Lecture_4_Radar_Principles(1) - Radar Radar Fundamentals...

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Unformatted text preview: Radar Radar Fundamentals Radar Fundamentals Radar Fundamentals Radar stands for? Radar scene First demonstrated in 1886 by Heinrich Hertz – Showed that radio waves could be reflected by metallic objects In 1904, a German engineer named C. Hulsmeyer patented an “obstacle detector and ship navigation device” Radar Fundamentals Radar Fundamentals Through electromagnetic waves, certain objects (ships/aircraft) can be detected at FAR greater range than the eye can see Radar permits a rapid, convenient, and accurate measurement of range to the object Radar Fundamentals Radar Fundamentals Radar can measure relative velocity in a simple way Range and range rate (change in range per unit time) can be fed to a fire control system with great precision Downsides? – Two closely spaced objects – Aircraft that are very close to terrain Radar Fundamentals Radar Fundamentals Radar best suited for dealing with isolated targets that appear against relatively featureless background – Ships at sea – Aircraft in the sky – Islands or large terrain features (shorelines) Two Basic Two Basic Radar Types Pulse Transmission Continuous Wave Pulse Transmission Pulse Transmission Transmits energy in a series of short pulses separated by non­transmission intervals or rest times – Target returns are processed during the rest time – Range is determined based on the total travel time for the pulse­return cycle This “pulse­echo” system most used Pulse Transmission Pulse Transmission During the “on” period the radar transmits short burst (pulse) of energy Pulse strikes an object, part of reflected energy returns to receiver – Processed and displayed Since transmitter is turned off after each pulse, it does not interfere with the receiver Pulse Transmission Pulse Transmission PRT Rest Time Rest Time Pulse Transmission Pulse Transmission Pulse Repetition Time (PRT) – Elapsed time between beginning of one pulse transmission and the beginning of the next – Total time for one cycle Must be sufficient duration to allow echo pulse to return from maximum range of the system – Otherwise, reception of return will be obscured by the succeeding pulse Pulse Transmission Pulse Transmission Pulse Width (PW) – Determines the minimum range at which a target can be detected – The active transmit time (μsec) – If target so close to transmitter that echo received before transmission is cut off and receiver turned on, echo not displayed Need short pulses to detect close targets Pulse Transmission Pulse Transmission Pulse Repetition Frequency (PRF) – Number of pulses transmitted per second (Hz) Radio Frequency (RF) – The working/operating frequency – The frequency of the carrier wave being modulated, forming the pulse train Rest Time (RT) – Non­transmit time or time between pulses Pulse Transmission Pulse Transmission PRT Rest Time PRT = PW+RT Rest Time PRT = 1/PRF Pulse Transmission Pulse Transmission Note: – Pulse width must be short to increase reception of nearby targets and yet contain sufficient power to ensure a return echo of enough magnitude from max range of system, LARGE TRANSMITTED POWER outputs are required to produce pulses with sufficient energy Duty Cycle Duty Cycle Duty Cycle Duty Cycle Peak Power Peak Power Pulse Xmsn Key Points Pulse Xmsn Key Points Varying the pulse width affects the range of the radar – Need short pulses for short range targets PW determines radar’s minimum range resolution The slower the PRF the greater the radar’s maximum range The faster the PRF the greater the radar’s accuracy Range Determination Range Determination Basic Radar Components Basic Radar Components Typical pulse­echo radar systems are composed of seven basic components Transmitter Transmitter Generate high­power pulses under control of a timer – Carrier wave is produced/modulated Receiver Receiver Converts incoming EM at a tuned frequency into electrical signals that can be used by a signal processor Power Supply Power Supply Furnishes all AC and DC voltages necessary for the operation of the system – Pulse power generated can be MUCH greater than the power supplied b/c energy can be stored during the rest time in capacitors Synchronizer Synchronizer aka the Timer Supplies signals that determine the timing of the transmitted pulses and coordinates action among all circuits in system Establishes PRF of the system Duplexer Duplexer Most radar sets use single antenna for transmission and reception aka the TR switch Allows antenna to be sequentially connected to the transmitter and the receiver Antenna Antenna Receives radio frequency energy from transmitter and radiates it in the form of a highly directional beam – Beamforming – "Tractor beam... Sucked me right in.“ Returning echoes are received by antenna and passed to receiver Display Display Visual indications of the received pulses displayed to operators – Range, azimuth/bearing, elevation – Difference between azimuth and bearing Radar Cross Section Radar Cross Section Radar Cross Section Radar Cross Section Defined as the area intercepting that amount of power which, when scattered equally in all directions, produces an echo at the radar Denoted σ Strictly a characteristic of the target Affects strength of radar return Generally, the larger the object, the larger the cross section Radar Cross Section Radar Cross Section Incident energy – Some gets absorbed as heat – Remainder is reradiated (scattered) in many directions Determined by target size, shape, skin material, aspect angle, polarization, radar carrier frequency Different shapes create different backscatter The Radar Range Equation The Transmit/Receive Capability Transmit/Receive Capability Simplex – one or the other – i.e. – car radio Half­Duplex – both, but not at the same time. – i.e. – “walkie­talkie” or BTB Full­Duplex – both and at the same time – i.e. – telephone system and most shipboard communications. Noise Noise Noise is bad on a communications circuit. Two types: – Broadband Noise – “White Noise” – Narrowband Noise – “Interference” Signal­to­Noise Ratio Signal­to­Noise Ratio Can be expressed in a pure number: – Signal power / Noise power More commonly expressed in Decibels. – Signal level is on a relative scale compared to the noise. – The more positive the dB number, the clearer the signal. – Unless you want to hide it!!! Modulation Modulation The process of encoding information on the “Carrier Wave”. – A simple Sine wave. The Sine wave has 3 independent parameters: – Amplitude – Frequency – Phase x Other Factors Affecting Other Factors Affecting Performance Scan Rate and Beam Width x x Pulse Repetition Frequency x x Determines radars maximum range(tactical factor). Carrier Frequency x x Narrow beam require slower antenna rotation rate. Determines antenna size, beam directivity and target size. Radar Cross Section x x This is what the radar can see, ie. reflected energy Function of target size, shape, material, angle and carrier frequency. Continuous Wave Radar Continuous Wave Radar x x Employs continual RADAR transmission Relies on the “DOPPLER AFFECT” Doppler Frequency Shifts Doppler Frequency Shifts Motion Away Echo Frequency Decreases Motion Towards Echo Frequency Increases Continuous Wave Radar Continuous Wave Radar Components Transmitter CW RF Oscillator Discriminator AMP Antenna OUT IN Mixer Antenna Indicator Pulse Vs. Continuous Wave Pulse Vs. Continuous Wave Pulse Echo x x x x Single Antenna Gives Range & Alt. Susceptible To Jamming Physical Range Determined By PW and PRF. Continuous Wave x Requires 2 Antennae x No 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 ...
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  • Spring '14
  • duty cycle, pulse transmission

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