Nexrad: Next Generation Weather Radar (WSR-88D)*
Nexrad, officially designated the WSR-88D (weather surveillance radar-1988, Doppler), is the second generation, operational meteorological radar. It will replace the non-Doppler meteorological radars of the National Weather Service, the Air Weather Service and the auxiliary weather capability of the Federal Aviation Administration's terminal radars.
The WSR-88D represents a major step from the earlier meteorological radars both in engineering technology and in meteorological measurements. As a fully coherent Doppler radar, it provides not only accurate reflectivity measurement and its attendant information on spatial location and distribution, but also measurement of the radial component of scatter motion and the velocity dispersion in the radar sample volume (spectrum width). The velocity information provides the meteorologist with a direct measure of an important storm dynamic parameter and enables recognition of a wide variety of severe storm features by an associated unique signature in the velocity field. 
Nexrad System Description
Physically, a WSR-88D unit is composed of functional groups, shown in Figure 1, including a radar data acquisition (RDA), a radar product generation (RPG) and one or more principal user processing (PUP) stations. The groups are inter-connected by communication lines and may be at separate sites. The figure also indicates the unit interfaces.
For descriptive purposes, the WSR-88D unit can be divided into two parts; the radar subsystem that is composed of an antenna, a transmitter, a receiver and ground clutter cancelers, and a dedicated signal processor; and a data analysis and display that is composed of the radar control processor, meteorological analysis and product generation processor, display processor, and associated communication ports and color displays.
The radar is a coherent chain transmitter S-band design, shown in Figure 2. Coherence is maintained by very stable oscillators or signal sources that operate continuously. These sources are used as the reference in extracting the Doppler shift of the return signal, which is proportional to the radial motion of the target from which the transmitter signal is backscattered. The trasmitter signal is generated at a power level of a few hundred mW and amplified to 750 kW by intermediate solid-state devices and a klystron amplifier. The klystron, the only vacuum tube device in the system, provides high gain amplification (53 dB) with negligible signal distortion or spurious signal emission. The antenna is a center-fed parabolic reflector having a diameter of 28 ft. and has a mainlobe one way 3 dB beamwidth of 0.95[degrees]. The receiver uses a frequency mixer to downconvert the received signal to an intermediate signal carrier at which most amplification, matched filtering and automatic gain control (AGV), are performed. A second frequency conversion is a synchronous detection that retains received signal amplitude and phase information. At this point, the signal has power proportional to echo radar reflectivity, and frequency equal to the Doppler shift. For convenience, the signal is decomposed into in-phase and quadrature components.
The digitized signals are compensated for AGC attenuation, converted to floating point format and passed to the clutter filters. The...
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