Transcript Elements of Simulation

Satellite Communications
Satellite subsystems
Global Positioning System (GPS), NASA
Lect 03
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Overview
Position control system
Attitude control system
Power control system
Environmental control system
Telemetry
Transponders
Antennas
Beam shaping
Reliability design
© 2012 Raymond P. Jefferis III
1
Satellite Subsystems Overview
• The communications mission of the satellite
is supported by subsystems to maintain its
position, orientation, electric power, and
internal environment.
• Fulfilling the mission may require
producing a shaped communications beam
to communicate.
Lect 03
© 2012 Raymond P. Jefferis III
2
Orbital Position Control
• A geosynchronous satellite must remain located
within a 3-dimensional box despite the effects of
gravitational anomalies and solar wind. (This is
necessary for accurate ground station location of
the satellite.)
• Station-keeping is effected by thruster “burns”
(For example: xenon ion engine)
• Fuel/energy is expended to do this, which limits
the effective life of the satellite.
Lect 03
© 2012 Raymond P. Jefferis III
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Schematic of Orbital Position Control
• Orbital position measured by
ground stations
• Thruster burns calculated
• Burn times sent to satellite
• Orbital corrections made by
thruster “burns”
Lect 03
© 2012 Raymond P. Jefferis III
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Simple Orbital Dynamics
v
d

dr

r
2
2
r

3
2
where μ = 3.986004418E5
r = orbital radius
v = orbital velocity
Note:
As orbital radius decreases, velocity increases.
Lect 03
© 2012 Raymond P. Jefferis III
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Orbital Maneuvers
In: Orbital and Celestial Mechanics Website
http://www.cdeagle.com
http://www.cdeagle.com/html/ommatlab.html
Recommended download:
Orbital Mechanics with MATLAB, Orbital Maneuvers
http://www.cdeagle.com/ommatlab/maneuvers.pdf
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 6
Attitude Control
• Earth station coverage requires that
satellite remain in a fixed orientation
with respect to the earth
• On-board sensors measure orientation
with respect to the earth, sun, and stars.
• Attitude corrections are made by control
thrusters in three axes. Little energy
required for attitude corrections.
Lect 03
© 2012 Raymond P. Jefferis III
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Attitude Measurements
Orientation measurements
• Sun orientation
• Star(s) orientation(s)
• Earth orientation
A-A´ angle and orientation
B-B´ angle and orientation
Lect 03
© 2012 Raymond P. Jefferis III
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Attitude Control
The governing differential equation is:
d 2
J 2 T
dt
Where J is the moment of inertia of the satellite
around the axis of interest, θ is the attitude angle
with respect to a fixed reference direction, and T is
the applied torque, supplied by thrusters.
Lect 03
© 2012 Raymond P. Jefferis III
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Simple Attitude Algorithm
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Estimate the required correction angles
Burn thrusters to provide torques
Let angles drift to calculated value
Cancel angular velocity with opposing the
thrusters
• Repeat until the alignment is correct
Lect 03
© 2012 Raymond P. Jefferis III
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Power Control
• Orients solar panels normal to solar radiation, for
maximum output
• Regulates system voltages and distributes current
loads
• Maintains battery conditioning by maintaining
charge and discharge cycles for expected outages
of 70 minutes (orbital darkness cycle)
• Limits discharge to 70%, for battery protection
Lect 03
© 2012 Raymond P. Jefferis III
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Solar Power
• Silicon solar cells produce electric power
from “incident radiation”
– Direct sunlight
– Sunlight reflected from earth (albedo)
• Power is proportional to incident energy
• Temperature affects conversion efficiency
• As solar cells age, power output is reduced
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 12
Solar Panel Characterization
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Short circuit currrent, Isc
Open circuit voltage, Voc
Maximum power point voltage, Vmpp
Maximum power point current, Impp
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 13
Simple Circuit Model
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 14
Simple Model Explanation
• Iph -> current delivered by photocell
• D -> Diode characteristic of photocell
(Some loss current, ID flows in diode)
• Rs -> Equivalent internal series resistence
of photocell
• Ip -> effective current delivered (Iph – ID)
• Vp-> effective photocell output voltage
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 15
Mathematical Model
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 16
Mathematica® Photocell Model Function
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 17
Photocell Output Voltage Calculation
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 18
Calculated Photocell Output
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 19
Current vs Voltage Output of Solar Cell
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 20
Maximum Power Point of Solar Cell
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 21
Class Work
• Plot graphs for photocell panels with the
following power outputs:
– 500 W/m2
– 250 W/m2
• See notes
Lect 03
© 2012 Raymond P. Jefferis III
Lect 00 - 22
Temperature Control
• Temperature regulation (0 - 75 degC)
• Temperature and its cycling stresses all
components, shortening operational life
• Satellites have multiple heat sources
– Solar cell power dissipated on board
– Direct absorption of solar radiation
• Waste heat can be dumped into space by:
– Radiation
– Evaporation (if fluid available)
Lect 03
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Heat Balance
Lect 03
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Telemetry
• Transmitted data about conditions in a
satellite
– Operational status information
• Subsystems data
• Utility data
– Environmental data
• Temperatures
• Pressures (propellant tanks)
Lect 03
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Telemetry Block Diagram
Lect 03
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Ground Station Telemetry
• Telemetry data received by
earth station(s)
• Satellite tracking data is
generated by the earth station
• Orbital control processing is
calculated on the earth
• External data & commands
are sent by earth station(s) to
the satellite using telemetry
channels
Lect 03
© 2012 Raymond P. Jefferis III
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Layered Telemetry Model
Waqas Afzal and Adnan
Mahmood, Proceedings of
the International
MultiConference of
Engineers and Computer
Scientists 2008 ,Vol II
IMECS 2008, 19-21 March,
2008, Hong Kong
Lect 03
© 2012 Raymond P. Jefferis III
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Transponders
• Receive weak communication signals on one
frequency
• Amplifly these weak signals
• Simultaneously retransmit the communication on
another frequency at much higher power.
• Electromagnetic isolation is required to prevent
transmitted signals from interfering with the
reception of weak signals from a ground station.
Lect 03
© 2012 Raymond P. Jefferis III
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Channel Isolation Methods
• Frequency separation between Reception
and Transmission channel frequencies
• Electronic filtering (bandpass amplifiers)
• Transmitting antennas oppositely polarized
(electromagnetically decoupled) from the
receiving antennas in each channel
• Circulators with high degree of isolation
Lect 03
© 2012 Raymond P. Jefferis III
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Double Conversion Transponder
H and V indicate Horizontal and Vertical polarization, respectively.
Lect 03
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Data Processing Transponder
H and V indicate Horizontal and Vertical polarization, respectively.
Lect 03
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In-Band Frequency Allocations
• Transmission and reception
frequencies are channelized
into discrete bands
• Band allocation (1 of n)
1 - 36 MHz Channel
(useful capacity)
2 - 2 MHz guard bands
• Bands on 40 MHz centers
Lect 03
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Transponder Frequency Plan
• The Intelsat GALAXY-11 communications
satellite uses the plan that follows.
• Each channel has 36 MHz bandwidth, with
a 2 MHz guard band on each end
• Transmit (XMT) and Receive (RCV) pairs
of frequencies are about 2500 MHz apart, to
provide enough isolation that simultaneous
reception and transmission can take place.
Lect 03
© 2012 Raymond P. Jefferis III
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G11 C-Band Transponders
Uplink (RCV)
Downlink (XMT)
Note 2225 MHz RCV/XMT separation on each channel.
Lect 03
© 2012 Raymond P. Jefferis III
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G11 Ku-Band Transponders
Uplink (RCV)
Downlink (XMT)
Note 2300 MHz RCV/XMT separation on each channel.
Lect 03
© 2012 Raymond P. Jefferis III
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Antennas
• Receive weak signals and couple them to a low
noise amplifier
• Transmit power signals and shape the beam for
planned reception patterns on the ground
Note: Satellite is stabilized in all axes
• Can be horizontally or vertically polarized.
Polarized signals are received best by similarly
polarized receiving antennas.
• Have “gain” due to focusing of energy
Lect 03
© 2012 Raymond P. Jefferis III
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Antenna Types
• Wire (monopoles and dipoles)
Low gain (4 - 8 dB) not focused
• Horn (tapered waveguide)
Intermediate gain (23 dB), 10˚ beam focus
Often used to feed dish antenna
• Reflecting (dish, many wavelengths in diameter)
High gain (45 dB), 3˚ beam focus
• Array (multiple phased antennas in pattern)
Adjustable gain and beam shape possible
Lect 03
© 2012 Raymond P. Jefferis III
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Dipole Antennas
• Broad radiation pattern (78 degrees)
• Low gain (2.15 dB over isotropic for halfwave antenna)
• Longer versions have more gain (radiation
pattern is altered)
• Low gain limits missions to only those that
can be accomplished with low orbit
Lect 03
© 2012 Raymond P. Jefferis III
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Reflector Antennas
• Narrow radiation pattern (for wavelength λ
and diameter D, and factor k),
BW  k( / D) (k = 60 for parabolic antenna)
• High gain (for diameter D, wavelength λ,
and area, A),
G  ( D /  )  4 A / 
2
2
BW  k( / D)
Lect 03
© 2012 Raymond P. Jefferis III
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Parabolic Dish Antenna
• Symmetric
• The feed faces
the reflector at its
focal point
Wikipedia
Lect 03
© 2012 Raymond P. Jefferis III
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Center Fed Parabolic Dish Antenna
Lect 03
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Offset Parabolic Dish Antenna
Wikipedia
Lect 03
• Asymmetric
• Feed is offset; faces
the reflector
• Reflector is shaped
above feed horn line
to compensate for
offset
© 2012 Raymond P. Jefferis III
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Offset-Fed Parabolic Dish
Lect 03
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Cassegranian Antenna
Wikipedia
Lect 03
• Symmetric
• Feed horn extends
through center of
reflector
• Hyperboloid
secondary reflector
positioned at focus of
primary reflector
© 2012 Raymond P. Jefferis III
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Cassegranian Antenna
Cassegrain radar antenna at Sondrestrom, Greenland ( Diameter: 32 m Normal operating frequency: 1290 MHz )
Photo by L. Chang (wikipedia)
Lect 03
© 2012 Raymond P. Jefferis III
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Double Reflector Antennas
• Cassegranian
– Feed horn through center of reflector
– Hyperboloid secondary reflector at the focus of the
primary reflector
• Gregorian
– Feed horn through center of reflector
– Ellipsoid secondary reflector at focus of the primary
reflector
• Offset Gregorian
– Gregorian with feed horn at edge of the primary
reflector
Lect 03
© 2012 Raymond P. Jefferis III
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Gregorian Antenna Feed
Flickr Photo: http://flickr.com/photos/ekirsche/87736375/
Lect 03
© 2012 Raymond P. Jefferis III
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Antenna Gain
• The angular dependence of radiation from
an antenna is its radiation pattern. It is
measured as radiated power per solid angle.
• The ratio of radiated power per solid angle
to that of an isotropic dipole is the gain of
the antenna.
Lect 03
© 2012 Raymond P. Jefferis III
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Antenna Power Flux Density
• Isotropic
• Radiated power per unit spherical area
• Equivalent to the square of the RMS E-field
voltage divided by the impedance of free
space, 377 Ohms.
• Ψ = P/4πr2 = E2/ZFS [Watts/m2]
Lect 03
© 2012 Raymond P. Jefferis III
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Antenna Aperture
• The received power, Pr , is equal to the
raditated flux density, Ψ, multiplied by the
effective aperture, Aeff , of the receiving
antenna
• Pr = Aeff ψ = ηAψ [Watts]
• Aeff is a fraction, η , of the actual antenna
aperture area because of edge effects and
other losses.
Lect 03
© 2012 Raymond P. Jefferis III
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Gain of Aperture Antenna
G  4A A / 
2
G = aperture antenna gain
 A = aperture efficiency
A = aperture area [m2]
 = operating wavelength [m]
For circular aperture,
G  A ( D / )2
Lect 03
G = aperture antenna gain
 A = aperture efficiency
D = aperture diameter [m]
 = operating wavelength [m]
© 2012 Raymond P. Jefferis III
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Example Calculation
• A circular antenna has D/ = 25 [wavelengths]
  A = 63%
• Gain = 0.63*(*25)2 = 3886 = 36 dB
Lect 03
© 2012 Raymond P. Jefferis III
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Beamwidth of Aperture Antenna
 3dB  58 / D (Large circular aperture)
 3dB  75 / D (Parabola)
θ = 3dB beamwidth in degrees
λ = operating wavelength
D = aperture diameter [m]
Ref: J. D. Kraus and R. J. Marhefka, Antennas for All Applications,
Third Edition, McGraw-Hill, 2002.
Lect 03
© 2012 Raymond P. Jefferis III
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Example Calculation
• If a circular antenna has D/ = 25 [wavelengths],
  3dB = 75/25 = 3 [degrees]
• Note that the same reflector diameter will yield a
gain of 6 dB at half the wavelength
Lect 03
© 2012 Raymond P. Jefferis III
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Antenna Beamwidth
D/ 
75
 3dB
where,
θ = 3dB beamwidth [degrees]
λ = operating wavelength [m]
D = aperture diameter [m]
Note:
For  = 3˚,
D/ = 25
Lect 03
© 2012 Raymond P. Jefferis III
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Approximate Gain vs Beamwidth
Run Mathematica(R) program: mAntGain
LECT 04
© 2012 Raymond P. Jefferis III
Lect 00 - 57
GALAXY-11 Calculation
Intelsat GALAXY-11 at 91W (NORAD 26038)
• 39.1 dBW on C-Band (20W, 24 ch, Bw: 36 MHz)
• 47.8 dBW on Ku-Band (75/140W, 40 ch, Bw: 36 MHz)
Lect 03
© 2012 Raymond P. Jefferis III
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Beamwidth of Ku-Band Antenna
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Antenna diameter: 1.8 [m]
Frequency:
12 [GHz]
Wavelength:
0.025 [m]
Beamwidth ≈ 75/(1.8/0.025) ≈ 1.05˚
Lect 03
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Example - Ku-Band antenna gain
• 3dB beamwidth = 3˚
• D/ = 25
  = 0.63
• G = 3886
• Gdb = 36
Lect 03
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Sample Calculation of Antenna Gain
eff = 0.63;
beamw = 3;
f = 12*10^9;
c = 2.99792458*10^8;
lam = c/f;
app = 75.0/beamw
diam = app*lam
G = eff*p^2*app^2
lG = 10*Log[10, G]
Lect 03
© 2012 Raymond P. Jefferis III
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Phased Array Antennas
• For N antenna sources phased ϕ degrees apart in
an array of aperture radius, a
• Physical spacing, typically  /4
• Resulting beam intensity and angle,  (from
Wikipedia) are:
   
 sin   a  

I  I0 
  a 
 

Lect 03
2
N 
 
sin
N
sin


 
  4
2 


 sin   sin     

 

4

2
© 2012 Raymond P. Jefferis III
 2 2 
  sin   
N  
1
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Transmitter Antenna Gain
For a circular antenna (parabolic dish),
Ae  A (d / 2)
G
4

2
Ae
 d 
G  A  
  
Lect 03
2
2
where,
Ae = Effective aperture [m2]
 A= aperture efficiency
d = aperture diameter [m]
G = aperture antenna gain
 = operating wavelength [m]
© 2012 Raymond P. Jefferis III
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Reliability
• Satellites cannot easily be maintained
• Reliability methods:
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–
–
–
Lect 03
Component qualification
Burn-in (100 - 1000 hours)
Redundancy
Component switching
© 2012 Raymond P. Jefferis III
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Component Qualification Conditions
• Components manufactured with 100% tested
materials
• Raw material tracked to component lots
• Component failure rates characterized
– Specified operating conditions (-85 to +125 ˚C)
– Many components tested (some destructively)
– Failure rates calculated
• Lot numbers qualified for further use
Lect 03
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Burn-in
• Most component failures occur early on
• Running under power (burn-in) causes weak
components to fail early
• Used to catch systematic problems - bad lots
• Does not reduce life of most components
• Burn-in times of 100 - 1000 hours is considered
optimal
Lect 03
© 2012 Raymond P. Jefferis III
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Measures Used To Provide Redundancy
• Multiple redundant pathways
(Repair by ground command)
• Median voting
(Self-repair)
• Switched alternative circuits
(Repair by ground command)
Lect 03
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Multiple Redundant Pathways
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All components operate simultaneously
Results can be rescaled for correct values
There is a common point of failure at output
Assumes advantageous failure modes!
Lect 03
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Parallel Amplifier Example
Output of C will either be double or half of its
correct value, assuming A or B fails in OFF mode
(advantageous failure mode)
Note: Permits repair by ground station command
(Gain change)
Lect 03
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Median Voting Circuit
• Its rules:
– Reject largest value
– Reject smallest value
– Take median value as true
• Rejects up-scale and down-scale failures
• Expensive!
• A voter circuit is a common point of failure
Lect 03
© 2012 Raymond P. Jefferis III
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Median Voting Schematic
Note: Self-repairing
Lect 03
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Simple Probability Calculations
• Given that a single channel has a failure
probability (p = 10-6), per unit time, the failure
probability is
p fail  p  106
• For three equal channel failure probabilities, p, the
probability of two simultaneous failures for (p =
10-6) is,
p fail  3p2 (1 p)  3*1012 *(1106 )  3*1012
Lect 03
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Conclusion
• The triple redundancy system failure
probability with voting is nearly the square
of the single-element system failure
• The voter circuit is a common point of
failure to be considered
• Up-scale or down-scale failure
(advantageous failure mode) is assumed
Lect 03
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Switched Alternative Circuits
• Two-way redundant paths built into signal path
• Switching between paths are provided to select
preferred component
• Both outputs analyzed on the ground
• Switching is effected to select the chosen
component
• Cheaper
– Uses fewer components
– Saves power
Lect 03
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Switching Circuit Example
Note: permits repair by ground station command,
when either Amplifier A or Amplifier B fails.
Lect 03
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End
Lect 03
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