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Basic Radar Theory - Tarpit

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  • pg 1
									 EEE381B
 Aerospace Systems
 & Avionics
Radar
Part 1 – Basic radar theory
Ref: Moir & Seabridge 2006, Chapter 3

Dr Ron Smith
Outline
1.     Principles of radar
2.     Radar antenna
3.     Radar modes
4.     Pulsed radar
5.     Doppler radar
6.     FM-CW radar
7.     Exercises

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1. Principles of radar [4]




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1.1 A radar operator view [4]




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1.2 Brief history of radar

   Conceived as early as 1880 by Heinrich Hertz
      Observed   that radio waves could be reflected off
        metal objects.
   Radio Aid to Detection And Ranging
   1930s
      Britainbuilt the first ground-based early warning
        system called Chain Home.
   1940
      Invention of the magnetron permits high power
        transmission at high frequency, thus making airborne
        radar possible.
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1.2.1 Brief history of radar

   Currently
      Radar   is the primary sensor on nearly all
       military aircraft.
      Roles include airborne early warning, target
       acquisition, target tracking, target illumination,
       ground mapping, collision avoidance,
       altimeter, weather warning.
      Practical frequency range 100MHz-100GHz.


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1.3 Airborne radar bands [1]




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1.3.1 Airborne radar bands [1]




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1.3.2 Airborne radar bands [1]




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1.4 Basic principle of radar[1]




      target   range, R = ct / 2

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1.4.1 Basic principle of radar[1]

   Two common transmission techniques:
      pulses
      continuous   wave




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2. Radar antenna
 A basic principle of radar is that it directs
  energy (in the form of an EM wave) at its
  intended target(s).
 Recall that the directivity of an antenna is
  measured as a function of its gain.
 Therefore antenna types most useful for
  radar applications include parabolic and
  array antenna.

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2.1 Parabolic (dish) antenna

   Early airborne radars typically
    consisted of parabolic
    reflectors with horn feeds.
      The   dish effectively directs the
        transmitted energy towards a
        target while at the same time
        “gathering and concentrating”
        some fraction of the returned
        energy.


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2.2 Planar (phased) array antenna

   Recent radars more likely
    employ a planar array
      It iselectronically steerable as
       a transmit or receive antenna
       using phase shifters.
      It has the further advantage of
       being capable of being
       integrated with the skin of the
       aircraft (“smart skin”).


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2.3 Radar antenna beam patterns

   The main lobe of the radar antenna beam is
    central to the performance of the system.
      The   side lobes are not only wasteful, they provide
        electronic warfare vulnerabilities.




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3. Airborne radar modes
   Airborne radars are designed for and used in
    many different modes. Common modes include:
      air-to-air search
      air-to-air tracking
      air-to-air track-while-scan (TWS)
      ground mapping
      continuous wave (CW) illumination
      multimode



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3.1 Air-to-air search [1]




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3.2           Air-to-air tracking [1]




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3.3           Air-to-air track-while-scan [1]




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3.4           Ground mapping [1]




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3.5           Continuous wave illumination




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3.6 Multimode   [1]




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4. Pulsed radar
   A pulsed radar is characterized by a high power
    transmitter that generates an endless sequence
    of pulses. The rate at which the pulses are
    repeated is defined as the pulse repetition
    frequency.
   Denote:
      pulse width, , usually expressed in sec
      pulse repetition frequency, PRF, usually in kHz
      pulse period, Tp = 1/PRF, usually in sec



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4.1 Pulsed radar architecture [1]




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4.1.1 A lab-based pulsed radar [4]




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4.2 Pulsed modulation [1]




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4.2.1 Pulsed radar bandwidth

   In the frequency domain, the transmitted and
    received signals are composed of spectral
    components centered on the radar operating
    frequency, f0, with a sin(x)/x shape.
   The practical limits of the frequency response is
    f0  1/,
   and therefore the bandwidth of the receiver must
    be at least:
    BW Rx ≥ 2/

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4.2.2 Pulsed radar average power

   Since a pulsed radar only transmits for a small
    portion of the time, the average power of the
    radar is quite low:
    Pav = Ppeak  / Tp

      For  example a pulsed radar with a 1 sec pulse width
        and a medium PRF of 4 kHz that transmits at a peak
        power of 10kW transmits an average power of:
        Pav = (10000 W) (0.000001 sec) (4000 /sec)
            = _____ W        = _____ dBW

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4.3 Pulsed radar range resolution

   The range resolution of a radar is its ability to
    distinguish two closely spaced targets along the
    same line of sight (LOS). The range resolution
    is a function of the pulse length, where pulse
    length, Lp = c.
      For  example, a 1 sec pulse width yields a pulse
        length of 0.3 km.
   Two targets can be resolved in range if:
      Lp < 2(R2 – R1)


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4.3.1 Pulsed radar range resolution     [4]




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4.3.2 Pulsed radar range resolution     [4]




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4.4 Pulsed radar range ambiguity

   The PRF is another key radar parameter and is
    arguably one of the most difficult design
    decisions.
   The range of a target becomes ambiguous as a
    function of half the pulse period; in other words
    targets that are further than half the pulse period
    yield ambiguous range results.
   Ramb = c / (2 PRF) = cTp / 2


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4.4 Pulsed radar range ambiguity [1]




                          This figure is very confusing.




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4.4.1 Range ambiguity

                                             Ramb



                     return time

                                             PRF


   A target whose range is:
     R       < Ramb = c / (2 PRF) = cTp / 2
                                                    0   10           20           30




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4.4.2 Range ambiguity

                                          Ramb



                                         return time

                                         PRF



   A target whose range is :
     R       > Ramb = c / (2 PRF) = cTp / 2
                                                       0   10           20           30




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4.4.3 Range ambiguity

                               Ramb




                              PRF



   Which target is which?                     ?
                                      0   10           20           30




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4.5 Angle resolution[4]




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5. Target tracking
   A target that is tracked is said to be “locked on”;
    key data to maintain on locked targets is:
      range,
      azimuth   and elevation angle.
   A frame of reference using pitch and roll from
    aircraft attitude indicators is required for angle
    tracking. Three angle tracking techniques are:
      sequential lobing
      conical scan
      monopulse

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5.1 Range tracking - range gating [1]




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5.2 Angle tracking – sequential lobing1




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5.3 Angle tracking – sequential lobing1




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5.4 Angle tracking – conical scan[1]




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5.5 Angle tracking – monopulse[1]




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5.6 Angle tracking – monopulse[1]




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6. In-class exercises




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6. 6.1 Quick response exercise # 1

   Explain the strange shapes on top of these
    two aircraft,
      E3     Sentry and AH-64 Longbow Apache [1]




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6.2 Quick response exercise # 2

   Given a 10.5 GHz intercept radar and a
    transmitter capable of providing a peak power
    of 44 dBW at a PRF of 2 kHz:
      What pulse  width yields an average power of 50W?
      What is the bandwidth in MHz and in % of this
       signal?




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6.3 Pulsed radar calculations

   Design the pulse parameters so as to achieve maximum
    average power for an unspecified Ku band pulsed radar
    given the following component specifications and system
    requirements:
        the receiver has a bandwidth of at least 0.5% across the band
        the required range resolution is 50m
        The required range ambiguity is 25 km
        For cooling purposes, ensure that the duty cycle of the
         transmitter does not exceed 0.2%




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References
1)      Moir & Seabridge, Military Avionics Systems, American Institute of
        Aeronautics & Astronautics, 2006. [Sections 2.6 & 2.7]
2)      David Adamy, EW101 - A First Course in Electronic Warfare, Artech
        House, 2000. [Chapters 3,4 & 6]
3)      George W. Stimson, Introduction to Airborne Radar, Second Edition,
        SciTch Publishing, 1998.
4)      Principles of Radar Systems, student laboratory manual, 38542-00, Lab-
        Volt (Quebec) Ltd, 2006.
5)      Mark A. Hicks, "Clip art licensed from the Clip Art Gallery on
        DiscoverySchool.com"




 Winter 2009                      EEE381B                         Basic radar theory - 49

								
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