Radar

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ELEC 4504 Avionics Engineering
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Radar
Secondary Radar – In the current civil aviation environment, transponders are installed on all commercial aircraft and most private ones as well. – The big advantage of secondary radar is that the transmitted signal has to travel only one way. – Thus the power loss is only 1/r2 – This results in less power required and a smaller antenna for the ground equipment
Low cross-section High cross-section
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ELEC 4504 Avionics Engineering
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Radar
Primary Radar – Radar Cross – Section 0.003 m2
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ELEC 4504 Avionics Engineering
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Radar
Primary Radar
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ELEC 4504 Avionics Engineering Nhomakorabea15
Radar
Secondary Radar – Secondary radar overcomes the shortcomings of primary radar in several areas – It was developed from a WWII system called IFF (identification friend or foe) – IFF transponders were installed on friendly aircraft – These transponders responded to radar interrogation by transmitting a coded signal which enabled the radar to determine which aircraft were enemy and which were not
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Radar
Basic Principles
– Transmits an electromagnetic signal modulated with particular type of waveform. (modulation depends on requirements of application) – Signal is reflected from target – Reflected signal is detected by radar receiver and analyzed to extract desired information
8/23/2011
ELEC 4504 Avionics Engineering
6
Radar
Basic Principles – Distance can be determined by measuring the time difference between transmission and reception – Angle (or relative bearing) can be determined by measuring the angle of arrival (AOA) of the signal (usually by highly directive antenna) – If there is a radial component of relative velocity between radar and target it can be determined from the Doppler shift of the carrier
8/23/2011
ELEC 4504 Avionics Engineering
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Radar
Secondary Radar – Secondary radars all transmit on a frequency of 1030MHz – The transponder replies on 1090 MHz – Thus the power loss is only 1/r2 – This results in less power required and a smaller antenna
8/23/2011
ELEC 4504 Avionics Engineering
3
Radar
• Basic Principles – Components • Transmitter • Antenna • Receiver • Processor
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ELEC 4504 Avionics Engineering
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ELEC 4504 Avionics Engineering
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Radar
Basic Principles – In a pulsed radar system, pulses are transmitted at a given rate called the pulse repetition frequency (PRF, designated fP) – If the PRF is too high, some echoes may arrive after the next pulse has been transmitted. This results in an erroneous range measurement. – The maximum unambiguous range (all echoes arrive before the next pulse) is c
8/23/2011
ELEC 4504 Avionics Engineering
12
Radar
Primary Radar
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ELEC 4504 Avionics Engineering
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Radar
Primary Radar
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ELEC 4504 Avionics Engineering
2 fP
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ELEC 4504 Avionics Engineering
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Radar
Primary Radar – This is the simplest form of radar. – The transmitter emits a train of pulses at a constant PRF and the receiver responds to the energy reflected from the skin of the aircraft. – The power density at a target of range r is: G ⋅ Pt 4πr 2 – Assuming that the target has an area σ, then the power intercepted by the target is P G ⋅ t 2 ⋅σ 4πr – We then assume (for simplicity) that the intercepted power is reradiated equally in all directions P 1 G ⋅ t 2 ⋅σ ⋅ 2 – Thus the power density back at the radar is 4πr 4πr or
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Radar
Primary Radar Advantages of Primary Radar – Does not require any equipment on board aircraft – Therefore detects all aircraft within range Disadvantages – Requires high power due to 1/r4 decrease in power – Large Antenna required – Provides only position information – Susceptible to “clutter” or unwanted replies from terrain or buildings or from antenna sidelobes
Radar
OUTLINE History Applications Basic Principles of Radar Primary Radar Secondary Radar Weather Radar Radar Altimeters
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ELEC 4504 Avionics Engineering
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Radar
History – Invented in 1900s (patented in 1904) and reinvented in the 1920s and 1930s – Applied to help defend England at the beginning of World War II (Battle of Britain) • Provided advance warning of air raids • Allowed fighters to stay on ground until needed – Adapted for airborne use in night fighters – Installed on ships for detecting enemy in bad weather (Bismarck)
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ELEC 4504 Avionics Engineering
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Radar
Modulation Types – Simple Pulse; one or more repetition frequencies – Frequency Modulation FM (radar altimeters) – Pulse with Chirp (pulse compression) – CW (continuous wave) - police radar (Doppler) – Pseudorandom code
Pt G ⋅ ⋅σ (4π )2 r 4
8/23/2011
ELEC 4504 Avionics Engineering
9
Radar
Primary Radar – Obviously most targets do not reradiate isotropically so the radar cross section does not bear much relationship to the physical cross-section of the target but is a measure of how much power is being directed in the direction of the radar. – e.g. the same object with different orientations:
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ELEC 4504 Avionics Engineering
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Radar
Commercial Aviation Radar Systems • • • • Primary Radar (ATC - position only) Secondary Radar (ATC – position, altitude, identification etc.)) Weather Radar Radar Altimeters