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11.12.2. Retarding the LOP 2 minutes of GS on a track of 70o would be the same as advancing it 2
minutes of GS on a track of 250o.
11.13. Motion of Observer Tables. A second method of conversion of LOPs to a common time is with
a Motion of the Observer table such as the one in Pub. No. 249. This table gives a correction to be
applied to the Ho or Hc so that the LOP plots in its converted position. The correction obtained from
Table 1 in all volumes of Pub. No. 249 is for 4 minutes of time. An additional table allows you to get the
correction for the number of minutes needed.
11.13.1. For example, suppose the LOP needs to be advanced for 11 minutes and the Ho of the body is
33o 29' and Zn is 080o. The track of the aircraft is 020o and the GS is 240 knots. In Table 1, Correction
for Motion of the Observer for 4 minutes of Time (Figure 11.10), the entering arguments is Rel Zn and
GS. Rel Zn is azimuth relative to course (Zn minus track or track minus Zn). Subtract the smaller angle
from the larger and enter the table with the answer. In this case, Zn – track = 080o – 020o = 060o (Rel
Zn) and GS is 240 knots. Entering this table with these arguments, the correction listed is +08' for 4
minutes of time.
11.13.2. Use the whiz wheel to calculate the total motion for 11 minutes. In this case, the 11-minute
correction totals 22'. By applying any other correction (refraction, sextant correction), a total adjustment
is derived. By changing the sign, this total may be applied to the Hc. To apply the correction to the Ho,
the sign of the adjustment would remain the same. Apply the adjustment to the intercept as the rules
state in Table 1. In each case, the resultant intercept would be the same.
11.13.3. Suppose the Hc was 33o57'. Applying the correction -22 yields 33o35'. Comparing this with our
Ho 33o29' results in an intercept of 6 NM away. If you decide to apply the correction to the Ho, 33o29' +
22' yields 33o5l'. Comparing this to the Hc 33o57' yields the same result, 6 NM away. When using the
Motion of the Observer table and when the fix time is earlier than the observation (LOP to be retarded),
the rule for the sign of the correction is also printed below Table 1.
11.14. Moving the Assumed Position. Another method of converting LOPs to a common time is to
move the assumed position. This method is recommended for shots 4 minutes apart computed to give all
three bodies a single assumed position. However, it is not limited to that type of computation. The
assumed position is moved along the best-known track at the best-known GS. For example, again
suppose the track is 330o and the GS 300 knots. LOPs are for 1500, 1504, and 1508 and a fix is desired
at 1508 (Figure 11.11). Since the first LOP would have to be advanced 40 NM (8 min at 300 knots), the
same result is realized by advancing the assumed position 40 NM parallel to the best-known track. The
1504 LOP must be advanced 20 NM; therefore, the assumed position is advanced 20 NM miles parallel
to the best-known track. The third shot requires no movement and it is plotted from the original assumed
position. It should be noted that the first shot is always plotted from the assumed position, which is
closest to destination. In this method, if observations are precomputed and the assumed position is
moved prior to shooting, the following procedure is used when shooting is off schedule. For every
minute of time that the shot is taken early, move the assumed position 15 minutes of longitude to the
east. For every minute of time that the shot is taken late, move the assumed position 15 minutes of
longitude to the west. In addition, the affected LOP must be moved along the best-known track for the
number of minutes of GS the observation was early or late. If the shot was early, advance the LOP; if the
shot was late, retard the LOP.
250 AFPAM11-216 1 MARCH 2001
Figure 11.10. Entering Arguments Are Relative True Azimuth and Groundspeed.
AFPAM11-216 1 MARCH 2001 251
Figure 11.11. Moving Assumed Positions.
11.15. Planning the Fix. In selecting bodies for observation, one should generally consider azimuth
primarily and such factors as brightness, altitude, etc., secondarily. If all observations were precisely
correct in every detail, the resulting LOPs would meet at a point. However, this is rarely the case. Three
observations generally result in LOPs forming a triangle. If this triangle is not more than 2 or 3 miles on
a side under good conditions and 5 to 10 miles under unfavorable conditions, there is normally no reason
to suppose that a mistake has been made. Even a point fix, however, is not necessarily accurate. An
uncorrected error in time, for instance, would require the entire fix to be moved eastward if observations
were early and westward if observations were late, at the rate of 1 minute of longitude for each 4
　
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