Radar Exam 2 Study Guide

Front

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

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

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

Front

Explain why the range length of a radar sample volume is h/2 where h is the pulse length.

Back

Front

Given typical cloud (or rain) droplet concentrations, estimate the typical number of rain drops in a radar sample volume.

Back

Number of drops = N_R × V

N_R = raindrop concentration (drops per cubic meter)

V = radar sample volume (cubic meters)

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

Front

Define in your own words and list a mathematical expression for the total backscattering cross-sectional area (σ_t) for a distributed target.

Back

σ_t 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

σ_t = V∑_vol σ_i

Front

Define the “time to independence” or “decorrelation time” and describe its significance for measuring received power (p_r) and hence estimating σ_t (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 σ_t , η, or z

Front

List the properties of the radar or meteorological targets that modify the decorrelation time. Describe how these properties affect the decorrelation time.

Back

1.) Wavelength (λ): shorter wavelengths means particles can travel less distance to re-shuffle

2.) Breadth of particle size distribution, N(D)

• -- If all drops same size, all particles fall at same speed or terminal fall speed (V_t) V_t = aD^b (a, b constant), and sample volume would decorrelate slowly

3.) Turbulence: more turbulence results in more small-scale mixing, re-shuffling of particles

4.) Others such as range averaging and antenna rotation rate

Front

Define the radar reflectivity (η) and list typical units. Show how η can be estimated from the radar
range equation for a beam-filling meteorological targets.

Back

Front

Define the radar reflectivity factor (z) and list typical units. Show (i.e., derive) how z can be
estimated from the radar range equation for a beam-filling meteorological targets
after making the Rayleigh scattering approximation. Define key assumptions associated with
assuming Rayleigh conditions.

Back