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RADAR  PULSE  CHARACTERISTICS
RADAR BEAM CHARACTERISTICS - 14271_60

Aerographers Mate, Module 03-Environmental Satellites and Weather Radar
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Range Ambiguity As  described  earlier,  the  pulse  repetition frequency largely determines the maximum range  of the radar set. If the period between successive pulses is too short, an echo from a distant target may return after the transmitter has emitted another pulse. This would make it impossible to tell whether the observed pulse is the echo of the pulse just transmitted or the echo of the preceding pulse. This produces a situation referred to as range ambiguity. The radar is unable to distinguish between pulses, and derives range information that is ambiguous  (unreliable). In theory, it is best to strike a target with as many pulses of energy as possible during a given scan. Thus, the higher the PRF the better. A high PRF improves resolution and range accuracy by sampling the position of the target more often. Since PRF can limit maximum range,   a   compromise   is   reached   by   selectively increasing the PRF at shorter ranges to obtain the desired  accuracy  of  measurements. The maximum unambiguous range (Rmax) is the longest range to which a transmitted pulse can travel and  return  to  the  radar  before  the  next  pulse  is transmitted. In other words, Rmax is the maximum distance  radar  energy  can  travel  round  trip  between pulses and still produce reliable information. The relationship  between  the  PRF  and  Rmax  determines the unambiguous range of the radar. The greater the PRF (pulses per second), the shorter the maximum unambiguous  range  (Rmax)  of  the  radar.  The maximum unambiguous range of any pulse radar can be  computed  with  the  formula:  Rmax  =  c/(2xPRF), where c equals the speed of light (186,000 miles per second). Thus, the maximum unambiguous range of a radar with a PRF of 318 would be 292 miles (254 nmi), 186,000/2 x 318 = 292. The factor of 2 in the formula Figure 2-8.—Radar range folding. accounts for the pulse traveling to the target and then back to the radar. Range Folding While it’s true that only targets within a radar’s normal  range  are  detected,  there  are  exceptions. Occasionally, a pulse strikes a target outside of normal range and returns during the next pulse’s listening time. This poses a complex problem known as range folding. Range  folding may cause operators to base crucial decisions on false echoes. The data received from this stray pulse could be misanalyzed and echoes may be plotted  where  nothing  exists.  The  data  may  look reliable and the radar may appear to be functioning properly, adding to the deception of normal operation. Refer to figure 2-8. Assume a pulse was emitted during   the  radar’s  previous  scan.  While  it  travels beyond normal range and strikes a target, the radar emits a second pulse. Since no targets exist within normal radar range, these pulses will pass each other in flight. The first pulse now returns while the radar is expecting the second pulse (during the listening time of the second pulse). The radar believes that the second pulse has struck a target 124 nmi from the antenna and displays.  an  echo  accordingly  (target  "X").  The operator is fooled by target "X" and issues a severe weather warning, when in fact, no clouds are present. Target  "X"  was  an  illusion,  a  reflection  of  a thunderstorm  located  372  nmi  from  the  antenna. Fortunately, the WSR-88D is equipped with a range unfolding mechanism that attempts to position all echoes properly. Pulse Volume As pulses travel they look like a cone with its point cut off (fig. 2-9). They expand with the beam and increase in volume. The volume of a pulse is the space Figure 2-9.—Radar pulse volume. Pulse volume increases with distance  from  the  antenna  as  the  pulse  expands  in  all directions. 2-7







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