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the sweep relative to the size of the target. Since the sweep is linear and begins with the emission of the
transmitted pulse, the point at which the echo brightens the sweep will be an indication of the range to
the object causing the echo.
7.3.11. The progressive positions of the pulse in space also indicate the corresponding positions of the
electron beam as it sweeps across the face of the CRT. If the radius of the scope represents 40 miles and
the return appears at three-quarters of the distance from the center of the scope to its periphery, the
target is represented as being about 30 miles away.
AFPAM11-216 1 MARCH 2001 177
7.3.12. In the preceding example, the radar is set for 40-mile range operation. The sweep circuits will
thus operate only for an equivalent time interval so that targets beyond 40 miles will not appear on the
scope. The time equivalent to 40 miles of radar range is only 496 microseconds (496 X 10-6 seconds).
Thus, 496 microseconds after a pulse is transmitted (plus an additional period of perhaps 100
microseconds to allow the sweep circuits to recover) the radar is ready to transmit the next pulse. The
actual pulse repetition rate in this example is about 800 pulses per second. The return will, therefore,
appear in virtually the same position along the sweep as each successive pulse is transmitted, even
though the aircraft and the target are moving at appreciable speeds.
7.3.13. At times, the PPI will not display targets across the entire range selected on the scope. In these
cases, atmospheric refraction and the line of sight (LOS) characteristics of radar energy have affected
the effective range of the set. The following formula can determine the radar's range in these situations
where D is distance and h is the aircraft altitude:
7.3.14. Azimuth measurement is achieved by synchronizing the deflection coil with the antenna. In the
basic radar unit, when the antenna is pointed directly off the nose of the aircraft, the deflection coils are
aligned to fire the trace at the 12 o'clock position on the scope. As the antenna rotates, the deflection coil
moves at the same rate. Relative target presentations are displayed as the sweep rotation is combined
with the range display.
Section 7C— Scope Interpretation
7.4. Basics. The PPI presents a map-like picture of the terrain below and around the aircraft. Just as map
reading skill is largely dependent upon the ability to correlate what is seen on the ground with the
symbols on the chart, so the art of scope presentation analysis is largely dependent upon the ability to
correlate what is seen on the scope with the chart symbols. Application of the concept of radar reflection
and an understanding of how received signals are displayed on the PPI are prerequisites to scope
interpretation. Furthermore, knowledge of these factors applied in reverse enables the navigator to
predict the probable radarscope appearance of any area.
7.5. Factors Affecting Reflection. A target's ability to reflect energy is based on the target's
composition, size, and the radar beam's angle of reflection (Figure 7.5). The range of the target from the
aircraft is definitive in the quantity of returned energy. The range of a target produces an inverse effect
on the target's radar cross-section. And there will be some atmospheric attenuation of the pulse
proportional to the distance that the energy must travel. Generally, all four factors contribute to the
displayed return. A single factor can, in some cases, either prevent a target from reflecting sufficient
energy for detection or cause a disproportionate excess of reflected energy to be received and displayed.
The following are general rules of radarscope interpretation:
7.5.1. The greatest return potential exists when the radar beam forms a horizontal right angle with the
frontal portion of the reflector.
7.5.2. Radar return potential is roughly proportional to the target size and the reflective properties
(density) of the target.
178 AFPAM11-216 1 MARCH 2001
Figure 7.5. Relative Reflectivity of Structural Materials.
7.5.3. Radar return potential is greatest within the zone of the greatest radiation pattern of the antenna.
7.5.4. Radar return potential decreases as altitude increases because the vertical reflection angle becomes
more and more removed from the optimum. (There are many exceptions to this general rule since there
are many structures that may present better reflection from roof surfaces than from frontal surfaces or in
the case of weather.)
7.5.5. Radar return potential decreases as range increases because of the greater beam width at long
ranges and because of atmospheric attenuation.
NOTE: All of the factors affecting reflection must be considered to determine the radar return potential.
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