Radar
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Transcript of Radar
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Radar
Many in a series of McGourty-Rideout Productions
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What is Radar?
• RADAR (Radio Detection And Ranging) is a way to detect and study far off targets by transmitting a radio pulse in the direction of the target and observing the reflection of the wave.
• It’s basically radio echo
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RADARRAdio Detection And Ranging
Radar observables:• Target range• Target angles (azimuth & elevation)• Target size (radar cross section)• Target speed (Doppler)• Target features (imaging)
Antenna
TransmittedPulse
TargetCross
Section
Propagation
ReflectedPulse
(“echo”)
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Radar Frequencies
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Radar Frequency Bands
Frequency
Wavelength 1 mm1 km 1 m 1 m 1 nm
1 MHz 1 GHzIR UV
109 Hz
0 1 2 3 4 5 6 7 8 9 10 11 12
30 20 10 8 6 5 4 39 7
Allocated Frequency (GHz)
Wavelength (cm)
X-BandC-BandS-BandL-BandUHF
VHF
Visible
1012 Hz
Ku K Ka W
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The Range
• Distance from the radar
• Measured from time delay between transmitted pulse and returned signal received
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The Range
• Remember, in general v=d/t and d=vt
• The range is just a distance
• Since radio waves travel at the speed of light (v = c = 300,000 km/sec )
range = c•time/2• Why divided by 2?
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The Range
• The “2” is because the measured time is for a round trip to and from the target. To determine the range, you only want the time to the object, so you take half!
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Radar Range Measurement
Transmitted
Pulse
Reflected
Pulse
Range
Target
• Target range = c2
where c = speed of light = round trip time
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Atmospheric Effects
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• Atmospheric attenuation
• Reflection off of earth’s surface
• Over-the-horizon diffraction
• Atmospheric refraction
Radar beams can be attenuated, reflected and bent by the environment
Radar beams can be attenuated, reflected and bent by the environment
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Radar
• The range and the direction of the target determine its location, which is what is needed for many radar applications such as air traffic control.
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How Strong Is It?
• The strength of the received echo can also be measured
• This will vary with the distance of the target, its size, its shape and its composition
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Types and Uses of Radar
• Search radars scan a large area with pulses of short radio waves
• Targeting radars use the same principle but scan a smaller area more often
• Navigational radars are like search radar, but use short waves that reflect off hard surfaces. They are used on commercial ships and long-distance commercial aircraft
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Types and Uses of Radar
• Mapping radar scans a large regions for remote sensing and geography applications
• Wearable radar which is used to help the visually impaired
• Air traffic control uses radar to reflect echoes off of aircraft
• Weather radar uses radar to reflect echoes off of clouds
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Types and Uses of Radar
• Weather radars use radio waves with horizontal, dual (horizontal and vertical), or circular polarization
• Some weather radars use the Doppler effect to measure
wind speeds
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Incoherent Scatter Radar- A Radar Application
• Used to study the Earth's ionosphere and its interactions with the upper atmosphere, the magnetosphere, and the solar wind
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Incoherent Scatter Echo
• Electrons in ionosphere are radar targets
• These electrons can scatter radio waves
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Radar Can Measure Pressure
• The strength of the echo received from the ionosphere measures the number of electrons able to scatter radio waves or what we call electron pressure
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Radar Can Measure Temperature
• Some electrons are moving due to heat - In this case the echo is scattered
• The echo will contain a range of frequencies close to the transmitter frequency
• As the temperature increases, the electrons move faster
• So radar can act like a thermometer and measure the temperature of the ionosphere
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Radar Can Measure Wind Speed• When an electron is
removed from an atom, the remaining charged atom is called an ion
• The ion gas can have a different temperature from the electron gas
• The electron/ion mixture is known as a plasma and is usually in motion (like our wind)
• So incoherent scatter radar can also measure wind speed
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