Atmospheric InstrumentationM. D. Eastin Fundamentals of Radar Systems.

Atmospheric Instrumentation M. D. Eastin

Fundamentals of Radar Systems


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


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


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


Historical OverviewEarly Imagery:

Radar image from 15 July 1960Hurricane Abbey near BelizeUS Navy photograph


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



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


Combined WSR-57 and WSR-74 Radar Network: Coverage below 10,000 feet AGL

Historical Overview



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



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



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)



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


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


fc


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



SMART RadarC-band

5 cmDoppler

NWS NEXRADS-band10.5 cmDoppler

NCAR S-PolS-band 10.7 cm

Polarimetric



Wyoming Cloud Radar

W-band3.15 mmDoppler

Polarimetric

NOAAHYDRO-X

X-band3.2 cm

DopplerPolarimetric

NOAA-KKa-band 8.7 mm

DopplerPolarimetric



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)


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


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


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer


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


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


TRANSMITTER

RECEIVER

ANTENNA

FrequencyMixer


Radar System Components

PULSEElectric

FieldSidelobes


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


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


Radar System Components

PULSEElectric

FieldSidelobes


Pulsemodulator


FrequencyMixer


Amplifier

PhaseDetector

DISPLAY

switch


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…)


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


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)


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 )


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


Signal CharacteristicsReceived Signal : Considerations

Frequency: Difference between the transmitted and received frequencies is the“Doppler shift” → Proportional to the radial velocity of the target

→ More on this later…

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


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


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.

Atmospheric InstrumentationM. D. Eastin Fundamentals of Radar Systems.

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