NEXRAD Technical Information
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The Weather Surveillance Radar, 1988 Doppler (WSR-88D),
also known as NEXRAD, is the most advanced operational
weather radar in the world. The fleet of 160 WSR-88D radars
operate 24/7 to support the weather warning and forecast
missions of the National Weather Service, FAA and DoD.
Additionally, real-time radar data is made available to the
nation’s academic and commercial weather enterprise.
Pictured to the left is the tower, which houses the antenna
(inside the radome).
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There are two major components of the radar (
RDA
and
RPG
):
RDA
The RDA (Radar Data Acquisition) samples the atmosphere to produce base
moments (reflectivity, velocity, and spectrum width) and Dual Pol variables
(differential reflectivity, correlation coefficient and differential phase)
representing the radar characteristics of the meteorological and biological
return within the radar coverage umbrella. The RDA is composed of four
primary components:
Transmitter
Transmitter characteristics-
Type: S-band, coherent chain (STALO/COHO), line modulator, klystron tube
amplifier (53 dB gain typical)
Frequency: 2700 to 3000 MHz
Power: 700 kw peak at klystron output
Transmitter to antenna loss: 1.5 to 2.5dB depending on tower height (5 to
30 meters) and site frequency
Average Power: 300 to 1300 watts
Pulse Widths: 1.57 and 4.71 microseconds (-6 dB points)
PRF short pulse: 318 to 1304 Hz
PRF long pulse: 318 to 452 Hz
Phase noise (system): phase and amplitude stability better than -57 dBc,
-60 dBc system goal
Short pulse output spectrum: -40 dB BW is 12.4 MHz, (-80 dB at +/- 62 MHz),
-80 dB at +/- 19.6 MHz for congested areas(congested areas require
transmitter output bandpass filter)
Antenna/Pedestal
Antenna characteristics-
Type: Parabolic dish (28 feet in diameter) with center feedhorn
Polarization: Dual Pol (simultaneous horizontal and vertical
transmit/receive)
Gain at 2850 MHz: 45.5 dB (including radome loss)
Beamwidth at 2850 MHz: 0.925 deg
First sidelobe: -29 dB (others less than -40 dB beyond 10 deg)
Radome: fiberglass foam sandwich frequency tuned, 39 foot truncated sphere
Radome two way loss: 0.24 dB at 2850 MHz
Pedestal Characteristics
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Pedestal Function
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Azimuth
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Elevation
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Steerability
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360 deg
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-1 to +45 deg
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Normal Scan
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360 deg
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+0.5 to +19.5
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Max rotation rate
|
30 deg/sec
|
30 deg/sec
|
Acceleration
|
30 deg/sec2
|
30 deg/sec2
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Mechanical Limits
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360 deg
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-1 to +60 deg
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Positioning Error (max)
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+/-0.2
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+/-0.2
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Pedestal Type: Elevation over Azimuth
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Receiver
Type: Coherent (stalo/coho), first downconvert to IF
Detection: digital IF with 16 bit analog to digital conversion of IF signal
at 100 MHz
Digital Matched Filter BW: 625 kHz, short pulse, 204 kHz, long pulse
Dynamic Range: 93 dB minimum required
Intermediate Frequency: 57.55 MHz
System noise figure: 270K (2.7dB)
Receiver Noise: -114dBm (Short Pulse), -118dBm (Long Pulse) Referenced to
Antenna Port.
Front end interference rejection filter: 0.5 dB BW: 700 kHz, 30 dB BW: 50
MHz, 60 dB BW: 200 MHz
Signal Detection Capabilities (at 0 dB SNR)
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Signal Description
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Signal Parameters
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Minimum required signal detection, short pulse
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-7.5dBZe at 50 km
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Typical Dectection (for Ze=200*R1.6)
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-10 dBZe at 50 km (rainfall of 0.01mm/hr)
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Minimum required signal detection, long pulse
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-23.0dBZe at 25 km
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Point target detection
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RCS = 4 cm2 at 100 km
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Signal Processor
Signal processor is PC based, with a Linux operating system
Clutter filters: spectral filter, clutter coefficient removal and weather
component restoration
Batch Mode Filtering: DC removal for overlaid echoes in Surveillance Mode
Range increment: 250 m
Azimuth increment: 1 deg and 0.5 deg
The WSR-88D RDA transmits short bursts (pulses) of electromagnetic energy
focused into a 1 degree beam by the antenna. During normal operation, the
antenna and thus the beam rotate continuously according to a prescribed
scanning program, or “volume coverage pattern (VCP).” The VCP directs the
beam through 360 degrees in azimuth about a vertical axis, and range from
0.5 to 19.5 degrees (0.2 to 19.5 degrees at a single coastal location)
above horizontal.
NOTE: The WSR-88D supports the execution of a variety of VCPs. Each VCP is
designed to optimize the likelihood of sampling particular meteorological
phenomena. Execution of a VCP is determined by the observed or expected
meteorological conditions, desired elevation density and data quality
expectations for the base data moments.
RPG
The RDA sends the base data moments (reflectivity, velocity, and spectrum
width) and Dual Pol variables (differential reflectivity, correlation
coefficient and differential phase) to the Radar Product Generator (RPG).
The RPG processes these data to produce base products and executes resident
algorithms to generate real-time, user-requested meteorological and
hydrological products. The RPG also manages all WSR-88D communications, as
well as, product and data distribution.
To optimize the WSR-88D RDA operations and data collection, RPG algorithms
analyze radar data strength, coverage and location to dynamically modify
the active scanning routines and data collection schemes. Additionally, the
RPG provides the human computer interface for WSR-88D command, control and
status monitoring.