For hundreds of years, mariners have navigated the seas keeping track of their positions by use of the sextant. This instrument measured the altitude of celestial bodies (their angular distance above the horizon), and the information derived from this measurement was used to determine 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. 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.
Sextants are subject to certain errors that must be compensated for when determining a line of position (LOP). Some of these errors are instrument errors while others are induced by the various inflight conditions. The first half of this category discusses the sextant and the second half explains sextant errors.
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.
The Periscopic Sextant
The periscopic sextant is an optical instrument that enables the navigator to determine true azimuth (Zn), relative bearing (RB), 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. [Figure 13-2]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 is 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 of collimation from better to worse.
Bubble size affects the accuracy of a sextant observation. The ideal situation for collimation is to have a small bubble for ease in determining the center. A bubble that is too small sticks to the lens, decreasing accuracy. A bubble that is too large moves like a creature from a science fiction movie, making it difficult to find the center. Experience shows that best results are obtained with a bubble approximately one and a half times the apparent diameter of the sun or moon, or about the size of a small washer ring. The field prism is geared to an altitude scale so that when the body is collimated, the altitude can be read from the scale.
An averaging mechanism is also incorporated that allows the navigator to take an observation over a period of time. The continuous motion of the aircraft affects the bubble and resultant artificial horizon. This movement resolves itself into a cycle in which the aircraft rolls, yaws, and pitches. To obtain an accurate reading, it is necessary to sight the body for a period of time during this cyclic movement and to average the results of a series of sightings. An averaging device has been incorporated in the sextant allowing an average reading to be obtained.
The sextant is actually a low-power periscope with a 15° field of view. [Figure 13-4] All lens surfaces in the sextant are coated to minimize light loss. To prevent condensation when the tip of the sextant is extended into cold air, the tube is filled with a dry gas and sealed. A desiccant, composed of silica gel, is used to remove moisture and check on the dryness of the gas inside the tube and is visible in the periscopic end of the sextant, or in some models, on the sextant body. When the silica gel is pink, there is moisture in the tube and the sextant should be replaced before flight.
Note: The numbers in parentheses refer to the parts indicated in Figure 13-4.