summery: P1-5

Question 1

Radio and radar operate using:

Answer 1

Electromagnetic radiation

Question 2

Electromagnetic radiation:

Answer 2

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.

Question 3

Electromagnetic radiation always travel at:

Answer 3

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.

Question 4

Important characteristics of electromagnetic radiation:

Answer 4

• Frequency
• Wavelength

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

Question 5

Electromagnetic radiation range from:

Answer 5

Very low frequency to very high frequencies

Question 6

Frequency of radio waves:

Answer 6

High

Question 7

radar is located:

Answer 7

upper end of EM spectrum

Question 8

located to the right of the EM spectrum:

Answer 8

infrared, visible and UV light

Question 9

higher frequency forms of EM radiation:

Answer 9

X-ray and Gamma waves

Question 10

Frequency used by radars:

Answer 10

100 MHz through 300 GHz

Question 11

Why are some frequencies designated by letters?

Answer 11

They are very frequent so it is convenient to assign letters

Question 12

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

Answer 12

Band designation

Nominal wavelength

L

30-15 cm

S

15-8 cm

C

8-4 cm

X

4-2.5 cm

Question 13

The speed of EM radiation depends on:

Answer 13

The material it is traveling in

Question 14

When EM radiation travels trough air and other material it travels

Answer 14

Slower than in vacuum

Question 15

Refractive index:

Answer 15

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

Question 16

Relationship between n, c and u:

Answer 16

• C is always greater than or equal u
• N is always greater than or equal 1

Question 17

Components of the refractive index:

Answer 17

(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

Question 18

Refractive index of air in the atmosphere near sea level:

Answer 18

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

Question 19

Refractive indexes as you go up the atmosphere

Answer 19

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

Question 20

The most important part of the refractive index is in:

Answer 20

The 4^th, 5^th and 6^th decimal places

Question 21

Refractivity:

Answer 21

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

Question 22

Equation of refractivity:

Answer 22

N=(n-1)x10⁶

Question 23

Refractivity index of the atmosphere depend on

Answer 23

• 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

Question 24

Explain each variable in the equation

Answer 24

• 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)

Question 25

**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)

Answer 25

* Right most term: only important in the ionosphere (ignored) * Numerical constants were determined empirically

Question 26

**How is reflectivity determined in the troposphere?**

Answer 26

From: * Temperature * Pressure * Vapor pressure

Question 27

**How are the factors that determine reflectivity in troposphere found?**

Answer 27

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

Question 28

**What is the standard value of refractivity decrease** **rate?**

Answer 28

-39 n-units/km

Question 29

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

Answer 29

* Refractivity decrease gradually (linear) * Refractivity decreases at a logarithmic rate (better fit to the real atmosphere)

Question 30

**Radar propagation is more dependent upon:**

Answer 30

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

Question 31

**Near the surface. The inversion produces:**

Answer 31

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

Question 32

**Instrument used to measure the refractive index:**

Answer 32

Refractometer

Question 33

**EM radiation travel at different speeds when:**

Answer 33

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

Question 34

**When are changes in refraction stronger?**

Answer 34

In vertical direction

Question 35

**The direction the wave bend depend on:**

Answer 35

Whether the lowest refractivity is on top or the bottom

Question 36

**In normal atmospheric conditions n is**

Answer 36

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

Question 37

**When are the waves bent downwards?**

Answer 37

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

Question 38

**It is more convenient to talk about radar…..**

Answer 38

Rays

Question 39

**Rays are:**

Answer 39

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

Question 40

**If waves bend:**

Answer 40

Rays bend

Question 41

**Rays are convenient in:**

Answer 41

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)

Question 42

**Snell’s law:**

Answer 42

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 = u_i/u_r = n_r/n_i 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.

Question 43

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

Answer 43

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

Question 44

**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:**

Answer 44

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