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