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Radar: Acronym for Radio Detection and RangingRadar is a remote
sensing technique: Capable of gathering information about objects
located at remote distances from the sensing device.Two
distinguishing characteristics:Employs EM waves that fall into the
microwave portion of the electromagnetic spectrum(1 mm < l <
75 cm)
2.Active technique: radiation is emitted by radar radiation
scattered by objects is detected by radar.
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Why microwaves?Microwaves can penetrate haze, fog and snow
readily, and rain and hail less readily, so radar can see through
these conditions.An elementary radar system
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What does a conventional radar measure?1. Distance to an object
or collection of objectsDetermined by the time it takes energy to
travel to the objects and return at the speed of light.2. Azimuth
and elevation angle to the object(s)Determined by the pointing
angles of the antenna.3. Physical properties of the
object(s)Determined by the magnitude of the backscattered power.r =
1 km Dt = 6.67 msr = 100 km Dt = 0.667 ms
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Pulse duration (t, ms) and pulse length (h, meters)Pulse
repetition period (msec) and pulse repetition frequency (s-1)Duty
Cycle (= t/Tr)Meteorological radars send out pulses of energy with
relatively long periods of listening between pulses. Pulses are
required, rather than continuous waves, to determine the distance
to the target.
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Resolution along the direction of the beam: half the pulse
length (h)The back of the pulse at a will arrive at b at the same
time that radiation scattered from objects at the front end of the
pulse at c will arrive back at b.
When energy arrives back at the radar, an instantaneous sample
will include all radiation scattered between locations b and c: the
sample volume is half the pulse length (h/2).
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Second Trip Echo: an echo from a pulse that is not the most
recent pulse
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Pulse repetition frequency (PRF): The frequency that pulses are
transmitted, measured in hertz (s-1)Pulse repetition period (Tr):
The time between pulses (typical value 1 ms)Maximum Unambiguous
Range (rmax): The maximum distance that an object can be located
such that a pulse arriving at the object can return to the radar
before another pulse is emitted.Definitions
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Maximum unambiguous range vs. pulse repetition frequency
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Second trip echoesNote weird velocities (characteristic of
distant storm)
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Why not use a low PRF, insuring a large rmax?Measurements are
not made with a single pulse, but ratherwith the average of many
pulses since the antenna is rotating,dwell time (observing the same
location) is an issue.
2.Measurement of Doppler velocities require a high PRFChange the
PRFUse a different PRF every 2-3 pulses, if echo moves, get rid of
it! This is the methodology employed by the 88-Ds
How can you eliminate second trip echoes automatically?
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Other quantities used to describe the transmitted
signal:Wavelength (l, cm, mm) and Frequency (ft, Ghz, Mhz)
Band designationFrequency rangeWavelength rangeCommon
FrequencyCommonWavelength(Ghz)(cm)(Ghz)(cm)UHF0.3-1.030-1000.4271L1.0-2.015-301.323S2.0-4.07.5-152.810.7C4.0-8.03.75-7.55.55.5X8.0-12.02.5-3.759.43.2Ku12.0-18.01.67-2.515.51.94K18.0-27.01.11-1.67241.25Ka27.0-40.00.75-1.11350.86Millimeter40-3000.1-0.7594
(W band)0.3
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Major wavelength choice issues:
Size of equipmentAttenuationSize of scatterers relative
towavelength (Rayleigh vs Mie scattering)Peak power (without arcing
inwaveguide e.g., 3 MW in unpressurized waveguide for S band, 0.4
MW for K band)K (0.8 cm) band radar antennaS (10 cm) band radar
antenna
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ModulatorStores powerBetween
pulsesMagnetronGeneratesMicrowaveswhen highvoltage pulse sent
fromModulator
FrequencyDeterminedby characteristicsof magnetronDuplexerFast
actingSwitch thatprotectssensitivereceiver fromhigh energypulse
frommagnetronSTALOOscillatorGeneratesa
steadyfrequencyCOHOOscillatesat lowerfrequencywith samephase
astransmittedpulse
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Quantities used to describe weather echoesWavelength (l l, cm,
mm) and Frequency (ft fD) Ghz, Mhz)fD is the Doppler shift, the
change in frequency that occursbecause scatterers are moving toward
or away from the radar.Doppler shift is typically no more than a
few kilohertz, whileThe transmitted frequency is typically
gigahertz!3,000,000,0003,000,001,000
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Quantities used to describe weather echoesReceived Power:
typical value: nanowattsCompare the received power with the
transmitted power:
Peak transmitted power: 106 wattsReceived power: 10-9
wattsReceiver must be very sensitive, and must be protected
frommain pulse of energy transmitted by the radar!
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ModulatorStores powerBetween pulsesKlystronAmplifierthat
createsmicrowaves atfrequencydeterminedby STALOand
COHODuplexerProtectsSensitiveReceiver fromHigh energyPulse
frommagnetronSTALOOscillatorGeneratesa
steadyfrequencyCOHOOscillatesat lowerfrequencywith samephase
astransmittedpulse
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Amplitude determination:Phase determination:
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Dynamic range of a receiverRatio (db) of input power that causes
the video output to reach its maximum (saturation) level, to the
lowest power that produces a detectable input.Dynamic range of
precipitation echoesRatio (db) of maximum echo power received from
a very intense storm close to the radar to the minimum power
received from the weakest cloud that can be detected at the
greatest range of interest.
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Dynamic range of a receiverDynamic range of precipitation
echoesWOULD LIKE THESE TO BE THE SAME!
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Linear receivers:Linear receivers: Output voltage is linear with
input powerSingle linear receiversTypically have only half of the
dynamic range of precipitation echoes, so two receivers are often
used in tandem with automatic switching depending on the magnitude
of the returned signal Receiver 1:Strong echoesReceiver 2:Weak
echoes
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Other types of receivers:Logarithmic and Square law receivers:
Output voltage is non-linear with input powerHave worse resolution
than linear receivers but cover full dynamic range of weather
echoes
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Additional components of radars
Power supplies: provide powerServo amplifiers and/or drive
motors: position antennaSelsyns or potentiometers: measure angular
coordinates of antennaWaveguides, rotary joints, slip rings:
transmit microwaves fromtransmitter to antenna while antenna is
rotatingDirectional couplers: allow sampling of transmitted signal
or to inject test signals into receiverRadomes: to protect antenna
from weather and windOther electronic components