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Radar > summery: P1-5 > Flashcards

summery: P1-5 Flashcards

1
Q

Radio and radar operate using:

A

Electromagnetic radiation

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2
Q

Electromagnetic radiation:

A

Has both electric and magnetic components. Each component vibrate at right angles to each other and both are right angles to the direction of propagation.

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3
Q

Electromagnetic radiation always travel at:

A

The speed of light where light itself is just a special form of electron agnetic radiation. It just happens to be at a frequency and wavelength which is detectable by our eyes.

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4
Q

Important characteristics of electromagnetic radiation:

A
  • Frequency
  • Wavelength

F=c/lamda where lamda is the wave length (in meters)

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5
Q

Electromagnetic radiation range from:

A

Very low frequency to very high frequencies

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6
Q

Frequency of radio waves:

A

High

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7
Q

radar is located:

A

upper end of EM spectrum

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8
Q

located to the right of the EM spectrum:

A

infrared, visible and UV light

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9
Q

higher frequency forms of EM radiation:

A

X-ray and Gamma waves

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10
Q

Frequency used by radars:

A

100 MHz through 300 GHz

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11
Q

Why are some frequencies designated by letters?

A

They are very frequent so it is convenient to assign letters

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12
Q

What are the nominal wavelength of L, S, C and X?

A

Band designation

Nominal wavelength

L

30-15 cm

S

15-8 cm

C

8-4 cm

X

4-2.5 cm

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13
Q

The speed of EM radiation depends on:

A

The material it is traveling in

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14
Q

When EM radiation travels trough air and other material it travels

A

Slower than in vacuum

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15
Q

Refractive index:

A

The ratio of the speed of light in a vacuum to the speed of light in a medium

N=c/u where c is the speed of light and u is the speed of light in the medium

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16
Q

Relationship between n, c and u:

A
  • C is always greater than or equal u
  • N is always greater than or equal 1
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17
Q

Components of the refractive index:

A

(two components)

  • (the one above) Simple, real component of the complex refractive index (m) m=n-ik where i is (radical -1) and k is related to the absorption coefficient of the medium
    • For a perfect dielectric (non conductor), k =0
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18
Q

Refractive index of air in the atmosphere near sea level:

A

1.0003 to 1.0004 this means that EM radiation travel 0.03% to 0.04% slower there than in vacuum

19
Q

Refractive indexes as you go up the atmosphere

A

Refractive index decrease with high (1.0003 near the surface and 1 at the top of the atmosphere)

20
Q

The most important part of the refractive index is in:

A

The 4th, 5th and 6th decimal places

21
Q

Refractivity:

A

Because it is hard to work with very small numbers you can subtract 1 from the index of refraction and multiply the result by 106. The result is in n units

22
Q

Equation of refractivity:

A

N=(n-1)x106

23
Q

Refractivity index of the atmosphere depend on

A
  • Temperature
  • Pressure
  • Vapor pressure
  • Number of free electrons present
    • In troposphere there aren’t enough free electrons for it to be important
    • The effect of free electrons is only important high in the atmosphere
    • Variation of free electrons allow wind profiling radars to detect wind in upper stratosphere and mesosphere
24
Q

Explain each variable in the equation

A
  • N: refractivity
  • T: temperature (in kelvin)
  • P: pressure (in mb or hpa)
  • E: vapor pressure of moist air (in mb or hpa)
  • Ne: number density of free electrons per m3
  • F: frequency of radar signals (in hz)
25
Q

Explain each variable in the equation

  • N: refractivity
  • T: temperature (in kelvin)
  • P: pressure (in mb or hpa)
  • E: vapor pressure of moist air (in mb or hpa)
  • Ne: number density of free electrons per m3
  • F: frequency of radar signals (in hz)
A
  • Right most term: only important in the ionosphere (ignored)
  • Numerical constants were determined empirically
26
Q

How is reflectivity determined in the troposphere?

A

From:

  • Temperature
  • Pressure
  • Vapor pressure
27
Q

How are the factors that determine reflectivity in troposphere found?

A

By soundings of the atmosphere (made twice a day)

  • N is calculated at each level from the sounding data
  • Gradient of n is found from n and height measurements
    • Gradient of n is the change of n over a given change in height h
    • Gradient of n/gradient of h is negative because n decrease with height
28
Q

What is the standard value of refractivity decrease rate?

A

-39 n-units/km

29
Q

What are the trends that models suggest regarding the refractivity rate?

A
  • Refractivity decrease gradually (linear)
  • Refractivity decreases at a logarithmic rate (better fit to the real atmosphere)
30
Q

Radar propagation is more dependent upon:

A

Gradient of refractivity rather than the absolute value of refractivity at any point

31
Q

Near the surface. The inversion produces:

A

A strong region of gradient that is +ve (+18 n-units/km). This would be a layer of subrefraction

32
Q

Instrument used to measure the refractive index:

A

Refractometer

33
Q

EM radiation travel at different speeds when:

A

Refractivity index are different at different places in the atmosphere. This causes the lower portions of the troposphere to be stratifies into horizontal layers.

34
Q

When are changes in refraction stronger?

A

In vertical direction

35
Q

The direction the wave bend depend on:

A

Whether the lowest refractivity is on top or the bottom

36
Q

In normal atmospheric conditions n is

A

Largest near the ground and decreases with height. Meaning that radar waves will travel faster aloft than near the surface.

37
Q

When are the waves bent downwards?

A

When n is largest near the ground and decreases with height. Meaning that radar waves will travel faster aloft than near the surface.

38
Q

It is more convenient to talk about radar…..

A

Rays

39
Q

Rays are:

A

Lines along which waves travel and are drawn perpendicular to the wave fronts

40
Q

If waves bend:

A

Rays bend

41
Q

Rays are convenient in:

A

Optics

  • To show how light travels through lenses
  • Determining the paths radar waves will follow in the atmosphere
    • (by knowing the refractivity index in the atmosphere at each level)
42
Q

Snell’s law:

A

Give the bending that light or EM radiation will undergo when it travels from one medium to another, each having its own refractive index

Sin i/sin r = ui/ur = nr/ni

Where i is the anglt of the incident and r is the angle of refraction. U is the speed of EM radiation and n is the refractive indices in the incident and refracted layers.

43
Q

To find the angle the ray will have in the second layer:

A
  • Know the starting angle of radar wave at one place
  • Refractive index of that layer
  • Refractive index at the next layer in the atmosphere
44
Q

To find the angle the ray will have in the second layer:

  • Know the starting angle of radar wave at one place
  • Refractive index of that layer
  • Refractive index at the next layer in the atmosphere

By doing the above it is possible to:

A
  • Calculate the path the rays will follow anywhere in the atmosphere
    • It is useful for determining how rays behave in the real world

Radar (8 decks)

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