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Review Doppler Radar (Fig. 3.1) A simplified block diagramPowerPoint Presentation

Review Doppler Radar (Fig. 3.1) A simplified block diagram

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ReviewDoppler Radar (Fig. 3.1)A simplified block diagram

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Electric field incident on scatterer

Reflected electric field incident on antenna

Voltage input to the synchronous detectors;

This pair of detectors shifts the frequency f to 0

jQ(t)

A

Echo phase at the output of the detectors and filters

ψe

I(t)

If the range r of the scatterer is fixed, phasor A is fixed.

But if scatterer has a radial velocity, Phasor A rotates

about the origin at the Doppler frequency fd.

Complex plane

(Phasor diagram)

r=cτs/2

cτ

λ

Pulsed Radar Principlec = speed of microwaves

= ch for H and = cv for V waves

τ = pulse length

λ = wavelength

= λh for H and λv for V waves

τs = time delay between

transmission of a pulse and

reception of an echo.

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For typical atmospheric conditions (i.e., normal) the propagation path

is a straight line if the earth has a radius 4/3rds times its true radius.

Normal and Anomalous PropagationSub refraction

Free space

Normal atmosphere: The

ray’s radius of curvature

Rc≈ constant)

Super refraction (trapping)

(anomalous propagation:

Unusually cold moist air near the ground)

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Angular Beam Formation propagation path(the transition from a circular beam of constant diameter to an angular beam of constant angular width)

Fresnel zone

Far field region

θ1= 1.27 λ/D (radians)

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Antenna propagation path(directive) Gaingt

The defining equation:

Eq. (3.4)

(W m-2) = power density incident on scatterer

r = range to measurement

= radiation pattern (= 1 on beam axis)

= transmitted power (W)

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Wavenumbers for H, V Waves propagation path

Horizontal polarization:

Vertical polarization:

where k = free space wavenumber = 3.6x106 (deg./km)

for λ = 10 cm

(e.g., for R= 100 mm h-1, = 24.4okm-1, = 20.7okm-1)

Therefore: ch< cv ; λh < λv ; kh>kv

Specific differential phase:

(for R = 100 mm h-1)

(KDP:an important polarimetric variable related to rainrate)

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Specific Differential Phase propagation path

(Fig.6.17)

Echo phase of H = h = 2khr

Eq. (6.60)

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Backscattering Cross Section, propagation pathσbfor a Spherical Particle

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Backscattered Power Density Incident on Receiving Antenna propagation path

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Echo Power propagation pathPrReceived

(3.20)

Aeis the effective area of the receiving antenna for radiation from the θ,φ direction. It is shown that:

(3.21)

If the transmitting antenna is the same as the

receiving antenna then:

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The Radar Equation propagation path(point scatterer/discrete object)

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n propagation pathth pulse

(n+1)th pulse

Ts

time

Apparent delay < Ts

True delay > Ts

Unambiguous Range ra- If targets are located beyond ra = cTs/2, their echoes from the nth transmitted pulse are received after the (n+1)th pulse is transmitted. Thus, they appear to be closer to the radar than they really are!
- This is known as range folding

- Ts = PRT
- Unambiguous range: ra = cTs/2
- Echoes from scatterers between 0 and raare called 1st trip echoes,
- Echoes from scatterers between ra and 2ra are called 2nd trip echoes, Echoes from scatterers between 2ra and 3ra are called 3rd trip echoes, etc

ra

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Ambiguous Doppler Shifted Echoes propagation path(Fig. 3.14)

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Δ propagation pathr= vrTs is the change in range of the scatterer between successive transmitted pulses

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Another PRT Trade-Off propagation path

- Correlation of pairs:
- This is a measure of signal coherency

- Accurate measurement of power requires long PRTs
- More independent samples (low coherency)

- But accurate measurement of velocity requires short PRTs
- High correlation between sample pairs (high coherency)
- Yet a large number of independent sample pairs are required

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Signal Coherency propagation path

- How large a Ts can we pick?
- Correlation between m = 1 pairs of echo samples is:
- Correlated pairs:
(i.e., Spectrum width must be much smaller than unambiguous velocity va = λ/4Ts)

- Increasing Ts decreases correlation exponentially
- also increases exponentially!

- Pick a threshold:
- Violation of this condition results in very large errors of estimates!

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8 m s propagation path-1

Signal Coherency and Ambiguities- Range and velocity dilemma: rava=cl/8
- Signal coherency: sv<va /p
- raconstraint: Eq. (7.2c)
- This is a more basic constraint on radar parameters than the first equation above
- Then, sv and not va imposes a basiclimitation on Doppler weather radars

- Example: Severe storms have a median sv ~ 4 m/s and 10% of the time sv > 8 m/s. If we want accurate Dopplerestimates 90% of the time with a 10-cmradar (l = 10 cm); then, ra ≤ 150 km. This will often result in range ambiguities

- This is a more basic constraint on radar parameters than the first equation above

Fig. 7.5

150 km

Spectrum width σv

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Unambiguous range ra

Echoes (I or Q) from Distributed Scatterers (Fig. 4.1) propagation path

t

c(s)≈ t

(t = transmitted

pulse width)

mTs

Weather signals (echoes)

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Weather Echo Statistics (Fig. 4.4) propagation path

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Spectrum of a transmitted rectangular pulse propagation path

ftf

If receiver frequency response is matched to the spectrum

of the transmitted pulse (an ideal matched filter receiver),

some echo power will be lost. This is called the finite bandwidth receiver

loss Lr. For and ideal matched filter Lr = 1.8 dB (ℓr= 1.5).

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Reflectivity Factor Z propagation path(Spherical scatterers; Rayleigh condition: D≤ λ/16)

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Differential Reflectivity propagation path

in dB units:

ZDR(dB) = Zh(dBZ) - Zv(dBZ)

in linear units:

Zdr = Zh(mm6m-3)/Zv(mm6m-3)

- is independent of drop concentration N0

- depends on the shape of scatterers

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Shapes of raindrops falling in still air and experiencing drag force deformation.

De is the equivalent diameter of a spherical drop.ZDR (dB) is the differential reflectivity in decibels

(Rayleigh condition is assumed). Adapted from Pruppacher and Beard (1970)

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The Weather Radar Equation drag force deformation.

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