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the position of the vessel. All celestial navigation follows this rule. Today's navigator measures the
altitude of the celestial bodies in much the same manner as Magellan or Columbus.
13.1.1. However, there is a difference between air and marine celestial navigation. Because marine
navigators are on the surface of the ocean, they can establish their horizon by referring to the natural
horizon. In an aircraft, this is impossible because altitude and aircraft attitude induce error. In the sextant
designed for air navigation, a bubble, like the one in a carpenter's level, determines an artificial horizon,
which is parallel to the celestial horizon. The bubble chamber is placed in the sextant so the bubble is
superimposed upon the field of view. Both the celestial body and the bubble are viewed simultaneously,
making it possible to keep the sextant level while sighting the body.
13.1.2. Sextants are subject to certain errors that must be compensated for when determining an LOP.
Some of these errors are instrument errors while others are induced by the various in-flight conditions.
The first half of this chapter discusses the sextant and the second half explains sextant errors.
13.2. The Bubble Sextant. The aircraft bubble sextant measures altitude above a horizontal plane
established by a bubble. Aviators use several types of bubble sextants, all of which are indirect sighting.
This means the navigator does not look directly toward the celestial body, but always looks in a
horizontal direction as shown in Figure 13.1. The image of the body is reflected into the field of view
when the field prism is set at the correct angle. In the bubble sextant, the bubble and body are visible in
the same field of view. The sextant system consists of four parts: the mount, the sextant, the electrical
cables and the carrying case.
13.3. The Mount. The mount, as shown in Figure 13.2, is fastened permanently to the top of the
fuselage of the aircraft. A shutter door is built into the mount to close the opening for the tube of the
periscopic sextant. This shutter door is controlled by the sextant port lever (1) on the mount. The mount
has a gimbal mechanism, which allows the sextant to be tilted from the vertical in any direction. This
permits a celestial body to be observed throughout the normal oscillations of an aircraft. A drain plug (2)
is provided at the low point in the shutter well for draining out water, which may have collected in the
mount.
NOTE: The numbers in parentheses in 13.3, 13.3.1, and 13.3.2 refer to the parts indicated in Figure
13.2.
AFPAM11-216 1 MARCH 2001 279
Figure 13.1. Body Is Not Sighted Directly.
13.3.1. The sextant is held in the mount by two locking pins (4), located in a movable collar on the
bottom of the mount. One pin locks the sextant into the mount and holds it in the retracted position; the
other pin locks the sextant in the extended position. These pins are spring-loaded and must be pulled out
to release the sextant. Located next to these locking pins is a friction clamping lever (3), which provides
the observer with the option of locking the sextant at a fixed azimuth or, when the tension is released,
the sextant may be rotated through 360o of azimuth. The azimuth scale (10) and azimuth counter (6) will
move when the azimuth crank (5) is rotated. The azimuth scale can be read against a lubber line or index
(11). The azimuth scale read against the lubber line and the azimuth counter reading should be the same.
13.3.2. Power is supplied from the aircraft through a cable connection (9) on the side of the mount. A
switch (8) on the side of the mount controls power to both the mount and the sextant. The mount has one
lamp that illuminates the azimuth counter window. Another cable (7) is connected to the socket on the
underside of the mount and supplies power to the sextant itself.
13.4. The Periscopic Sextant. The periscopic sextant is an optical instrument which enables the
navigator to determine true azimuth (Zn), relative bearing (RB), and altitude angle of a celestial body
and aircraft true heading (TH). The sextant provides an angle of observation from below the horizon to
directly overhead, as compared to an artificial horizon.
280 AFPAM11-216 1 MARCH 2001
Figure 13.2. Periscopic Sextant Mount.
13.4.1. Proper collimation techniques and the correct size bubble are essential ingredients of accurate
celestial observations. Collimation is effected when the body is placed in the center of the bubble. For
greatest accuracy, the bubble should be in the center of the field, with the body in the center of the
bubble. The error will be small if the bubble is anywhere on the vertical line of the field, as long as it
does not touch the top or bottom of the bubble chamber. Figure 13.3 shows examples from better to
　
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