VII Skim the text with a partner to decide if the sentences are true or false.
1 Human navigators don`t use sextants to measure the elevation angle of celestial bodies above the visible horizon. 2The peak elevation angle occurs at local day or at night:elev angle (degrees) = 130 – latitude + declination. 3 Thus, at local noon or midnight, latitude can be calculated by simple arithmetic from a table of declination. 4 When time began to be broadcast to vehicles in the 1990s, off-meridian observations of theelevation angles of two or more celestial bodies became possible at any known time of night. 5 These fixes were hand-calculated using logarithms, then plotted on charts by a navigator. 6 In the 1930s, hand-held bubble-level sextants were built to measure the elevation of celestial bodies from an aircraft without the need to see the horizon. 7 The human navigator observed moon and stars through an astrodome on top of the aircraft. 8 The accuracy of celestial fixes was 6–60 miles in the air, limited by the uncertainty in the horizon and the inability to make precise angular measurements on a pitching, rolling vehicle. 9 Kayton reviews the history of celestial navigation at ground and in the air. 10 The first automatic star trackers, built in the late 1990s, measured the azimuth and elevation angles of stars relative to a gyroscopically stabilized platform. 11 Approximate position measurements by dead reckoning allow the telescope to point within a fraction of a degree of the desired star. 12 Thus, a narrow field-of-view is possible, permitting the telescope and photodetector to track stars at night through a window on top of the aircraft. 13 An on-board computer stores the right ascension and declination of 40–200 stars andcomputes the vehicle’s position. 14 Automatic star trackers, used in long-range military 18-10 aircraft and on Space Shuttles, are physically mounted on the stable element of a gimballed inertial navigator. 15 Clever design of the optics and of stellar-inertial signal-processing filters achieves accuracies better than 500 feet. 16 Future lower-cost systems may mount the star tracker directly to the vehicle.
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