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. 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. Figure 5-4 shows how this is done. The desired time of the fix is 1100Z.

Figure 5-4. Adjusting lines of position for fix.

Figure 5-4. Adjusting lines of position for fix. [click image to enlarge]


Determine the time to advance the 1055Z LOP (5 minutes). Multiply this time by the aircraft GS (300 knots).

  • Measure the distance computed in the first step in the direction of the track of the aircraft (045°).
  • 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.
  • 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. 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.

Figure 5-5. Bisector method. [click image to enlarge]

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 45°, 90°, and 135° 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.

Figure 5-6. The running fix. [click image to enlarge]

Accuracy of a Fix

The accuracy of a fix can sometimes be improved by the use of a little foresight. If the track of the aircraft is known more accurately than the GS, the course line should be adjusted since any error in the GS has little effect on it. If, however, you desire to adjust a speed line under these conditions, the accuracy of the fix is in doubt. Similarly, if the GS is known more accurately than the track, the speed line should be adjusted to the time of the course line. The line that is affected least by the information in doubt is the line that should be adjusted.