270° OF PHASE SHIFT (AWAY) SEENAS 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° ofphase shift (away) SEEN AS 90° of phase shift (toward).intervals are those velocities from zero up to andincluding the Nyquist velocity. The Nyquist co-interval is the entire range of detectable velocities bothnegative and positive. For example, if the Nyquistvelocity is 25 knots, then the Nyquist interval is anyvelocity from 0-25 knots, and the Nyquist co-intervalis -25 through +25 knots.Since the WSR-88D has a fixed wavelength of10.7 cm, we can compute the Nyquist velocity (V_{max})for any given PRF from the following formula: V_{max} =(PRF) x (Wavelength) ÷ 4. For example, a WSR-88Dradar operating with a PRF of 1000 would have aNyquist velocity of 52 knots. This is found bymultiplying 1000 by 10.7 and dividing by four (2675cm/sec). This can then be converted to 26.75 metersper second (100 centimeters in a meter). Multiply thisvalue by 1.94 to convert to knots. From this formula,you can see that higher PRFs yield higher maximumdetectable velocities, and that lower PRFs willincrease the chances of velocity aliasing.Doppler DilemmaWe learned earlier that Doppler radar is subject torange folding. This resulted when the radar detected aprevious pulse while listening for the most recentpulse. Reducing the pulse repetition frequency (PRF)and allowing for a longer listening time will alleviatethe problem of range folding. However, as justdiscussed, low PRFs may then lead to the problem ofvelocity aliasing. These two difficulties combine todefine what is known as the Doppler dilemma. Forexample, in order for the WSR-88D to detect radialvelocities of 200 mph without aliasing, the PRF wouldhave to be increased to about 4,000 pulses per second.However, this would reduce the maximumunambiguous range of the radar to about 20 nmi. Tohave an unambiguous range of 100 nmi, the PRF would2-21

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