Radar

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Radar Principles & SystemsWith your facilitator, LT Mazat

I. Learning ObjectivesA. The student will comprehend the basic operation of a simple pulse radar system. B. The student will know the following terms: pulse width, pulse repetition frequency, carrier frequency, peak power, average power, and duty cycle. C. The student will know the block diagram of a simple pulse radar system and will comprehend the major components of that system.

D. The student will comprehend the basic operation of a simple continuous wave radar system. E. The student will comprehend the concept of doppler frequency shift. F. The student will know the block diagram of a simple continuous wave radar system and will comprehend the major components of that system, including amplifiers, power amplifiers, oscillators, and waveguides.

G. The student will comprehend the use of filters in a continuous wave radar system. H. The student will know the fundamental means of imparting information to radio waves and will comprehend the uses, advantages, and disadvantages of the various means. I. The student will comprehend the function and characteristics of radar/radio antennas and beam formation.

J. The student will comprehend the factors that affect radar performance. K. The student will comprehend frequency modulated CW as a means of range determination. L. The student will comprehend the basic principles of operation of pulse doppler radar and MTI systems.

Two Basic Radar Typesx

Pulse Transmission Continuous Wave

x

Pulse Transmission

Range vs. Power/PW/PRFMinimum Range: If still transmitting when return received RETURN NOT SEEN. Max Range:AveragePower PeakPower

=

PW PRT

= PW *PRF

As PW PRF

min Rh

max Rh

x 2.x

Pulse repetition frequency (PRF)

a. Pulses per second x b. Relation to pulse repetition time (PRT) x c. Effects of varying PRFx x

(1) Maximum range (2) Accuracy

x 3.x

Peak power

a. Maximum signal power of any pulse x b. Affects maximum range of radar

x

4. Average powerx x

a. Total power transmitted per unit of time b. Relationship of average power to PW and PRT a. Ratio PW (time transmitting) to PRT (time of entire cycle, time transmitting plus rest time) b. Also equal to ratio of average power to peak power

x

5. Duty cyclex

x

x

C. Discuss the determination of range with a pulse radar.

Determining Range With Pulse Radar

Range =c = 3 x 108 m/sec t is time to receive return

c*t 2

divide by 2 because pulse traveled to object and back

Pulse Transmissionx x

x

x

Pulse Width (PW) x Length or duration of a given pulse Pulse Repetition Time (PRT=1/PRF) x PRT is time from beginning of one pulse to the beginning of the next x PRF is frequency at which consecutive pulses are transmitted. PW can determine the radars minimum detection range; PW can determine the radars maximum detection range. PRF can determine the radars maximum detection range.

x

D. Describe the components of a pulse radar system.x x x x x x x

1. 2. 3. 4. 5. 6. 7.

Synchronizer Transmitter Antenna Duplexer Receiver Display unit Power supply

Pulse Radar Block Diagram

Transmitter Synchronizer RF Duplexer Power Supply Echo TR (Switching Unit) ATR Antenna

Display

Video

Receiver

Antenna Bearing or Elevation

Continuous Wave Radarx

Employs continual RADAR transmission Separate transmit and receive antennas Relies on the DOPPLER SHIFT

x

x

Doppler Frequency Shifts

Motion Away: Echo Frequency Decreases

Motion Towards: Echo Frequency Increases

Continuous Wave Radar ComponentsTransmitter CW RF Oscillator Antenna OUT

Discriminator

AMP

Mixer

IN Antenna

Indicator

Pulse Vs. Continuous WavePulse Echox x x x

Single Antenna Gives Range, usually Alt. as well Susceptible To Jamming Physical Range Determined By PW and PRF.

Continuous Wave x Requires 2 Antennae x Range or Alt. Info x High SNR x More Difficult to Jam But Easily Deceived x Amp can be tuned to look for expected frequencies

RADAR Wave Modulation4

Amplitude Modulation

Vary the amplitude of the carrier sine wave Vary the frequency of the carrier sine wave Vary the amplitude of the pulses Vary the Frequency at which the pulses occur

4

Frequency Modulation

4

Pulse-Amplitude Modulation

4

Pulse-Frequency Modulation

Modulation

Antennaex

Two Basic Purposes:x Radiates x Provides

RF Energy Beam Forming and Focus

x

Must Be 1/2 of the Wave Length for the maximum wave length employed Wide Beam pattern for Search, Narrow for Track

x

Beamwidth Vs. AccuracyBeamwidth vs Accuracy

Ship A

Ship B

Azimuth Angular MeasurementAzimuth Angular MeasurementRelative Bearing = Angle from ships heading. True Bearing = Ships Heading + Relative Bearing NShips Heading Angle

Target Angle

Determining AltitudeDetermining Altitude

e ang nt R Sla

Altitude Angle of Elevation

Altitude = slant range x sin0 elevation

x

Linear Arraysx

Concentrating Radar Energy Through Beam FormationUses the Principle of wave summation (constructive interference) in a special direction and wave cancellation (destructive interference) in other directions. Made up of two or more simple half-wave antennas.

x

x

Quasi-opticalx

Uses reflectors and lenses to shape the beam.

Basic Dipole Antenna and Beam FormingHalf-Wave Dipole Antenna

Basic Beam Formed

Wave Shaping Linear ArrayBeam FormingSide

Bottom

Types of Linear Arrays

Broadside

Endfire Array

Wave Shaping Linear Array Parasitic Element

Reflector ShapeParaboloid - Conical Scan used for fire control - can be CW or Pulse x Orange Peel Paraboliod - Usually CW and primarily for fire control x Parabolic Cylinder - Wide search beam - generally larger and used for longrange search applications - Pulsex

Wave Shaping -Quasi-Optical SystemsReflectors Lenses

Wave Guidesx x

x

Used as a medium for high energy shielding. Uses A Magnetic Field to keep the energy centered in the wave guide. Filled with an inert gas to prevent arcing due to high voltages within the waveguide.

Questions?

Please read Ch 9.

Radar Principles and Systems Part II

Factors That Affect Radar Performancex x x x x x x x

Signal Reception Receiver Bandwidth Pulse Shape Power Relation Beam Width Pulse Repetition Frequency Antenna Gain Radar Cross Section of Target

x x x x

Signal-to-noise ratio Receiver Sensitivity Pulse Compression Scan Ratex x

Mechanical Electronic

x x

Carrier Frequency Antenna aperture

Radar Receiver Performance Factorsx x x x

Signal Reception Signal-to-Noise Ratio Receiver Bandwidth Receiver Sensitivity

Signal Reception

Only a minute portion of the RF is reflected off the target. Only a fraction of that returns to the antenna. The weaker the signal that the receiver can process, the greater the effective range .

Signal-to-Noise Ratiox x

Measured in dB!!!!! Ability to recognize target in random noise.x x

Noise is always present. At some range, noise is greater that targets return.

x

x

Noise sets the absolute lower limit of the units sensitivity. Threshold level used to remove excess noise.

Receiver Bandwidthx

x

Is the frequency range the receiver can process. Receiver must process many frequenciesx

x

Pulse are generated by summation of sine waves of various frequencies. Frequency shifts occur from Doppler Effects. Increases the signal-to-noise ratio(good) Distorts the transmitted pulse(bad)

x

Reducing the bandwidthx x

Receiver Sensitivityx

Smallest return signal that is discernible against the noise background.x Milliwatts

range.

x

An important factor in determining the units maximum range.

Pulse Effects on Radar PerformancePulse Shape x Pulse Width x Pulse Compression x Pulse Powerx

Pulse ShapeDetermines range accuracy and minimum and maximum range. x Ideally we want a pulse with vertical leading and trailing edges.xx Very

clear signal easily discerned when listening for the echo.

Pulse WidthDetermines the range resolution. x Determines the minimum detection range. x Can also determine the maximum range of radar. x The narrower the pulse, the better the range resolution.x

Pulse CompressionIncreases frequency of the wave within the pulse. x Allows for good range resolution while packing enough power to provide a large maximum range.x

Pulse PowerThe Ummph to get the signal out a long way. x High peak power is desirable to achieve maximum ranges. x Low power means smaller and more compact radar units and less power required to operate.x

Other Factors Affecting Performancex

Scan Rate and Beam Widthx

Narrow beam require slower antenna rotation rate. Determines radars maximum range(tactical factor). Determines antenna size, beam directivity and target size.

x

Pulse Repetition Frequencyx

x

Carrier Frequencyx

x

Radar Cross Section (What the radar can see(reflect))x

Function of target size, shape, material, angle and carrier frequency.

Summary of Factors and CompromisesSummary of Factors and CompromisesFactorPulse Shape

DesiredSharp a