SS 3011 Space Technology and Applications

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SS 3011 Space Technology and Applications
Spacecraft Communication Systems
Relay Satellite
Mission Satellite
Control Center
Ground Station
Sellers Chapter 11, pp 382-388,
Chapter 15, pp. 617-629.
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Spacecraft Building Blocks
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Communication System Architecture
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How do These Remotely Located Systems Communicate?
Electromagnetic Radiation! EM radiation --
transverse waves produced by moving charges.
A charge can radiate electromagnetic
radiation only if it is undergoing
accelerated motion. Electromagnetic radiation
also can be described as discrete packets
known as photons. Light is a general
term referring to electromagnetic radiation in
the visible part of the spectrum.
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What Produces ElectromagneticWaves?
As objects are heated up, electrons are
stripped from Lattice and randomly
accelerated Thus hot objects glow (emit EM
radiation) Naturally occurring EM
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Example Remote Sensing Mission
Naturally occurring EM
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How are EM Waves Manufactured
Dipole Antenna
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Antenna Theory (contd)
Accelerating Charge Induces an Electrical
Field Electric Field Induces a Magnetic
Field If charge is accelerated back and forth
along the Antenna at a prescribed Frequency .
. Electromagnetic radiation at that
Prescribed Frequency is produced
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Antenna Theory (contd)
Maxwells Equations
Process Described By Maxwells equations
Any Questions?
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Antenna Theory (contd)
Maxwell showed that these equation implicitly
required the existence of electromagnetic wave
traveling at the speed of light. He also
proposed a physical ether theory. He abandoned
attempts to formulate a specific mechanical
model, instead using the formalism of Lagrangian
dynamics. His theory of electromagnetic
fields led directly to the existence of
electromagnetic waves.
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Electromagnetic Waves
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Electromagnetic Wave Energy
High Frequency waves are more energetic than
low frequency waves
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Electromagnetic Spectrum
High energy end of spectrum
Low energy end of spectrum
Radio Frequency (RF) band
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(revisited)
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Radio Waves
Why do we use RF Spectrum for communications?
Atmospheric transmissivity
Atmospheric is nearly transparent to long-wave
radiation
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Encoding and Modulation
Artificially produced EM waves are used to
transmit Information over long distances by
using encoding and modulation
Encoding embeds a message into a mathematical
code Morse code, example Of pulse-width-encodin
g
100
1111
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1
Morse Code
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Encoding and Modulation (contd)
Modulation, in communications, is a process in
which some characteristic of a wave (the carrier
wave) is made to vary in accordance with an
information-bearing signal wave (the modulating
wave) Demodulation is the process by which
the original signal is Recovered from the wave
produced by modulation. The original,
Un-modulated wave may be of any kind, such as
sound or, most often, electromagnetic radiation,
including optical waves. The carrier wave
can be a direct current, an alternating current,
or a pulse chain. In modulation, it is processed
in such a way that its amplitude, frequency, or
some other property varies.
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Encoding and Modulation (contd)
Modulation embeds this code onto an
electromagnetic carrier wave
Amplitude Modulation
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Amplitude Modulation
Amplitude modulation (AM) is the modulation
method used in the AM radio broadcast band.
AM modulation varies the STRENGTH of the radio
signal according according to the information
encoded into the carrier wave. This form of
modulation is not a very efficient way to send
information the power required is relatively
large because the carrier, which contains no
information, is sent along with the information
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Encoding and Modulation (continued)
Carrier wave
Frequency Modulation
Modulation Wave (Baseband)
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Frequency Modulation
In Frequency modulation the instantaneous
frequency of a sinusoidal carrier wave is caused
to depart from the center frequency by an amount
proportional to the instantaneous value of the
modulating signal. The baseband signal is the
original information bearing signal by a
transducer, such as a microphone, telegraph key,
or other signal-initiating device, prior to
initial modulation. Baseband frequencies are
usually characterized by being much lower in
frequency than the frequencies that result when
the baseband signal is used to modulate a the
carrier wave.
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Frequency Modulation (contd)
Example Modulating a Test Tone onto a Carrier
Wave
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Frequency Modulation (contd)
Proportional to the amount of information you can
encode
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Modulation /De-Modulation Systems
Baseband Spacecraft data encoded onto carrier
signal by Modulator Signal is amplified for
broadcast Antenna Broadcasts data to ground
(telemetry) Ground receiver amplifies weak
spacecraft signal Demodulator re-creates and
decodes the baseband signal.
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Communication Link Budgets

• How big should we make the antennas?
• How powerful do the transmitters need to be?
• How sensitive must the receivers be?
• How accurately do the antennas need to track?

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Transmitter Power(Isotropic Power Flux Density)
Transmitted power
DiPole Antenna
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Antenna Gain

• Isotropic (dipole) antenna radiates equally in
  all directions.
• Dish Antennas focus the radiation in a desired
  direction.
• Dependent on the size of the antenna and the
  wavelength of the signal

Antenna Efficiency (.5 - .9)
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Antenna Gain Analogy
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Antennae
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Antennae (contd)
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Effective Isotropic Radiated Power(EIRP)

• Three factors (transmitter power, line loss, and
  antenna gain) are often combined into one number,
  the Effective Isotropic Radiated Power, or EIRP

Transmitter power output
Antenna Gain
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EIRP
EIRP Maps
into
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Received Signal Strength
Received Signal Power
Transmitted Signal Power
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Received Signal Strength (contd)
Receiver Antenna Effective Area
Effective power spread of Sphere of radius R
Free Space loss term
Receiver gain
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Free Space Loss

• The intensity of a signal is inversely
  proportional to the square of the distance from
  the transmitter
• As the beam travels out into space it spreads out
  so the power is spread over a wider area.
• Not a true attenuation just an Inverse-square
  loss

R is distance from transmitter
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Atmospheric Attenuation

• Atmospheric attenuation losses depend heavily
• on the signal frequency

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Other Factors (losses)

• Line losses in transmitter and receiver
  hardware
• Antenna pointing losses--the gain of an antenna
  is not constant across its beamwidth. In a well
  designed antenna it peaks at the boresight.

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Putting it all Together

• The Received Signal is equal to the power
  transmitted multiplied by all of the gain/loss
  factors

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Noise?

• Hot objects radiate in all frequencies, and the
  hotter
• they are, the more they radiate.
• Major Source of Noise in Communication Systems

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Black Body Radiation Curve
Solar Radiation
The hotter the object, the more EM Radiation it
emits at shorter wave- Lengths.
Huh? lets re-visit this later
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Wiens Displacement Law (radiant frequency)
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Emitted Radiation (contd)
Emitted Radiant Energy (magnitude) -- as
object heats up, it radiates energy back
into space
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Noise (revisited)

• Because Thermal radiation is the dominant source
  for noise in Communication Systems, the system
  Noise is modeled as a function of temperature.
• The usual noise equation is N k T Bw, where
• k is Boltzmanns constant, 1.3810-23 Joules/K
• T is the system temperature (noise) rating in
  degrees Kelvin
• Bw is the bandwidth of the receiver - the range
  of frequencies it is designed to receive.

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Signal to Noise Ratio
What Effects Signal-to-noise ratio?

• Changes in transmitter distance
• Changes in receiver or transmitter antenna size
• Changes in carrier frequency
• Changes in Signal Bandwidth
• Changes in Receiver Temperature rating (Noise)

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To Improve Signal-to-Noise Ratio
Increase Signal Strength Reduce the Signal
bandwidth Reduce the receiver temperature
rating All things being equal Higher frequency
generally gives a better signal-to-noise ratio
(huh?)
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Improve Signal-to-Noise Ratio ?
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Improve Signal-to-Noise Ratio ? (contd)
Increase Transmitted Signal Strength
Increase the Sizes of the Antennae Reduce the
Signal bandwidth Reduce the receiver
temperature rating Higher frequency carrier
generally gives a better signal-to-noise ratio
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Improved Signal to Noise Ratio (contd)
OK . So Why dont we use Gamma rays for
Communication?
Atmospheric transmissivity
Because High energy EM waves are almost
completely attenuated by the atmosphere
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Improved Signal to Noise Ratio (contd)
OK . How about Visible light?
Sometimes we do!
Atmospheric transmissivity
Remote Sensing/Reconnaissance
Fiber Optic Communications
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Telescopes One Way CommunicationDevices
All remote sensors are basically one of two
variations on a Telescope
Reflecting telescope (Hale (Mt. Palomar),
Radar, Radio telescopes, DSS)
Primary Mirror
Eyepiece
Refracting telescope (very cumbersome and
expensive)
Objective lens
Eyepiece
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Hubble Space Telescope
2.4 m
Catadioptric Design
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Disadvantages of Using Visible Spectrumfor
Remote Communications
Solar Radiation
The Sun Emits most of its energy in the visible
spectrum
Tremendous Noise Source
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Gain (re-visited)

• The gain of an element is the ratio of the power
  out to the power in.

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A Series of Elements

• The Gain of the series is the product of the
  gains of the individual elements

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Decibels

• Engineers hate to multiply when they can add or
  subtract instead, particularly very large or
  small numbers
• Defined to be 10 times the log of the power or a
  ratio of powers

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Example

• Your car phone is advertised to have a 1 watt
  transmitter. Actual measurement at the antenna
  input connector is 0.82 watts. What is the gain
  of the antenna cable?

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Points to note

• Gains are always positive, but if they are less
  than one, they are actually losses
• When expressed in dB, Gains are added and losses
  are subtracted.
• Power is always positive, so a negative dBW is
  just a very small power.
• You also may see dBm, which is decibel-milliwatts.
  The conversion is 103. 30dBm is 1 dBW

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Questions

• How many dBs are represented by gains of 0.25,
  0.5, 0.66, 1, 2, 5, 10, 100?
• What is the gain of -3db, -10db, -20db, 3 db, 6
  db, 10 db, 30 db?

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Example EIRP

• Your ground station design has a 50 watt
  transmitter, the allocated frequency is 2 GHz
  (2109 ) and you intend to use a 1 meter dish
  antenna. Assuming a h of 0.7 and only 1 dB of
  line losses, what EIRP can you expect?

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Example EIRP (cont)

• What is the gain of the 1 meter antenna

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Space Loss Example

• For our 2 GHz transmitter example, what will be
  the free space loss if we are trying to
  communicate with satellites at an altitude of
  1,000 km? Assume that the slant range from the
  effective horizon will be approximately 1,400 km

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Example (cont)

• For simplification (and because they are usually
  small factors) we will ignore pointing errors and
  atmospheric attenuation, but we need to model the
  receiving antenna. Assume a half meter dish, but
  an efficiency of only 0.55. We will also assume
  a 1db line loss.

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Example (cont)
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Noise Example

• In our continuing example, let us assume a noise
  temperature of 1000 degrees Kelvin, (this is
  hotter that it will actually be, but gives us
  some design margin, and a very poor receiver) and
  a bandwidth of 100 Khz.

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Overall Link Margin

• The signal to noise margin is

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Desired Signal-to-Noise Ratio?
Curves available for S/N required to support
desired Bit Error Rates (BER) for various
modulations.
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Desired Signal-to-Noise Ratio?

• The Link budget
• How much received signal power is enough?
• The answer depends on the Signal-to-Noise
  ratio.
• Depends on modulation technique and acceptable
• bit error rates
• Rule of thumb is a received signal power at about
  
• 10 dB more than the noise.

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What is the Signal-to-NoiseMagnitude Ratio of 10
db?
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Example Iridium Global Star