GROUND   CLUTTER   FILTERING.—The process of reducing the effects of ground clutter begins as the data (radar returns received at the RDA) leaves the   RDA’s   signal   processor.   A   ground   clutter suppression algorithm is then applied. This algorithm is designed to suppress power (reduce reflectivity) from echoes that return a lot of energy but are not moving (buildings, trees etc.). Two clutter suppression maps are created during this process: one map is used by  the  RDA  to  determine  which  echoes  are  of  a permanent nature (buildings, trees), and a second map that is used by the operator to select areas for specific clutter suppression. Once built, these maps can be referenced by the radar to apply suppression to clutter that has been identified with each new scan of the radar.  Obviously,  this  process  increases  product reliability. RADAR PRODUCT GENERATOR (RPG) The RPG is considered the "brains" of the WSR- 88D. It is a bulky mainframe computer, housed in what resembles an oversized wall locker (fig. 2-28). An alphanumeric input terminal allows user interface. The RPG   creates   all   WSR-88D   radar   products   by performing  sophisticated  analyses  of  data  through  a multitude  of  computer  programs  called  algorithms. The  RPG  provides  hundreds  of  new  and  unique products every 5 to 10 minutes. Because of the vast quantities   available,   products   themselves   are categorized as either base or derived products. Base products  are  near  real-time  images.  They  display targets as seen by the radar during its most recent scan. Derived products, on the other hand, are specialty products. Derived products evolve from base products that  have  been  modified  or  enhanced  to  produce special  results.  Derived  products  are  often  used specifically to examine features that are not easily seen on base products. We will discuss the more important base and derived products later. Once  products  are  built,  the  RPG  stores  and distributes  them  according  to  predefined  system commands. In some cases the RPG is instructed to generate  audible  alarms  (alerts)  when  special  weather conditions  are  detected.  Once  alerted,  users  may examine conditions more thoroughly by using all available products. Conditions that trigger these alerts are user definable and can be tailored to meet the mission   of   your   base.   Strong   wind   velocities, rotational  shear,  or  intense  reflectivity  values  (from thunderstorms)  are  among  the  more  popular  criteria used to trigger alerts. The RPG may be located many miles from the RDA, but remains critical to the production process. Without  it,  NO  radar  products  are  available.  It  is important to mention the links that exist between these components.   Considering   the   volume   of   data collected, the communications link between the RPG and RDA is crucial. Without it, the system is non- functional. For this reason, the RDA and RPG are connected via a "wideband" communications link, which acts as an information superhighway on which massive volumes of data can flow freely (refer to fig. 2-25). Like the RDA, only a single RPG exists for each WSR-88D system. It supports all users who subscribe for products of that system. PRINCIPAL USER PROCESSOR (PUP) Up to this point we’ve discussed the RDA (which transmits and receives radar energy) and the RPG (which builds products from that energy). Now let’s discuss how you will gain access to these products. The PUP  is  a  sophisticated  computer  workstation  that allows human interface with the WSR-88D (fig. 2-29). Here,  the  observer  or  forecaster  can  display  or manipulate products freely and perform such functions as zooming (to study tornadic activity), or screen roaming (to examine other weather features). Products can also be time-lapsed. This allows the PUP operator to extrapolate movement of features. Products may be stored locally on the PUP’s hard disk, or sent to the printer for a color hard copy. PUPS are connected to the RPG via a narrow band (ordinary telephone lines). Like other components, PUPS may be geographically separated from the host system (RDA/RPG) by many miles. A minimum of one PUP  must  exist  to  constitute  a  complete  system. However, since the WSR-88D is designed to support several  users,  numerous  PUPS  can  be  configured  to access a single RPG; the actual number of users varies. Each  PUPS  distance  from  their  respective  RPG  depends solely  on  their  geographical  location.  Common  PUP sites  include  military  installations,  National  Weather Service   forecast   centers,   Federal   Aviation Administration  airport  offices,  and  the  Air  Force Weather  Agency  (AFWA)  to  name  a  few.  PUP hardware includes the following: 2-27