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inconsequentially.
7.6.5. Cultural Returns. The overall size and shape of the radar return from any given city can usually
be determined with a fair degree of accuracy by referring to a current map of the area (Figure 7.7).
However, the brightness of one cultural area as compared to another may vary greatly and this variance
can hardly be forecasted by reference to the navigation chart. In general, due to the collection of dense
materials therein, urban and suburban areas generate strong returns, although the industrial and
commercial centers of the cities produce a much greater brightness than the outlying residential areas.
Many isolated or small groups of structures create radar returns. The size and brightness of the radar
returns these features produce are dependent on their construction. If these structures are not plotted on
the navigation charts, they are of no navigational value. However, some of them give very strong
returns, such as large concrete dams, steel bridges, etc.; and, if any are plotted on the chart and can be
properly identified, they can provide valuable navigational assistance.
7.6.6. Weather Returns. 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
AFPAM11-216 1 MARCH 2001 181
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, which 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 which 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:
Figure 7.8. Weather Returns.
7.6.6.1. Brightness varies considerably, but the average brightness is greater than a normal ground
return.
7.6.6.2. Returns generally present a hazy, fuzzy appearance around their edges.
7.6.6.3. Returns often produce shadow areas similar to mountain shadows because the radar beam does
not penetrate clouds completely.
7.6.6.4. 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.
7.6.6.5. Returns can appear in the altitude hole when altitude delay is not used and the distance to the
cloud is less than the altitude.
7.6.7. Effects of Snow and Ice:
7.6.7.1. The effects of snow and ice are similar to the effect of water.
182 AFPAM11-216 1 MARCH 2001
7.6.7.2. If a land area is covered to any great depth with snow: (1) some of the radar beam will reflect
from the snow and (2) some of the energy will be absorbed by the snow. The overall effect is to reduce
the return that would normally come from the snow-blanketed area.
7.6.7.3. Ice will react in a slightly different manner, depending upon its roughness. If an ice coating on a
body of water remains smooth, the return will appear approximately the same as a water return.
However, if the ice is formed in irregular patterns, the returns created will be 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 PPI display will often be the
opposite of the anticipated display.
7.7. Inherent Scope Errors. Another factor which 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.
7.7.1. Beam-Width Error. Beam-width error is not overly significant in radar navigation (although it
must be taken into account in radar bombing). 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
　
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