Week 2 - Session 4 1

Radar SystemsOverview

Week 2 – 4th Session

23-Jan-14

Week 2 - Session 4 2

This Lecture

• The physics of electromagnetic waves• Interaction of EM waves with matter

23-Jan-14

Week 2 - Session 4 3

The Physics of EM Waves

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Week 2 - Session 4 4

Intensity of Spherical Waves

The Physics of EM WavesIntensity

Wavefront curvature ˂ λ/16 over a given aperture of dimension D ⇒ planar wave

This condition is met if the distance from the source to the aperture is at least 2D2 / λ.

This is called the far-field or plane wave approximation

23-Jan-14

Week 2 - Session 4 5

Polarization

The Physics of EM Waves

23-Jan-14

𝐸=𝐸0 cos(𝑘𝑧−𝜔𝑡+𝜑 )

radians/m and radians/s

Week 2 - Session 4 623-Jan-14

Interaction of EM Waves with MatterDiffraction

Diffraction is the bending of EM wavesas they propagate through an aperture or around the edge of an object.

Diffraction depends on:• Size of aperture (antenna), a • Wavelength, λ

Week 2 - Session 4 723-Jan-14

Interaction of EM Waves with MatterDiffraction

• Angular shape of wave as it exits the aperture: a sin (x) / x (sinc) function

• Main lobe half-power (-3dB) beamwidth, θ3 of sinc function:

• Diffraction responsible for the formation of:• Antenna pattern• Antenna beam (main lobe)• Sidelobes

Week 2 - Session 4 8

Interaction of EM Waves with Matter

Example 1

A multi-element antenna

Multi-element linear array of radiating elements with in-phase signals and resulting main beam pattern

Example 2

Diffraction

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Week 2 - Session 4 9

Interaction of EM Waves with Matter

radians𝑭𝒐𝒓 𝒕𝒚𝒑𝒊𝒄𝒂𝒍 𝒓𝒂𝒅𝒂𝒓 𝒅𝒆𝒔𝒊𝒈𝒏𝒑𝒂𝒓𝒂𝒎𝒆𝒕𝒆𝒓𝒔Diffraction

Idealized one-dimensional antenna pattern

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Week 2 - Session 4 10

Interaction of EM Waves with MatterAtmospheric Attenuation

Rain, fog and clouds further attenuate EM waves

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Week 2 - Session 4 11

Interaction of EM Waves with MatterAtmospheric Refraction

Difference in wavelength for wavefronts in two materials

Refraction is the bending of EM waves at the interface of two different dielectric materials.

Speed of EM wave is function of ‘optical density’ of the material

Speed decreases in glass, but frequency does not change, therefore, wavelength must decrease

Index of refraction: n = c/v

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Week 2 - Session 4 12

Interaction of EM Waves with MatterAtmospheric Refraction

Refraction is encountered in radar signals directed upward (or downward) through the atmosphere.

As the index of refraction reduces with altitude, therefore, EM wave will bend back towards the earth.

Adverse effects:

• Target location• Tracking accuracy

Benefits:

• OTH detection• Increase in detection range

Severe ray bending is called ducting.

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Week 2 - Session 4 13

Interaction of EM Waves with MatterReflection

In radar technology, scattering phenomenology is quantified by the target parameter radar cross section, σ.

RCS is determined by interception, reflection and directivity.

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Week 2 - Session 4 14

Next Lecture

• Basic radar configurations and waveforms• Noise, Signal-to-Noise ratio, and detection• Basic radar measurements• Basic radar functions• Radar applications

23-Jan-14