JP 3-50 National Search and Rescue Manual Vol I - US Navy

easily sighted down-sun, whereas a craft's silhouette is
likely to be sighted first if viewed up-sun. Thus, color and
brightness contrast are most influential when the target is
down-sun.
B. Target motion influences detection range by contributing to the
"something different" in a scanner's visual field and by disturbing the
water. As a vessel moves faster, wake is larger and the detection range
from the air increases due to the effective increase in target size.
Any movement by an object in light seas is likely to attract attention.
Conversely, a stationary target can sometimes be detected among
whitecaps because target position remains the same while the whitecaps
"blink" on and off.
C. The relative amount of time that a target is exposed to the
observer affects target detectability. Target shape, particularly
freeboard, can influence duration of exposure because waves and swells
may hide the target intermittently. Small search objects are especially
difficult to detect in high seas and swell because of this effect.
D. Target characteristics influencing detectability and,
therefore, visual sweep width are summarized in Figure 6-11.
E. Search altitude affects a number of aspects of the visual
detection process. As altitude decreases, the search target passes
through the field of vision more rapidly than at higher altitudes,
low-freeboard targets are more difficult to sight, and surface
irregularities become more pronounced. Also, at lower altitudes pilots
tend to concentrate more on their instruments and flying than they would
if they had more of a safety margin. These effects are most pronounced
at altitudes below 500 feet. For a surface search craft, the freeboard
and the length of the target are more important than its beam. However,
as altitude increases, the beam and length of the search target become
more important because they determine the size of the target as viewed
from overhead. See Figure 6-12.
Figure 6-11. Target Detectability Factor
6-15