same time, the first real-time doppler radar display, The
Plan Shear Indicator, was developed by the Air Force
Cambridge Research Laboratories.
In the early 1970s, a unique new class of
high-powered, sophisticated S-band doppler weather
radar appeared, incorporating integrated circuitry and
advanced computer processing. Two were built by the
NSSL. Others were built by the National Center for
Atmospheric Research and by the University of Chicago
in cooperation with the Illinois State Water Survey in
Champaign, Illinois. By the mid 1970s, technological
advances allowed real-time processors to be linked to
color displays.
Joint Doppler Operations Project (JDOP)
Studies conducted by NOAA Environmental
Research Laboratories for the National Weather Service
(NWS) during 1975 and 1976 showed that it was not
feasible to convert existing network radars to suitable
doppler systems. As a result, the historic Joint Doppler
Operations Project (JDOP) was conducted from 1977 to
1979 at the NSSL by the Air Force, the Federal Aviation
Administration (FAA), and the NWS. This was the true
birthplace of the WSR-88D.
JDOP demonstrated the meteorological utility of
operational doppler radars and also benchmarked the
engineering requirements for the NEXRAD. The
NEXRAD program formally began with the
establishment of the Joint System Program Office
(JSPO) at NWS headquarters in 1979. Since this time,
NEXRAD has been officially renamed the (WSR-88D).
A detailed discussion of the WSR-88D PUP and its
capabilities is beyond the scope of this text. For a
detailed discussion of capabilities and procedures
refer to the technical manual, Operation Instructions
Principal User Processor (PUP) Group/
Doppler Meteorological Radar WSR-88D,
NAV EM400-AF-OPI-010/WSR-88D.
We will now discuss velocity-aliased data, followed
by a discussion on it’s recognition.
VELOCITY ALIASED DATA
Doppler radar uses the change in frequency between
the outgoing signal and the returning signal (doppler
shift) to determine radar velocities. However,
limitations in velocity measurements do exist. We now
will take a look at an example to see what can go wrong.
Trains are designed to go as fast forward as in
reverse.
The speedometer shows both forward and
reverse speed. Refer to figure 12-4 throughout this
discussion. Direct reading of speeds between 0-49 mph,
whether forward or reverse is quite easy. As with a
train’s speedometer, radar has limits to speeds that it can
measure without error. These speeds that can be
measured without error are known as Unambiguous
Velocities.
In part (a), the train is traveling at 40 mph in a
forward direction.
The speedometer indicates the
correct speed. In part (b), the speed of the train has
increased by 20 mph, so that the train is now traveling
at 60 mph, but, the speedometer indicates 40 mph in
reverse. The maximum forward speed was exceeded on
Figure 12-4.-Speedometer.
12-4
