ch4: radar equations for point targets Flashcards
Radar is often used to:
- Show location of storms near the radar
- Measure the strength of the returned power
- Estimate rain rate and other parameters of the storm
In order to use radar quantitatively we must know:
- The value of radar parameters
What happens after power is radiated for isotropic antenna?
- Power radiated moves away from the antenna at the speed of light
- Forming a spherically expanding shell of energy
Area covered by a single expanding pulse of energy is equal to:
- The area on the surface of a sphere at the corresponding distance
Power density:
- Power per unit area (transmitted power divided by this area
Difference between real antenna and isotropic radiator in power radiation. Why?
- 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
Similarity between real antenna and isotropic radiator in power radiation.
- Radar will transmit the same amount of power
- Average power density over the entire sphere would remain constant
……… value of antenna gain is used
- Linear
For most targets detected by radar, the power intercepted is:
- 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
When a target radiates its energy some of the energy will be
- Received back at the radar
The physical size of the target is not the size the target appears to the radar. To overcome this problem:
- We define a new parameter called the backscattering cross sectional area of the target
The backscattering cross sectional area of a target:
- A function of
- The size
- Shape
- Kind of matter making up the target
- Wave length of the radar viewing it
Cons of backscattering cross sectional area:
- Cannot always be calculated analytically especially for complex targets
- Met targets are simple.
- Hydrometers are approximately spheres
Explain the conditions for large and small spherical targets? Define raylight and Mie regions.
- 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
When a sphere is large compared to the wavelength of the radar, the backscattering cross-sectional area of the target is
- Equal to the geometric area
In reighlight region the backscattering cross sectional area of a sphere is
- Proportional to the sixth power of the diameter
Why is rayleight region an important part of meteorological radar use?
- Because many meteorological targets are really small compared to the wavelength of a radar
Standard targets:
- It is occasionally useful to aim the radar at a target with precisely known characteristics. Those target are sometimes called standard targets.
Why are spheres useful as standard targets?
- Because they have the same backscattering cross sectional area no matter what direction they are from the radar
If the sphere had been smaller or larger it would have
- A cross sectional area smaller than its geometric area
If the sphere is large compared to the wavelength of the radar:
- The backscattering area is the same as its geometric area
Give examples of standard targets:
- Spheres
- Flat-plate reflectors dihedrals and tihedrals
Flat plate reflectors work as intended when:
- They are oriented such that they are perpendicular to the radar beam
Dihedral reflector:
- If a flat plate reflector is folded making one side a 90 degree angle with the other side
For dihedral reflectors to work properly:
- They must be oriented so that the folded axis is perpendicular to the radar beam
Trihedral:
- Also known as corner reflector
- Formed by putting three mutually perpendicular surfaces together
Advantage of corner reflectors. Why?
- 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
When properly oriented, all three kinds of flat plate type reflectors give
- Very strong returns
- Can be used to measure the antenna gain of radar
What was the primary motivation driving the development of radar?
- Detection of aircrafts
…… might act as a point target to a radar
- Buildings
