EEE381B Aerospace Systems

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EEE381BAerospace Systems Avionics

• Radar
• Part 3 Continuous Wave Radars
• Ref Moir Seabridge 2006, Chapter 3
• Dr Ron Smith

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Outline

• Introduction
• Doppler radar
• Frequency-modulated CW radar
• Terrain-following radar (TFR)
• CW illumination
• Exercises

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1. Introduction

• Pulsed radar is typically used to detect targets,
  determining range and bearing. These radars
  generally require high-power, are quite complex
  and thus expensive.
• Continuous wave (CW) radars typically determine
  target velocity, and can achieve considerable
  ranges without the high peak power. These radars
  are typically simpler, more compact and less
  costly.

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2. Doppler radar 4

• Recall that the Doppler effect is the change in
  frequency that occurs when a source and a target
  are in relative motion.
• The Doppler affect can be used in a CW radar in
  order to determine velocity.

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2.1.1 Doppler radar theory4

• Depicted below is a Doppler radar with transmit
  wavelength ?t and period Tt. As a closing target
  approaches at velocity v, the radar will observe
  a shift in return wavelength, ?r as a function of
  v.
• ?r ?t 2vTt

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2.1.1 Doppler radar theory5

• Depicted below is a Doppler radar with transmit
  wavelength ?t and period Tt. As a closing target
  approaches at velocity v, the radar will observe
  a shift in return wavelength, ?r as a function of
  v.
• ?r ?t 2vTt

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2.1.2 Why 2vTt ? 4
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2.1.3 Doppler radar line of sight 4
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2.1.4 Doppler radar velocity 4

• Substituting frequency for wavelength and
  considering direction of target to line of sight,
  yields a general expression for Doppler velocity.
• v c(1- ft / fr ) / (2 cos(? ))

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2.2 Doppler navigator radar
lamda configuration
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3. FM-CW radar

• An unmodulated CW radar is incapable of detecting
  range, as there is no reference point in the
  transmitted or returned signal for measuring
  elapsed time.
• By frequency modulating the CW signal,
  differences between the transmitted and received
  frequencies can be used to estimate range.
• The further the target, the larger the frequency
  difference.

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3.1.1 FM-CW radar theory 4

• The modulation parameters are frequency
  deviation, ?f, and modulation period, Tm .

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3.1.2 FM-CW radar theory 4
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3.1.3 FM-CW radar theory 4

• closing target

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3.1.4 FM-CW radar theory 4

• Given an FM-CW radar with triangular frequency
  modulation of fm and frequency deviation ?f, the
  range of a stationary target can be derived as
  follows
• fb tr dft/dt, where the round-trip transit
  time, tr 2R/c,
• and the changing transmit frequency, dft/dt
  4fm?f.
• Therefore
• fb (8Rfm?f/c), or R cfb/(8fm?f)

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3.1.5 FM-CW radar theory 4

• Recall that the range resolution of a radar is a
  measure of its ability to distinguish closely
  spaced targets. The range resolution of a FM-CW
  radar is a function of its modulating bandwidth,
  and is c/(4?f).
• The range ambiguity is the range beyond which the
  radar yields ambiguous range results. The range
  ambiguity of a FM-CW radar is a function of its
  modulating frequency, and is cTm.
• This is usually well beyond the signal range.

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3.2 FM-CW radar architecture 4
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4. CW Radar applications 1

• Radar altimeter
• Section 3.7.1 and Section 8.6.11
• Terrain-following radar
• Section 3.7.2
• CW illumination
• Section 3.7.2

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4.1 Radar altimeter

• Triangular FM-CW radar is commonly used in
  aircraft to determine the instantaneous altitude
  above the terrain it is flying.

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4.2 Terrain-following radar 1
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4.3 CW illumination 1

• Used in conjunction with semi-active missiles.
  The aircraft radar illuminates the target,
  while the missile uses the received return signal
  to track the target.
• What are the advantages and disadvantages?

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5. In-class exercises
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5.1 Quick response exercise 1

• Recalling the radar range equation, why is it
  possible for a CW radar to achieve much greater
  ranges than a pulsed radar?
• Can you think of an application in sports where a
  simple Doppler radar may be employed?

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5.2 Doppler calculation

• Just after take-off you realize that you are
  following a military CC-138 (Twin Otter) in a
  Cessna 152. Your air speed is 190 km/hour. You
  estimate the that the Twin Otter is at an
  approximate 15? angle above you.
• You have a home-made 10.6 GHz Doppler radar
  installed on the Cessna oriented straight ahead.
• If the beat frequency on your Doppler radar is
  1517 Hz, what is speed of the Twin Otter?
• What range resolution can you get with this crude
  radar?

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5.3 Radar altimeter calculation

• An aircraft is equipped with an FM-CW radar
  altimeter with a modulation frequency of 1.0 kHz
  and a frequency deviation of 0.60 MHz.
• Compute the beat frequency as a function of
  range.
• If the system has a measured beat frequency of 60
  kHz, what is the aircraft altitude?
• What is the range resolution of the altimeter?
• What frequency variation in MHz is required to
  give a range resolution of 10m?

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References

• Moir Seabridge, Military Avionics Systems,
  American Institute of Aeronautics Astronautics,
  2006. Sections 2.6 2.7
• David Adamy, EW101 - A First Course in Electronic
  Warfare, Artech House, 2000. Chapters 3,4 6
• George W. Stimson, Introduction to Airborne
  Radar, Second Edition, SciTch Publishing, 1998.
• Principles of Radar Systems, student laboratory
  manual, 38542-00, Lab-Volt (Quebec) Ltd, 2006.
• Georgia State University, hyperphsyics,,
  http//hyperphysics.phy-astr.gsu.edu/Hbase/sound/r
  adar.html
• Mark A. Hicks, "Clip art licensed from the Clip
  Art Gallery on DiscoverySchool.com"