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NEXRAD Technical Information

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).

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

Pedestal Function

Azimuth

Elevation

Steerability

360 deg

-1 to +45 deg

Normal Scan

360 deg

+0.5 to +19.5

Max rotation rate

30 deg/sec

30 deg/sec

Acceleration

30 deg/sec2

30 deg/sec2

Mechanical Limits

360 deg

-1 to +60 deg

Positioning Error (max)

+/-0.2

+/-0.2

Pedestal Type: Elevation over Azimuth

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)

Signal Description

Signal Parameters

Minimum required signal detection, short pulse

-7.5dBZe at 50 km

Typical Dectection (for Ze=200*R1.6)

-10 dBZe at 50 km (rainfall of 0.01mm/hr)

Minimum required signal detection, long pulse

-23.0dBZe at 25 km

Point target detection

RCS = 4 cm2 at 100 km

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.