Satellite Communication

Spacecraft subsystems

Spacecraft

Spacecraft subsystem overview

Satellite Subsystems(Cont..)

Power system Solar cells Used by transmitter and other electrical systems Communication sub system Major part Small weight and volume Composed of one or more antennas which receive and

transmit over wide bandwidths at microwave frequencies Receiver – transmitter unit is known as transponder 2 types1. linear or bend pipe transponder

Amplifies the received signal and retransmits it at a usually different lower frequency

2. Base band processing transponderUsed only with digital signal. Converts the received signal to baseband, processes it retransmits

Satellite antennas Most are designed to operate in a

single band Such as Cu band or K band

4 or more antennas can be used for multiple frequencies

AOCS (Attitude & orbit control system)

AOCS (Attitude & orbit control system) At GEO orbit altitude the moon’s

gravitational force is about twice as strong as the sun’s

Moon orbit is inclined to the equatorial plane by approximately 5 degrees

The plane of the earth’s rotation around the sun is inclined to 23 degrees to the equatorial plane

Net gravitational force on the satellite tends to change the inclination of the satellite.

Approximately 0.86 degrees per year from the equatorial plane.

LEO satellites are less effected by this gravitational pull from the sun and moon

At the equator there are bulges of about 65m at longitudes 162 degress East and 348 degrees East.

Satellite is accelerated towards one of two stable points on GEO orbit at the longitude of 75 degree E and 252 degrees E

Attitude and Orbit Control System(AOCS)

Attitude and orbit change reasons Gravitational field of Sun and Moon

Irregularities in Earths gravitational field

Solar pressure from Sun

Variations in Earths magnetic field

Solar pressure and Earths magnetic field can generate eddy currents in satellites metallic structure

The Earth is not a perfect sphere Satellite is accelerated towards the two stable

points To maintain accuracy of satellite position, it is

accelerated in between by using thrusters, which can be controlled from Earth via TTC&M

Fine positioning

Two ways to make the satellite stable in orbit when it is weightless.

Satellite can be rotated at a rate between 30 and 100 rpm to create gyroscopic force that provides stability (spinner satellites)

Satellites can be stabilized by one or more momentum wheels, called three-axis stabilized satellites.

Attitude Control System

Two ways to make a satellite stable in its orbit

1. Create a gyroscopic force by rotating the body of the satellite between 30 to 100 rpm Gyroscope a wheel mounted such that the

force gravity acts on centre of mass, and no torque is acting on the wheel. Without torque acting on the wheel, without torque to change its direction of motion, a spinning gyroscopic wheel will remain pointing in same direction

Gyroscopic force provides stability of spin axis and keeps it pointing in the same direction. Such satellites are known as spinners

Eg: Hughes (Boeing 376)

2. The satellite can be stabilized by one or more momentum wheels. This is called 3 axis stabilized satellite.

Eg: Hughes (Boeing) 701 The momentum wheel is usually a solid metal

disk driven by an electric motor Either there must be one momentum wheel for

each of the three axes , or a single momentum wheel can be mounted on gimbals and rotated to provide a rotational force about any of the 3 axes

As per principle of angular momentum, increasing the speed of the momentum wheel causes the satellite to precess in the opposite direction

Orbit insertion & Maintenance- GEO Two types of motors used on satellites. Traditional bipropellant thruster

Bipropellants used are Mono-methyl Hydrazine and Nitrogen tetraoxide

They are hypogolic, i.e., they ignite simultaneously on contact without any catalyst or heater

Arc jets or ion thrusters High voltage is used to accelerate ions

Fuel stored in GEO satellite is used for two purposes

Apogee kick motor (AKM) that injects the satellite into its final orbit

Maintain the satellite in that orbit over its lifetime.

Definition of axis

TTC&M

Telemetry Modes

Tracking

• Determines the current orbit of the satellite

Command

Typical TTC&M system

Power systems-1

Power systems-2

Power systems-3

Communication subsystems

Repeaters and Transponders

Types of payloads/Transponders

Transponders

In Transponders, two amplifiers are used is series so that though one fails other will make up

Output back off the degree to which output backup power is reduced below its peak output to reduce intermodulation products

In FDMA systems, back off is 2 to 7 db

Output amplifier is usually a solid state power amplifier

Travelling wave tube amplifier is used for higher output powers

M by N redundancy providing M amplifiers instead of N in a ring

Eg: 16 by 10 redundancy Transponder lasts till the last time of

satellite..

Basic Transponder elements

On Board Processors Methods to conserve uplink bandwidth

Use different modulation techniques on uplink and downlink

Provide a baseband processor on the satellite On board processing is advantageous to

switch between uplink(eg. MF-TDMA) and downlink(eg. TDM) so that small earth stations may access each other via directly via the satellite

The processor can provide data storage and also perform error correction independently

Spacecraft Antennas

Wire antennas: monopoles and dipoles

Horn antennas Reflector antenna Array antenna

Antenna Theory

Wire antennas Used primarily at VHF and UHF to

provide communications for TTC&M systems

Omnidirectional coverage

Horn antennas: Used at microwave frequencies when

relatively wide beams are required, as for global coverage

A flared section of waveguide provides an aperture several

wavelengths wide Also provides a good match between

the waveguide impedance and free space

Horns are used as feeds for reflectors either singly or in clusters

Reflector antennas: Usually illuminated by one or more

horns Provide a larger aperture than

provided with a horn alone Horns and reflectors are examples of

aperture antennas that launch a wave into free space from a waveguide

Important terms Antenna pattern:

Plot of field strength in the far field of the antenna when the antenna is driven by the transmitter

Usually measured in decibels (db) Gain:

Measure of antennas capability to direct energy in one direction rather than all around

Reciprocity: Antenna has same gain and pattern

at a given frequency whether receiving or transmitting

To provide a separate beam for each earth station would also require one antenna feed per earth station if multiple-feed antenna with a single reflector were used. A compromise between one beam per station and one beam for all stations has been used in many satellites by using zone – coverage beams

Footprint

Spot Beams Small geographic area

Zonal Coverage Less than one-third of

the earth’s surface Earth Coverage

One-third of the earths surface with approximate antenna beam width of 17 degrees.(hemi beams)

Outer space at geostationary distances is a harsh environment

Total vacuum; Sun irradiates the satellite with 1.4 K W heat and light on each square meter

When surfaces are in shadow, heat is lost to the infinite sink of space and surface temp falls to zero

Electronic devices cant withstand such temp Temperature must stay b/n 0 to 75 degree

celcius Quality control or quality assurance

Space Qualification

Quality control or quality assurance

Each satellite component is tested individually to ensure that it meets the specification

Testing in 3 parts Mechanical model contains structural and

mechanical parts Ensures all moving parts operate correctly in

vaccum Thermal modelEntire satellite will be

modelled in a chamber- shake and brake test Electrical modelchecks all electronic parts

Space qualification makes the GEO satellites expensive

Many electronic and mechanical components have less life time Hence back up or redundant unit will

be provided If one unit fails, back up device

automatically takes charge

ReliabilityFamiliar to automobile users.Chances for failure in starting and ending years is more compared to others

Redundancy

Next lecture earth station technology

Reference : Satellite communications engineering , Wilburt L Pritchard Robert A Nelson and Henri G Suyderhoud