Cloud returns that appear on the scope are of interest for two reasons. First, since the brightness of a given cloud return is an indication of the intensity of the weather within the cloud, intense weather areas can be avoided by directing the pilot through the areas of least intensity or by circumnavigating the entire cloud return. [Figure 7-8] Second, cloud returns obscure useful natural and cultural features on the ground. They may also be falsely identified as a ground feature that can lead to gross errors in radar fixing. Clouds must be reasonably large to create a return on the scope. However, size alone is not the sole determining factor.
The one really important characteristic that causes clouds to create radar returns is the size of the water droplets forming them. Radar waves are reflected from large rain droplets and hail that fall through the atmosphere or are suspended in the clouds by strong vertical air currents. Thunderstorms are characterized by strong vertical air currents; therefore, they give very strong radar returns. Cloud returns may be identified as follows:
- Brightness varies considerably, but the average brightness is greater than a normal ground return.
- Returns generally present a hazy, fuzzy appearance around their edges.
- Returns often produce shadow areas similar to mountain shadows, because the radar beam does not penetrate clouds completely.
- Returns do not fade away as the antenna tilt is raised, but ground returns do tend to decrease in intensity with an increase in antenna tilt.
- Returns can appear in the altitude hole when altitude delay is not used and the distance to the cloud is less than the altitude.
Effects of Snow and Ice
The effects of snow and ice are similar to the effect of water. If a land area is covered to any great depth with snow:
- Some of the radar beam reflects from the snow, and
- Some of the energy is absorbed by the snow.
The overall effect is to reduce the return that would normally come from the snow-blanketed area.
Ice reacts in a slightly different manner, depending upon its roughness. If an ice coating on a body of water remains smooth, the return appears approximately the same as a water return. However, if the ice is formed in irregular patterns, the returns created are comparable to terrain features of commensurate size. For example, ice ridges or ice mountains would create returns comparable to ground embankments or mountains, respectively. Also, offshore ice floes tend to disguise the true shape of a coastline so that the coastline may appear vastly different in winter as compared to summer. This phenomenon is termed arctic reversal, because the resultant display is often the opposite of the anticipated display.
Inherent Scope Errors
Another factor that must be considered in radarscope interpretation is the inherent distortion of the radar display. This distortion is present to a greater or lesser degree in every radar set, depending upon its design. Inherent scope errors may be attributed to three causes: width of beam, the length (time duration) of the transmitted pulse, and the diameter of the electron spot.
Beam-width error is not overly significant in radar navigation. Since the distortion is essentially symmetrical, it may be nullified by bisecting the return with the bearing cursor when a bearing is measured. Reducing the receiver gain control also lessens beam-width distortion.
Pulse-length error is caused by the fact that the radar transmission is not instantaneous but lasts for a brief period of time. There is a distortion in the range depiction on the far side of the reflector, and this pulse-length error is equal to the range equivalent of one-half of the pulse time. Since pulse-length error occurs on the far side of the return, it may be nullified by reading the range to, and plotting from, the near side of a reflecting target when taking radar ranges.
Spot-size error is caused by the fact that the electron beam that displays the returns on the scope has a definite physical diameter. No return that appears on the scope can be smaller than the diameter of the beam. Furthermore, a part of the glow produced when the electron beam strikes the phosphorescent coating of the display radiates laterally across the scope. As a result of these two factors, all returns displayed on the scope appear to be slightly larger in size than they actually are. Spot-size distortion may be reduced by using the lowest practicable receiver gain, video gain, and bias settings and by keeping the operating range at a minimum so that the area represented by each spot is kept at a minimum. Further, the operator should check the focus control for optimum setting.
For navigational purposes, these errors are often negligible. However, the radar navigator should realize that they do exist and that optimum radar accuracy demands that they be taken into account. They are usually most significant when the target is a thin, no-show (river), when it is very reflective but small, or when it is in close proximity to another show target. Thin no-shows are erased except for their wider points. With tiny, but very reflective targets, the crosssection of the return would normally be negligible on the display. Their extremely strong reflectance, coupled with the inherent errors, causes them to appear larger and of seemingly more significance on the indicator. When show targets are close to each other, these errors cause them to blend together, diminishing the scope resolution. Generally, the combined effects of the inherent errors cause reflecting targets to appear larger and nonreflecting targets to dwindle. [Figure 7-9]