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

DRe to determine SRU error (Y = Fixe + DRe).
E. Total Probable Error (E) is calculated using one of the
following formulas:
1. E = X2+Y2 for the initial 4 hours of a mission when drift can
be disregarded, and in most inland missions.
2. E = De2+X2+Y2 when drift forces are relevant.
F. Recalculation of E is necessary when one of the following
happens:
1. The drift changes. A target in water continually drifts. The
SMC periodically recomputes datum, leading to a change in De.
To illustrate, suppose a surface position is the initially
reported position. Drift and drift error are zero. When
datum is computed 4 hours later, d1 = 16 miles, and de1 = 4.8
miles, making De = de1 = 4.8 miles. Six hours later datum 2
is computed with d2 = 24 miles and de2 = 7.2 miles, making De
= de1 + de2 = 4.8 + 7.2 = 12 miles. The process is continued
throughout the mission.
2. The SRUs change. The SMC recomputes E each time the SRU is
changed if the Fixe(Y) changes.
3. The initial position changes. Once errors in the initial
position are computed, they usually are not changed unless
later information shows initial assumptions to be in error.
522 Search Radius
The search radius (R) is the radius of a circle centered on a
datum, having a length equal to E plus an additional safety length to
help ensure that the target is in the search area. For ground and
underwater searches, R is measured in yards. On other searches, R is
measured in nautical miles. R is usually increased after successive
searches to increase the chance of the target being in the search area.
Table 5-4 lists Safety Factors (fs) used sequentially to gradually
enlarge the search area.
TABLE 5-4. Search Radius Safety Factors
Search fs
1st 1.1
2nd 1.6
3rd 2.0
4th 2.3
5th 2.5
Subsequent Searches 2.5
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