Satellite Communications

Overview

Basics of Satellite Basic Satellite Communication Link Types of Satellite Frequency Bands Antennae Power Amplifiers

Basics of Satellite

Basics…. Satellite A smaller object orbiting a larger one. Communications

satellites receives radio signals from earth stations and retransmit them to other earth stations.

Transponder A unit in a satellite that receives an uplink signal, converts its

frequency, amplifies it and retransmits it to the ground. Communications satellites generally have 10 to 40 transponders.

Basics…. Earth Station Ground equipment used in conjunction with an antenna for

receiving or transmitting radio frequency signals to or from a satellite.

Uplink Transmission of signal from an earth station to the satellite.

Downlink The retransmission of a signal received by a satellite

transponder back down to earth stations.

Antenna Device that concentrates a beam of electromagnetic waves so

as to send and receive signals.

Basics…. Azimuth The angle of rotation (horizontal) that a ground based parabolic

antenna must be rotated through to point to a specific satellite in a geosynchronous orbit.

Elevation The upward tilt to a satellite antenna measured in degrees

required to aim the antenna at the communications satellite. When. aimed at the horizon, the elevation angle is zero. If it were tilted to a point directly overhead, the satellite antenna would have an elevation of 90 degrees.

Basics…. Polarization A technique used by the satellite designer to increase the

capacity of the satellite transmission channels by reusing the satellite transponder frequencies.

Beacon Low-power carrier transmitted by a satellite which supplies the

controlling engineers on the ground with a means of monitoring telemetry data, tracking the satellite, or conducting propagation experiments.

Basics…. Amplifier A device used to boost the strength of an electronic signal.

Aperture A cross sectional area of the antenna which is exposed to the

satellite signal.

Apogee The point in an elliptical satellite orbit which is farthest from

the surface of the earth.

Attenuation The loss in power of electromagnetic signals between

transmission and reception points.

Basics…. Beam width

The angle or conical shape of the beam the antenna projects.

Feed hornA satellite TV receiving antenna component that collects the signal reflected from the main surface reflector and channels this signal into the low-noise amplifier (LNA).

Gain A measure of amplification expressed in dB

LNB A combination Low Noise Amplifier and down converter built

into one device attached to the feed.

Basic Satellite Communication Link

How it works Satellite communications are comprised of 2

main components: 1.The Satellite The satellite is composed of three separate units, namely the

fuel system, the satellite and telemetry controls, and the transponder.

The transponder includes the receiving antenna to pick-up signals from the ground station, a broad band receiver, an input multiplexer, and a frequency converter which is used to reroute the received signals through a high powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals.

How it works… 2. The Ground Station This is the earth segment. The ground station's job is two-fold.

In the case of an uplink, or transmitting station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately

converting signals received through the parabolic antenna to base band signal.

Why we use Satellite 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 transmits the signals to the satellite. Up

link frequency is the frequency at which Ground Station is communicating with Satellite.

The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

Consider the light bulb example:

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.

Advantages of Satellites

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.

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

Frequency Bands

Geostationary Earth Orbit(GEO) These satellites are in orbit 35,786 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.

GEO…. 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

Disadvantages GEO satellites, centered above the equator, have difficulty for

broadcasting signals to near polar regions Launching of satellites to orbit are complex and expensive.

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. 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…. 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.

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.

Medium Earth Orbit(MEO) A MEO satellite is in orbit somewhere between 8,000 km

and 18,000 km above the earth’s surface. MEO satellites are similar to LEO satellites in

functionality. MEO satellites are visible for much longer periods of time

than LEO satellites, usually between 2 to 8 hours. MEO satellites have a larger coverage area than LEO

satellites.

MEO…. 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.

Frequency Bands C band – uplink 5.925-6.425 GHz; downlink 3.7-

4.2 GHz The C band is primarily used for voice and data communications

as well as backhauling. Because of its weaker power it requires a larger antenna, usually above 1.8m (6ft). However, due to the lower frequency range, it performs better under adverse weather conditions on the ground.

Frequency Bands X band – uplink 7.9- 8.4 GHz, downlink 7.25 –

7.75 GHz The X band is used mainly for military communications and

Wideband Global SATCOM (WGS) systems.This band is less susceptible to rain fade than the Ku Band due to the lower frequency range, resulting in a higher performance level under adverse weather conditions.

Frequency Bands Ku band– uplink 14 GHz; downlink 10.9-12.75

GHz Ku band is used typically for consumer direct-to-home access,

distance learning applications, retail and enterprise connectivity. The antenna sizes, ranging from 0.9m -1.2m (~3ft), are much smaller than C band because the higher frequency means that higher gain can be achieved with small antenna sizes than C-band. Networks in this band are more susceptible to rain fade, especially in tropical areas.

Frequency Bands Ka band – uplink 26.5-40GHz; downlink 18-20

GHZ The Ka band is primarily used for two-way consumer broadband

and military networks. Ka band dishes can be much smaller and typically range from 60cm-1.2m (2' to 4') in diameter. Transmission power is much greater compared to the C, X or Ku band beams. Due to the higher frequencies of this band, it

can be more vulnerable to signal quality problems caused by rain fade.

Antennae

Introduction Definition Satellite Antenna

Introduction

An antenna is an electrical conductor or system of conductors Transmission - radiates electromagnetic energy into

space Reception - collects electromagnetic energy from

space In two-way communication, the same antenna

can be used for transmission and reception

Definition An antenna is a circuit element that provides a transition

form a guided wave on a transmission line to a free space wave and it provides for the collection of electromagnetic energy.

In transmit systems the RF signal is generated, amplified, modulated and applied to the antenna

In receive systems the antenna collects electromagnetic waves that are “cutting” through the antenna and induce alternating currents that are used by the receiver

Satellite Antenna Satellites very often use the same antenna to receive

and transmit RF signals. This idea is practical because the satellite receives the uplink signal at a higher frequency and generally sends it back out on the opposite polarization at a lower frequency. When receiving a signal, the antenna routes the 500MHz RF to the appropriate wideband receiver determined by the band and polarization. When transmitting, the satellites antenna feed horn determines the polarization of the signal and directs it onto the antenna to be reflected back to earth.

Cassegrain Antenna Cassegrain antenna is an antenna in which the feed

radiator is mounted at or near the surface of a concave main reflector and is aimed at a convex subreflector. Both reflectors have a common focal point. Energy from the feed unit illuminates the secondary reflector, which reflects it back to the main reflector, which then forms the desired forward beam.

In TV Broadcasting Cassegrain antenna is used.

Cassegrain Antenna Picture illustrate parabolic antenna theory

High Power Amplifiers The high power amplifier (HPA) in anearth station facility provides the RFcarrier power to the input terminals of theantenna that, when combined with theantenna gain, yields the equivalentisotropic radiated power (EIRP) requiredfor the uplink to the satellite.

Types Of Amplifiers SSPA (Solid State Power Amplifier) TWTA (Travelling Wave Tube Amplifier) KPA (Klystron Power Amplifier)

SSPA A solid state power amplifier (SSPA) uses a gallium

arsenide (GaAs) metallic semiconductor field effect transistor (FET) as the amplifier gain element.

The maximum continuous output power of a single microwave FET can be from a few watts to several tens of watts. The limiting factor is the generation of heat.

A typical GaAs FET at C-band might have a maximum output power of between 30 W and 45 W, while at Ku-band it is 15 W.

TWTA The traveling wave tube amplifier (TWTA) consists of the

traveling wave tube (TWT) itself and the power supply. The TWT can have either a helix or coupled-cavity design. In a TWT, amplification is attained by causing a high

density electron beam to interact with an electromagnetic wave that travels along a "slow-wave structure", which usually takes the form of a helical coil.

KPA The klystron power amplifier (KPA) is a narrowband

device capable of providing high power and high gain with relatively high efficiency and stability.

In a klystron tube an electron beam is formed by accelerating electrons emitted from a heated cathode through a positive potential difference. The electrons enter a series of cavities, typically five in number, which are tuned around the operating frequency and are connected by cylindrical "drift tubes".

Optimum bunching of electrons occurs in the output cavity. Large RF currents are generated in the cavity wall by the density modulated beam, thereby generating an

amplified RF output signal.

KPA…. Klystron Power Amplifier

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