frequency range was called the L band, 2 to 3 GHz: the S band;
3 to 6 GHz: the C band; 7 to 9 GHz: the X band; 10 to 17 GHz: the
Ku band; and 18 to 22 GHz: the Ka band. At the dawn of the Satellite
Age during the mid-1960s, microwave engineers decided to carry forward
the existing radar terminology and apply it to the communications
satellite bands as well.
first commercial satellite systems used the C band frequency range
of 3.7 to 4.2 GHz. By the late 1960s, many telephone companies around
the world had numerous terrestrial microwave relay stations operating
within the 3.7 to 4.2 GHz frequency range. The amount of power that
any C-band satellite could transmit had to be limited to a level
that would not cause interference to terrestrial microwave links.
The first commercial Ku band satellites
made their appearance in the late 1970s and early 1980s. Relatively
few terrestrial communications networks were assigned to use this
frequency band; Ku-band satellites could therefore transmit higher-powered
signals than their C-band counterparts without causing interference
problems down on the ground.
Ku-band satellite antennas have a much
narrower beam width, the corridor through which the dish looks up
at the sky, than C-band parabolic antennas of a given diameter.
There is a direct relationship between wavelength and antenna beam
width: the shorter the wavelength, the narrower the beam width.
From the graph above, we can see that
a 60cm C-band antenna could potentially receive three satellites
within its main beam if the satellites are separated by 3 degrees
in longitude. In North America, where 2 degree spacing has become
the norm, a 60cm C-band antenna could potentially have signals from
four satellites falling within its main beam.
to any satellite's orbital location, as well as to the intervals
between adjacent satellites, are made in degrees of longitude. Keep
in mind, however, that the geostationary orbit is a circle and the
reference point for the calculation of degrees longitude is the
Earth's center. Since we all live on the surface of the Earth, the
apparent spacing between two satellites will be greater than the
actual spacing in degrees of longitude. The precise amount of variance
between actual and apparent spacing is a function of site latitude
and the difference between site longitude and satellite longitude.
For the purpose of calculating the charts which appear in this article
I have assumed an apparent spacing of 3.4 degrees between satellites
which are separated by three degrees in longitude.
The next graph shows the equivalent
performance of the various antenna apertures when receiving Ku-band
satellite signals. The 30 to 60cm dish becomes possible because
of the dramatic reduction in parabolic antenna beam width that takes
place when we use the higher satellite frequency bands.
|Ku-band Satellite TV: We've
Got the Power! |
strength is expressed in decibels referenced to one Watt of power
(dBW). An increase of 3 dBW represents a doubling of power; 10 dBW
represents a ten-fold increase; and 20W dBW a one-hundred-fold increase.
C-band satellites typically transmit
signal levels ranging from 33 to 38 dBW. The strongest signals fall
within the center of each satellite's coverage beam, with signal
intensity decreasing outward from there. Depending on the location
of the receiving site within the satellite's primary coverage beam,
the antenna apertures required to receive crystal clear TV pictures
typically range from 2.0 to 3.7m in diameter. Today's Ku-band satellites
transmit nominal signal levels ranging from 47 to 52 dBW, a 14 dBW
increase in power over what most C-band satellites can deliver.
Receiving antennas as small as 30cm in diameter can therefore be
used to receive Ku-band satellite signals. This significant reduction
in antenna size lowers the cost of the receiving equipment and simplifies
the system installation requirements.
|Blame It On
The Rain |
is one major drawback to satellites downlinking signals at frequencies
greater than 10 gigahertz: the length of these microwaves is so
short that rain, snow or even rain-filled clouds passing overhead
can reduce the intensity of the incoming signals. At these higher
frequencies, the length of the falling rain droplets are close to
a resonant sub-multiple of the signal's wave length; the droplets
therefore are able to absorb and de-polarize the microwaves passing
through the Earth's atmosphere.
In places such as Southeast Asia or
the Caribbean, torrential downpours can lower the level of the incoming
Ku-band satellite signal by 20 dB or more; this may severely degrade
the quality of the signals or even interrupt reception entirely.
The duration of rain outages, however, is usually very short and
typically occurs in the afternoons or early evenings rather than
during the prime time evening viewing hours. For most Ku-band satellite
TV viewers, these service interruptions will only amount to the
loss of a few hours of viewing time over the course of any year.
To help counteract the effects of rain
fade, Ku-band system designers typically use a larger antenna than
what would be required under clear sky conditions. This increase
in antenna aperture gives the system several dB of margin so that
the receiving system will continue to function in light to moderate
rain storms. Satellite TV viewers in arid regions such as central
or the Middle East will rarely experience rain outages. In the Middle
East, however, satellite dish owners may experience outages caused
by intense sand storms. The presence of any atmospheric particulate,
even sand, can have an adverse effect on satellite TV reception.
|The Saga of S-Band
Although the International Telecommunication Union has assigned
S-band frequency spectrum for direct-to-home TV transmissions, few
organizations have so far elected to use this spectrum for satellite
broadcasting. One limiting factor has been the bandwidth available:
just 100 MHz of spectrum from 2.5 to 2.6 gigahertz. India and Arabsat
have included S-band transponders on their C-band satellites to
make it cost effective to launch S-band payloads into geostationary
orbit. In 1997, Indonesia will launch Indostar 1, the world's first
dedicated S-band satellite (carrying one transponder transmitting
70 Watts of power) to an orbital assignment of 107.7 degrees east
longitude. Digital video compression will make it technically and
economically feasible for Indostar to broadcast a multi-channel
TV package to subscribers in Indonesia.
Indostar intends to use antennas ranging
from 70cm to 1m in diameter for DTH reception. As Figure 9 illustrates,
DTH systems such as Indostar will only work if there are no other
adjacent satellites using the same frequency spectrum. Even a 1.2m
antenna would have problems in a 3 degree spacing environment. S-band
does offer the advantage of minimal rain fade problems, a significant
consideration for satellite broadcasters in the world's highest
rain rate region. But in terms of fulfilling global satellite communications
needs in the next Century, S-band's contribution will be minimal.