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Atmospheric Instrumentation M. D. Eastin
Fundamentals of Radar Systems
Atmospheric Instrumentation M. D. Eastin
Outline
Radar Systems
• Historical Overview• WRS-57• WSR-74• WSR-88D
• Modern Radar Systems• Data Usages• Designation by Pulse Wavelength
• Typical Components
• Signal Characteristics• Transmitted• Received
Atmospheric Instrumentation M. D. Eastin
Historical OverviewEarly Development:
1904 Christian Hulsmeyer developed a device that could remotely detect ships beyond the human visual range – the first “radar” device
1917 Nikola Tesla outline how a “radar” device could be used for tracking ships bytransmitting pulses at regular intervals
1930s Pulsing “radar” developed by British, German, French, and US militaries fordefense – the Allies thought the Germans were developing “death rays”
1940s Science of radar meteorology was born during World War II
1940 A “radio detection and ranging” (radar) device was officially developed by the US Navy
1941 A 10-cm (S-Band) defense radar along the southern coast of England was used to track a thunderstorm with large hail over a distance of 7 miles.
1943 First operational weather radar – Panama Canal Zone
Atmospheric Instrumentation M. D. Eastin
Historical Overview
Early Theoretical Work:
1940 J. W. Ryde – a scientist at British Electric – develop the theory of Rayleighand Mie scattering (which describes how electromagnetic waves interact withparticles and gases) while working on the frosting of light bulbs to enhance their diffusion of light through a room.
1947 Ray Wexler published the first scientific paper on the “radar equation” and itsnumerous complexities, including how one can relate the radar signal returnstrength (i.e., radar reflectivity) and rainfall intensity (a Z-R relationship)
1951 David Atlas demonstrated that 3-cm (C-band) radars – while smaller in sizeand thus cheaper to manufacture – suffer from attenuation (or a loss in beampower due to absorption), and can not detect targets at far ranges (> 80 km)
However, 10-cm (S-band) radars suffer much less attenuation, and can detecttargets at long ranges (> 300 km) – NWS and airlines develop 10-cm radars.
1951 Herb Ligda coined the term “mesoscale” and declared the radar as its primary means of observation
1953 J.W. Brantley presented the first practical theory of Doppler radar
Atmospheric Instrumentation M. D. Eastin
Historical OverviewEarly Imagery:
Radar image from 15 July 1960Hurricane Abbey near BelizeUS Navy photograph
Atmospheric Instrumentation M. D. Eastin
Genealogy of Modern Radar Systems:
1957 -- Weather Surveillance Radar (WSR-57)
• First operational network in U.S.• Total of 66 pulse radars (10-cm or S-band)• Scanned at single elevation angle (0.5°)• Maximum range = 915 km• Reflectivity only (non-Doppler)• Electronics based on vacuum tubes• Remained in operation into the 1990s
Historical Overview
Atmospheric Instrumentation M. D. Eastin
Genealogy of Modern Radar Systems:
1974 -- Weather Surveillance Radar (WSR-74)
• Added to existing network of WSR-57 radars• “Filled gaps” for better severe weather detection
(but many gaps remained → next slide)
• Total of 62 pulse radars (5-cm or C-band)(combined network was 128 radars)
• Scanned at a single elevation angle (0.5°)(but could be adjusted by forecaster)
• Maximum range = 579 km• Reflectivity only (non-Doppler)• Electronics based on transistors• Remained in operation into the 1990s
• Charlotte (CLT) radar was in operationfrom 1978 to 1996 (never replaced)
Historical Overview
Atmospheric Instrumentation M. D. Eastin
Combined WSR-57 and WSR-74 Radar Network: Coverage below 10,000 feet AGL
Historical Overview
Atmospheric Instrumentation M. D. Eastin
Genealogy of Modern Radar Systems:
1988 -- Weather Surveillance Radar (WSR-88D)
• Next generation radar network (NEXRAD)• Complete replacement of WSR-57 / WSR-74
• Fewer gaps for better severe weather detection • Total of 160 pulse radars (10-cm or S-band)• Doppler radar (single polarization) • Can scan at multiple elevation angles
(9 possible volume coverage patterns)• Faster scanning at single elevation angle• Maximum range = 460 km (reflectivity)
= 230 km (velocity)• Electronics based on digital microprocessors• Automated detection algorithms for numerous
aspects of severe weather (hail, mesocyclones)
Historical Overview
Atmospheric Instrumentation M. D. Eastin
Genealogy of Modern Radar Systems:
2008 -- Weather Surveillance Radar (WSR-88D) -- Super Resolution
• Enhancement to existing network• Azimuthal resolution decreased (1.0°→ 0.5°)• Maximum range of velocity data increased (230 → 300 km)• Increases the range at which tornadic mesoscale rotations can be detected• Allows for faster lead times on tornado warnings
Historical Overview
Super Resolution Legacy Resolution
Atmospheric Instrumentation M. D. Eastin
Genealogy of Modern Radar Systems:
2013 -- Weather Surveillance Radar (WSR-88D) -- Dual Polarization
• Enhancement to existing network• Adds vertical polarization to the current horizontal radar beam• Allows the radar to better distinguish between rain, hail, snow, birds, and insects• Permits increased accuracy of storm-total precipitation amounts• Allows forecasters to more readily identify severe hail cores/shafts
Historical Overview
Radar Reflectivity
(single polarization)(and)
(dual polarization)
Hydrometeor Type
(dual polarization)(ONLY)
Atmospheric Instrumentation M. D. Eastin
Atmospheric Instrumentation M. D. Eastin
Current Usage:
• 3-D convective storm structure and evolution• 3-D convective wind structure and evolution• Quantitative precipitation measurements• Data assimilation into regional and global numerical models
• Onset of convection near pre-exiting surface boundaries → severe weather watches• Vertical wind profiles in and near convection → evolution of near storm vertical shear• Mesocyclone detection → tornado warnings• Hail detection → severe hail warnings• Low-level gust front intensity – severe wind warnings • Turbulence and wind shear detection → aircraft operations• Detection of low-level melting / freezing layers → influences aircraft de-icing operations
• Melting layers in stratiform precipitation → winter storm forecasting• Vertical wind profiles in stratiform precipitation → winter storm forecasting
• Hurricane structure
Modern Radar Systems
Atmospheric Instrumentation M. D. Eastin
Radar Type – Wavelength Band Designation:
• Radar systems are designed to transmit electromagnetic (EM) pulse waves (or radar beams) in short bursts and then “listen” for a return echo from the desired (and undesired) targets •EM pulse waves travel at the speed of light and are characterized by wavelength / frequency
where: c = speed of light [ m s-1 ] λ = wavelength [ m ] Gigahertz (GHz) = 109 Hz
f = frequency [ s-1 or Hertz (Hz) ] Megahertz (MHz) = 106 Hz
Modern Radar Systems
fc
Atmospheric Instrumentation M. D. Eastin
Modern Radar SystemsRadar Type – Wavelength Band Designation:
Wavelength Choice Impacts:
• Hydrometer size that can be detected → cloud or precipitation drops• Rader beam width → azimuthal resolution of observations• Antenna size → Financial cost of the radar system• Maximum detectable range → Number of individual radars in a network
PrecipitationRadars
CloudRadars
Atmospheric Instrumentation M. D. Eastin
Modern Radar Systems
SMART RadarC-band
5 cmDoppler
NWS NEXRADS-band10.5 cmDoppler
NCAR S-PolS-band 10.7 cm
Polarimetric
Atmospheric Instrumentation M. D. Eastin
Modern Radar Systems
Wyoming Cloud Radar
W-band3.15 mmDoppler
Polarimetric
NOAAHYDRO-X
X-band3.2 cm
DopplerPolarimetric
NOAA-KKa-band 8.7 mm
DopplerPolarimetric
Atmospheric Instrumentation M. D. Eastin
Modern Radar Systems
NOAAWP-3DX-band3.22 cmDoppler
Doppler on WheelsX-band3.2 cm
Doppler
-15 0 15 30 45
0 5 10 15 20 25 Range (km)
ReflectivityFactor (dBZ)
Atmospheric Instrumentation M. D. Eastin
Radar System ComponentsA Typical Pulse Radar System: Four Basic Components
• Transmitter• Antenna• Receiver• Display (future lectures)
• Radar systems are designed to transmit electromagnetic pulses (at microwave wavelengths) in short bursts from the antenna and then switch to the receiver and “listen” for any returns associated with that pulse
• Returns are then amplified and displayed as radar reflectivity
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
Atmospheric Instrumentation M. D. Eastin
Radar System ComponentsA Typical Pulse Radar System:
Transmitter:
• A microwave tube (“Klystron”) produces pulses of power at a desired frequency (or wavelength – 10 cm – S-band)
• A pulse modulator controls the timing of each pulse. Typical pulse durations are ~1 μs with each pulse separated by a few milliseconds to allow time for unique returns at large ranges
Pulse Repetition Frequency (PRF)
• Sets the timing between each pulse• Fixed (operational radars)• User-controlled (research radars)
Duplexer: Switch which allows the same antenna to transmit pulses and receive returns
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
Atmospheric Instrumentation M. D. Eastin
Radar System ComponentsA Typical Pulse Radar System:
Antenna:
• Output from the antenna is a pulse modulated microwave-frequency sine wave.
• Waves travel along a microwave transmission line (or “waveguide”) through the duplexer to the antenna
• The antenna concentrates waves into the desired shape – often a narrow cone (or “beam”) for most meteorological radars
• Transmitted beams travel through the environment until they strike an object (meteorological or not!)
• A very small portion of the beam is reflected back toward the antenna
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
Atmospheric Instrumentation M. D. Eastin
Radar System Components
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
A Typical Pulse Radar System:
Antenna:
Sidelobes:
• No radar antenna is perfectly built!• Small construction flaws allow for a portion of the transmitted signal to escape through “holes” as the beam is being formed• Can also strike environmental targets and have power reflected back
Half-power Beam Width:
• Function of radar design and range• Radius of a conical cross-section (i.e. a circle) at a given range
Atmospheric Instrumentation M. D. Eastin
Radar System Components
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
A Typical Pulse Radar System:
Receiver:
• The echo power is very small compared the transmitted power
• Echoes are first converted to an “intermediate frequency” by mixing the unique return echo frequency with the constant transmitted frequency
• Intermediate waves are then amplified by a known amount before being sent to the Doppler phase detector and display unit
Reflectivity:
• Amplitude difference between echo and known amplification
Doppler winds:
• Related to frequency difference between transmitted wave and echo (later…)
Atmospheric Instrumentation M. D. Eastin
Signal CharacteristicsTransmitted Signal:
Quantity Symbol Units Units Typical Value Comments
Frequency ft hertz MHz, GHz 3000 MHz c = ftλ
Wavelength λ meter cm 10 cm c = ftλ
Pulse Duration τ second μs 1 μs
Pulse Length h meter m 300 mLength of pulse as it travels
through the atmosphereh = cτ
Pulse Repetition Frequency
F s-1 s-1 400 s-1
Pulse RepetitionPeriod
Tr second ms 2.5 msTime between pulses
Tr = 1 / F
Peak Power Pt watt kW, MW 1 MW 1 MW = +90 dBm(reference is 1 milliwatt)
Pulse Energy W joule J 1 J Integral of the average power over one complete pulse
Average Power Pavg watt kW 400 WPower averaged over one
complete pulse repetition periodPavg = WF
Atmospheric Instrumentation M. D. Eastin
Signal CharacteristicsTransmitted Signal: Considerations
Wavelength: Choice is a function of the target to be studied and budget
Larger wavelengths → Precipitation detection Require larger antennas ($)
Pulse Duration: Choice is a function of sensitivity and range resolution
Longer durations → Better sensitivity (i.e. less error in a given dBZ) Poorer range resolution (i.e. no detailed structure)
PRF: Choice dictates the maximum range at which a target can be detected ( after a pulse has been transmitted, the radar must wait long enough ) ( to allow echoes from the most distant detectable targets to return ) ( “second trip echoes” → Returns observed after the next pulse )
Larger frequencies → Greater range→ Multiple echoes of same target (better sensitivity)→ Less motion by radar between consecutive pulses (better angular resolution of target)
Atmospheric Instrumentation M. D. Eastin
Signal CharacteristicsTransmitted Signal: Considerations
Peak Power: The power of the return echo from a target increases with the transmitted power of the pulse → larger peak powers are desired
Pulse Energy: Radar sensitivity increases with pulse energy → larger magnitudes desired
Average Power: Directly related to peak power and pulse energy → larger values desired
( Quantity most often “calibrated” for modern radars ) ( Most radars achieve accuracies of < 0.1 dBZ )
Atmospheric Instrumentation M. D. Eastin
Signal CharacteristicsReceived Signal:
Quantity Symbol Units Units Typical Value Comments
Frequency fr hertz MHz, GHz ~3000 MHzDiffers from the transmitted
frequency by the Doppler shift (usually less than a few kHZ)
Wavelength λr meter cm ~10 cm c = frλr
Pulse Repetition Frequency
F s-1 s-1 400 s-1 Same as transmitted PRF
Pulse RepetitionPeriod
Tr second ms 2.5 ms Same as for transmitted pulse
Received Power Pr watt mW, nW 10-6 mW 10-6 mW = -60 dBm
Time of Arrival Δt second ms 1 ms Measured from the time of the transmitted pulse
Atmospheric Instrumentation M. D. Eastin
Signal CharacteristicsReceived Signal : Considerations
Frequency: Difference between the transmitted and received frequencies is the“Doppler shift” → Proportional to the radial velocity of the target
Received Power: Many orders of magnitude smaller than the transmitted powerLarger values denote a greater “total” cross-section by the target(s)
Minimum Detectable Signal (MDS) → weakest return power that candiscriminated from the ever present background noise
Time of Arrival: Used to determine target’s range from the radar following:
where: r = target’s range (m) c = speed of light (m/s) Δt = elapsed time between the transmitted
pulse and received pulse (s)2
tcr
Atmospheric Instrumentation M. D. Eastin
Summary
Radar Systems
• Historical Overview• WRS-57• WSR-74• WSR-88D
• Modern Radar Systems• Data Usages• Designation by Pulse Wavelength
• Typical Components
• Signal Characteristics• Transmitted• Received
Atmospheric Instrumentation M. D. Eastin
References
Atlas , D., 1990: Radar in Meteorology, American Meteorological Society, 806 pp.
Crum, T. D., R. L. Alberty, and D. W. Burgess, 1993: Recording, archiving, and using WSR-88D data. Bulletin of the American Meteorological Society, 74, 645-653.
Doviak, R. J., and D. S. Zrnic, 1993: Doppler Radar and Weather Observations, Academic Press, 320 pp.
Fabry, F., 2015: Radar Meteorology Principles and Practice, Cambridge University Press, 256 pp.
Federal Meteorological Handbook No. 11, 1991a: Doppler radar meteorological observations, Part A, System concepts, responsibilities, and procedures. FCM-H11A-1990. Office of the Federal Coordinator for Meteorological Services and Supporting Research, Rockville, Maryland, 58 pp.
Federal Meteorological Handbook No. 11, 1991b: Doppler radar meteorological observations, Part B, Doppler radar theory and meteorology. FCM-H11B-1990. Office of the Federal Coordinator for Meteorological Services and
Supporting Research, Rockville, Maryland, 228 pp.
Federal Meteorological Handbook No. 11, 1991c: Doppler radar meteorological observations, Part B, WSR-88D products and algorithms. FCM-H11C-1991. Office of the Federal Coordinator for Meteorological Services and Supporting Research, Rockville, Maryland, 210 pp.
Lazo, J. K., R. E. Morss, and J. L. Demuth, 2009: 300 billion served – Sources, perceptions, and values of weather forecasts. Bulletin of the American Meteorological Society, 90, 785-798.
Reinhart, R. E., 2004: Radar for Meteorologists, Wiley- Blackwell Publishing, 250 pp.