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18A Core I Test 1 Final > Radar Fundamentals > Flashcards

Radar Fundamentals Flashcards

1
Q

Define wavelength

A

Physical distance of one complete wave

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2
Q

Define frequency

A

Number of cycles that the RF energy completes per second

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3
Q

Lower freq radar characteristics

A 
Long wavelength
Large antenna
Components generate high transmit power
Low atmospheric attenuation 
Good long range detection capability 
Best long range for EW Radars
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4
Q

Med freq radar characteristics

A

Smaller wavelength allow smaller antenna
Components cannot handle much power—less detection range
Some atmospheric attenuation
Used by ASR, HF,GCI,SAM and ADA Radars acquisition radars

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5
Q

High freq radars characteristics

A

Shorter wavelength and smaller antennas
Smaller components allow less power—shorter detection range
High precision
Atmospheric attenuation more of a problem
Fire control radars- SAM and ADA

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6
Q

Define amplitude

A

The electromagnetic signal strength

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7
Q

Define Phase

A

A compete 360 degree cycle of an EM wave

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8
Q

Define polarization

A

Orientation of the electrical field in an EM wave

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9
Q

Define Beam Width

A

An angular size of the mainbeam, normally expressed in degrees

Technically the width in degrees between the points where the mainbeam decreases to half power

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10
Q

Define Pulse Width (Obj 9)

A

The time radar is transmitting each pulse (Tao)

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11
Q

Define Pulse Length (Obj 9)

A

The distance between the leading and trailing edge of a Pulse

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12
Q

Define PRF (Obj 9)

A

The rate at which pulses are transmitted

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13
Q

Discuss polarization

A

Linear: horizontal, vertical and slant

Circular: right hand, left hand waves whose polarization rotates thru 360 degrees in every wavelength

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14
Q

Discuss power

A

Strength of a radar signal:

Signal hitting a tgt: 1/r^2

Signal received by a radar: 1/r^4

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15
Q

Discuss reflection

A

The process of reradiating an incident radio wave

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16
Q

Discuss refraction

A

The bending of an Em wave

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17
Q

Discuss diffraction

A

Phenomenon observed when a radio wave spreads around objects whose size is comparable to its wavelength and bends around the edges of larger objects; it increases with wavelength

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18
Q

Discuss subrefraction ducting

A

Decreases radar LOS by bending the radar beam upward, decreasing its detection range

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19
Q

Discuss superrefraction ducting

A

Increase radar detection range by bending the beam downward, increasing the radar horizon and overcoming the masking caused by the earth’s curvature

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20
Q

Discuss the impact of the antenna on the radar pattern, BW, and angular resolution

A

Antenna determines beam shape and size

Larger antenna = narrower BW

Narrower BW = higher angular resolution in AZ/EL

21
Q

Characteristics of parabolic antennas

A

Large sidelobes
Easiest and cheapest to manufacture
Transmitter/receiver at focus of parabola

22
Q

Discuss the characteristics of mechanically scanned planar array antennas (Obj 6)

A

Flat faced antenna mechanically scanned in AZ/EL

Consists of an array of many Indiv radiators of EQUAL PHASE distributed over a flat surface

Slots cut in the walls of a complex of waveguides behind the antennas face

Designed to distribute the radiated power across the array so as to MINIMIZE SIDELOBES

23
Q

Discuss ESA characteristics. (Obj 7)

A

Mounted in a fixed position

Beam is steered by individually controlling the Phase of the radio waves transmitted and received by each radiating segment

24
Q

Discuss passive ESA (Obj 7)

A

Operates in conjunction With the same type of central transmitter as an MSA

Beam is steered by an electronically controlled Phase shifter placed behind each radiating element

25
Q

Discuss active ESA (Obj 7)

A

Instead of a Phase shifter, a small T/R module is placed behind each radiating element

26
Q

Discuss ESA limitations (Obj 7)

A

Field of Regard (FOR)
- Apparent size of the antenna decreases when
viewed from angles off-bore sight

Electronically complex

27
Q

Advantages of Continuous Wave (CW) Radar (Obj 8)

A

High Average Output Power (AOP) with greater detection range

28
Q

Disadvantages of CW Radar (Obj 8)

A

Separate transmitter and receiver

Unable to determine range

29
Q

Discuss Average Output Power (Obj 9)

A

Peak power averaged over the PRI

30
Q

Discuss pulse ranging (Obj 9)

A

Range accuracy is good

Radar transmits an RF signal, RF reflects off a tgt, RF echoes are received

31
Q

Discuss range resolution (Obj 9)

A

Is determined by tau

Pulse length/2

32
Q

Discuss range rate (Obj 9)

A

Computed on the basis of change in the measured range with time

*Not the best method to determine tgt speed

33
Q

Discuss resolution cell (Obj 9)

A

The smallest amount of 3D airspace in which radar cannot distinguish between multiple tgts

  • AZ/EL resolution is based on BW and range to the tgts
  • range resolution is based on tau
34
Q

Advantages of pulsed radar (Obj 10)

A

Only one antenna required
Good range accuracy
Simple electronics
Good Ground mapping and weather detection

35
Q

Disadvantages of pulses radar (Obj 10)

A

Lower AOP limits range detection

Cannot filter out ground clutter without increased processing

Not very accurate in velocity measurements

36
Q

How does CW/Doppler measure tgt velocity (Objective 11)

A

Measures the shift in freq of an EM wave radiated reflected or received by an object in motion

37
Q

How CW/Doppler cancels ground clutter (Objective 11)

A

Filtered out using Doppler processing

38
Q

Discuss the impact of mainbeam and sidelobe clutter on tgt detection (Objective 11)

A

Mainbeam clutter is good for ground mapping, bad when searching for aircraft
- normally a factor only during lookdown

  • Sidelobes are lower in amplitude due to power output
  • Sidelobes have less Doppler shift due to angular difference between velocity of the radar and LOS to the ground
39
Q

Characteristics of PD radars (Objective 13)

A

Pulsed radar allows for one antenna; simpler, lighter system

Accurate range measurement (*time Domain)

Doppler allows for accurate measurement of tgt velocity, all aspect detection and tracking and ground clutter rejection (*frequency Domain)

Combine the capabilities of pulsed and Doppler radars

40
Q

*Advantages of CW/Doppler radar (Objective 12)

A

Can filter out ground clutter

High AOP increase detection range

Very accurate velocity measurement

Simple

Good against high aspect angle tgts

41
Q

*Disadvantages of CW/Doppler radar (Objective 12)

A

Susceptible to Doppler notch

Two antenna required; more weight and space

No range information

Degraded capabilities against beam and stern targets

42
Q

Impact operating PRF has on MPRF and HPRF PD radars (Objective 13)

A

MPRF

  • good all aspect detection
  • decreased detection range compared to HPRF (lower AOP)

HPRF

  • good long range (higher AOP) detection for high-aspect tgts
  • difficult to determine range to tgt
    • Time to listen for return target echoes limited
  • PD allows look-down, shoot-down capability
  • Complicated electronics
43
Q

*Advantages of PD radars (Objective 14)

A

Only one antenna required

Filters out ground clutter

High output power

Good all aspect capability (MPRF)

44
Q

*Disadvantage of PD radars (Objective 14)

A

Complicated electronics

Susceptible of Doppler notch

Range measurement difficult with an HPRF radar

45
Q

Discuss radar ground mapping considerations (Objective 15)

A

Objective is to make radar maps of sufficiently fine resolution so that topographical features and objects on the ground can be recognized

Resolution distance (dr) = range resolution; function of tao.

Real beam antenna: Based on BW and range to target

To improve (dr), PW must be decreased

    - Decreasing PW decreased AOP - negatively impacts radar detection range
    - How much PW can be decreased is a trade-off in desired detection range

da is limited to the physical size of the antenna

- larger antenna improves da
- physical limit to aircraft's radar size
46
Q

Discuss radar acquisition techniques (Objective 16)

A

Circular
Sector
Spiral
Raster

47
Q

*Discuss SAR considerations (Objective 15)

A
  • Side looking radar system which utilizes the forward motion of the platform to simulate an extremely large antenna or aperture electronically
  • Requires time and compute power.
  • Radar must be pointed to the side.
  • The beam of the antenna must be wide enough for the area/target to fall within the beam for every position of the antenna in the entire length of an array

da min = 1/2 length of real antenna (for a radar position at a fixed angle relative to the flight path

48
Q

Discuss tracking techniques (Objective 16)

A

Monopulse - Angular traking information derived from one reflected pulse

Conical

49
Q

Angular resolution

A

Based off BW and range to the targets

18A Core I Test 1 Final (5 decks)

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