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SPECIFICATIONS OF NEXRAD
There are three major components of the NEXRAD radar: The
Radar Data Acquisition (RDA), the Radar Products Generator
(RPG), and the Principal User Processor (PUP).
RDA - Radar Data Acquisition
The RDA is the information gatherer of the system--essentially the radar itself--and is
composed of four primary components:
- Transmitter
- 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
- 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 - Radar Products Generator
The RPG 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 - Principal User Processor
The PUP 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.
Sources:
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