NEXRAD Technical Information

The NEXRAD, also known as the Weather Surveillance Radar, 1988 Doppler (WSR-88D) is the most advanced operational weather radar in the world; it gives meteorologists the ability to see "inside" a storm. Using the NEXRAD, forecasters can predict the weather with the aid of data such as reflectivity, velocity, and spectrum width of anatmospheric disturbance. Since this data was unavailable in the past the NEXRAD has the capability to save more lives than radars of the past.  Pictured to the left is the tower, which houses the antenna (inside the radome).

There are three major components of the radar (RDA, RPG, and PUP):

RDA

The RDA (Radar Data Acquisition) is the information gatherer of the system, and 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: 750 kw peak at klystron output
Transmitter to antenna loss: site dependent, 2 dB typical
Average Power: 300 to 1300 watts
Pulse Widths: 1.57 and 4.5 microseconds (-6 dB points)
PRF short pulse: 318 to 1304 Hz
PRF long pulse: 318 to 452 Hz
Phase noise (system): -54 dBc required, -60 dBc typical
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: center fed paraboloid of revolution 28 feet in diameter
Polarization: linear horizontal
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	15 deg/sec2	15 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, instantaneous automatic gain control and matched filtering, second convert for synchronous detection (I and Q input to A/D Converter)
dynamic Range: 95 dB
Intermediate Frequency: 57.55 MHz
3 dB bandwidth: 0.630 MHz
6 dB bandwidth: 0.798 Mhz
System noise figure: 4.6 dB (540 Kelvin)
Receiver Noise: -113 dBm ref to antenna
Front end interference rejection filter: 0.5 dB BW: 700 kHz, 30 dB BW: 50 MHz, 60 dB BW: 200 MHz
Optional interference detection, log amplifier based
 

·         Signal Processor

A/D Converter sample interval: 1.66 microsec, 602 kHz (approx)
A/D Converter number of bits: 12
Clutter Filter: infinite impulse response (5 pole elliptic)
suppression: 30 to 50 dB, user selectable
notch half widths: 0.5 to 4 m/sec
range increment: 250 m
azimuth increment: 1 deg

RPG

The RPG (Radar Products Generator) takes base data (reflectivity, velocity, and spectrum width) from the RDA and generates user-requested meteorological and hydrological products.
 

·         Reflectivity Base Product

Reflectivity computation: linear average return power
Reflectivity estimate standard deviation: less than 1 dB typical
Number of pulses averaged: 6 to 64
range increment: 1000 m
azimuth increment: 1 deg
Max range for reflectivity: 460 km

 

Signal Detection Capabilities (at 0 dB SNR)
Signal Description	Signal Parameters
Minimum required signal detection, short pulse	-7.5dBZe at 50 km
Typical Detection (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

 ·         Velocity Base Product

velocity computation: complex covariance argument (pulse pair estimator)
velocity estimate standard deviation: less than 1 m/sec (at spectrum width of 4 m/sec)
number of pulses averaged: 40 to 200
range increment: 250 m
azimuth increment: 1 deg
max range for velocity: 230 km
 

·         Spectrum Width Base Product

Spectrum width computation: autocorrelation, single lag
Spectrum width estimate standard deviation: less than 1 m/sec (at spectrum width of 4 m/sec)
Number of pulses averaged: 40 to 200
Range increment: 250 m
Azimuth increment: 1 deg
Max range for spectrum width: 230 km

PUP

The PUP (Principal User Processor) is the piece of equipment which brings all the products to the meteorologist. Using a graphic tablet and a magnetic puck, the meteorologists have the ability to display whatever products they wish on dual 20 inch color monitors.

Note: Transmitter, Antenna/Pedestal, Receiver, Signal Processor, Reflectivity Product, Velocity Product, and Spectrum Width Product characteristics were prepared by ROC Engineering Branch, May 29, 1998