AZIMUTHDefined as the horizontal

pointing angle of an antenna. It is usually measured in a clockwise direction in degrees from north. The angle of elevation and the azimuth both depend on the latitude of the earth station and the orbiting satellite. For geosynchronous satellite in an equatorial orbit, the procedure is follows.

For a good map, determine the longitude and latitude of the earth station.

From the table, determine the longitude of the satellite of the interest.

Calculate the difference, in degrees (L) between the longitude of the satellite and the longitude of the earth station.

Then from Figure 7-6, determine the azimuth and elevation angle for the antenna. This figure is geosynchronous satellite in the equation orbit.

N

AzimuthElevation

Earth

Elevation

Horizontal Line

NorthAzimuth Angle

Beam Direction

ORBITAL CLASSIFICATIONThere are two primary classification

for communication satellite.1.) Spinners2.) Three-axis stabilizer satellites.

Spinners satelliteUse the angular momentum of its

spinning body to provide roll and yaw stabilization.

Three-axis Stabilizerthe body remains fixed relative

to earth’s surface while an internal subsystem provide roll and yaw stabilization.

SPACE CRAFTINTELSAT IV SARELLITE is composed of a

transponder subsystem, antenna subsystem, telemetry and command subsystem, control system and power supply subsystem.

The external view of INTELSAT IV SATELLITE, the solar array is attached to the outer wall of the spacecraft.

The space craft is divided into the spun section and spin section both sections are interconnected with each other through BAPTA (bearing and power transfer assembly).

The Despun section, antenna and communication equipments are mounted.

The Spin section accommodates the apogee motor, power supply unit, fuel tank, axial jets and sensors.

The despun and spin section are spin-rotated in reverse sections with each other in order to constantly let the communication antennas of the satellite point toward the earth.

DESIGN AND LAUNCH OF COMMUNICATION SATELLITE

Launcher firing

Antennas and solar arrays are folded

Note 1: for modern satellite using a liquid fuel ( the oxygen is already mixed to a material.

Note 2: the launch site for GEO SAT should be near the equator for power and limit.

First, the satellite is launched onto the parking orbit using the first and second stage rockets. The parking orbit is a circular orbit located at the altitude of about 200 to 300 km. from the earth.

The satellite is then rocketed onto the transfer orbit by the propellant of the third stage.

The transfer orbit is an elliptical orbit with the perigee on the parking orbit and its apogee nearly on the geostationary orbit. After achieving the desired orbit, the satellite is separated from the launch vehicle to travel alone in space

The satellite in the transfer orbit is then, adjusted in the geostationary orbit at the apogee by operating a rocket called the Apogee Kick Motor (AKM) built in the satellite.

The satellite does not automatically becomes a geostationary satellite which stands stationary at the target location, but rather it is put on a drift orbit which is located slightly away from the geostationary orbit, then moved to a predetermined location, and finally reaches the point of geostationary orbit through the counter-thrusting operation of a small trusted.

As we seen far, communications satellite are launched to be put in the geostationary satellite orbit located at an altitude of about 36,000 km. above the equator.

Satellite is fed with electric power by solar array. It is also provided with chemical batteries in preparation for the solar eclipse. The solar array consists of a lot of solar cells connected in parallel series . The solar cells convert photon energy radiated from the sun directed to electrical energy.

Example:Intelsat IV uses a total of 45,800 silicon solar cells. For the chemical battery, Intelsat IV-A satellites employ a nickel-cadmium battery. This battery has a high charging efficiency, is strong against overcharge and exhaust little gas. The battery electrolyte is alkaline and free from corrosive property.

REGULATOR

CHARGINGCONTROL

UNIT

SOLAR ARRAY INPUT

STORAGE CELL

Solar array supplies power directly to each unit in the satellite. In case of solar eclipse, however power is supplied from the chemical batteries. After the solar eclipse, chemical batteries are charged with the solar energy

Eclipse

Eclipse Principle

BEAM OF ANTENNAThe frequency bands and electric power are limited resources of a communication satellite. We need to use more effective of these resources. The communication satellite has some kinds of beam for efficient use in the frequency bands. We can use the same frequency band on the same satellite by using different beams for different area. The global beam, hemispheric beam and spot beam are shown in the figure.

COMMUNICATION ANTENNAS

Three types of antennas are provided:a.) Global beam antennasb.) Spot beam antennasc.) Telemetry and Command Antennas

GLOBAL BEAM TRANSMITTING AND RECEIVING ANTENNAS

Global beam antennas can cover all earth stations seen from the geostationary satellite. These antennas are horn reflector types. Their bandwidth is about 17 degrees which can cover 1/3 of the whole earth surface. Radio signal transmitted from the earth stations are received by the global beam-receiving antenna. Two global beam-receiving antennas are provided, working and standby antennas.

A. SPOT BEAM ANTENNA

It is parabolic type of antenna with a higher gain and narrower beam than global beam. The bandwidth of spot beam antenna is 4.5 degrees, and its direction can be changed on command from the earth station. It is suitable for an earth station handling large traffic of communication.

B. TELEMETRY AND COMMAND ANTENNAS

This antenna is a biconical type to receive commands from the earth and to send telemetry signals to the earth until the satellite is placed at the stationary position.

Telemetry Horn antenna is used after the satellite is stationed at the stationary position and become operational. This antenna is used for transmitting telemetry signals like supervisory alarms, tracking and positioning of antennas.

SATELLITE SYSTEM LINK MODELS

Satellite system consists of three basic sections

a.) an uplinkb.) a satellite transponderc.) a downlink

A. UPLINK MODELthe primary component within the

uplink section of a satellite system is earth station transmitter. A typical earth station transmitter consists of an

-IF modulator-IF to RF microwave up-converter-High power amplifier (HPA)-And some means of band limiting

the final output spectrum (an output BFP)

Figures show the block diagram of a satellite earth station transmitter. The IF modulator converts the input baseband signals to the either an FM, a PSK or a QAM modulated Intermediate Frequency. The up-converter (Mixer and BPF) adequate input sensitivity and output power to propagate the signal to the satellite transponder. HPA commonly used are klystron and TWT.

B. SATELLITE TRANSPONDER

A typical transponder consists of an - Input band limiting device (BPF)- An Input low noise amplifier- A frequency translator- A low level power amplifier- A output BPF

Figure shows a simple block diagram of a satellite transponder. This transponder is a RF to RF repeater. Other transponder configurations are IF and Baseband repeaters similar to those used in microwave repeaters. An input BPF limits the total noise applied to the input of the LNA. (A common device used as an LNA is a tunnel diode. The output of the LNA is fed to a frequency translator that converts the frequency to a low band down link frequency. The low-level power amplifier which is commonly a TWT, amplifies the RF satellite channel requires a separate transponder.

Some Transponder have different bandwidth, it depends on the designer who has an idea on the number of transponder and a bandwidth of each transponder.

C. DOWNLINK MODELAn earth station receiver consists of an - Input BPF- LNA - RF to IF down converter

Down converterFrom satellitetransponder

BPF lLNAMIXE

RBPF DEM

LO

RF

4 or 12 GHz

Figures shows a simple block diagram of a typical earth station receiver. The BPF limits the input noise power to the LNA. The LNA is a highly sensitive, RF to IF down converter is a mixer/band pass filter combination which converts the received RF signals to an IF frequency.

HIGH POWER AMPLIFIER (HPA)In order to send signals to the satellite located

36,000 km. above the ground against the free space loss, a high gain power amplifier is required. Two types of often used for the HPA.

A. Traveling Wave Tube Amplifier(TWT)B. Klystron Amplifier (KLY)

The major characteristic difference between these two types of electron is the amplifying frequency bandwidth.

The amplification frequency bandwidth of a TWT is about 500 MHz and that of KLY is about 50 MHz in the 6 GHz band.

A TWT has an advantage of a wide bandwidth and disadvantage such as low power efficient complicated construction and high cost. A KLY has the disadvantage of a narrow bandwidth and advantage such as high power efficiency, simple construction and low cost.

TWT KLYSTRON

Wide Bandwidth Narrow Bandwidth

Inefficiency High efficiency

Complicated Construction Simple Construction

High Cost Low Cost

A TWT has a wide amplification bandwidth and usually used for a common amplification system.

A KLYSTRON has a narrow bandwidth and is used for individual amplification system.

d. EARTH STATIONThe earth station’s facilities are

divided by the following.

1. Communication facility2. terrestrial transmission facility3. Power supply facility4. control building

The communication facility consist of the following system:

1. Antenna subsystem2. Transmit amplifier subsystem3. Receive amplifier subsystem4. GCE ( Ground Communication System) subsystem

5. terminal equipment subsystem6. Communication control subsystem

HPA

LNA

GCE

GCE

Muxfor

Satellite

MUXfor

Domestic

Control

4.1 Antenna subsystemIn the antenna subsystem, there are

mainly two types of antenna for satellite communications. One is horn antenna, another is a reflector antenna. In the satellite communications systems the reflector antennas are usually used, used as a primary radiator for the reflector antenna.

the communication satellite is located at the height of 36,00 km from the surface of the globe. This means that there is much free space loss

horn antenna

reflector antenna

The free space loss can be calculated by using the following equation.

Ls = (4πd/ λ)²d: distance λ: wavelength

Usually the frequencies of 6 GHz are used (for uplink) and 4 GHz (for downlink). Try to calculate the free space loss in 6,000 GHz between the surface of the globe and communication satellite with a given parameter distance of 36,000 km.

λ = c/fc: the velocity of the lightf: frequency

λ = c/f = 3x 10⁸ / 6 x 10⁹ = 0.05 (m)

Ls = (4πd/ λ)² = (4 x π x 36 X 10⁸m / 0.05m)²= 8.186 x 10¹⁵

[Ls] = 10 x log (8.186 x 10¹⁵)= 199.1 (dB)

Therefore, we need high gain antenna for satellite communication system.

The gain of antenna is expressed by the following equation,

G = (4π / λ)² x A x ηA: physical aperture areaη (Greek small letter eta): aperture efficiency

“Aη” is called the effective aperture area when is used by a disc of diameter D, then equation can be expressed by the following equation.

G = (πD / λ)² x η

When we get much gain, we save the power of HPA (High Power Amplifier), because transmitting power is expressed by the following equation.

PT = GT x PHPAPHPA: The output power of HPA

PT is called E.I.R.P. (Equivalent Isotropically Radiated Power). GT is one factor to indicate antenna performance. It means transmission gain of the antenna.

Bur G/T (Gain per noise Temperature) is also useful factor for the antenna in receiving. T means noise temperature of the antenna system including antenna, feed system and receiver because at the earth station it will become a weak signal from satellite. The satellite does not have a big antenna and HPA with big power. The satellite can not transmit a signal with big power. Therefore the earth station shall have a big gain receiving system with low noise temperature. If the receiving system has a big gain, it is not enough for the receiving system. Because if this antenna has a big noise, we can not get the signal by noise.

The noise power represented by temperature ‘T’ from ‘KTB’, where ‘k’ is Boltzmann’s constant and ‘B’ is the bandwidth. It is useful to express noise because noise power is different by bandwidth.

There are many kinds of antennas. The preferable to use is the parabolic antenna,. It will be described as the cassegrain antenna.

F1

P1

Hyperboloid

P2

Paraboloid

F2

The main reflector is paraboloid and the sub-reflector is hyperboloid. The paraboloid has a focus of F1. the hyperboloid has a focuses F1 and F2. The wave radiated from F2. It is reflected at the point P1, after then also reflected at the point P2. Then the wave becomes parallel to the axis of the reflector system.