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sight (LOS) to a known fixed object. The direction of the LOS is the bearing of the object from the
aircraft. A line plotted in the direction of the bearing is an LOP. At the time of the observation, the
aircraft was on the LOP.
5.3.4. Relative Bearings (RB). An RB is the angle between the fore-and-aft axis of the aircraft and the
LOS to the object, always measured clockwise from 000o at the nose of the aircraft through 360o. In
Figure 5.3, the RB of the object is shown as 070o. You must convert this to a true bearing (TB) before
you can plot it. To do this, you simply add the RB to the true heading (TH) the aircraft was flying when
you obtained the bearing. (Subtract 360o if the total exceeds this amount.) Thus:
RB + TH = TB (RuB THe TuB)
Where:
RB is the relative bearing,
TH is the true heading, and
TB is the true bearing.
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Assuming the aircraft was on a TH of 210o when the bearing was taken, the corresponding TB of the
object is 280o. (070o RB + 210o TH = 280o TB)
Figure 5.3. True Bearing Equals Relative Bearing Plus True Heading.
5.4. Plotting the LOP. As previously stated, two intersecting LOPs determine the position of the
aircraft. The only other possible point from which to begin plotting the LOP is the object on which you
took the bearing. The procedure is to use the reciprocal of the TB of the object, thus drawing an LOP
toward the aircraft. In actual practice, it is not necessary to compute the reciprocal of the bearing; the TB
is measured with the plotter, and the LOP is drawn toward the opposite end of the plotter. To establish
an LOP by RB, the navigator must know (1) the position of the source (object) of the bearing, (2) the TH
of the aircraft, (3) the RB of the object, and (4) the exact time at which the TH and RBs were taken.
Section 5B— Fixes
5.5. Adjusting LOPs for a Fix. Sometimes it is impossible for a navigator to obtain more than one LOP
at a given time. If two LOPs are for two different times, their intersection does not constitute a fix
because the aircraft moved between the time it was on the first LOP and the second LOP. The
illustration in Figure 5.4 shows a bearing taken at 1055Z and another at 1100Z. At 1055Z when the
navigator took the first bearing, the aircraft was somewhere along the 1055Z LOP (single-barbed LOP)
and, at 1100Z, it was somewhere along the 1100Z LOP. The intersection of these two lines, as plotted,
does not constitute a fix. For an intersection to become a fix, the navigator must either obtain the LOPs
at the same time or adjust them to a common time by using the motion of the aircraft between the
observations. The usual method of adjusting an LOP for the motion of the aircraft is to advance one line
to the time of the other. The illustration in Figure 5.4 shows how this is done. The desired time of the fix
is 1100Z.
5.5.1. Determine the time to advance the 1055Z LOP (5 minutes). Multiply this time by the aircraft GS
(300 knots).
5.5.2. Measure the distance computed in the first step in the direction of the track of the aircraft (045o).
154 AFPAM11-216 1 MARCH 2001
Figure 5.4. Adjusting Lines of Position for Fix.
5.5.3. Draw a line through this point parallel to the 1055Z LOP (double-barbed LOP). This represents
the advanced LOP. The intersection of the advanced LOP and the 1100Z LOP is the fix. The advanced
LOP is usually plotted on the chart with two arrowheads, while the unadvanced LOP is marked with a
single arrowhead.
5.5.4. When three LOPs are involved, the procedure is exactly the same as for two. The resolution of
three LOPs, however, may result in a triangle instead of a point, and the triangle may be large enough to
vary the position of the fix. The technique many navigators use is to place the fix at the center of the
triangle. The illustration in Figure 5.5 shows a technique for finding the center of the triangle by
bisecting the angles of the triangle. The point of intersection of the bisectors is equal distance from all
three LOPs and is the fix position.
Figure 5.5. Bisector Method.
AFPAM11-216 1 MARCH 2001 155
5.6. The Running Fix. It is possible to establish an aircraft position by a series of bearings on the same
object. For best accuracy, these RBs are taken when the object is approximately 45o, 90o, and 135o from
the aircraft. The navigator then advances or retards the LOPs to a common time. The result is a running
fix. The accuracy is based on the aircrafts distance from object and the amount of time it takes to go
from the first bearing to the last bearing since you must move two of the LOPs for the aircrafts track and
GS. The running fix is illustrated in Figure 5.6.
Figure 5.6. The Running Fix.
5.7. Accuracy of a Fix. The accuracy of a fix can sometimes be improved by the use of a little foresight.
　
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