Distinguish between a point and distributed target. Determine when to utilize the point or distributed target forms of the radar equation.

Radar Exam 2 Study Guide

Flashcard
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Science
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University
•
Hard
Josie Nelson
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63 questions
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1.
FLASHCARD QUESTION
Front
Back
Point target:
A radar target that is small compared with the pulse volume, which is the cross-sectional area of the radar beam multiplied by half the length of the radar pulse.
Plane, drone, bird, building, tower
Radar equation:
Target size << resolution cell
Return from a single object
Output desired: Radar cross-section σ of target
Distributed target:
A radar target that is large compared with the pulse volume, which is the cross-sectional area of the radar beam multiplied by one-half the length of the radar pulse
Clouds, precipitation
Radar equation:
Target fills multiple resolution cells
Return from area/volume of scatterers
Output desired: Reflectivity Z
2.
FLASHCARD QUESTION
Front
Describe and derive an approximate form of the radar sample volume (V) for a uniformly illuminated main lobe. Given the range (r), circular parabolic beamwidth (θ), pulse width (τ), estimate V.
Back
Contains all objects from which backscattered microwaves return to radar simultaneously
Derivation:
ΔR = cτ/2
A ≈ R^2θΦ
V = A × ΔR
V ≈ R^2θΦcτ/2
3.
FLASHCARD QUESTION
Front
Explain why the range length of a radar sample volume is h/2 where h is the pulse length.
Back
4.
FLASHCARD QUESTION
Front
Given typical cloud (or rain) droplet concentrations, estimate the typical number of rain drops in a radar sample volume.
Back
Number of drops = NR × V
NR = raindrop concentration (drops per cubic meter)
V = radar sample volume (cubic meters)
5.
FLASHCARD QUESTION
Front
List an expression for the sample volume for a Gaussian shaped beam and explain why it is different than for a uniformly illuminated beam.
Back
Gaussian shaped beam:
V = πr2 θΦh / 16 ln(2)
Uniformly illuminated beam:
V = π [(rθ/2) (rΦ/2)] ⋅ h/2
A Gaussian-shaped beam doesn't spread energy uniformly across the beam cross-section. Instead, the intensity is strongest at the center and drops off smoothly toward the edges
6.
FLASHCARD QUESTION
Front
Define in your own words and list a mathematical expression for the total backscattering cross-sectional area (σt) for a distributed target.
Back
σt represents how much radar energy is scattered back in the direction of the radar by all the individual scatterers (like raindrops or cloud particles) within the radar sample volume
σt = V∑vol σi
7.
FLASHCARD QUESTION
Front
Define the “time to independence” or “decorrelation time” and describe its significance for measuring received power (pr) and hence estimating σt (η or z).
Back
Time it takes hydrometeors to rearrange themselves so measurements are independent of one another
If pulse intervals < decorrelation time --> correlated noise --> poor averaging
Pulse intervals >= decorrelation time --> statistically sound averaging --> better power estimate --> more accurate σt , η, or z
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