The display 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 display 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.
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 is 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:
- The greatest return potential exists when the radar beam forms a horizontal right angle with the frontal portion of the reflector.
- Radar return potential is roughly proportional to the target size and the reflective properties (density) of the target.
- Radar return potential is greatest within the zone of the greatest radiation pattern of the antenna.
- 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.)
- 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.