location of the targets. This kind of beam is created with
a parabolic reflector, the end of the waveguide being at
the focal point of the parabola.
Radars display targets on the scope as if the targets
are at the center of the beam, even though the target may
have been illuminated by energy that has been scattered
outside the beam.
This means that any object
illuminated sufficiently by the radar energy to return to
the antenna some of that energy will be shown on the
indicators as being directly in front of the antenna, while
it actually maybe several degrees to the side. Although
this sometimes leads to inaccurate interpretation of the
radar-scope information, the problem usually concerns
strong targets fairly close to the radar that are masked in
the ground clutter.
Pulse Length and Pulse Repetition Frequency
The length (h) of a radar pulse in space is
determined by the product of the pulse duration (~) and
the speed of light (c):
For instance, a pulse of l-second duration would
have a length of
Beam Resolution and Target Distortion
Resolution describes the ability of the radar to show
objects separately. There are two distinct resolution
problems:
1. Range resolution—The ability to distinguish
between two targets in the same direction from the radar,
but at different ranges.
2. Beam-width resolution—The ability to
distinguish between two targets at the same range, but
in different directions.
Both resolution problems arise from the fact that the
radar pulse occupies considerable space, and any part of
the pulse may illuminate a target sufficiently for
detection. If two targets are detected at the same time,
the radar will present only one echo on the scope.
Range Effect on Signal Strength
and Echo Definition
The cross-sectional area of the radar beam is
proportional to the range from the radar, becoming
larger as the range increases. Accordingly, the energy
incident on a unit area of the beam cross section
decreases with range, being inversely proportional to the
square of the range.
This is often called range
attenuation, although the term attenuation is more
properly applied to the dissipation of energy by the
medium through which it passes.
Now let’s look at the history of doppler weather
radar, as well as a discussion of principles,
characteristics, and phenomena associated with doppler
radar. Information on doppler radar maybe found in the
Federal Meteorological Handbook No. 11 (FMH-11),
Doppler Radar Meteorological Observations, parts B,
C, and D. Additional information maybe found in The
Doppler Radar Glossary, Thunderstorm Morphology
and Dynamics, and Doppler Radar Principles,
KWXN-5002, KWXN-1005, and KWXN-1002, which
are practical training publications produced by the
United States Air Force Training School at Keesler Air
Force Base, Mississippi.
DOPPLER RADAR
LEARNING OBJECTIVES: Discuss the
history of doppler radar. Recognize
velocity-aliased data, range-folded data, and
ground clutter and assess their impact on radar
interpretation. Evaluate doppler velocity and
wind shear patterns.
Interpret radar
presentations of cloud layers and the bright
band.
In the following we will be discussing a brief history
of Doppler Radar from the first real-time Doppler
display in 1967, to the present day Weather Surveillance
Radar 1988 – Doppler (WSR-88D).
HISTORY
In 1967, the first simultaneous observations of
atmospheric flow patterns by two doppler radars were
made. This was performed in central Oklahoma by the
National Severe Storms Laboratory (NSSL) and Cornell
Aeronautical Laboratory and concurrently in England
by the Royal Radar Establishment. Data in these studies
was stored in real time and analyzed later. At about the
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