Fundamentals of Radar and Display
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Transcript of Fundamentals of Radar and Display
Radar Meteorology M. D. Eastin
Fundamentals of Radar and Display
Radar Meteorology M. D. Eastin
Fundamentals of Radar and Display
Outline
• Radar Components• Signal Characteristics• Display – NEXRAD• Display – Other Types• Display – Phenomena
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
A Typical Pulse Radar System: Four Basic Components
• Transmitter• Antenna• Receiver• Display
• System is designed to transmit microwave pulses in shorts bursts from the antenna, and then activate the receiver to “listen” for any returns associated with that pulse
• Returns are then amplified and displayed as radar reflectivity
Duplexer: Switch which allows the same antenna to transmit pulses and receive returns
Radar Meteorology M. D. Eastin
Radar Components
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
Radar Meteorology M. D. Eastin
Radar 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)
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
Radar Meteorology M. D. Eastin
Radar 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
Radar Meteorology M. D. Eastin
Radar ComponentsA 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
PULSEElectric
FieldSidelobes
DuplexerKlystronAmplifier
Pulsemodulator
STALOMicrowaveOscillator
FrequencyMixer
COHOMicrowaveOscillator
Amplifier
PhaseDetector
DISPLAY
switch
Half-power beamwidth
TRANSMITTER
RECEIVER
ANTENNA
FrequencyMixer
Radar Meteorology M. D. Eastin
Radar ComponentsA 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 wave 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…)
Radar Meteorology 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 Pav watt kW 400 WPower averaged over one
complete pulse repetition periodPav = WF
Radar Meteorology 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 large antennas ($)
Pulse Duration: Choice 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)
Radar Meteorology 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 → large peak powers are desired
Pulse Energy: Radar sensitivity increases with pulse energy → large magnitudes desired
Average Power: Directly related to peak power and pulse energy → large values desired
( Quantity most often calibrated for modern radars )( Most radar achieve accuracies of < 0.1 dBz )
Radar Meteorology M. D. Eastin
Signal CharacteristicsTransmitted Signal: How to Express Power
Ratio of two powers:
(decibels)
where P1 = Observed power P2 = Reference power (constant)
2
1log10P
Pdb
mw
Pdbm
1log10 1
Radar Meteorology M. D. Eastin
Signal CharacteristicsReceived Signal (Radar Echoes):
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
Radar Meteorology M. D. Eastin
Signal CharacteristicsReceived Signal (Radar Echoes): 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 (r) from the radar following:
2
tcr
Radar Meteorology M. D. Eastin
Display - NEXRAD
Radar Meteorology M. D. Eastin
Display - NEXRADPlan-Position Indicator (PPI) Scanning Strategy: Single Elevation Angle
Data collected on a cone are projected onto a plane
Echoes close to the radar are at a low
elevation
Echoes far from the radar are at a high
elevation
Radar Meteorology M. D. Eastin
Display - NEXRADVolume Scanning Strategies:
Precipitation mode scan geometry Severe weather scan geometry
Saves time…fewer elevations
Radar Meteorology M. D. Eastin
Display - NEXRAD
Clear air mode scanning geometry
Fewer elevations, slower antennarotation achieves greater sensitivityfor clear air turbulence, clouds,Insects, drizzle, or light snowfall.
Volume Scanning Strategies:
Radar Meteorology M. D. Eastin
Display - NEXRADRadar Reflectivity:
• A measure of the power scattered back to the radar from objects in the path of a radar beam• Proportional to the sum of the sixth power of the diameter of all the particles illuminated by a pulse provided the particles are smaller than the radar wavelength (more on this later…)
65 55 45 35 25 15 5
Reflectivity Factor (dBZ)
Radar Meteorology M. D. Eastin
Display - NEXRADPrecipitation Mode:
• Used once liquid precipitation is observed
Radar Meteorology M. D. Eastin
Display - NEXRADClear-Air Mode:
• Used for snow and detecting the onset of deep convection
Radar Meteorology M. D. Eastin
Display - NEXRADBase Reflectivity:
• Echo intensity at the lowest PPI scan level (0.5°) measured in dBZ
Radar Meteorology M. D. Eastin
Display - NEXRADComposite Reflectivity:
• Maximum echo intensity at any PPI scan level measured in dBZ
Radar Meteorology M. D. Eastin
Display - NEXRADStorm Total Precipitation:
• Time integral of base reflectivity after NWS selected start time (measured in inches)• Primary tool to predict flash flooding
Radar Meteorology M. D. Eastin
Display - NEXRADVertically Integrated Liquid (VIL):
• Integral of reflectivity (or water mass) through a column (measured in kg /2)• Used to estimate the presence of hail and hail size (large VIL = large hail)
Radar Meteorology M. D. Eastin
Display - NEXRADRadial Velocity:
• Observed velocity component along the radar beam direction (measured in knots)
Radar Meteorology M. D. Eastin
Display - NEXRADStorm-relative Radial Velocity:
• Velocity component with the component of the storm motion along the radar beam removed• Best display for detecting mesocyclones, tornado vortex signatures, or microbursts
Radar Meteorology M. D. Eastin
Display - NEXRADCombined Radar Reflectivity and Radial Velocity:
• Used to detect most severe weather
Radar Meteorology M. D. Eastin
NEXRAD DataAvailable Data:
• All NEXRAD data since 1992 has been archived and is publically available through the NCDC at: http://hurricane.ncdc.noaa.gov/pls/plhas/has.dsselect
Level-II: Radar reflectivity and radial velocity at original sampling resolutionRaw volumetric data
Level-III: Derived products most used by forecasters
Base reflectivityComposite reflectivityBase radial velocityBase storm-relative radial velocityVertically-integrated liquid (VIL)Echo tops (ET - maximum height of 10 dBZ echo)Storm total precipitationand many more….
Radar Meteorology M. D. Eastin
Display – Other TypesRange-Height Indicator (RHI) Scanning Strategy:
• Radar is scanned in elevation at a fixed azimuth• Volume scans are accomplished by rotating slowly in azimuth while scanning rapidly in elevation
Radar Meteorology M. D. Eastin
Display – Other TypesExamples of RHI Scans:
• Sequence of RHI scans showing development of shallow cumulus along the south Florida coast
Note ground clutter and echo from tall buildings
(echo from radar side lobe)
Radar Meteorology M. D. Eastin
Display – Other TypesExamples of RHI Scans:
• Vertical cross-section through a squall line reconstructed via RHI slices through a PPI volume
Radar Meteorology M. D. Eastin
Display – Other TypesTime-Height Scanning Strategy:
• Radar is pointed vertically as storm passes over
Radar Meteorology M. D. Eastin
Display – Other TypesHorizontal cross-sections:
• Radar data is interpolated from cylindrical to Cartesian coordinates and displayed in Cartesian space
• Often done when constructing analyses from multiple Doppler radars (more on this later…)
Radar Meteorology M. D. Eastin
Display – Other TypesVertical cross-sections:
• Radar data is interpolated from cylindrical to Cartesian coordinates and displayed in Cartesian space
• Cartesian grid can be sliced similar to RHI scan
• If constructed from multiple Doppler radars, the vertical wind component can be estimated and displayed
Radar Meteorology M. D. Eastin
Display – Other TypesRadar Composites:
• Composite reflectivity from multiple PPI scans are projected onto a single display to show regional or national precipitation distributions• The rain-snow distinction determined by surface observations (not the radar)
Radar Meteorology M. D. Eastin
Display – PhenomenaBeam Blockage:
• Caused by tall buildings, trees, water towers, and cell towers near radar…
BlockedBeam
Radar Meteorology M. D. Eastin
Display – PhenomenaNon-Meteorological Targets:
• Insects and bats often rest during the day and travel at night → take-off at sunset• Birds rest at night and travel during the day → take-off at sunrise
Birds departingAt 1114 UTC
Radar Meteorology M. D. Eastin
Display – PhenomenaNon-Meteorological Targets:
• Ground clutter, aircraft, etc…
Columbia shuttleBreak-up
Ground clutter and diverted aircraft
Radar Meteorology M. D. Eastin
Display – PhenomenaBright Band:
• Enhancement of radar reflectivity at the melting level as large aggregate snowflakes develop a thin film of water on their surface before they collapse to a smaller drop
D istan c e (km )
R ef le ct iv ity fa cto r (d B Z )
S tr a t i fo rm a rea C on v ec t io n
B BAltitude (km)
Radar Meteorology M. D. Eastin
Display – PhenomenaConvective Storms: PPI Displays
Radar Meteorology M. D. Eastin
Fundamentals of Radar and Display
Summary:
• Radar Components• Signal Characteristics• Display – NEXRAD• Display – Other Types• Display – Phenomena