the speedometer, therefore, speeds of 0-49 mph are unambiguous. Radar speeds that exceed the maximum unambiguous  velocity  are  said  to  be  aliased,  and referred to as aliased  velocities. Velocity aliasing occurs when frequencies too high to  be  analyzed  with  the  given  sampling  interval  appear as a frequency less than the Nyquist frequency (the highest frequency that can be determined in data that has been sampled). In other words, wind speed greater than the  unambiguous  velocity  (Nyquist  co-interval  [the entire range of detectable velocities]) for the current pulse  repetition  frequency  (PRF)  are  wrapped  around into  the  incorrect  Nyquist  co-interval.  A  sophisticated velocity-dealiasing technique is implemented in the WSR-88D  (referenced  in  part  D  of  the  FMH-11), However, it is expected that improperly dealiased data will  occasionally  occur. Recognition of Velocity-Aliased Data Data that are incorrectly dealiased can be difficult to  ascertain.  However,  understanding  the  limitations  of the  algorithm  should  help  to  recognize  improperly dealiased  data. If the suspected data is in an area isolated from other data and there is a large variation in the subsynoptic or mesoscale wind field, the algorithm may not be able to initially  assign  the  correct  velocity.  Other  instances  of incorrect dealiasing may occur when there are shifts in the inward- and outward-bound velocities along the radials of data that do not fit those allowed by the algorithm. In these cases, the actual values maybe off by a factor of twice the unambiguous velocity of the PRF in use at the time. Typical unambiguous velocities for the WSR-88D, in the Precipitation mode, range from 40 to 60 kts. Occasionally, groupings of data appear along a small set of ranges that could not be successfully dealiased.  These  should  be  obvious. Assessing Impacts of Velocity Aliasing Incorrectly dealiased velocity data can seriously impact  certain  WSR-88D  algorithms  and  products. Mean  Radial  Velocity  products  will  be  difficult  to analyze when contaminated with incorrectly dealiased data. To determine the extent of the dealiasing problem, it is recommended that earlier displays of these products be examined to determine if there is time or space continuity. In addition, other elevation angles of the Mean   Radial   Velocity   products   may   be   used   to determine if there is vertical continuity. Algorithms  and  products  that  ingest  mean  radial velocity data can output incorrect results when such data are  used.  In  the  case  of  the  mesocyclone  detection algorithm,  there  will  likely  be  a  lack  of  vertical continuity of incorrectly dealiased data. Consequently, only  uncorrelated  shears  should  result  from  using aliased data. In the rare event of a tornadic vortex signature  being  output  in  the  vicinity  of  an  identified mesocyclone  because  of  vertical  continuity  of incorrectly dealiased data, other products should be examined  to  verify  the  existence  of  a  severe thunderstorm. It is expected that incorrectly dealiased data will not have  a  large  impact  on  Combined  Shear  products because of the amount of averaging of data done by the algorithm. RANGE-FOLDED  DATA Second trip echoes (range folding) occurs when the radar hears a previous pulse, while listening for the most recent  pulse. Due to the sensitivity and narrow beam width of the WSR-88D, precipitation echoes beyond 250 nmi will occasional y appear in closer range due to range folding. However, far more significant are the range ambiguities in the doppler velocity and width fields caused by the WSR-88D’S  pulsed  doppler  sampling  interval (referenced  in  part  B  of  the  FMH-11).  For  any  pulsed doppler system, the product of the unambiguous range and  the  doppler  Nyquist  interval  is  a  constant  function of the wavelength of the radar and the speed of light. Decreasing the PRF allows for a longer listening time, thus  increasing  the  unambiguous  range,  but,  this  lower PRF  creates  a  problem  in  determining  radial  velocities. THE DOPPLER DILEMMA High  PRFs  are  required  for  high  velocity measurements and low PRFs are required for long ranges.  The  solution  to  this  dilemma  lies  in  finding  a balance between the effects of velocity aliasing and range  folding.  This  dilemma  is  caused  by  physical restrictions based on the laws of nature. To solve this dilemma, the WSR-88D will use several methods to work  around  these  restrictions.  One  method  is  to operate  at  variable  PRFs;  the  second  is  to  collect refractivity  information  at  low  PRFs  and  velocity information at high PRFs. The two sets of information collected  are  compared,  then  processed  to  estimate  true radial  velocities  and  ranges. 12-5