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

E. When only a datum area exists, search areas are developed
differently:
1. If the area is small, a search radius is computed as before
and added to the radius of the datum area.
2. If the datum area is a reasonable size for searching and
probably contains the target, it can be used as the first
search area. An example is a military aircraft disappearing
in a prescribed operating area. When the datum area is based
on reports of low flying or crashed aircraft, planners should
avoid the tendency to excessively limit the size of the search
area around the reported site. A radius of 15 miles from the
observation
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