ch4: radar equations for point targets

Question 1

Radar is often used to:

Answer 1

• Show location of storms near the radar
• Measure the strength of the returned power
  
  • Estimate rain rate and other parameters of the storm

Question 2

In order to use radar quantitatively we must know:

Answer 2

• The value of radar parameters

Question 3

What happens after power is radiated for isotropic antenna?

Answer 3

• Power radiated moves away from the antenna at the speed of light
• Forming a spherically expanding shell of energy

Question 4

Area covered by a single expanding pulse of energy is equal to:

Answer 4

• The area on the surface of a sphere at the corresponding distance

Question 5

Power density:

Answer 5

• Power per unit area (transmitted power divided by this area

Question 6

Difference between real antenna and isotropic radiator in power radiation. Why?

Answer 6

• The amount of power along the center of the beam axis at some distance is greater if real antenna is used
• Because the increased power is the gain of the antenna times the power that would have been there if an isotropic antenna had been used
  
  • But now more power will be on the center of the beam axis while less power will occur in other directions

Question 7

Similarity between real antenna and isotropic radiator in power radiation.

Answer 7

• Radar will transmit the same amount of power
• Average power density over the entire sphere would remain constant

Question 8

……… value of antenna gain is used

Answer 8

• Linear

Question 9

For most targets detected by radar, the power intercepted is:

Answer 9

• Reradiated isotopically back to space
• Some targets radiate stronger in some direction than another
  
  • Ignored
• Some targets absorb some of the incident energy converting it to internal heat
  
  • Ignored

Question 10

When a target radiates its energy some of the energy will be

Answer 10

• Received back at the radar

Question 11

The physical size of the target is not the size the target appears to the radar. To overcome this problem:

Answer 11

• We define a new parameter called the backscattering cross sectional area of the target

Question 12

The backscattering cross sectional area of a target:

Answer 12

• A function of
  
  • The size
  • Shape
  • Kind of matter making up the target
  • Wave length of the radar viewing it

Question 13

Cons of backscattering cross sectional area:

Answer 13

• Cannot always be calculated analytically especially for complex targets
  
  • Met targets are simple.
  • Hydrometers are approximately spheres

Question 14

Explain the conditions for large and small spherical targets? Define raylight and Mie regions.

Answer 14

• Large:
  
  • d/lamda > 10 (some specify D/lamda > 16)
• Small:
  
  • d/lamda <0.1 (some specify D/lamda < 1/16)
• reylight region: size of sphere is small compared to the wavelength of the radar
• Mie Region: size of sphere is intermediate compared to the wavelength of the radar

Question 15

When a sphere is large compared to the wavelength of the radar, the backscattering cross-sectional area of the target is

Answer 15

• Equal to the geometric area

Question 16

In reighlight region the backscattering cross sectional area of a sphere is

Answer 16

• Proportional to the sixth power of the diameter

Question 17

Why is rayleight region an important part of meteorological radar use?

Answer 17

• Because many meteorological targets are really small compared to the wavelength of a radar

Question 18

Standard targets:

Answer 18

• It is occasionally useful to aim the radar at a target with precisely known characteristics. Those target are sometimes called standard targets.

Question 19

Why are spheres useful as standard targets?

Answer 19

• Because they have the same backscattering cross sectional area no matter what direction they are from the radar

Question 20

If the sphere had been smaller or larger it would have

Answer 20

• A cross sectional area smaller than its geometric area

Question 21

If the sphere is large compared to the wavelength of the radar:

Answer 21

• The backscattering area is the same as its geometric area

Question 22

Give examples of standard targets:

Answer 22

• Spheres
• Flat-plate reflectors dihedrals and tihedrals

Question 23

Flat plate reflectors work as intended when:

Answer 23

• They are oriented such that they are perpendicular to the radar beam

Question 24

Dihedral reflector:

Answer 24

• If a flat plate reflector is folded making one side a 90 degree angle with the other side

Question 25

For dihedral reflectors to work properly:

Answer 25

• They must be oriented so that the folded axis is perpendicular to the radar beam

Question 26

Trihedral:

Answer 26

• Also known as corner reflector
• Formed by putting three mutually perpendicular surfaces together

Question 27

Advantage of corner reflectors. Why?

Answer 27

• Don’t need to be aimed toward the radar with great accuracy because
  
  • The reflected radar signal will always return directly along the path of the incident signal

Question 28

When properly oriented, all three kinds of flat plate type reflectors give

Answer 28

• Very strong returns
• Can be used to measure the antenna gain of radar

Question 29

What was the primary motivation driving the development of radar?

Answer 29

• Detection of aircrafts

Question 30

…… might act as a point target to a radar

Answer 30

• Buildings