EC 723 Satellite Communication Systems

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EC 723 Satellite Communication Systems

• Mohamed Khedr
• http//webmail.aast.edu/khedr

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Grades
Load Percentage Date
Midterm Exam 30 Week of 3 December 2007
Final Exam 30
Participation 10
Report and presentation 30 Starting week 11th

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Textbook and website

• Textbook non specific
• Website http//webmail.aast.edu/khedr

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Syllabus
Week 1 Overview
Week 2 Orbits and constellations GEO, MEO and LEO
Week 3 Satellite space segment, Propagation and satellite links , channel modelling
Week 4 Satellite Communications Techniques
Week 5 Satellite error correction Techniques
Week 6 Multiple Access I
Week 7 Multiple access II
Week 8 Satellite in networks I
Week 9 INTELSAT systems , VSAT networks, GPS
Week 10 GEO, MEO and LEO mobile communications INMARSAT systems, Iridium , Globalstar, Odyssey
Week 11 Presentations
Week 12 Presentations
Week 13 Presentations
Week 14 Presentations
Week 15 Presentations

• Tentatively

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Exploded view of a spinner satellite based on the
Boeing (Hughes) HS 376 design. INTELSAT IVA
(courtesy of Intelsat).
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a) A spinner satellite, INTELSAT IV A (courtesy
of Intelsat).
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(b) A three-axis stabilized satellite, INTELSAT V
(courtesy of Intelsat).
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SPACECRAFT SUBSYSTEMS

• Attitude and Orbital Control System (AOCS)
• Telemetry Tracking and Command (TTC)
• Power System
• Communications System
• Antennas

More usually TTCM - Telemetry, Tracking,
Command, and Monitoring
TelemetryAutomatic transmission and measurement
of data from remote sources by wire or radio or
other means
We will look at each in turn
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Typical tracking, telemetry, command and
monitoring system.
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Bathtub curve for probability of failure.
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AOCS

• AOCS is needed to get the satellite into the
  correct orbit and keep it there
• Orbit insertion
• Orbit maintenance
• Fine pointing
• Major parts
• Attitude Control System
• Orbit Control System

Look at these next
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ORBIT MAINTENANCE - 1

• MUST CONTROL LOCATION IN GEO POSITION WITHIN
  CONSTELLATION
• SATELLITES NEED IN-PLANE (E-W) OUT-OF-PLANE
  (N-S) MANEUVERS TO MAINTAIN THE CORRECT ORBIT
• LEO SYSTEMS LESS AFFECTED BY SUN AND MOON BUT MAY
  NEED MORE ORBIT-PHASING CONTROL

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FINE POINTING

• SATELLITE MUST BE STABILIZED TO PREVENT NUTATION
  (WOBBLE) Move unsteadily
• THERE ARE TWO PRINCIPAL FORMS OF ATTITUDE
  STABILIZATION
• BODY STABILIZED (SPINNERS, SUCH AS INTELSAT VI)
• THREE-AXIS STABILIZED (SUCH AS THE ACTS, GPS,
  ETC.)

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DEFINITION OF AXES - 1

• ROLL AXIS
• Rotates around the axis tangent to the orbital
  plane (N-S on the earth)
• PITCH AXIS
• Moves around the axis perpendicular to the
  orbital plane (E-W on the earth)
• YAW AXIS
• Moves around the axis of the subsatellite point

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DEFINITION OF AXES - 2
Earth
o
Equator
s
Yaw Axis
Roll Axis
Pitch Axis
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TTCM

• MAJOR FUNCTIONS
• Reporting spacecraft health
• Monitoring command actions
• Determining orbital elements
• Launch sequence deployment
• Control of thrusters
• Control of payload (communications, etc.)

TTCM is often a battle between Operations (who
want every little thing monitored and Engineering
who want to hold data channels to a minimum
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TELEMETRY - 1

• MONITOR ALL IMPORTANT
• TEMPERATURE
• VOLTAGES
• CURRENTS
• SENSORS
• TRANSMIT DATA TO EARTH
• RECORD DATA AT TTCM STATIONS

NOTE Data are usually multiplexed with a
priority rating. There are usually two telemetry
modes.
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TELEMETRY - 2

• TWO TELEMETRY PHASES OR MODES
• Non-earth pointing
• During the launch phase
• During Safe Mode operations when the spacecraft
  loses tracking data
• Earth-pointing
• During parts of the launch phase
• During routine operations

NOTE for critical telemetry channels
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TRACKING

• MEASURE RANGE REPEATEDLY
• CAN MEASURE BEACON DOPPLER OR THE COMMUNICATION
  CHANNEL
• COMPUTE ORBITAL ELEMENTS
• PLAN STATION-KEEPING MANEUVERS
• COMMUNICATE WITH MAIN CONTROL STATION AND USERS

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COMMAND

• DURING LAUNCH SEQUENCE
• SWITCH ON POWER
• DEPLOY ANTENNAS AND SOLAR PANELS
• POINT ANTENNAS TO DESIRED LOCATION
• IN ORBIT
• MAINTAIN SPACECRAFT THERMAL BALANCE
• CONTROL PAYLOAD, THRUSTERS, ETC.

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COMMUNICATIONS SUB-SYSTEMS

• Primary function of a communications satellite
  (all other subsystems are to support this one).
• Only source of revenue
• Design to maximize traffic capacity
• Downlink usually most critical (limited output
  power, limited antenna sizes).
• Early satellites were power limited
• Most satellites are now bandwidth limited.

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Typical satellite antenna patterns and coverage
zones. The antenna for the global beam is usually
a waveguide horn. Scanning beams and shaped beams
require phased array antennas or reflector
antennas with phased array feeds.
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Typical coverage patterns for Intelsat satellites
over the Atlantic Ocean.
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Contour plot of the spot beam of ESAs OTS
satellite projected onto the earth. The contours
are in 1 dB steps, normalized to 0 dB at the
center of the beam.
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Radio Propagation Atmospheric Losses

• Different types of atmospheric losses can perturb
  radio wave transmission in satellite systems
• Atmospheric absorption
• Atmospheric attenuation
• Traveling ionospheric disturbances.

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Radio PropagationAtmospheric Absorption

• Energy absorption by atmospheric gases, which
  varies with the frequency of the radio waves.
• Two absorption peaks are observed (for 90º
  elevation angle)
• 22.3 GHz from resonance absorption in water
  vapour (H2O)
• 60 GHz from resonance absorption in oxygen (O2)
• For other elevation angles
• AA AA90 cosec ?

Source Satellite Communications, Dennis Roddy,
McGraw-Hill
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Radio PropagationAtmospheric Attenuation

• Rain is the main cause of atmospheric attenuation
  (hail, ice and snow have little effect on
  attenuation because of their low water content).
• Total attenuation from rain can be determined by
• A ?L dB
• where ? dB/km is called the specific
  attenuation, and can be calculated from specific
  attenuation coefficients in tabular form that can
  be found in a number of publications
• where L km is the effective path length of the
  signal through the rain note that this differs
  from the geometric path length due to
  fluctuations in the rain density.

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Radio PropagationTraveling Ionospheric
Disturbances

• Traveling ionospheric disturbances are clouds of
  electrons in the ionosphere that provoke radio
  signal fluctuations which can only be determined
  on a statistical basis.
• The disturbances of major concern are
• Scintillation
• Polarisation rotation.
• Scintillations are variations in the amplitude,
  phase, polarisation, or angle of arrival of radio
  waves, caused by irregularities in the ionosphere
  which change over time. The main effect of
  scintillations is fading of the signal.

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Signal PolarisationWhat is Polarisation?

• Polarisation is the property of electromagnetic
  waves that describes the direction of the
  transverse electric field. Since electromagnetic
  waves consist of an electric and a magnetic field
  vibrating at right angles to each other it is
  necessary to adopt a convention to determine the
  polarisation of the signal. Conventionally, the
  magnetic field is ignored and the plane of the
  electric field is used.

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Signal PolarisationTypes of Polarisation

• Linear Polarisation (horizontal or vertical)
• the two orthogonal components of the electric
  field are in phase
• The direction of the line in the plane depends on
  the relative amplitudes of the two components.
• Circular Polarisation
• The two components are exactly 90º out of phase
  and have exactly the same amplitude.
• Elliptical Polarisation
• All other cases.

Linear Polarisation
Circular Polarisation
Elliptical Polarisation
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Signal PolarisationSatellite Communications

• Alternating vertical and horizontal polarisation
  is widely used on satellite communications to
  reduce interference between programs on the same
  frequency band transmitted from adjacent
  satellites (one uses vertical, the next
  horizontal, and so on), allowing for reduced
  angular separation between the satellites.

Information Resources for Telecommunication
Professionals www.mlesat.com
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Signal PolarisationDepolarisation

• Rain depolarisation
• Since raindrops are not perfectly spherical, as a
  polarised wave crosses a raindrop, one component
  of the wave will encounter less water than the
  other component.
• There will be a difference in the attenuation and
  phase shift experienced by each of the electric
  field components, resulting in the depolarisation
  of the wave.

Polarisation vector relative to the major and
minor axes of a raindrop. Source Satellite
Communications, Dennis Roddy, McGraw-Hill
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Signal PolarisationCross-Polarisation
Discrimination

• Depolarisation can cause interference where
  orthogonal polarisation is used to provide
  isolation between signals, as in the case of
  frequency reuse.
• The most widely used measure to quantify the
  effects of polarisation interference is called
  Cross-Polarisation Discrimination (XPD)
• XPD 20 log (E11/E12)

• To counter depolarising effects circular
  polarising is sometimes used.
• Alternatively, if linear polarisation is to be
  used, polarisation tracking equipment may be
  installed at the antenna.

Source Satellite Communications, Dennis Roddy,
McGraw-Hill
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Illustration of the various propagation loss
mechanisms on a typical earth-space path
The ionosphere can cause the electric vector of
signals passing through it to rotate away from
their original polarization direction, hence
causing signal depolarization.
the sun (a very hot microwave and millimeter
wave source of incoherent energy), an increased
noise contribution results which may cause the
C/N to drop below the demodulator threshold.
The absorptive effects of the atmospheric
constituents cause an increase in sky noise to be
observed by the receiver
Refractive effects (tropospheric scintillation)
cause signal loss.
The ionosphere has its principal impact on
signals at frequencies well below 10 GHz while
the other effects noted in the figure above
become increasingly strong as the frequency of
the signal goes above 10 GHz
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Atmospheric attenuation
Attenuation of the signal in
Example satellite systems at 4-6 GHz
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rain absorption
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fog absorption
e
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atmospheric absorption
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elevation of the satellite
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Signal TransmissionLink-Power Budget Formula

• Link-power budget calculations take into account
  all the gains and losses from the transmitter,
  through the medium to the receiver in a
  telecommunication system. Also taken into the
  account are the attenuation of the transmitted
  signal due to propagation and the loss or gain
  due to the antenna.
• The decibel equation for the received power is
• PR EIRP GR - LOSSES
• Where
• PR received power in dBW
• EIRP equivalent isotropic radiated power in
  dBW
• GR receiver antenna gain in dB
• LOSSES total link loss in dB
• dBW 10 log10(P/(1 W)), where P is an arbitrary
  power in watts, is a unit for the measurement of
  the strength of a signal relative to one watt.

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Link Budget parameters

• Transmitter power at the antenna
• Antenna gain compared to isotropic radiator
• EIRP
• Free space path loss
• System noise temperature
• Figure of merit for receiving system
• Carrier to thermal noise ratio
• Carrier to noise density ratio
• Carrier to noise ratio

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Signal TransmissionEquivalent Isotropic Radiated
Power

• An isotropic radiator is one that radiates
  equally in all directions.
• The power amplifier in the transmitter is shown
  as generating PT watts.
• A feeder connects this to the antenna, and the
  net power reaching the antenna will be PT minus
  the losses in the feeder cable, i.e. PS.
• The power will be further reduced by losses in
  the antenna such that the power radiated will be
  PRAD (lt PT).

(a) Transmitting antenna Source Satellite
Communications, Dennis Roddy, McGraw-Hill
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Antenna Gain

• We need directive antennas to get power to go in
  wanted direction.
• Define Gain of antenna as increase in power in a
  given direction compared to isotropic antenna.

• P(?) is variation of power with angle.
• G(?) is gain at the direction ?.
• P0 is total power transmitted.
• sphere 4p solid radians

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Signal TransmissionLink-Power Budget Formula
Variables

• Link-Power Budget Formula for the received power
  PR
• PR EIRP GR - LOSSES
• The equivalent isotropic radiated power EIRP
  is
• EIRP PS G dBW, where
• PS is the transmit power in dBW and G is the
  transmitting antenna gain in dB.
• GR is the receiver antenna gain in dB.
• LOSSES FSL RFL AML AA PL,
  where
• FSL free-space spreading loss in dB PT/PR
  (in watts)
• RFL receiver feeder loss in dB
• AML antenna misalignment loss in dB
• AA atmospheric absorption loss in dB
• PL polarisation mismatch loss in dB
• The major source of loss in any ground-satellite
  link is the free-space spreading loss.

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More complete formulation

• Demonstrated formula assumes idealized case.
• Free Space Loss (Lp) represents spherical
  spreading only.
• Other effects need to be accounted for in the
  transmission equation
• La Losses due to attenuation in atmosphere
• Lta Losses associated with transmitting antenna
• Lra Losses associates with receiving antenna
• Lpol Losses due to polarization mismatch
• Lother (any other known loss - as much detail
  as available)
• Lr additional Losses at receiver (after
  receiving antenna)

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Transmission Formula

• Some intermediate variables were also defined
  before
• Pt Pout /Lt EIRP Pt Gt
  
• Where
• Pt Power into antenna
• Lt Loss between power source and antenna
• EIRP effective isotropic radiated power

• Therefore, there are many ways the formula could
  be rewritten. The user has to pick the one most
  suitable to each need.

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Link Power Budget
Tx
EIRP
Transmission HPA Power Transmission Losses
(cables connectors) Antenna Gain
Antenna Pointing Loss Free Space Loss Atmospheric
Loss (gaseous, clouds, rain) Rx Antenna Pointing
Loss
Reception Antenna gain Reception Losses
(cables connectors) Noise Temperature
Contribution
Rx
Pr
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Translating to dBs

• The transmission formula can be written in dB as
• This form of the equation is easily handled as a
  spreadsheet (additions and subtractions!!)
• The calculation of received signal based on
  transmitted power and all losses and gains
  involved until the receiver is called Link Power
  Budget, or Link Budget.
• The received power Pr is commonly referred to as
  Carrier Power, C.

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Link Power Budget
Now all factors are accounted for as additions
and subtractions
Tx
EIRP

• Transmission
• HPA Power
• Transmission Losses
• (cables connectors)
• Antenna Gain

• Antenna Pointing Loss
• Free Space Loss
• Atmospheric Loss (gaseous, clouds, rain)
• - Rx Antenna Pointing Loss

• Reception
• Antenna gain
• Reception Losses
• (cables connectors)
• Noise Temperature Contribution

Rx
Pr
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Easy Steps to a Good Link Power Budget

• First, draw a sketch of the link path
• Doesnt have to be artistic quality
• Helps you find the stuff you might forget
• Next, think carefully about the system of
  interest
• Include all significant effects in the link power
  budget
• Note and justify which common effects are
  insignificant here
• Roll-up large sections of the link power budget
• Ie. TXd power, TX ant. gain, Path loss, RX ant.
  gain, RX losses
• Show all components for these calculations in the
  detailed budget
• Use the rolled-up results in build a link
  overview
• Comment the link budget
• Always, always, always use units on parameters
  (dBi, W, Hz ...)
• Describe any unusual elements (eg. loss caused by
  H20 on radome)

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Simple Link Power Budget
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Why calculate Link Budgets?

• System performance tied to operation thresholds.
• Operation thresholds Cmin tell the minimum power
  that should be received at the demodulator in
  order for communications to work properly.
• Operation thresholds depend on
• Modulation scheme being used.
• Desired communication quality.
• Coding gain.
• Additional overheads.
• Channel Bandwidth.
• Thermal Noise power.

We will see more on these items in the next
classes.
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Closing the Link

• We need to calculate the Link Budget in order to
  verify if we are closing the link.
• Pr gt Cmin ? Link Closed
• Pr lt Cmin ? Link not closed
• Usually, we obtain the Link Margin, which tells
  how tight we are in closing the link
• Margin Pr Cmin
• Equivalently
• Margin gt 0 ? Link Closed
• Margin lt 0 ? Link not closed

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Carrier to Noise Ratios

• C/N carrier/noise power in RX BW (dB)
• Allows simple calculation of margin if
• Receiver bandwidth is known
• Required C/N is known for desired signal type
• C/No carrier/noise p.s.d. (dbHz)
• Allows simple calculation of allowable RX
  bandwidth if required C/N is known for desired
  signal type
• Critical for calculations involving carrier
  recovery loop performance calculations

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System Figure of Merit

• G/Ts RX antenna gain/system temperature
• Also called the System Figure of Merit, G/Ts
• Easily describes the sensitivity of a receive
  system
• Must be used with caution
• Some (most) vendors measure G/Ts under ideal
  conditions only
• G/Ts degrades for most systems when rain loss
  increases
• This is caused by the increase in the sky noise
  component
• This is in addition to the loss of received power
  flux density

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System Noise Power - 1

• Performance of system is determined by C/N ratio.
• Most systems require C/N gt 10 dB.
• (Remember, in dBs C - N gt 10 dB)
• Hence usually C gt N 10 dB
• We need to know the noise temperature of our
  receiver so that we can calculate N, the noise
  power (N Pn).
• Tn (noise temperature) is in Kelvins (symbol K)

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System Noise Power - 2

• System noise is caused by thermal noise sources
• External to RX system
• Transmitted noise on link
• Scene noise observed by antenna
• Internal to RX system
• The power available from thermal noise is
• where k Boltzmanns constant
• 1.38x10-23 J/K(-228.6 dBW/HzK),
• Ts is the effective system noise temperature,
  andB is the effective system bandwidth

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Thank you