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

C. For open-ocean incidents, mathematical methods exist to
determine R because drift determination is more quantifiable. The
method described in the following paragraph is used primarily for air
and water craft, but could also be used for inland or coastal region
planning. See Vol. II, Chapter 3 for further search area calculation
information.
521 Total Probable Error
A. Total Probable Error (E) is a mathematical tool for determining
search area based on the probable errors in estimation of drift (De),
initial position of the incident (X), and navigational capability of the
SRU (Y).
B. Total Drift Error (De) accounts for errors in estimating drift
and is used when determining E. It is the arithmetic sum of the
individual drift errors from the time of the incident until datum.
1. Individual Drift Error (de) should be computed for each datum
and is assumed to be three-tenths of total drift. Errors less
than one mile are disregarded. When the first datum is
computed, De equals de on the first search plan. However, as
the mission progresses, De becomes de1, plus de2, etc.
2. Drift Errors for datum minimax calculations are determined by
graphical or algebraic solutions.
a. In the graphical solution, de is found for both dmin
and dmax by multiplying their distances by the drift error
confidence factor (CF) of 0.3, as shown in Figure 5-9. Using
the dmin position as a center, and the minimum de as a radius,
a circle is drawn. Using the dmax position as a center, and
the maximum de as a radius, another circle is drawn. A third
circle is drawn tangent to the other two circles with the
center located on a straight line connecting the dmin position
and dmax position. This circle's radius is the minimax drift
error (de minimax), used as De on the first search plan. See
Figure 5-9. Three circles need not actually be drawn; once
the dmin position and dmax position have been determined, the
problem can be laid off on an extended line joining the two
positions.
b. The algebraic method obtains de minimax by first
plotting minimum and maximum datums. The distance between the
two is added to de min and de max, and divided in half to
derive de minimax.
de minimax = (Distance + de min + de max)/2