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322 AFPAM11-216 1 MARCH 2001
15.10.2. In Figure 15.12, a PLOP has been plotted from the following information:
D1 at a fix at 1000 hrs
D2 at an air position at 1045 hrs
Zn = - 20 NM
Constant TH of 90o
Figure 15.11. Plotting the PLOP.
Figure 15.12. Solution of Bellamy Drift by Using PLOP.
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15.10.3. Next, construct an MPP on the PLOP. This is done by swinging the arc, with a radius equal to
the ground distance traveled, from the fix at the first D reading to intersect the PLOP. The ground
distance traveled can be found by multiplying the best known groundspeed (groundspeed by timing,
metro groundspeed, etc.) by the time interval between readings. The mean track is shown by the line
joining D1 and the MPP. The mean drift is the angle between true heading and the mean track (8oR).
Thus, the Bellamy drift is 8o right.
15.11. MB-4 Solution of Bellamy Drift. Compute Bellamy drift on the slide rule side of the DR
computer by placing the ZN over the ground distance and reading the Bellamy drift angle opposite 57.3
(Figures 15.13 and 15.14). This can be set up in a formula as follows:
EXAMPLE:
Given: ZN = +12.1
Time = 0:30
GS = 190 Knots
Find: Ground Distance = 95NM
Drift = 7o left
Figure 15.13. Computer Solution of Bellamy Drift.
324 AFPAM11-216 1 MARCH 2001
Figure 15.14. Mathematical Solution of Bellamy Drift.
Section 15B— Limitations of Pressure Differential Techniques
15.12. Limitations. Pressure navigation is limited by a few meteorological considerations. The basic
accuracy of the LOP in average conditions is about 5 to 10 miles. It will rapidly become worse under the
following conditions: tightly circulating pressure systems of highs and lows, flying through a front, or
carelessness in reading or computing the information. Bellamy drift has another limitation. To determine
drift you must stay on one heading long enough to take two readings about 20 minutes apart.
15.13. Summary. ZN is a displacement in NM perpendicular to the EAP. Compute ZN on the MB-4
using the equation:
Determine ETAS by using the EAD and time. Measure EAD along a straight line between the two
points in question. In the Northern and Southern Hemispheres, the sign of the ZN is the sign of the drift
correction. Use airplot in conjunction with a fix position to plot the PLOP, and plot it parallel to the
EAP. If the absolute altimeter fails, use pressure by temperature as a backup. With this method, use
temperature and pressure altitude to find equivalent D readings. If you change altitudes, restart pressure
at the new altitude, or correct the last D reading prior to the altitude change with a pastagram. Another
expression of the PLOP is Bellamy drift, used as a backup source of drift angle. Figure 15.15 shows a
fix determined by a PLOP and a celestial LOP.
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Figure 15.15. Fix Using PLOP and Celestial Line of Position.
326 AFPAM11-216 1 MARCH 2001
Chapter 16
NAVIGATION SYSTEMS
Section 16A— Introduction
16.1. Basics. Navigation systems are computer systems that determine position and calculate navigation
information. They free the operator from the intense mathematical calculations characteristic of DR
navigation to allow additional time for military related activities. Simple navigation systems rely on an
initial position and basic instrument data, like doppler groundspeed and drift, magnetic heading, and
variation, to compute and constantly update the DR position. These systems require the operator to
spend time to maintain system accuracy. Complex systems integrate information from a variety of
sources using complex statistical algorithms to produce constantly updated, highly accurate position and
navigation information. Some components of a navigation system may be stand-alone systems. This
chapter examines navigation systems in general and the most common systems in detail—Inertial
Navigation Systems (INS) and Global Positioning Systems (GPS).
Section 16B— Navigation Systems
16.2. Basics. In the same way an autopilot frees a pilot from the manual operations of flying, a
navigation system relieves you of many manual operations required to direct the aircraft. When sensors
are tied into a navigation system, the system automatically uses their data to compute present position
for the navigator. This reduces the workload and frees you to make the decisions that are beyond the
capability of computers and more effectively employ the weapon system. During a flight from the
United States to a foreign country, the aircraft may pass over areas of land, water, and icecaps. You may
have to deal with conditions of overcast, undercast, day, night, altitude changes, turn points, and air
　
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