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plotted.
12.7.4. Determine the intercepts and azimuth for each LOP. Plot these data from the respective assumed
positions.
12.7.5. Resolve the LOPs to a common time, preferably that of the transit LOP.
NOTE: At 30o N latitude, the linear speed of the sun is approximately 780 knots. Thus, on westerly
headings in high-speed aircraft, the DR distance involved before encountering a 30o change in azimuth
will be considerable.
12.8. Subpoint Method. When the observer is within approximately 4o of the subpoint of the body, the
subpoint method of solution is normally used. This is because the radius of the circle of equal altitude is
so small that a straight line does not approximate the arc and a straight line will not give an accurate
LOP. The procedure is:
12.8.1. Plot the subpoints of the body for the time of the observations (using GHA and/or Dec).
12.8.2. Find the co-altitude of the shots and convert it to NM ([90o – Alt] x 60 NM).
12.8.3. Advance the first subpoint and retard the third along the DR track, using best-known track and
GS.
12.8.4. Set the distance found from the co-altitude and strike it off from the resolved subpoints (with a
compass or pair of dividers). Do this for each observation.
NOTE: The resulting intersection or triangle will give an on-time fix. If the LOPs form a triangle, the
aircraft position is probably within the triangle.
264 AFPAM11-216 1 MARCH 2001
12.8.5. The subpoint method is convenient because Pub. No. 249 isn't used— only the Air Almanac. This
method can also be used with a star near your assumed position and may be necessary if, for some
reason, your Volume 1 is unavailable. The stars Dec and GHA are needed to determine if the observer is
within 4o of the subpoint. The Air Almanac may be used to find the Dec and sidereal hour angle (SHA)
of the star. The SHA of the star is added to the GHA of Aries to find the GHA of the star.
12.9. Eliminating Motions with the Bracket Technique. For sun observations, you can eliminate
motion calculations by using a shooting schedule of 3 minutes early, on fix time and 3-minutes late.
With this schedule, the 3-minute early and 3-minute late shots have the same magnitude of motion, but
an opposite sign. Therefore, these motions cancel each other out and do not need to be computed. The
on-time shot has no motions. Therefore, the three intercepts can be averaged for a single LOP. At night,
shooting the same star 4 minutes early and late, with a different star shot on time can employ a similar
method. In this case, the intercepts for the same star's 4-minute early or late shots can be averaged. This
reduces workload, but only two LOPs are obtained.
12.10. DR Computer Modification. Rather than eliminating motions, your DR computer can be
modified so both observer and body motions can be computed at one time, without entry into the Pub.
No. 249. Make a GS and latitude scale as shown in Figure 12.8. After constructing these, the DR
computer can be modified for quick and accurate computations of 1-minute motion adjustments.
12.10.1. Tape the GS scale (0 through 900) along the centerline of the grid scale. Match zero to zero,
300 to 50 and 600 to 100 as shown in Figure 12.8. Then, tape the latitude scale along the zero grid line
so that 90o falls on the centerline and the scale extends to the left as shown. Check the accuracy of your
placement: 30o latitude should fall 13 divisions left of centerline. Juggle the scale as necessary to
provide the greatest accuracy between 30o and 45o.
Figure 12.8. MB-4 Motions Modification.
12.10.2. To use the modified MB-4 computer for motion adjustments:
12.10.2.1. Set true north under the index. If computing for grid, set polar angle (PA) under the index. In
the NW and SE hemisphere quadrants PA equals convergence angle (CA). In the NE and SW quadrants
AFPAM11-216 1 MARCH 2001 265
PA=360-CA. Next place the grommet over the zero grid line. Mark a cross (+) at the assumed latitude
(Figure 12.9).
Figure 12.9. Celestial Motions – Step One.
12.10.2.2. Set track (or grid track) under the index and position the slide so the GS is under the
grommet. Place a dot on the zero point of the grid scale (Figure 12.10).
12.10.2.3. Place the Zn (or grid Zn) of the body under the index. Position the slide so the cross or the
dot, whichever is uppermost, is on the zero line of the grid (Figure 12.11).
266 AFPAM11-216 1 MARCH 2001
Figure 12.10. Celestial Motions – Step Two.
AFPAM11-216 1 MARCH 2001 267
Figure 12.11. Celestial Motions – Step Three.
NOTE: The vertical distance between the zero line and the low mark is the combined 1-minute motion.
Each line of the grid equals 1 minute of arc (1 mile). If the cross is on the zero line, the motion is
　
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