03-2Radar System Design Example

8/7/2019 03-2Radar System Design Example

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RadarSystemDesign

Example


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Discuss factors that affect radar performance.

. gna recep on

2. Signal-to-noise ratio3. Receiver bandwidth

9. Beam width10. Pulse repetition

4. Receiver sensitivity5. Pulse shape6. Pulse com ression

11. Carrier frequency

12. Antenna gain (directivity

7. Power relation8. Scan rate

and power)13. Antenna aperture14. Radar cross section of

.b. Electronic target

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RadarSystemDesignTradeoffs

Size:highfrequencieshavesmallerdevices.

transmitpower:generallyfavorslowerfrequencies

antennagain HPWB:sma ig gain avors ig requencies

atmosphericattenuation:smallerlossalowfrequencies

ambientnoise:lowestin110GHzrange

Dopplershift:greaterathighfrequencies

Polarizationaffects:

c utteran groun re ections

RCSofthetargetsofinterest

antennadeploymentlimitations

Waveformselectionaffects:signalbandwidth:(determinedbypulsewidth)

PRF: setst eunam guousrange

averagetransmitterpower:(determinesmaximumdetectionrange)


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Radar S stem Desi n Exam le

Pfa , B, Tfa, (S/N)T , Gint , R, R,

t, , , ,

max, ,

min

k, Ts , Bn , antenna beamwidths

,constant is fixed

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Radar S stem Desi n Exam le

Begins with a specification

.

Calculate related quantities

.

Does performance meet specification?

.

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Radar Desi n Exam le

-

General requirements:

Pulse power should be as low as possible

Small boats close in

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Stop: Decide what is important

Stop: Decide where to start

Make a trial design, then adjust parameters

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X band: 9.6 GHz

t

Range resolution = 15 m

PPI synthesized display

d = . , fa =

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Broadvertical

beam

arrow

horizontal

Antenna beam requirements

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.

Calculate Probability of False Alarm Pfa a a

For R = 15 m, = 1 s x 15 / 150 = 0.1 s

= . =

Pfa = 1 / (1.4 x 106 x 3600) ~ 2 x 10-11

. T

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Figure 2.6Pd

0.999

Illustration only

Pfa = 10-6

.Pfa = 10

-11

0.900.80

10 dB 12 dB 14 dB 16 dB

0.50

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.

Pfa = 2 x 10-11 Pd = 0.8

~. .

Assume constant RCS target

.

Integration gain - How many pulses/target?

= , int ~

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.

Use Radar Equation

-min t int - 4 Rmax - losses

m n Smin = k Ts Bn in dBW + (S/N)T

s N = -228.6 + 30 + 71.4 = -127.1 dBW

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= - =.

Smin = -127.1 + 15.0 = -112.1 dBW

= 0.03125 m = -15.1 dBmeter

min = t int -- 4 Rmax - losses

- -. .- 4 Rmax losses

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RF - two way loss 4 dB

Filter mismatch loss 1 dB

Radome loss ?

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Radar Design Example

Radar Equation = 10 m2 = 10 dBmeter2

Losses = 7.0 dB

-112.1 = 40 + 2 G 30.2 + 10 + 11 - 33- 4 Rmax 7.0

Solve for G with R = 40 km = 46 dBm

2 G = -112.1 - 40 + 30.2 - 21+ 33 + 184 + 7

G = 40.6 dB

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We need an antenna with gain of 40.6 dB tomeet specified performance

Set vertical beamwidth = 15o

horizontal beamwidth = 1o G = 33,000 / (15 x 1) = 2200 = 33.4 dB

Find dimensions of antenna

3 dB ~ 75 / D degrees

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3 dB ~ 75 / D degrees D = 75 / 1 = 75 x 0.03125 m = 2.34 m

DH = 7.7 feet - may be too large

Set horizontal beamwidth = 1.5o

DH = 1.56 m = 5.1 ft, G = 31.6 dB

DV = 75 / 15 = 5 x 0.03125 m = 0.16 m

V

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Stotted waveguide

antennaLength = 75

Flare

Aperture

height

Power in

o o

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Radome

Antenna

RxTx T/R cell

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Review design of X band radar: Antenna:

D = 1.56 m = 5.1 ft, G = 31.6 dB

We wanted G = 40.6 dB to meet ran e andtarget RCS requirements

We must com romise!

We need to find 18 dB in radar equation

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Check antenna rotation rate first We set 3 dB beamwidth = 1.5o Hor lane

We need 20 hits/target

Must decide on rf

Max range is 40 km, set Run = 100 km

= =, .

20 pulses = 13.33 ms

o . . .

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Review Radar Equation: S = P + 2 G + 2 + + G - 33

- 4 Rmax losses

What can we change? Smin is set by Noise Power and (S/N)T Wavelen th and losses are fixed

We want G = 31.6 dB, set by dimensions

Available: P R G

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Antenna Rotation Rate

Screen updates every 5 seconds may wantfaster u dates

Hits/target = 18.8 / 12 x 20 = 31

= =n . .

We can increase integration gain by 3.8 dB

.

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Trade-off Study

, , max

We need to find 18 dB t

We are still 11 dB below original specification

We must detect target at 40 km range.

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Trade-off Study

> ,

Then RCS = 10 dBm2 + 11 dB = 21 dBm2

Range km 40 10 3 1

- - -

Target m2 126 0.5 0.004 0.5 cm2

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Trade-off Study

> ,

Then RCS = 10 dBm2 + 11 dB = 21 dBm2

Range km 40 10 3 1

- - -

Target m2 126 0.5 0.004 0.5 cm2

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Trade-off Study

Radar sensitivity - range in nautical miles Ran e nm 21.6 5.4 1.62 0.54

Target m2 126 0.5 0.004 0.5 cm2

What is RCS of a t ical tar et?

Large ship: 10,000 m2

2

Open boat: 0.2 m2 to 1.0 m2

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Revised Design

Recalculate Noise power and Smin N = -137.1 dBW

(S/N)T = 15 dB

S = -137.1 + 15.0 = -122.1 dBW We can trade pulse width for Tx power

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Revised Design

Alternative strategy: Reduce Tx ower b 10 dB to 5 kW

Lowers cost of transmitter and PSU

Short ran e erformance will suffer when = 0.1 s

Tabulate ran e, ran e resolution, tar et RCS

(Scale RCS of target in proportion to newparameters)

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Revised Design

Alternative strategy: Reduce Tx ower b 10 dB to 5 kW

Lowers cost of transmitter and PSU

Short ran e erformance will suffer when = 0.1 s

Tabulate ran e, ran e resolution, tar et RCS

(Scale RCS of target in proportion to newparameters)

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Revised Design

5 kW transmitter with 1 / 0.1 s pulse length Ran e Pulse width Resolution RCS min

nm s m m2

21.6 1.0 150 126 10 1.0 150 5.8

. .

3 0.1 15 0.5

. .

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Alternative o tion

Antenna will be most costl art of this radar

Horizontal beamwidth = 1.5o = = =. . , .

We could offer low cost option with smaller

antenna Horizontal beamwidth = 3 o

D = .7 m ~ 2. f = 2 . B

In Radar equation: Const x G2 / R4

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Economy Version

Economy version with 2.5 ft antenna Ran e Pulse width Resolution RCS min

nm s m m2

20 1.0 150 500 10 1.0 150 46

.

3 0.1 15 4

. .

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Econom Marine Radar

2.5 ft antenna 5 kW transmitter

Performance is marginal on open boat at 1nm

Increase rotation rate to 18.8 rpm to get morehits per target and extra 3.8 dB integration gain

min = t + + + + int -- 4 Rmax losses

int

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Econom Marine Radar

2.5 ft antenna 5 kW transmitter

Performance is marginal on open boat at 1nm

Increase rotation rate to 18.8 rpm to get morehits per target and extra 3.8 dB integration gain

min = t + + + + int -- 4 Rmax losses

int

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Economy Marine Radar

Economy version with 2.5 ft antenna, 19 rpm Ran e Pulse width Resolution RCS min

nm s m m2

20 1.0 150 210 10 1.0 150 20

.

3 0.1 15 1.6

. .

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Econom version

range 1 nm with X-band radar Screen u date rate is 5 seconds

Transmit power 5 kW (pulse)

Antenna: 3 degree horizontal beam

Range resolution 150 m / 15 m for >/< 10 nm

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Other Issues

Atmospheric loss about 1 dB in clear air -

Radome or antenna cover - salt spray

Clutter: Backscatter from sea surface

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Summar of Radar Desi n

Neither meets the original specification

Two versions satisfy objectivesof specification

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03-2Radar System Design Example