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Radar Systems Design > Week 2 > Flashcards

Week 2 Flashcards

1
Q

Constructive interference

A

Adding two waves with a shift of 0 degrees

2 times the amplitude

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2
Q

Destructive interference

A

Adding two waves with a shift of 180 degrees

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3
Q

An isotropic point source generates

A

Spherical waves

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4
Q

Validity of plane wave assumption

A

For a plane wave, intensity and phase are constant across planar surfaces that are oriented perpendicular to the direction of propagation

At great enough distances from the source, the spherical wavefronts will appear approximately planar across an aperture of fixed dimension D

Phase variation across D of less than 1/16 cycle (22.5 deg) is small enough to treat the waves as planar

Valid in the far field region which is R > 2D^2/λ m

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5
Q

Antenna

A

Transducer that converts bound waves in a waveguide to unbound waves in free space and vice versa

Characterized by gain and beamwidth

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6
Q

Directional antennas

A

Transmit and receive EM waves over a small angular region (most radars use these)

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7
Q

Antenna gain

A

The increase in power density at a given point in space when the test antenna is used in place of an isotropic antenna

Varies with angle

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8
Q

If quotes as just “gain”

A

The maximum (on axis, bore sight) value is inferred

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9
Q

Antenna pattern

A

The variation of antenna gain with angle

Have a main lobe and sidelobes

Usually presented as gain vs angle in a plane

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10
Q

Beamwidth

A

Angular extent between the 3dB points of the main beam

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11
Q

Sidelobes

A

All directional antennas have sidelobes in their patterns that are defined by nulls

The forward direction produces a main beam because of constructive interference

Radar targets can appear in the sidelobes of the antenna - gives an erroneous direction

Jammers can radiate noise and false signals into antenna sidelobes

Sidelobes can be reduced but not eliminated

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12
Q

Nulls

A

Directions in which no energy is radiated

Due to destructive interference of transmissions from across the antenna aperture

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13
Q

Why do we have lobes

A

Interference

Because we have a physical dimension antenna, we are going to have constructive interference

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14
Q

Radar targets

A

Scatter EM waves with patterns that are similar to antenna patterns

Pattern depends on size and shape of target

Scattering of EM waves described by the RCS of the target

RCS will vary with direction

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15
Q

Rayleigh scatterer

A

If target has linear dimension d &laquo_space;λ then it is more ‘point-like’ and scatters the wave in all directions (more isotropic)

Ex: raindrop with d = 6 mm seen by 10 cm radar

In the optical domain, Rayleigh scattering accounts for the blue color of the sky

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16
Q

Optical scatterer

A

If target has linear dimension d&raquo_space; λ then it is more ‘large’ and scatters the wave in preferred directions (more directive, unless it is a sphere)

Ex: automobile seen with λ=3 cm (x-band radar)

17
Q

Larger targets

A

Have specific scatter patterns

Ex: flat plate a,b&raquo_space; λ (behaves like an antenna with aperture a,b; energy reflected in direction of ray reflection)

18
Q

Corner reflector

A

Has a large backscatter RCS over a wide angular range (RCS will be large)

Most aircraft are seen as several corner reflectors

19
Q

Stealth technology

A

Targets such as aircraft can be designed to have low RCS in certain directions

Key elements:

  • avoid corner reflectors
  • use rounded surfaces
  • use flat plates to direct energy away
20
Q

RCS

A

Based on an equivalence to the reflection of radio signals by a metal sphere

21
Q

To declare that a target is detected

A

Pr >= Smin

22
Q

Smin

A

Set by the noise in the receiver

23
Q

Typical value of (S/N)min

A

15 dB = 31.6

24
Q

How high should SNR be to confidently detect target

A

10-20 dB

Want to avoid false targets

25
Q

Receiver noise

A

Rx noise is thermal in origin and is assumed to be spread evenly in frequency (constant PSD)

White noise

Dependence of noise power on Bn creates an incentive to make the receiver BW smalle

26
Q

Radar sensitivity

A

Being able to detect a target at greater range amounts to greater sensitivity

Sensitivity increases with the energy in a pulse

27
Q

AN nomenclature for US systems

A

Developed by US military in WWII

AN/xxx-nn
-A = army, N = navy, nn = identifier of version of radar

x-letters designate

  • type of installation (S: ship, T: ground, transportable, F: fixed ground)
  • type of equipment (P: radar)
  • purpose (S: search)
28
Q

Svalbard ISR radar

A

498-502 MHz
Pt = 1 MW
32 m and 42 m dishes
44 dBi gain

29
Q

Over the horizon radar system

A

Utilizes ionospheric refraction (bending) on HF frequencies (3-30 MHz) to see out to enormous ranges (thousands of km)

30
Q

Multiple pulses

A

Integrating the returns from the multiple pulses improves the ability to detect the target

  • helps because signal tends to add up systematically while noise adds up randomly
  • integration can be coherent or noncoherent

Receiver performs the function of adding up the returned pulses

31
Q

Coherent integration

A

Adding in both amplitude and phase

Gives more improvement but requires more complicated hardware

32
Q

Integration gain

A

A boost to the S/N ratio over that expected from a single pulse

Improves the S/N in the radar receiver

33
Q

Losses

A

Anything that detracts from ideal S/N

34
Q

System losses

A 
Microwave plumbing losses
Between tx and antenna, rx and antenna
Waveguide run
Rotating joint
T-R joint (duplexes)
Waveguide components
35
Q

Beam shape loss / scanning loss

A

Surveillance radar must rotate antenna

As the antenna beam sweeps over a target the target is hit with multiple pulses

The pulses strike the target at different points of the antenna pattern

The hits that occur away from the axis have an antenna gain that is less than Gmax
-will incur a loss as compared to having the pulses all hit at the peak of the antenna pattern

Usually only count pulses that strike between the 3dB points of the antenna pattern

For N>10, loss is typically 1.6 to 2 dB

36
Q

Radome loss

A

If antenna has an enclosure (radome), there will be RF loss (1.2 dB two-way)

37
Q

Rx filter mismatch loss

A

Radar pulse has sinX/X spectrum

Rx bandwidth filter is ~ rectangular (causes loss since sinc function hard to implement in hardware)

Typically 0.9 dB

38
Q

Reason for B ~ 1/τ

A

Larger filter to get more power through - get more noise

Shrinking filter - reduces noise but choking off signal

Radar Systems Design (6 decks)

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