Satellite Communications

Course: Communication Systems

Dr. Muhammad Saleem Awan

Satellites

• 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 them.

• Since the Soviet Union launched Sputnik in 1957, hundreds of artificial satellites have been sent into orbit.

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 satellite.

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.

Overview

• 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

communication• 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 area. (Footprint)

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

precise. Higher Bandwidths are available for use.

COMSAT ADVANTAGES

TERRESTRIAL COMMUNICATIONS SATELLITE COMMUNICATIONS

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 environments

• mapping satellite - takes pictures of the Earth and make exact maps (in the absence of clouds)

• 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 (cont.)

• Other impairments to satellite communication: The distance between an earth station and a satellite (free

space loss). 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 and fog.

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 GEO LEO MEO Molniya Orbit HAPs

• 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 earth.

Geostationary Earth Orbit (GEO)

Satellites are positioned every 4-8 degrees.

Approx. 300 GEO satellites are in orbit.

GEO (cont.)

Instruments usually have:

Small telescope or antenna. A scanning mechanism. One or more detectors that detect either visible, infrared, or

microwave radiation.

• Advantages 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.

GEO (cont.)

• Disadvantages 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 communication.

GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions

Altitude (375-1000 miles) Revolution time: 90 min - 3

hours. Advantages:

Reduces transmission delay Eliminates need for bulky

receiving equipment. Disadvantages:

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 the surface.

• 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

LEO (cont.)

• Advantages 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.

LEO (cont.)

• Disadvantages A network of LEO satellites is needed, which can

be costly LEO satellites have to compensate for Doppler

shifts cause by their relative movement. Atmospheric drag effects LEO satellites, causing

gradual orbital deterioration.

Middle-Earth-Orbiting (MEO)

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.

MEO (cont.)

• Advantage A MEO satellite’s longer duration of visibility and

wider footprint means fewer satellites are needed in a MEO network than a LEO network.

• Disadvantage 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.

Other Orbits

• 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 GEO satellite.

Useful in near polar regions.

Other Orbits (cont.)

• High Altitude Platform (HAP) One of the newest ideas in satellite

communication. 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.

Hubble Telescope Classification: LEO 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 solar minimum.

Space Debris

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.

GPS Satellites

Name: NAVSTAR Manufacturer: Rockwell International Altitude: 10,900 nautical miles Weight: 1900 lbs (in orbit)Size: 17 ft with solar panels

extended Orbital Period: 12 hours Orbital Plane: 55 degrees to

equatorial plane Planned Lifespan: 7.5 years Current constellation: 24 Block II production satellites

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 worksSatellites 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 second. )

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 every second.

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 professional mariners

Air: navigation by general aviation and commercial aircraft

COMMUNICATION SYSTEM APPLICATIONS

• COMPONENTS– USER/TRANSMITTER SEGMENT– TRANSMISSION MEDIUM– USER/RECEIVER SEGMENT

• RELAY OR TRANSMISSION OF INFORMATION– TEXT– VOICE– VIDEO– DATA

COMMUNICATION SYSTEMLINKS

POINT A

POINT A

POINT A

POINT B

POINT B

POINT B

SIMPLEX(one way only)

HALF DUPLEX(one way or the other)

FULL DUPLEX(both ways simultaneously)

GEO-SYNCHRONOUS ORBIT

THREE SATELLITES IN NEAR-EQUATORIAL ORBITS CAN PROVIDE CONTINUOUS GLOBAL COVERAGE...

...EXCEPT FOR THE POLES

GEO-SYNCHRONOUS SATCOM

HIGHLY INCLINED, ELLIPTICAL ORBIT

HIGHLY INCLINED, ELLIPTICAL ORBITS COVER THE POLES, BUT...

...DO NOT PROVIDE CONTINUOUS GLOBAL COVERAGE

HIGHLY ELLIPTICALORBITS

NEAR-EARTH ORBIT

NEAR-EARTH SATCOM SYSTEMS USE MANY SMALL, CHEAP SATELLITES TO PROVIDE WIDE AREA COVERAGE

NEAR-EARTH ORBITS

SATCOM VS TERRESTRIAL COMM.GENERAL ADVANTAGES

• 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

Frequency Bands

• Different kinds of satellites use different frequency bands. L–Band: 1 to 2 GHz, used by MSS S-Band: 2 to 4 GHz, used by MSS, NASA, deep space

research 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

+10

-10

-30

-50

Link distance in km3.0 km

+30

+50 dBm

+40

+20

0

-20

-40

System power figure (virtual) at link distance 0

(Maximum) received optical power over link distance

Minimum link distance

Hysteresis

Received optical powerat the installed link

System margin

System dynamics

Rec

eived

opt

ical

pow

er

Receiver overload limit

Receiver sensitivity

Link budget model gets valid(receiver optics completely illuminated)

Frequency BandsAll satellites have some basic elements

A structural subsystem, or bus. The bus is a metal or composite frame on whichthe other elements are mounted. Because it bears the stresses of launch, the bus isgenerally resilient. It may be painted with reflective paint to limit the solar heat itabsorbs, which could also provide some protection from laser attacks.

A thermal regulation subsystem. This system keeps the active parts of thesatellite cool enough to work properly. Active satellite components such as thecomputer and receiver can generate a large amount of heat. Sunlight incidenton the satellite’s surface also generates heat, although the satellite’s surface canbe made highly reflective to minimize heat absorption.

A power source. Power is often supplied by arrays of solar cells (“solarpanels”) that generate electricity, which is stored in rechargeable batteriesto ensure a power supply while the satellite is in shadow. Technologicalimprovements in battery technology have led to new battery types with highspecific energy (energy stored per unit mass) and high reliability. A computer control system. The on-

board computer monitors the state of thesatellite subsystems, controls its actions, and processes data. High-value satellitesmay incorporate sophisticated anti-jamming hardware that is operated bythe computer. If someone gained control of the satellite’s computer, the satellitecould be made useless to its owners. Computer systems are also sensitiveto their electromagnetic environment and may shut down or reboot duringsolar storms or if barraged by high levels of electromagnetic radiation.

A communications system (TT&C). Communications form the link between the satellite and its ground stations or other satellites. This system generally consists ofa receiver, transmitter, and one or more radio antennae.

- Jamming- Spoofing

An attitude control system. This system, which keeps the satellite pointed inthe correct direction, may include gyroscopes, accelerometers, and visualguidance systems. Precise control is required to keep antennas pointed in theright direction for communication, and sensors pointed in the right directionfor collecting data. If the attitude control system were not functioning, thesatellite 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 areused to tell the satellite what to do.

A propulsion subsystem. The satellite’s propulsion system may include theengine that guides the spacecraft to its proper place in orbit once it has beenlaunched, small thrusters used for station keeping and attitude control, andpossibly larger thrusters for other types of maneuvering. GROUND STATIONS

Satellites are monitored and controlled from their ground stations. One type ofground station is the control station, which monitors the health and status ofthe satellite, sends it commands of various kinds, and receives data sent by thesatellite. The antenna that the control station uses to communicate with thesatellite may be located with the station