lecture 1_satellite comms

EEE 545:SATELLITE

COMMUNICATIONS

Danson Njue

Course Outline

Orbital Aspects: Kepler law-orbit fundamentals-orbit shape-satellite speed and period-angle of indication-station keeping-attitude control-orbital elements-orbital perturbation-SSP-satellite launching –transfer orbit-antenna look angles-LEO, MEO, GEO-Sun synchronous orbit -constellation

Link Design: Frequency of operation-bands-propagation effects on the signal-attenuation, frequency rotation, ice and snow effects –depolarization-sun transit outage-eclipse-EIRP-Power budget equation-Uplink power-Downlink power-C\No,G\T-t system noise-thermal noise-intermodulation noise

Space and Earth Segment: Space craft subsystem-power-attitude control-telemetry tracking and command-transponder-stabilization subsystem-thermal protection-payload-bus- antenna subsystem-earth segment-low noise amplifiers-high power transmitters-TWTA, Klystron amplifiers-redundancy configuration-Cassegrain antenna

Satellite Services: Satellite bandwidth-frequency division multiplexing-time division multiplexing-multiple access-FDMA, TDMA, CDMA-INSAT INMARSAT, INTELSAT, weather forecasting, mobile satellite service, satellite navigation

Reference Books

• The Satellite Communication Applications Handbook ;

Author: Bruce R. Elbert;

• Satellite Communication Engineering; Author: Michael O.

Kolawole;

• Satellite Communications; Author: Dennis Roddy

Introduction (1)

• The word satellite originated from the Latin word ‘Satellit’

which means an attendant who is constantly going round

and attending to the master

• A satellite is basically a body that around another larger

body in a mathematically predictable path called an orbit

• There are about 750 satellite in space, most of which are

used for communication. A communication satellite refers

to a station in space that is used to transmit

telecommunication, radio and TV signals

• The first satellite was launched in 1957

The origin of satellite

• The concept of using satellite was first proved by the

Naval Research Lab in the US in 1940s. It involved the

use of the moon to establish a very low data rate link

between Washington and Hawaii

• In Oct-1957, Russia launched SPUTNIK, the first artificial

spacecraft to orbit the earth which transmitted telemetry

information for 21 days

• In 1958, America launched an experimental satellite

known as the EXPLORER

• In 1963, two satellites (Intelsat and Molniya) were

launched and in 1965, they were used to transmit Video

and Voice signals

The Concept of satellite communications

(1)

When using a satellite for

long distance

communications, the

satellite acts as a repeater

An earth station transmits

the signal up to the satellite

(uplink), which in turn

retransmits it to the

receiving earth station

(downlink)

Different frequencies are

used for uplink/downlink.

The Concept of satellite communications

(2)

The Concept of satellite communications

(3) Space Segment

The following takes place at the space segment;

• Satellite Launching Phase

• Transfer Orbit Phase

• Deployment

• Operation • TT&C - Tracking Telemetry and Command Station

• SSC - Satellite Control Center;

• OCC - Operations Control Center

• SCF - Satellite Control Facility

• Retirement Phase

The Concept of satellite communications

(4)

Satellite

The ground segment consists of a collection of facilities, users and applications. The Earth/Ground station can consist of either mobile or fixed stations

Ground Segment

The Concept of satellite communications

(5) • Relay satellite; Used for communicating with two stations that

are too far away to use conventional means

• Uplink: Refers to the link from a ground station up to a satellite

• Downlink: Refers to the link from a satellite down to one or more ground stations or receivers

NB: Some satellite companies sell uplink and downlink services to;

• Television stations, corporations, and to other telecommunication carriers

• A company can specialize in providing uplinks, downlinks, or both

Keplers laws of planetary motion (1)

• Sir Johannes Kepler observed the planetary motion and

derived three laws based on the geometry of ellipses.

• Major axis: Longest

diameter of an ellipse

• Minor axis: Shortest

diameter of an ellipse

Keplers laws of planetary motion (2)

Law 1:

Planets move in elliptical orbits with the sun at one focus of the ellipse

Keplers laws of planetary motion (3)

Law 2:

A line joining the planet from the center of the sun sweeps out equal areas in

equal times (periods).

A B

For a 30-day period, areas A and B

are equal

Keplers laws of planetary motion (3)

Law 3:

The squares of the periods of two planet’s orbits are proportional to each other as

the cubes of their semi-major axis

In other words, (Period of orbit)2 is proportional to (semi-major axis of orbit)3

p2a3; Therefore P2 = ka3, where k is some constant number.

We can find k if p is expressed in (Earth’s orbit) years and a is expressed in terms

of the distance between Sun & Earth. This distance is called 1 Astronomical Unit, or

1 A.U. T

Substituting above values into the equation yields k=1; Therefore, p2=a3

3

2

3

1

2

2

2

1

a

a

P

P

Satellite transmission bands

Frequency band Downlink Uplink

C 3.7GHz-4.2GHz 5.925GHz-6.425GHz

Ku 11.7GHZ-12.2GHz 14GHz-15.5GHz

Ka 17.7GHZ-21.2GHZ 27.5GHZ-31GHZ

Note; The Ka and Ku bands are reserved exclusively for satellite

communication but are subject to rain attenuation. The C-band is the most

frequently used satellite band

• The size of satellite dishes (antennas) depends on the transmission

frequency - =c/f.

• As wavelength increases (and frequency decreases), larger antennas

(satellite dishes) are necessary to gather the signal effectively and

minimize losses.

Applications of satellites

• Satellite TV broadcasting

• High speed broadband networks

• Voice Telephony networks

• Corporate networks for multinational businesses

• Weather forecasting

• Global Positioning Systems (GPS)

Advantages of Satellite Communication

• Can be used to provide large geographical coverage area • Very viable for provision of service to remote or

underdeveloped areas • Highly flexible • Easy to install new circuits • Circuit costs independent of distance • Broadcast possibilities • Very viable for temporary applications • Very viable for niche applications • Mobile applications • Can be used for terrestrial network "by-pass" • User has control over own network • Satellite can be used to achieve highly redundant systems

(1:N multipoint standby diversity)

Disadvantages of Satellite Communication

• Large up-front capital requirements

• Interference and propagation delay

• Congestion of frequencies and orbits

Operation of satellites

• A Earth Station sends

message in GHz range.

(Uplink)

• Satellite receives and

retransmit signals back.

(Downlink)

• Other Earth Stations

receive message in useful

strength area. (Footprint)

Satellite Orbits

• Low Earth Orbits (LEOs)

• Middle Earth Orbits (MEOs)

• Geosynchronous Earth Orbits (GEOs)

Low-Earth-Orbit (LEO) • 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 Orbits can be categorized into; Little, Big, and Mega (Super) LEOs.

Little LEOs

• 0.8 GHz range

• Small, low-cost applications

• Mainly used for short, narrowband communications such

as vehicle tracking, environmental monitoring and two-

way data communication.

Big LEOs

• 2 GHz or above range

• Can offer global services, which can be subject to

regulatory requirements.

• Used for technology devices such as high-speed, high-

bandwidth data communications, and video conferencing.

They carry voice and high-speed data services.

• The main uses are data communications and real-time

voice delivery to hand-held devices.

Mega (Super) LEOs

• Wider frequency range; 20-30 GHz range

• They share the same advantages and drawbacks of other

LEOs and are intended to operate with inter-satellite links

to minimize transmission times and avoid dropped

signals.

• Mainly used for broadband data applications since they

are optimized for packet-switched data rather than voice.

Middle-Earth-Orbits (MEO) • MEOs orbits between the altitudes of 5,600 and 9,500

miles- 8000km to 20,000km above the earth surface. Average distance from the moon is about 240,00 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.

Geosynchronous-Earth-Orbit (GEO) - 1

• Orbit is synchronous with the earth’s rotation.

• From the ground the satellite appears fixed.

• Altitude is about 23,000 miles and provide coverage to 40% of planet per satellite.

• The typical service life expectancy of a geostationary satellite is 10-15 years.

• NB: Because geostationary satellites circle the earth at the equator, they are not able to provide coverage at the Northernmost and Southernmost latitudes.

Geosynchronous-Earth-Orbit (GEO)- 2

• Geostationary satellites are commonly used for communications and weather-observation.

• The altitude is chosen so that it takes the satellite 24 hours to orbit the Earth once, which is also the rotation rate of the Earth. This produces the cloud animations seen on TV weather bulletins.

• Can take images approximately every minute.

Geosynchronous-Earth-Orbit (GEO)- 3

Advantages and disadvantages of GEO satellites

Advantages:

Weather images can be

displayed.

Television broadcasts are

uninterrupted.

Used to track major

developments such as

hurricanes 24 hours a day.

Disadvantages:

It takes longer for the signal

to get to earth and back to

satellite.

Increased difficulty of

telephone conversations.

GEOs are not positioned in

the farthest northern and

southern orbits.