The Text Book makes an odd choice in calling DBS BSS (BSS is actually the abbreviation used for Boeing Satellite Service). These distinctions aren’t set in stone. For example, FSS satellites used to be used for broadcasting.
4 BASIC APPLICATION OF ANY COMMUNICATION SYSTEM IS THE TRANSFER OF INFORMATION. THIS INFORMATION MAY BE TEXT, SUCH AS A DOCUMENT FILE, VOICE INFORMATION, VIDEO OR STRICTLY RAW DATA SUCH AS POSITION INFORMATION FROM A GPS SATELLITE
5 WHEN DISCUSSING A BASIC COMMUNICATION SYSTEM, IT IS IMPORTANT TO UNDERSTAND BASIC CAPABILITIES INHERENT TO COMMUNICATION LINKS THE FIRST TYPE OF LINK IS CALLED SIMPLEX. PUT SIMPLY, IT IS ONE WAY COMMUNICATION FROM POINT A TO POINT B. POINT B IS NOT ABLE TO SEND ANY SIGNALS TO POINT A EXAMPLES ARE RADIO, TV, OR A GPS RECEIVER THE SECOND TYPE OF LINK IS CALLED HALF DUPLEX. THIS SYSTEM ALLOWS 2-WAY COMMUNICATION ON A LIMITED BASIS. THE LIMITATION HERE IS THAT ONLY ONE POINT CAN TRANSMIT WHILE THE OTHER POINT IS RECEIVING. EXAMPLES ARE HAND-HELD RADIOS (WALKIE-TALKIES), TELEGRAPH THE FINAL METHOD IS DUPLEX, WHICH ENABLES TO USERS TO SEND AND RECEIVE SIGNALS SIMULTANEOUSLY. EXAMPLES ARE TELECOMMUNICATIONS
6 3 SATELLITES IN GEOSYNCRONOUS ORBITS SPACED EQUALLY APART, ARE ABLE TO PROVIDE CONTINUOUS COVERAGE ON THE EARTH’S SURFACE, WITH THE EXCEPTION OF THE POLES
Course: Communication Systems
Dr. Muhammad Saleem Awan
• Satellites are man-made objects or vehicles
intended to orbit the Earth, the moon, or
another celestial body.
• Probes are satellites that move outward and skim
by other worlds, sometimes actually landing on
• Since the Soviet Union launched Sputnik in 1957,
hundreds of artificial satellites have been sent
History of Satellites
The First Satellites
The theory of satellites was simple enough - shoot something out into space at the
right speed and on the correct trajectory and it will stay up there, orbiting Earth,
for years - if not forever.
If the orbit is the right distance in space the satellite will keep pace with the
rotation of the Earth.
Pioneer Satellites (1957)
Early in October 1957 communications stations started picking up a regular
beeping noise coming from space.
The signals were coming from Russia's Sputnik 1, the world's first man-made
It was January 1958, before a Jupiter rocket successfully launched Explorer 1, the
first American satellite.
History of Satellites
Early Bird (1965)
The world's first commercial communications satellite was Early Bird, built
for the Communications Satellite Corporation (COMSAT) by Hughes.
The satellite was launched on April 6, 1965, and placed in commercial
service after moving into geosynchronous orbit 22,300 miles above the
equator. That meant it was always on station to provide line of sight
communications between Europe and North America.
Early Bird didn't have a battery - and worked only when its solar panels
were exposed to the sun.
• Basics of Satellites
• Types of Satellites
• Capacity Allocation
Basics: How do Satellites Work
• Two Stations on Earth want to communicate through radio
broadcast but are too far away to use conventional means.
• The two stations can use a satellite as a relay station for their
• One Earth Station sends a transmission to the satellite. This
is called a Uplink.
• The satellite Transponder converts the signal and sends it
down to the second earth station. This is called a Downlink.
How do Satellites Work
Other Earth Stations receive
message in useful strength
Basics: Advantages of Satellites
• The advantages of satellite communication
over terrestrial communication are:
The coverage area of a satellite greatly exceeds
that of a terrestrial system.
Transmission cost of a satellite is independent of
the distance from the center of the coverage area.
Satellite to Satellite communication is very
Higher Bandwidths are available for use.
Basics: Disadvantages of Satellites
• The disadvantages of satellite communication:
Launching satellites into orbit is costly.
Satellite bandwidth is gradually becoming used up.
There is a larger propagation delay in satellite
communication than in terrestrial communication.
Satellites & Probes
Satellites and probes can be classified into three
groups, as follows:
• 1. those that observe the Earth
• 2. those that peer into space
• 3. the spacecraft that travel to other planets
Types of Satellites
• Communications satellite - provides fast radio,
telephone and TV communications
• Weather satellite - takes photographs of Earth,
sending them down in the form of
• Radio waves; forecasters show satellite pictures
on nightly weather reports
• Remote sensing satellite - studies the Earth’s
surface and send back vital data about global
• mapping satellite - takes pictures of the Earth
and make exact maps (in the absence of
• surveillance satellite - takes pictures of the
Earth from 100 miles and higher (22,300
miles); recorders and cameras can listen in on
walkie-talkie radio conversations, make out
peoples faces and buildings, and even figure
out the building composition
Types of Satellites
Basics: Factors in satellite communication
• Other impairments to satellite communication:
The distance between an earth station and a satellite (free
Satellite Footprint: The satellite transmission’s strength is
strongest in the center of the transmission, and decreases
farther from the center as free space loss increases.
Atmospheric Attenuation: caused by air and water can
impair the transmission. It is particularly bad during rain
Basics: How Satellites are used
• Service Types
Fixed Service Satellites (FSS)
• Example: Point to Point Communication
Broadcast Service Satellites (BSS)
• Example: Satellite Television/Radio
• Also called Direct Broadcast Service (DBS).
Mobile Service Satellites (MSS)
• Example: Satellite Phones
Types of Satellites
• Satellite Orbits
• Frequency Bands
Geostationary Earth Orbit (GEO)
• These satellites are in orbit 35,863 km above
the earth’s surface along the equator.
• Objects in Geostationary orbit revolve around
the earth at the same speed as the earth
rotates. This means GEO satellites remain in
the same position relative to the surface of
Geostationary Earth Orbit (GEO)
Satellites are positioned
every 4-8 degrees.
Approx. 300 GEO
satellites are in orbit.
Instruments usually have:
Small telescope or antenna.
A scanning mechanism.
One or more detectors that detect either visible, infrared,
or microwave radiation.
A GEO satellite’s distance from earth gives it a large
coverage area, almost a fourth of the earth’s surface.
GEO satellites have a 24 hour view of a particular area.
These factors make it ideal for satellite broadcast and
other multipoint applications.
A GEO satellite’s distance also cause it to have
both a comparatively weak signal and a time delay
in the signal, which is bad for point to point
GEO satellites, centered above the equator, have
difficulty broadcasting signals to near polar
Altitude (375-1000 miles)
Revolution time: 90 min - 3
Eliminates need for bulky
Smaller coverage area.
Shorter life span (5-8 yrs.)
than GEOs (10 yrs).
Low Earth Orbit (LEO)
Low Earth Orbit (LEO)
• LEO satellites are much closer to the earth than
GEO satellites, ranging from 500 to 1,500 km above
• Because they orbit so close to the Earth, they must
travel very fast (17,000 mph) so the gravity can’t
pull them back into the Earth’s atmosphere.
• LEO satellites don’t stay in fixed position relative to
the surface, and are only visible for 15 to 20
minutes each pass.
• A network of LEO satellites is necessary for LEO
satellites to be useful
A LEO satellite’s proximity to earth compared to a
GEO satellite gives it a better signal strength and
less of a time delay, which makes it better for
point to point communication.
A LEO satellite’s smaller area of coverage is less of
a waste of bandwidth.
A network of LEO satellites is needed, which can
LEO satellites have to compensate for Doppler
shifts cause by their relative movement.
Atmospheric drag effects LEO satellites, causing
gradual orbital deterioration.
MEOs orbits between the altitudes
of 5,600 and 9,500 miles.
These orbits are primarily reserved
for communications satellites that
cover the North and South Pole.
Unlike the circular orbit of the geostationary satellites,
MEOs are placed in an elliptical (oval-shaped) orbit.
Approximately a dozen medium Earth orbiting satellites
are necessary to provide continuous global coverage 24
hours a day.
A MEO satellite’s longer duration of visibility and
wider footprint means fewer satellites are needed
in a MEO network than a LEO network.
A MEO satellite’s distance gives it a longer time
delay and weaker signal than a LEO satellite,
though not as bad as a GEO satellite.
• Molniya Orbit Satellites
Used by Russia for decades.
Molniya Orbit is an elliptical orbit. The satellite
remains in a nearly fixed position relative to earth
for eight hours.
A series of three Molniya satellites can act like a
Useful in near polar regions.
Other Orbits (cont.)
• High Altitude Platform (HAP)
One of the newest ideas in satellite
A blimp or plane around 20 km above the earth’s
surface is used as a satellite.
HAPs would have very small coverage area, but
would have a comparatively strong signal.
Cheaper to put in position, but would require a lot
of them in a network.
Orbit: 375 miles, 600 km.
Revolution time: 100 min.
Speed: 17,000 miles/hr
Concerns: Orbit decay from
gravity and solar output. During
“solar maximum”, the densities
at all altitudes are enhanced, and
the drag effects on satellites are
much larger than during times of
According to the U.S. Space
Command (USSC), there are
more than 8,000 objects
larger than a softball now
circling the globe.
Of these, over 2000 are
satellites (working and not).
GPS: What is it ?
A constellation of 24 satellites
The Global Positioning System (GPS) is a
worldwide radio-navigation system formed
from a constellation of 24 satellites and
their ground stations.
They are constantly moving, making two
complete orbits in less than 24 hours.
These satellites are traveling at speeds of
roughly 7,000 miles an hour.
Manufacturer: Rockwell International
Altitude: 10,900 nautical miles
Weight: 1900 lbs (in orbit)
Size: 17 ft with solar panels
Orbital Period: 12 hours
Orbital Plane: 55 degrees to
Planned Lifespan: 7.5 years
Current constellation: 24 Block II
The spacing of the satellites are arranged
so that a minimum of five satellites are in
view from every point on the globe.
GPS: How it works
Satellites are reference points for locations on Earth
The whole idea behind GPS is to use
satellites in space as reference points
for locations here on earth.
GPS satellites use a "triangulate,"
system where the GPS receiver
measures distance using the travel
time of radio signals.
By using triangulation, we can
accurately measure our distance and
find out position from three satellites
position anywhere on earth.
EX. THE BIG PICTURE
If a particular satellite is 11,000 miles
above it. Then we know that it’s radius
is 11,000 miles!
EX. THE BIG PICTURE
Basic calculations measuring distance
Velocity * Time = Distance
Velocity = speed of light (186,000 miles per
Time = a lot of analysis and calculations!
GPS: Problems in the System
Even though the satellites
positions are constantly
monitored, they can't be watched
The atomic clocks they use are
very, very precise but they're not
perfect. Minute discrepancies can
occur, and these translate into
travel time measurement errors.
The signal may not actually get to
the ground station receivers first.
It may bounce off various objects
before it gets to the receivers.
Satellites are precise but are not perfect.
GPS: Who Uses GPS ?
GPS has a variety of applications
Land: diverse uses; ex. surveying, recreational. Etc
Sea: navigation by recreational boaters, commercial fishermen, and
Air: navigation by general aviation and commercial aircraft
COMMUNICATION SYSTEM APPLICATIONS
– USER/TRANSMITTER SEGMENT
– TRANSMISSION MEDIUM
– USER/RECEIVER SEGMENT
• RELAY OR TRANSMISSION OF INFORMATION
(one way only)
(one way or the other)
(both ways simultaneously)
THREE SATELLITES IN NEAR-EQUATORIAL ORBITS CAN PROVIDE CONTINUOUS
...EXCEPT FOR THE POLES
HIGHLY INCLINED, ELLIPTICAL ORBIT
ELLIPTICAL ORBITS COVER
THE POLES, BUT...
...DO NOT PROVIDE CONTINUOUS
NEAR-EARTH SATCOM SYSTEMS USE MANY SMALL, CHEAP
SATELLITES TO PROVIDE WIDE AREA COVERAGE
SATCOM VS TERRESTRIAL COMM.
• LINE OF SIGHT NOT REQUIRED BETWEEN USERS
• COMMUNICATION COST & QUALITY LESS SENSITIVE TO
DISTANCE AND TERRAIN EFFECTS
• MOBILE COMM LINKS BEYOND LINE OF SIGHT
• DIRECT COMMUNICATIONS TO REMOTE AREAS, NO NEED
FOR INTERMEDIATE GROUND RELAYS
• WIDE AREA OR FOCUSED BROADCAST OPTIONS
• LARGE CAPACITY FOR MESSAGE TRAFFIC
• MINIMUM RESTRAINT ON USERS
• Different kinds of satellites use different frequency
L–Band: 1 to 2 GHz, used by MSS
S-Band: 2 to 4 GHz, used by MSS, NASA, deep space
C-Band: 4 to 8 GHz, used by FSS
X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial
imaging, ex: military and meteorological satellites
Ku-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)
K-Band: 18 to 26.5 GHz: used by FSS and BSS
Ka-Band: 26.5 to 40 GHz: used by FSS
0 0.5 1.0 1.5 2.0 2.5
Link distance in km
System power figure (virtual) at link distance 0
(Maximum) received optical power over link distance
Minimum link distance
Received optical power
at the installed link
Receiver overload limit
Link budget model gets valid
(receiver optics completely illuminated)
All satellites have some basic elements
A structural subsystem, or bus. The bus
is a metal or composite frame on which
the other elements are mounted. Because
it bears the stresses of launch, the bus is
generally resilient. It may be painted with
reflective paint to limit the solar heat it
absorbs, which could also provide some
protection from laser attacks.
A thermal regulation subsystem. This
system keeps the active parts of the
satellite cool enough to work properly.
Active satellite components such as the
computer and receiver can generate a
large amount of heat. Sunlight incident
on the satellite’s surface also generates
heat, although the satellite’s surface can
be made highly reflective to minimize heat
A power source. Power is often supplied
by arrays of solar cells (“solar
panels”) that generate electricity, which is
stored in rechargeable batteries
to ensure a power supply while the satellite
is in shadow. Technological
improvements in battery technology have
led to new battery types with high
specific energy (energy stored per unit
mass) and high reliability. A computer control system. The on-
board computer monitors the state of the
satellite subsystems, controls its actions,
and processes data. High-value satellites
may incorporate sophisticated anti-
jamming hardware that is operated by
the computer. If someone gained control of
the satellite’s computer, the satellite
could be made useless to its owners.
Computer systems are also sensitive
to their electromagnetic environment and
may shut down or reboot during
solar storms or if barraged by high levels of
A communications system (TT&C).
Communications form the link between the
satellite and its ground stations or other
satellites. This system generally consists of
a receiver, transmitter, and one or more
An attitude control system. This system,
which keeps the satellite pointed in
the correct direction, may include
gyroscopes, accelerometers, and visual
guidance systems. Precise control is
required to keep antennas pointed in the
right direction for communication, and
sensors pointed in the right direction
for collecting data. If the attitude control
system were not functioning, the
satellite is unlikely to be usable
TT&C - Telemetry refers to the information the satellite sends the control
station about the status of its various components and how they are operating.
Tracking refers to knowing where the satellite is; for example, the time for a
signal to travel between the satellite and ground can be used to accurately
determine the distance to the satellite. Command refers to the signals that are
used to tell the satellite what to do.
A propulsion subsystem. The satellite’s
propulsion system may include the
engine that guides the spacecraft to its
proper place in orbit once it has been
launched, small thrusters used for station
keeping and attitude control, and
possibly larger thrusters for other types of
maneuvering. GROUND STATIONS
Satellites are monitored and controlled
from their ground stations. One type of
ground station is the control station, which
monitors the health and status of
the satellite, sends it commands of various
kinds, and receives data sent by the
satellite. The antenna that the control
station uses to communicate with the
satellite may be located with the station