have to be approximately 810 pulses per second. This
would cause the maximum unambiguous velocity to
decrease to 45 mph.
The Doppler Dilemma is caused by physical
restrictions based on the laws of nature. One of the
ways the WSR-88D works around this dilemma is to
operate at variable PRFs, collecting reflectivity
information at low PRFs and velocity information at
high PRFs. The two sets of information collected are
compared and processed to estimate true radial
velocities and ranges.
Range Versus Height
The WSR-88D samples through 360° at a series of
fixed elevation angles. At each elevation angle, the
distance outward along the radar beam represents an
increase in height above the ground. In other words,
the further you move away from the antenna, the
higher the beam is off the ground. You must remember
that the very center of a product display represents only
the height of the radar tower, while the outer edges of
the display are thousands of feet above the ground.
You can now see how the WSR-88D gives you a
pseudo three-dimensional display.
Q33.
Q34.
Q35.
Q36.
Q37.
Q38.
Q39.
Q40.
Q41.
Q42.
Q43.
REVIEW QUESTIONS
Doppler is used to measure what property?
What are some of the advantages of the WSR-
88D over conventional weather radars?
What happens to a return frequency when a
target moves toward a radar?
What is a "phase shift "?
The phase of a wave is measured in what units?
Which three measurements are required to
compute radial velocity?
How is the detected velocity of a target affected
as the target moves more perpendicular to a
Doppler radar antenna?
What is "velocity aliasing"?
What is meant by the term "Nyquist velocity"?
What would be the Nyquist velocity of a WSR-
88D radar operating with a pulse repetition
frequency (PRF) of 1100?
The "Doppler dilemma" is a combination of
what two difficulties?
WSR-88D SYSTEM FUNDAMENTALS
LEARNING OBJECTIVES: Identify the major
components of the WSR-88D. Identify the
purpose of the RDA, RPG, UCP, and PUP.
Recognize the various WSR-88D system users
and their capabilities. Distinguish between
wideband and narrowband communication
links. Recognize the basic configuration of the
National Weather Radar Network. Identify the
various data archive levels. Identify the major
volume coverage patterns of the WSR-88D.
The following text provides an overview of the
WSR-88D system layout, communication
configuration, and data flow. The WSR-88D is much
more than just a radar-it is actually a carefully
integrated system of basic radar components,
sophisticated computer hardware and software, and a
unique communications network. Minimum system
configuration includes four major components: an
RDA (Radar Data Acquisition), an RPG (Radar
Product Generator), a UCP (Unit Control Position) and
a PUP (Principal User Processor). While these
components might be separated by many miles, they
remain linked to each other through an intricate
communication network (fig. 2-25).
RADAR DATA ACQUISITION (RDA)
The RDA consists of an antenna and all
subcomponents necessary to process backscattered
(reflected) energy into useful radar information. The
RDA is considered to be the radar’s eyes and ears
because it provides a detailed snapshot of the radar’s
surrounding environment. It strategically scans
thousands of square miles, and controls transmission
and receipt of all radar energy. Any accumulation of
data is then passed on to the RPG for in-depth analysis.
Radar Moments
Since all atmospheric scatterers provide valuable
information, the ability to see them is useful in
forecasting, observing, and warning weather
customers. Obviously we want to see as many of these
scatterers as possible. The RDA is responsible for this
detection process. The RDA performs signal
processing of Doppler weather radar data and transfers
this data based upon three radar moments. A radar
moment is a measurement of a scatterer’s reflectivity,
velocity, and/or spectrum width at a specific period in
time.
2-22
