Lecture_5 - Radar Principles Systems II Systems Archer...

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Unformatted text preview: Radar Principles & Systems II Systems Archer Factors That Affect Factors That Affect Radar Performance x x x x x x Pulse Shape Pulse Width PRF Pulse Compression Power Radar Cross Section of Target x x x x x Beam Width Signal Reception Signal­to­noise ratio Receiver Sensitivity Receiver Bandwidth Pulse Shape Pulse Shape Determines range accuracy and minimum and maximum range x Determines resolution/definition of target x “Perfect pulse” would have vertical leading and trailing edges x x This would require in infinite number of waves, i.e. infinite bandwidth Pulse Width Pulse Width Determines the range resolution, min detection ranges x Good range resolution, or the ability to separate two targets at nearly the same range, requires a narrow pulse width x Pulse Repetition Frequency Pulse Repetition Frequency x PRF important in tactical use of radars x Long­range search radars require a pulse rate slow enough to allow echoes from targets at the maximum range to return to the receiver before transmitter is pulsed again x Higher pulse rates used in aircraft interception radars where max range is less b/c of power limitations Pulse Compression Pulse Compression Increases 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 Power Pulse Power x High peak power is desirable to achieve maximum ranges x The more energy transmitted, the greater the return signal strength and subsequently the higher the probability of detection x Low power means smaller and more compact radar units and less power required to operate Beamwidth Beamwidth x Target search vs. tracking or close­in interception x Early warning radar used to search wide area, wide beam pattern—accuracy is sacrificed for max detection probability x Tracking/guidance requires extreme accuracy, so radar beam has a narrow width propagated at higher frequencies x Narrowness of beam determines accuracy with which radar can measure bearing/elevation Beamwidth Vs. Accuracy Beamwidth Vs. Accuracy Beamwidth vs Accuracy Ship A Ship B Beamwidths determine the angular accuracy of the radar! Signal Reception Signal Reception Only a tiny amount of RF energy is reflected off target x Even less of that energy makes it back to receiver x x The weaker the signal that the receiver can process, the greater the effective range Signal­to­Noise Ratio Signal­to­Noise Ratio x Ability to recognize target in random noise x Noise is always present x At some range, noise is greater that target’s return Noise sets the absolute lower limit of the unit’s sensitivity x Atmospheric disturbances, stray signals x Signal­to­Noise Ratio Signal­to­Noise Ratio To cope with this problem, the operator To can set a threshold level can x If signals are below this threshold level, If they will not be displayed they x x If threshold level is set too low - you get many false detentions x If set to high - could mask out the real contact x Must compromise Receiver Bandwidth Receiver Bandwidth x x Is the frequency range the receiver can process Receiver must process many frequencies x x x Pulses are generated by summation of sine waves of various frequencies Frequency shifts occur from Doppler Effects Reducing the bandwidth x x Increases the signal­to­noise ratio (good) Distorts the transmitted pulse (bad) Receiver Sensitivity Receiver Sensitivity Smallest return signal that is discernible against the noise background x An important factor in determining the unit’s maximum range x Summary of Factors and Compromises Summary of Factors and Compromises Summary of Factors and Compromises Factor Desired Why Trade-off Required Pulse Shape Sharp a rise as possible Better range accuracy Require infinite bandwidth, More power /longer range more complex Tall as possible Requires larger equipment More power Pulse Width Short as possible Closer minimum range More accurate range Pulse Repetition Freq. Short Better range accuracy Reduces maximum range Better angular resolution Better detection probability Pulse Compression Uses technique Greater range Shorter minimum range Reduces maximum range More complex circuitry Summary of Factors and Compromises Summary of Factors and Compromises Summary of Factors and Compromises Factor Desired 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 Why Trade-off Required Better signal-to-noise ratio Distorts pulse shape Types of Combined Types of Combined Radar Systems x Frequency Modulated CW Radar x x Pulse Doppler x x x Carrier wave frequency within pulse is compared with a reference signal to detect moving targets. Can use Bandpass Filter to remove signal from signals that do not have a specific Doppler shift. Moving Target Indicator (MTI) System x x Use for radar altimeters and missile guidance. Signals compared with previous return to enhance moving targets. (search radars) Frequency Agile Systems (change freq. with time) Modulating the Modulating the Radar’s Wave 4 Amplitude Modulation – Vary the amplitude of the carrier sine wave 4 Frequency Modulation – Vary the frequency of the carrier sine wave 4 Pulse-Amplitude Modulation – Vary the amplitude of the pulses 4 Pulse-Frequency Modulation – Vary the Frequency at which the pulses occur ...
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  • Spring '14
  • Carrier wave, maximum range

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