270° OF PHASE SHIFT (AWAY) SEEN AS 90° OF PHASE SHIFT (TOWARD)
Figure 2-24.—Several phase shifts. (A) 90° of phase shift (away), (B) 90° of phase shift (toward), (C) 180° of phase shift, (D) 270° of
phase shift (away) SEEN AS 90° of phase shift (toward).
intervals are those velocities from zero up to and
including the Nyquist velocity. The Nyquist co-
interval is the entire range of detectable velocities both
negative and positive. For example, if the Nyquist
velocity is 25 knots, then the Nyquist interval is any
velocity from 0-25 knots, and the Nyquist co-interval
is -25 through +25 knots.
Since the WSR-88D has a fixed wavelength of
10.7 cm, we can compute the Nyquist velocity (Vmax )
for any given PRF from the following formula: Vmax =
(PRF) x (Wavelength) ÷ 4. For example, a WSR-88D
radar operating with a PRF of 1000 would have a
Nyquist velocity of 52 knots. This is found by
multiplying 1000 by 10.7 and dividing by four (2675
cm/sec). This can then be converted to 26.75 meters
per second (100 centimeters in a meter). Multiply this
value by 1.94 to convert to knots. From this formula,
you can see that higher PRFs yield higher maximum
detectable velocities, and that lower PRFs will
increase the chances of velocity aliasing.
Doppler Dilemma
We learned earlier that Doppler radar is subject to
range folding. This resulted when the radar detected a
previous pulse while listening for the most recent
pulse. Reducing the pulse repetition frequency (PRF)
and allowing for a longer listening time will alleviate
the problem of range folding. However, as just
discussed, low PRFs may then lead to the problem of
velocity aliasing. These two difficulties combine to
define what is known as the Doppler dilemma. For
example, in order for the WSR-88D to detect radial
velocities of 200 mph without aliasing, the PRF would
have to be increased to about 4,000 pulses per second.
However, this would reduce the maximum
unambiguous range of the radar to about 20 nmi. To
have an unambiguous range of 100 nmi, the PRF would
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