location of the targets. This kind of beam is created with a parabolic reflector, the end of the waveguide being at the focal point of the parabola. Radars display targets on the scope as if the targets are at the center of the beam, even though the target may have been illuminated by energy that has been scattered outside   the   beam. This   means   that   any   object illuminated sufficiently by the radar energy to return to the antenna some of that energy will be shown on the indicators as being directly in front of the antenna, while it actually maybe several degrees to the side. Although this  sometimes  leads  to  inaccurate  interpretation  of  the radar-scope information, the problem usually concerns strong targets fairly close to the radar that are masked in the ground clutter. Pulse Length and Pulse Repetition Frequency The  length  (h)  of  a  radar  pulse  in  space  is determined by the product of the pulse duration  (~) and the speed of light (c): For instance, a pulse of l-second duration would have a length of Beam Resolution and Target Distortion Resolution describes the ability of the radar to show objects separately. There are two distinct resolution problems: 1. Range resolution—The ability to distinguish between two targets in the same direction from the radar, but at different ranges. 2.   Beam-width   resolution—The   ability   to distinguish  between  two  targets  at  the  same  range,  but in  different  directions. Both resolution problems arise from the fact that the radar pulse occupies considerable space, and any part of the  pulse  may  illuminate  a  target  sufficiently  for detection. If two targets are detected at the same time, the radar will present only one echo on the scope. Range Effect on Signal Strength and Echo Definition The  cross-sectional  area  of  the  radar  beam  is proportional to the range from the radar, becoming larger  as  the  range  increases.  Accordingly,  the  energy incident  on  a  unit  area  of  the  beam  cross  section decreases with range, being inversely proportional to the square  of  the  range. This  is  often  called  range attenuation, although the term  attenuation is  more properly applied to the dissipation of energy by the medium  through  which  it  passes. Now let’s look at the history of doppler weather radar,   as   well   as   a   discussion   of   principles, characteristics,  and  phenomena  associated  with  doppler radar. Information on doppler radar maybe found in the Federal Meteorological Handbook No. 11 (FMH-11), Doppler Radar Meteorological Observations, parts B, C, and D. Additional information maybe found in  The Doppler Radar Glossary, Thunderstorm Morphology and   Dynamics,    and  Doppler   Radar   Principles, KWXN-5002, KWXN-1005, and KWXN-1002, which are practical training publications produced by the United States Air Force Training School at Keesler Air Force  Base,  Mississippi. DOPPLER  RADAR LEARNING  OBJECTIVES:  Discuss   the history  of  doppler  radar.  Recognize velocity-aliased data, range-folded data, and ground clutter and assess their impact on radar interpretation.  Evaluate  doppler  velocity  and wind   shear   patterns. Interpret   radar presentations of cloud layers and the bright band. In the following we will be discussing a brief history of  Doppler  Radar  from  the  first  real-time  Doppler display in 1967, to the present day Weather Surveillance Radar  1988  –  Doppler  (WSR-88D). HISTORY In  1967,  the  first  simultaneous  observations  of atmospheric flow patterns by two doppler radars were made. This was performed in central Oklahoma by the National  Severe  Storms  Laboratory  (NSSL)  and  Cornell Aeronautical  Laboratory  and  concurrently  in  England by the Royal Radar Establishment. Data in these studies was stored in real time and analyzed later. At about the 12-3