Radar Principles Systems Two Basic Radar Types u

Radar Principles & Systems

Two Basic Radar Types u Pulse Transmission u Continuous Wave

Pulse Transmission

Range vs. Power/PW/PRF • Minimum Range: If still transmitting when return received RETURN NOT SEEN. • Max Range: As PW PRF min Rh max Rh

u 2. Pulse repetition frequency (PRF) u a. Pulses per second u b. Relation to pulse repetition time (PRT) u c. Effects of varying PRF (1) Maximum range u (2) Accuracy u u 3. Peak power u a. Maximum signal power of any pulse u b. Affects maximum range of radar

u 4. Average power a. Total power transmitted per unit of time u b. Relationship of average power to PW and PRT u u 5. Duty cycle a. Ratio PW (time transmitting) to PRT (time of entire cycle, time transmitting plus rest time) u b. Also equal to ratio of average power to peak power u u C. Discuss the determination of range with a pulse radar.

Determining Range With Pulse Radar c = 3 x 108 m/sec t is time to receive return divide by 2 because pulse traveled to object and back

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

u D. Describe the components of a pulse radar system. u 1. u 2. u 3. u 4. u 5. u 6. u 7. Synchronizer Transmitter Antenna Duplexer Receiver Display unit Power supply

Pulse Radar Block Diagram Transmitter Synchronizer RF ATR Duplexer Power Antenna (Switching Unit) Supply Echo Display Video TR Receiver Antenna Bearing or Elevation

Continuous Wave Radar u Employs continual RADAR transmission u Separate transmit and receive antennas u Relies on the “DOPPLER SHIFT”

Doppler Frequency Shifts Motion Away: Echo Frequency Decreases Motion Towards: Echo Frequency Increases

Continuous Wave Radar Components Transmitter CW RF Oscillator Discriminator AMP Mixer Antenna OUT IN Antenna Indicator

Pulse Vs. Continuous Wave Pulse Echo u Single Antenna u Gives Range, usually Alt. as well u Susceptible To Jamming u Physical Range Determined By PW and PRF. Continuous Wave u Requires 2 Antennae u Range or Alt. Info u High SNR u More Difficult to Jam But Easily Deceived u Amp can be tuned to look for expected frequencies

RADAR Wave Modulation G Amplitude Modulation – Vary the amplitude of the carrier sine wave G Frequency Modulation – Vary the frequency of the carrier sine wave G Pulse-Amplitude Modulation – Vary the amplitude of the pulses G Pulse-Frequency Modulation – Vary the Frequency at which the pulses occur

Amplitude modulation Modulation Pulse-amplitude modulation Pulse frequency modulation Freq. mod.

Antennae u Two Basic Purposes: u Radiates RF Energy u Provides Beam Forming and Focus u Must Be 1/2 of the Wave Length for the maximum wave length employed u Wide Beam pattern for Search, Narrow for Track

Beamwidth Vs. Accuracy

Azimuth Angular Measurement

Determining Altitude

Concentrating Radar Energy Through Beam Formation u Linear Arrays u Uses the Principle of wave summation (constructive interference) in a special direction and wave cancellation (destructive interference) in other directions. u Made up of two or more simple half-wave antennas. u Quasi-optical u Uses reflectors and “lenses” to shape the beam.

Reflector Shape u Paraboloid - Conical Scan used for fire control - can be CW or Pulse u Orange Peel Paraboliod - Usually CW and primarily for fire control u Parabolic Cylinder - Wide search beam - generally larger and used for long-range search applications Pulse

Wave Shaping -Quasi-Optical Systems Reflectors Lenses

Wave Guides u Used as a medium for high energy shielding. u Uses A Magnetic Field to keep the energy centered in the wave guide. u 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 Performance u u u u Signal Reception Receiver Bandwidth Pulse Shape Power Relation Beam Width Pulse Repetition Frequency Antenna Gain Radar Cross Section of Target u u Signal-to-noise ratio Receiver Sensitivity Pulse Compression Scan Rate Mechanical u Electronic u u u Carrier Frequency Antenna aperture

Radar Receiver Performance Factors u Signal Reception u Signal-to-Noise Ratio u Receiver Bandwidth u 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 Ratio u Measured in d. B!!!!! u Ability to recognize target in random noise. u Noise is always present. u At some range, noise is greater that target’s return. u Noise sets the absolute lower limit of the unit’s sensitivity. u Threshold level used to remove excess noise.

Receiver Bandwidth u Is the frequency range the receiver can process. u Receiver must process many frequencies u Pulse are generated by summation of sine waves of various frequencies. u Frequency shifts occur from Doppler Effects. u Reducing the bandwidth u Increases the signal-to-noise ratio(good) u Distorts the transmitted pulse(bad)

Receiver Sensitivity u Smallest return signal that is discernible against the noise background. u Milliwatts u An range. important factor in determining the unit’s maximum range.

Pulse Effects on Radar Performance u Pulse Shape u Pulse Width u Pulse Compression u Pulse Power

Pulse Shape u Determines range accuracy and minimum and maximum range. u Ideally we want a pulse with vertical leading and trailing edges. u Very clear signal – easily discerned when listening for the echo.

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

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

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

Other Factors Affecting Performance u Scan Rate and Beam Width u u Pulse Repetition Frequency u u Determines radars maximum range(tactical factor). Carrier Frequency u u Narrow beam require slower antenna rotation rate. Determines antenna size, beam directivity and target size. Radar Cross Section (What the radar can see(reflect)) u Function of target size, shape, material, angle and carrier frequency.

Summary of Factors and Compromises Factor Desired Why Trade-off Required Pulse Shape Sharp a rise as possible Tall as possible Better range accuracy More power /longer range Require infinite bandwidth, more complex Requires larger equipment/more power Pulse Width Short as possible Closer minimum range More accurate range Reduces maximum range Pulse Repetition Freq. Short Better range accuracy Better angular resolution Better detection probability Reduces maximum range Pulse Compression Uses technique Greater range Shorter minimum range More complex circuitry Power More Greater maximum range Requires larger equipment & power Beam Width Narrow Greater angular accuracy Slow antenna rate, Detection time Carrier Frequency High Greater target resolution Detects smaller targets Smaller equipment Reduces maximum range Receiver Sensitivity High Maximizes detection range More complex equipment Receiver Bandwidth Narrow Better signal-to-noise ratio Distorts pulse shape

Specific Types of Radar u Frequency Modulated CW Radar u u Pulse Doppler u u Carrier wave frequency within pulse is compared with a reference signal to detect moving targets. Moving Target Indicator (MTI) System u u Use for radar altimeters and missile guidance. Signals compared with previous return to enhance moving targets. (search radars) Frequency Agile Systems u Difficult to jam.

Specific Types of Radar u SAR / ISAR u Phased Array - Aegis u Essentially 360° Coverage u Phase shift and frequency shift allow the planar array to “steer” the beam. u Also allows for high / low power output depending on requirements.

Questions? sio n W as te is a r T m s n d o h Ec Ec ho