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PRINCIPLES OF DOPPLER RADAR
RADIAL VELOCITY

Aerographers Mate, Module 03-Environmental Satellites and Weather Radar
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frequency  shift,  giving  the  motion  and  speed  of  the target. While frequency of electromagnetic energy is modified by moving targets, the change is usually too slight to measure precisely. Therefore, Doppler radar focuses on the phase of electromagnetic energy, as this aspect  experiences  a  greater  degree  of  displacement and  increases  the  likelihood  of  detecting  motion. Using phase shifts instead of frequency changes can be compared to viewing an insect under a magnifying glass. Features that might otherwise go unseen take on a new dimension and become observable. PHASE SHIFT The  phase  of  a  wave  is  a  specific  point  or benchmark  along  that  wave.  A  phase  shift  is  an observable  repositioning  of  this  benchmark  between successive transmissions. A pulse Doppler radar, in its simplest form, provides a reference signal by which changes  in  the  frequency  phase  of  successively received pulses may be recognized. The known phase of the transmitted signal allows measurement of the phase  of  the  received  signal.  The  Doppler  shift associated  with  the  echo  from  which  the  return originated is calculated from the time rate of change of phase. The phase of a wave, measured in degrees, where 360 degrees equals one wavelength, indicates the current position of the wave relative to a reference position. For example, look at figure 2-18. At time T1 (fig. 2-18, view A), the position of the wave along the vertical line was as shown, while at time T2 (fig. 2-18, view B), the position of the wave along the vertical line was as shown. Notice that the wavelength did not change from T1 to T2. However, the wave’s position relative to the vertical line changed 1/4 wavelength, or 90 degrees. This change is the phase shift. Figure 2-18.—Wavelengths and phase shifts. (A) T-l is wave reference position. (B) T-2 wave’s position has changed 90° from reference  position  (T-l). 2-17







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