Satellite Comm LecIII

8/13/2019 Satellite Comm LecIII

1/45

Satellite Communications

Part II-Satellite Subsystems

Lecturer Madeeha Owais12/26/2008 1NUST-SEECS


2/45

Learning Objectives

Satellite Spacing in Orbit Communication Satellite Classifications

Satellite System Link Model

Communication Satellite Subsystems

12/26/2008 2NUST-SEECS


3/45

Geo-stationary satellites must share a limited space andfrequency spectrum within a given arc of geostationary orbit

Each satellite is assigned a longitude in the geostationary arcabove the equator

The position in the slot depends on the frequency band used.

Satellites operating at or near the same frequency must besufficiently separated in space to avoid mutual interference


Satellite Spacing in Orbit


4/45

Required spatial separation is dependent on the fewvariables:

Beam widths and side lobe radiation of both earth stationand satellite antennas

RF carrier frequency

Encoding or modulation technique used Acceptable limits of interference

Transmit carrier power

Initially,3 to 6 of spatial separation for geo-sats

Now has been reduced to 2 to make available extra slots. Some positions in GEO orbit, such as mid-atlantic and mid-

pacific position, have higher demand than others




5/45




6/45

Spinner Satellites

Less common type

Mostly used in relatively high-altitude

geosynchronous or Molniya orbits

Intelsat VI Satellite, DSP (Defense

Support Program) Satellite of USA

Intelsat VI Satellite

DSP Satellite

Communication Satellite Classifications

Courtesy of Boeing Satellite Systems. All rights reserved.


7/45

Three-Axis Stabilized Satellites

Their body is roughly box-shaped and

have deployable solar-array panels

These keep their bodies stable through

inertia except for a slow motion about

one axis to keep their payload

antennas and sensors continuouslypointing towards Earth. The solar

panels are counter-rotated to track the

sun.

Examples: Defense Meteorological

Satellite Program (DMSP), Japanese

Earth Resources Satellite (JERS),Russian Communication Satellite,

Communication Satellite Classifications

Courtesy of Orbital Sciences Corp. All rights reserved.


8/45

Satellite System Link Model

CSE 426-F2007 NDG Notes 8

Solarpanels

Basic Sections:

Uplink

Transponder

Downlink


9/45

Primary component

Earth Station Transmitter

IF modulator

IF-to-RF microwave up-converter

High Power Amplifier(Klystrons or Travelling-wave tubes )

Band pass filter


Uplink Model


10/45

12/26/2008 10NUST-SEECS

Transponder

Transponder (Transmitter + Responder) Model

To be discussed in coming slides..


11/45

Primary component

Earth Station Receivers

Input BPF

LNA(tunnel diode)

RF-to-IF down-converter(mixer+BPF)

12/26/2008 11NUST-SEECS

Downlink Model


12/45

12/26/2008 12NUST-SEECS

Communication Satellite Subsystems

References:Satellites Communications by

Timothy Pratt Chapter 3,Titled:Satellites


13/45

The major subsystems required on the communication satellite are:

Attitude and orbit control system(AOCS):It consists of rocket motor

that are used to move the satellite back to the correct orbit when

external forces causes it to drift. Telemetry,Tracking,Command and Monitoring(TTC & M):This is

partly on the satellite and partly on the controlling earth station.

A dedicated earth station is used for this purpose.

Used for launch sequence deployment, monitoring of commandactions, report spacecraft health, control of thrusters and payload etc

Power Systems: Mainly solar cells

Communications Subsystem: These are major components

(represent small part of volume, weight and cost of sat in orbit) of a

communication satellite.Includes transponders and antennas

Satellite Antennas: Type depends on functionality and coverage

12/26/2008 13NUST-SEECS


14/45

Attitude and Orbital Control AOC system keeps the satellite pointed towards the desired location on the earth.

Attitude control means controlling the angular orientation

AOCS is needed to get the satellite into the correct orbit and keep it there.

Several factors make the space craft tend to rotate and wobble(nutation) and change

orbit(e.g gravitational forces from sun,moon ,solar pressure,variations in earths

magnetic field) for which orbit maintenance is required

Different forms of stabilization for fine pointing are used depending upon type of

the satellite

12/26/2008 14NUST-SEECS

http://en.wikipedia.org/wiki/Nutation


15/45

CSE 426-F2007 NDG Notes 15

Few Basic Definitions

Roll, Pitch, and Yaw


16/45

Spinner Satellites use the angular

momentum of its spinning body to providestabilization

Entire craft is rotated at 30-100RPM to

provide gyroscopic force

Keeps satellite point in same direction

DSP Satellite

Stabilization of Spinner

http://www.gyroscopes.org/behaviour.asp

http://en.wikipedia.org/wiki/Gyroscope
http://www.gyroscopes.org/behaviour.asphttp://en.wikipedia.org/wiki/Gyroscopehttp://en.wikipedia.org/wiki/Gyroscopehttp://www.gyroscopes.org/behaviour.asp


17/45

Three-Axis Stabilized Satellites

keep their body fixed relative to

Earths surface and an internal

subsystem provides roll and yaw

stabilization

momentum wheel, which is a

solid metallic disk driven by an

electric motor is used . By

spinning the disk, the stability of

the satellite is maintained.

Stabilization of Three-Axis Stabilized Sat


18/45

Telemetry and Monitoring (T&M)

in TTC & M Collects data from many sensors and send them to the control

earth station for reporting of spacecraft health

Pressure in fuel tanks

Current drawn by each subsystem

Critical voltages and currents

Temperature

Status and position of switches

Devices used to maintain attitude

Low date rate is used to allow the receiver at the earth stationto have narrow band-width and maintain high C/N ratio

12/26/2008 18NUST-SEECS


19/45

Tracking(T) in TTC & M

The determination of the current orbit and position of the

spacecraft

Velocity and Acceleration sensors are employed

The control earth station can observe the doppler shift of the

telemetry carrier to determine the rate of change of range.

Triangulation can be used from measurements from several

earth stations observing the satellite

12/26/2008 19NUST-SEECS


20/45

Command(C) in TTC & M

Secure and effective command structure is vital for the

successful launch and operation of a communication satellite.

During launch sequence backup command system is used to:

Switch on full power so that handover to the main TTC & M is possible

Deploy antennas and solar panels

Point antennas to desired location

In orbit it is used for:

Making changes in attitude and orbit correction

Maintain spacecraft thermal balance

Controlling the communication system

12/26/2008 20NUST-SEECS


21/45

Command(C) in TTC & M

Safeguards against errors in the received commands are builtin command structure

Command originates at the control terminal by converting a control

code into a command word which is sent in a TDM frame.

Validity is checked and sent back via the telemetry link where it is

checked again on computer

If the command word is received correctly, an execute instruction will

be sent to satellite

The entire process takes 5-10sec.And minimizes the risk of

malfunctioning

12/26/2008 21NUST-SEECS


22/45

Typical Tracking,Telemetry and

Command System

12/26/2008 22NUST-SEECS


23/45

Power System

All communication satellites obtain their power from solar cell Solar radiation falling on a geostationary spacecraft has

intensity of 1.39 kw/m2

Efficiency of solar cells falls with time due to aging and

etching of surface. Spacecrafts carry batteries to power the subsystems during

launch and eclipses

12/26/2008 23NUST-SEECS


24/45

Communications Subsystem-1 A communication satellite exists to provide a platform in orbit

for relaying voice, video and data communications

All other subsystems on the satellite exist to support the comm

system

Only source of revenue

Design is to maximize the traffic capacity

Downlink design is the critical part due to limited transmitter

power and antenna size and gain.

Received power level rarely exceeds 10-10

W Satisfactory performance->S/N in receiver must be between

5dB and 25dB depending on the bandwidth of transmitted

signal and modulation used.

12/26/2008 24NUST-SEECS


25/45

Communications Subsystem-2

Early Communication Satellite(Power Limited) Transponders B.W=250 or 500Mhz

Transmitter output power=1 to 2 W

Later generations(Bandwidth limited)

Transmitter output power up to 200W

Increased total channel capacity by re-use of frequency

Techniques employed for Re-use

Spatial Frequency Reuse->Multiple directional antenna beams at the

same frequency

Polarization Reuse->Orthogonal polarization at same frequency

Geo-sats have achieved effective B.W=2250 MHz within a 500MHz

band at 6/4 GHz by using spatial and polarization frequency reuse

12/26/2008 25NUST-SEECS


26/45

12/26/2008 NUST-SEECS 26


C i i S b 4


27/45

Repeater/Transponder

12/26/2008 27NUST-SEECS

The total Repeater bandwidth(up to 500MHz-1.5GHz) is split intosub-bands(a few tens of MHz).

Each sub-band is then amplified by a Transponder.


C i i S b 5


28/45

Transponder Frequency Plan

12/26/2008 28NUST-SEECS

Transponder arrangement of RCAs SATCOM satellite

24 active transponders by adopting orthogonal polarization frequency re-use

500MHz B.W divided into channels of 36MHz

Center frequencies are spaced 40MHz apart to allow guard bands

3800


C i i S b 6


29/45

Transponder Technology

12/26/2008 29NUST-SEECS

Two Types

Transparent(Bentpipe) TranspondersOnboard Processing(Regenerative)Transponder


Transparent

Processin

g

C i ti S b t 7


30/45


12/26/2008 30NUST-SEECS

Functionality of Transparent(Bentpipe) Transponder

Frequency TranslationAmplification


C i ti S b t 8


31/45

12/26/2008 31NUST-SEECS


Correct

mistake in

book

C i ti S b t 9


32/45


12/26/2008 32NUST-SEECS

Functionality of Onboard Processing(OBP) Transponder

Uplink is demodulated, data recovered prior to processingAfter processing, data is reformatted for transmission on

downlink



33/45

Satellite Antennas

12/26/2008 NUST-SEECS 33

Wire Antennas:monopoles and dipoles

Horn Antennas

Reflector Antennas

Array Antennas


34/45

Satellite Antenna Radiation Patterns: Footprints

Footprint: Geographical representation of satellite antenna radiation

pattern. It is the area on Earths surface that the satellite can receive

from or transmit to.

Shape of footprint depends on:

Orbital Path,

Height,

Antenna used

12/26/2008 34NUST-SEECS

Same EIRP Contour Lines Super-imposed on a Geo-graphical Map


35/45

12/26/2008 NUST-SEECS 35

http://personal.ee.surrey.ac.uk/Personal/L.Wood/publications/lloyd-wood-iwssc-08-tutorial.pdf


36/45

Footprint Categories

Spot Zonal

Hemispherical

Earth(Global)

12/26/2008 36NUST-SEECS


37/45

Footprint Categories

Spot and Zonal Beams: Concentrated power to very small geographical areas

Have high EIRPs

Blanket less than 10% of earths surface

Hemispherical Beam

Blanket 20% of Earths surface

Have EIRP that are 3dB lower than spot beams

Earth(Global)

Beamwidth of approximately 17

Coverage of upto 42% of earths surface

Power levels are considerably low

Require large receive dishes for adequate signal detection

12/26/2008 37NUST-SEECS


38/45

12/26/2008 NUST-SEECS 38


39/45

Relations to Remember

An aperture antenna has a gain G given by

G = A4A/2

where A=area of the antenna aperture in meters

=wavelength in meters

A=aperture efficiency

If aperture is circular,G= A

(D/)2

where D=diameter of circular aperture in meters

3 dB Beam width and aperture dimension are related by:

3dB=75 /D degrees

For antennas with A=60% :

G=33,000/(3dB )2

where G is not in decibels and beam width is in degrees.Value of constant varies

from 28,000 to 35000 for different sources.33,000 typical for reflectors39


40/45

Coverage of

Geostationary Satellite

12/26/2008 NUST-SEECS 40


f


41/45

Coverage of

Geosynchronous Satellite

12/26/2008 NUST-SEECS 41


C f


42/45

Coverage of

Molniya

12/26/2008 NUST-SEECS 42

http://personal.ee.surrey.ac.uk/Personal/L.Wood/constellations

F i f I idi


43/45

12/26/2008 NUST-SEECS 43

Footprint of Iridium

http://personal.ee.surrey.ac.uk/Personal/L.Wood/constellations/


44/45


45/45

Reading Assignment

Chapter 3-Timothy Pratt Book

pg 57-92,exclude maths on pg 90

http://personal.ee.surrey.ac.uk/Personal/L.Wood/constellations/

Recommended site visit for Everyone!

Documents

Satellite Comm LecIII