PHASE SHIFT

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